US2327956A - Magnetic recording and reproducing - Google Patents

Description

MAGNETIC RECORDING AND REPRODUGING Filed Deo. 3,6, 1.940 6 Slzleezs-Shee'h 2 ATTORNEY MAGNETIC RECORDING AND BEPRODUCING Filed. Decu 16, 1940 6 Sheems-Sheet AIV/2l A/V/l 'IH 69 .IH @a fe e T wg @eff/mf S MbJbW/Vl@ ATTORNEY MAGNETIC RECORDING AND REPBODUCING Filed Dec. l5, .1940 6 Sheets-Sheet llllllllllllllf INVENTOR owp MEQ,

ATTORNEY Aug., 249 M4@ s, J. SEGUN MAGNETIC RECORDING AND REPRODUCING Filed Decu 16, 194,0 6 SheetsSheei; 5

@agua @12M m VH www fm am 5% m ug., 24, 1943 s, J, BEGUN 2,327,956

MAGNETIC RECORDING AND REPRODUGING Filed. Dec.. 16 1940 6 Sheets-Smet 6 Sm BY MMR-1M ATTORNEY Patented Aug. 24, 1943 UNITED STATES PATENT OFFICE MAGNETIC RECORDING AND REPRODUCIN G Semi Joseph Begun, Cleveland, Ohio y Application December 16, 1940, Serial No. 370,417

17 Claims. (Cl. 179-1003) This invention relates to magnetic recording and reproducing, and it has among its objects .improved novel magnetic recording and reproducing methods and arrangements which are particularly useful for applications, such as time delaysystems, although the various features of the invention are not limited thereto.

'lh'e foregoing and other objects of the invention will be best understood from the following description of exempliilcations thereof, reference being had to the accompanying drawings wherein Fig. l. is a diagrammatic view of the circuits and the operating elements of an endless magnetic recording arrangement designed to supply artificial reverberation in accordance with the principles of the invention;

Figs. to 4 are diagrammatic views similar to Fig. l illustrating other exemplications of the invention;

Fig. 5 is an elevational View illustrating the structural features of an endless tape magnetic recording device designed in accordance with the principles of the invention;

Fig. 6 is a top view of Fig. 5 with' parts shown in section;

Fig. 7 is an elevational view of the switching unit associated with the recording device of Fig. 5;

Fig 8 is a side view of the switching unit shown in Fig. '7;

Fig. 9 is an elevational view of another form of endless magnetic recording device exemplifying another form of the invention;

Fig. lo is a side view of the devices shown in Fig. 9;

Fig. ll is a cross-sectional view along line l I i i of Fig. 10;

Fig. 12 is a cross-sectional View along line |2-l2 of Fig. 10;

Fig. i3 is a perspective view illustrating the method of winding the endless magnetic tape on the device of Figs. 9 and 10; and

Fig. 14 is a perspective view illustrating th'e driving arrangement of the device shown in Figs. 9 and l0.

Time delay systems have been long known in the communication field. Thus, for instance, Mills Patent 1,647,242 describes how a time delay network consisting of inductances and condensers may be utilized for supplying artificial reverberation in the reproduction of sound for broadcasting, sound recording and the like. It has also been long recognized that if a. longer time delay, for instance, of the order of 1,/4 of a second or more is required, time delay systems, formed of electric network elements only, are very expensive and bulky particularly if used in the lower frequency range, such as at audiofrequencies, requiring magnetic core inductances.

It has also been suggested that under conditions requiring such longer time delays, sound recording sh'ould be used instead of electric network elements as the time delay medium. Thus, for instance, Arnold Patent 1,859,423 proposes the production of reverberation effects by recording the sound and reproducing the record with a plurality of pickups located at closely succeeding intervals along the sound record track and contrellably attenuating and combining the outputs oi the pickups so as to reproduce the recorded sound with delayed echo and reverberation effects.

Although disc recording and other long known sound recording media, such as magnetic recording, which enable instantaneous playback of recorded sound, make it possible to add articial reverberation eiects to sound sequences, prior efforts to use sound recording for applications, such as artificial reverberation systems, never came into use because the discontinuity between the end and beginning of a sound track formed on such recording medium make it impossible to use such time delay systems for supplying the reverberation effects during prolonged continuous periods.

Although an endless magnetic recording tape might have seemed to be useful as a continuously operating sound recording medium suitable for artificial reverberation systems, the discontinuity at the junction or the soldering point which has to be provided in order to join the ends of a length of magnetic tape into an endless tape would, on passing through the large number of closely spaced playbacks required to supply the prolonged artificial reverberation time delay effects, induce in each playback head disturbing high electric potentials, thereby introducing into the playback reverberation channel a series of disturbing clicks. -As a result, such endless magto devise an articial reverberation system utilizing recording on an endless phosphorescent record surface as the time delay medium. However, although phosphorescent sound recording media have the advantage that they may be readily designed to form a continuous endless sound track without any discontinuity, the practical adaptation of such phosphorescent surface record media for artificial reverberation encountered great difilculties and have never reached practical acceptance.

One phase of the invention is based on the discovery that an endless magnetic tape with a soldering Junction discontinuity may be used for supplying artificial reverberation during prolonged continuous periods, and that such endless tape magnetic recording medium may be designed so as to suppress the disturbing effect of the soldering junction without in any way diminishing the effectiveness of the endless tape recording medium in producing artificial reverberation eifects.

If sound proceeds from a source located in a room and strikes a wall surface, it is partly reflected therefrom and travels until it reaches another surface where it is again partly reflected.

the recording head may be used, or the output of the amplifier which drives the recording head is combined with the blended output of the series of playback heads which reproduces the recorded sound with an exponential attenuation eiiect.

In the simplest form of an endless magnetic tape recording machine designed to supply reverberation in accordance with the invention, an endless magnetic recording tape is continuously moved during each cycle of its total recording length through an obliterating head, a recording head, and then in sequence through a series of closely spaced playback heads so that each successive playback head reproduces the recorded signal with a greater time delay, and the output of each playback head is exponentially attenuated with relation to the preceding playback head, so that the combined output of all the At each sound impact, some of the energy of the sound is absorbed by the wall surface and the reflected sound is weaker than the original. Such action results in a succession of tones of diminishing intensity which give the listener the impression that the original sound dies out slowly, and is known as reverberation. The reverberation time of a room is usually defined as the time necessary for the sound of a source, that is suddenly stopped, to decay 60 decibels.

In many applications, it is desired to change the reverberation time of a room. Orchestra music, for instance, has more appeal if it is richer in reverberation, and the voice of a singer is more pleasing if it is heard with less reverberation.

The absorption of the sound in a room will depend on the vapor content of the air, particularly for high frequencies, and on the absorption characteristics of the walls, and the fumiture in the room. Accordingly, the sound decay will vary for different frequencies. In general, the rate at which the sounds of the diil'erent frequencies decay has an exponential characteristic, and the higher frequencies have a faster decay time than the lower frequencies.

A satisfactory artificial reverberation device should, therefore, operate in relation to a constant frequency impulse as follows: At the termination of the sound impulse, the device should bring about a gradual decay of the impulse from full amplitude to inaudibility.

The magnetic recording device of the invention is designed to function as a time-delay medium operating in accordance with the following principles: A recording head and a plurality of magnetic playback heads are placed successively along portions of an endless magnetic tape, so that the sucessive playback heads reproduce each recorded sound impulse with a greater time delay, and the output of the successive heads is exponentially attenuated so that the combined output of all the playback heads shall blend and, on being converted into sound. create the illusion of an exponentially decaying true reverberation effect.

For reproducing sound with reverberation, the output of the amplifier which output of an additional playback head placed immediately behind sible the exponential decay of natural reverbera-A tion However, assuming that such conditions would be possible, and that the outputs of the playback heads are not attenuated, their mixed output will always contain two output components which are reproducing voltages oi' the same amplitude, out of phase, thereby cancelling the signal. This reasoning applies for any frequency. An arrangement of this character, even if the output of the successive playback heads is subjected to successively increased attenuation, would actually operate as a frequency discriminating system which attenuates high frequencies more than low frequencies. Accordingly, such system would be undesirable.

With respect to the spacing between the playback heads, the yfollowing considerations are of great importance: If a record of a short sudden sound impulse is played back with time delay intervals between successive playback heads, a listener will distinguish individual repetitions of the short signal rather than the blending of the successive playback outputs if the time delay interval between two adjacent heads is too large. I have found that in order to blend the time dc= layed outputs of the successive pickup heads in a manner that would create the illusion of natural reverberation, it is important that at least the first few playback heads should be spaced as closely as possible following the recording head, and that the distance between them, measured in the time delay intervals, should be of the order between about 1/zo to 1/m of a second or even closer, if possible.

I have found that by taking advantage of the fact that even a small studio has a certain small reverberation time, a universally adaptable endless magnetic recording tape reverberation device may be provided if the playback heads are placed sufficiently close to play back the recorded signal with successive time delays of the order of between about 1,oth and 1,40th of a second between the first group of successive playback heads, which contribute the major part of the reverberation output with an attenuation up to about l2 decibels relatively to the original signal level, although relatively good results are obtween the playback heads which deliver the recorded signal output with an attenuation up to about 6 to 8 decibels.

Satisfactory operating conditions have been obtained by spacing the rst ten playback heads so that each picks up the recorded signal with a time delay of approximately /Qth of a second relatively to the preceding magnetic head, and. that the additional playback heads picks up the recorded sound with successive time delays ci -sth of a second, and that the last playback head picks up the signal about one second after it was recorded.

By increasing the attenuation of the output of the nrst ten playback heads in 11/2 decibel steps, that is, attenuating the output of the rst head with relation to the original signal lll/ decibels, and the second head 3 decibels, and each successive head upto the tenth head .l1/2 decibels per step, the output level of the tenth head is l5 decibels lower than the original signal level. The output of the next iour or ve playback heads may be attenuated 3 decibels per step so that the last head is about 30 decibels attenuated below the original signal level.

The outputs of the diierent heads are then combined in a mixer amplifier and the output in this amplifier again is combined in proper relationship either with the original signal or with the signal supplied by a playback head placed in ahead ci the `hrst reverberation playback head.

In designing such a system, the following precautions must be observed: The signals contributed from each cf the pickup heads must be mixed so that one does not interfere with the signal obtained from another playback head. 'io this end, the output of each playback head may be impressed on two series connected resistance elements, one being a fixed decoupling resistor, the other being a variable tap resistor, so as to permit proper adjusting of the'attenuation while making it impossible for a signal picked up by one playback head from being impressed on another head and cause it to record the impressed signal.

ln the arrangement shown, satisfactory operating conditions were obtained by connecting the first nve pairs ci' magnetic heads in series across a potentiometer element formed of a xed dew coupling resistance and an adjustable tap resistor. The variable tap resistance section of each potentiometer unit is designed so that in the successive stages it gives at its terminals a smaller output voltage in the ratio described above, the overall resistance of each potentiometer remaining the same. l

Since in an endless magnetic tape time delay recording device required for producing rever-y beration eiects, the soldering junction must pass through a large number of magnetic heads, it causes not only electrical disturbances, but the mechanical impact of the soldering junction on the thin pole pieces of the magnetic heads imposes thereon also serious mechanical strains which aect their magnetic properties.

I have discovered that the disturbing effect of the soldering point may be completely eliminated without in any way diminishing the illusion of the artificial reverberation produced by an endless magnetic tape, if the output of the successive playback heads is cut ofi during the period while the soldering point passes through them. particularly if the time required for the soldering point to pass through the succession of playback heads is less than about 50 to 100 times the total recording period of the endless tape.

I have found that for best performance, the endless magnetic tape used' in such recording arrangement should have a recording length ot' at least of the order of 1/2 of a minute, but preferably at least of the order of one minute.

l have also found that itis not necessary to cut off the output of each playback head individually at the moment when the soldering joint passes through it, and that very good results are obtained ii the output of all adjacently placed playback heads used for producing the reverberation effect are cutoi momentarily for a short instant when the soldering joint reaches the pole piece of the iii-st playback head and provision is made to gradually bring up the level of the output as the soldering joint moves from the heads which operate with the smallest attenuation and are closest to the recording head toward the farther removed playback heads which operate with the maximum attenuation, since the much greater level of the first group of playback heads is not perceptibly affected by their output while thepiier is cutoff completely only While the solder ing joint is moving through the rst few playback heads which contribute the major portion ofi/the output level, and after the soldering joint has passed through them and moves through the playback heads which contribute only the minor fraction of the output level, the gain of the am pliiier is gradually restored, restoring the effectiveness of the nrst few magnetic heads which contribute the major part of the output level, and the restored gain is not suilicient to permit the soldering joint to cause any disturbing effects while it is passing through the last few heads, their amplined disturbing signal being kept sufficiently down below the level of the signal contributed by the first playback heads.

According to another phase of the invention, the eect of cutting out the output of the playback heads on the overall reverberation eiect is further reduced by distributing the magneticv playback heads required for producing the desired reverberation effect on a plurality of different sections of the endless magnetic tape, so as to provide each tape section with its subgroup of playback heads and its own recording head. which are cooperating to supply complementary parts of the overall reverberation effect.

Thus, if an endless magnetic tape, having 20 magnetic playback heads reproducing the recorded signal with time delays of 1/20th of a second, is to be used for supplying articiai reverberation, the magnetic heads are distributed not on one continuous section of the endless tape, but, for instance, on four sections as follows: On the first section, a recording head with playback heads reproducing the signal with l, 5, 9, 13 and 17 times greater delay than the minimum time delay of l/20th of a second; on the second section, a recording head with playback heads reproducing the signal with 2, 6, l0, 14 and 18 times greater delay; on the third section, a recording head with playback heads reproducing the signal with 3, 7, il, l5 and 19 times greater delay; and on the fourth section, a recording head with playback heads reproducing the signal with a, 8, l2, 16 and 20 times greater delay.

have found that an endless magnetic tape reverberation equipment Will supply an illusion of a 60 decibel reverberation time, although only enough playback heads are used for playing back the recorded signal with a gradual attenuation down to 30 decibels only, dispensing with additional playback heads that would be required in order to attenuate the original signal to full 60 decibels.

An endless tape magnetic recording time delay machine designed for supplying artificial reverberation may also be used for producing artificial echos or other sound effects. Thus, by providing an endless magnetic tape recording machine designed for supplying articial reverberation with one or more recording heads located a distance ahead of the reverberation recording head used for producing artiiicial reverberation by a distance corresponding to the desired echo time, such machine will operate as an echo machine if the additionally provided recording heads are used in lieu of the reverberation recording head. For instance, if an echo effect of one second is desired, the additional recording head is displaced relatively to the iirst of the succession of playback heads used for producing artificial reverberation by a distance corresponding to one second.

When using such endless magnetic tape recording device for producing echo effects, the limitations with regard to the exponential attenuation oi the successive pickup heads do not apply, and the ratio of the output level of the playback heads relatively to the recorded signal may be so chosen as to give the desired echo or other sound effects.

The output channel of the playback heads used for producing reverberation, echo and other sound eiiects may be provided with suitable frequency discriminating networks so as to make lt possible to modify the frequency characteristics of the output in any well known Way.

An endless magnetic tape reverberation equipment of the invention permits ready change of the reverberation time and may be operated to supply a short reverberation time, for instance, if the voice of a singer is to be transmitted, and a longer reverberation time, for instance, if the music of an orchestra accompanying the singer is to be transmitted. In such case, one microphone setup may be used for picking up the voice of the singer, and another microphone setup may be used for picking up the music of the orchestra, and two different reverberation effects, one with a shorter reverberation time and one with a longer reverberation time, are combined with the outputs of the two microphone setups.

A single endless magnetic tape driven by a single motor drive system may be used for operating one or more of such reverberation systems operating with diierent reverberation times.

A11 endless magnetic tape recording equipment exemplifying one practical form of a time delay arrangement which in actual operation proved to be very effective in supplying a perfect illusion of actual reverberation, is illustrated diagrammatically in Fig. l. In the form shown, a microphone MC, representing a setup of several microphones, impresses its electric output, generated by the sound to which it is exposed, into electric energy impressed through input lead 2l on the input side of a pre-amplifier IA. 'I'he pre-amplier IA-impresses the amplied microphone output through a direct channel lead 22 by way of an intermediate constant impedance attenuator 23 and an attenuator decoupling network ATD-l to an output lead 24 which is connected, for instance, to a modulator transmitter equipment MO'IR of a broadcasting station, so as to transmit a radio broadcast modulated by the sound to which the microphone MC is exposed.

In addition, the pre-amplifier IA also impresses the ampliiied microphone output through a reverberation channel lead 25 on a time delay or artiiical reverberation equipment RE, the output of which is impressed on amplifier PMA, the output of which is impressed through a'reverberation channel output lead 26 and a constant impedance amplifier 21 connected in series with an attenuator and decoupling network ATD-2 on the common output lead 24, so that the radio broadcast transmitter shall be modulated by the combined output of the direct transmission channel and the reverberation transmission channel.

In the form shown, the time delay or reverberation equipment RE comprises an endless magnetic tape 3| driven by a guide roller 33 engaging a portion of the tape and driven by a suitably energized electric motor 34, for instance, so as to impel the tape 3| at; a uniform speed, for instance, in clockwise direction indicated by an arrow. The endless magnetic tape recording arrangement is preferably of the type disclosed in my copending application Serial No. 197,851, tiled March 24, 1938 as a continuation-in-part of my application Serial No. 82,962, tiled June 1, 1936, which will issue as Patent 2,224,854, or it may be of the type disclosed in my copending application Serial No. 359,177, led September 23, 1940.

An endless magnetic recording medium, such as the tape Si, has to be formed by joining the ends of a long tape of magnetic steel, through soldering, for instance, at a soldering junction 35, although the various features of the invention are not limited to an endless magnetic tape of this particular material. The endless tape is guided and driven at a uniform speed and, in the form of arrangement shown, three sets of obliterating and recording heads OA, OB1 OC, RA, RB, RC, respectively, are placed along diierent sections of the endless tape so that upon passing through the magnetic heads OA, RA, a new signal will be recorded thereon by the recording head RA, and after an interval, upon passing through the obliterating and recording heads, OB, RB, another signal will be recorded thereon by the recording head RB, and after a further interval, another signal will be similarly recorded thereon by the magnetic head RC.

As explained hereinabove, to produce the desired articial reverberation effect, a set or array of playback heads has to be placed following one of the recording heads, such as RA, so that the recorded signal shall be reproduced by the successive playback heads with successively greater time delays. For instance, assuming that a series of successive time intervals are indicated in Fig. 1 by the distances between a series of time interval double-dot dash lines T- to T-25, extending transversely to the indicated directions of the motion of the tape, and a set of playback heads Pl to Pl4, such as indicated on the drawings is placed successively in the direction of the motion of the tape immediately following the recording head RA, the successive playback heads will reproduce the recorded signal with a successively greater time delay represented by the distance of the successive time interval linesI T-l to T-25 from the zero line T-ll.

According to one phase of the invention, the same effect is obtained by distributing the set of time-delay playback heads Pl to P|4 into a number of subgroups or subarrays, for instance, as shown, in three subgroups, and arranging them so that, with the three recording heads RA. RB, RC recording simultaneously the same signal, the signal recorded by the recording head RA is re produced only by the playback heads PI, P2, Pl, P8, Pl2 with time delays indicated by the time interval lines Tl, T-2, T-fl, T8, T-I'l;

that the signal recorded by the recording head RB is reproduced only with the playback heads P3, P4, P9, Pill, Pl3 so that they play back with time delays indicates by the time interval lines T-3, JT-4, 'lL-9, IL-l0, 'Ilm-2i; and that the signal recorded by the recording head RC is ren produced only by the playback heads P5, P6, PI i, PM with time de'iays indicated by the time interval lines TM5, T-S, IL-E3, 'IL-25, the combined output of all playback heads Pi to Pit being equivalent to that obtained if all playback heads are placed so that they reproduce the signal, for instance, recorded by the head RA, with the indicated successively greater time delays IL-i to 'Tm-25. Taking the time interval represented by the distance between lines T-il and 'i1-i as /zoth oi a second, the array of the time intervals Tim@ to T-b represents the time intervals oi a practical reverberation eduipment which proved successful in actual operation.

Such distribution of the playback heads along diiferent endless magnetic tape sections for cooperation with different recording heads so as to reproduce the same signal with successively greater time delays, or, in general, to give the same overall effect as if all the playback heads were arranged to reproduce the signal of a single recording head, has many advantages.

Thus, the frictional forces exerted by the pole pieces of the playback heads are distributed on a greater length of tape and thereby reduces the strain imposed on the tape, and keeping below the magnitude that would impair its magnetic as Well as mechanical properties.

This arrangement makes it also possible to impart independent driving forces to each tape section passing through the individual magnetic heads, for instance, by applying the force of a driving roller to each of the sections of the tape passing through the diierent subgroups of playback heads, thereby reducing the total driving tension which has to be imposed upon the tape.

Furthermore, the greater spacing with which the successive playback heads of each subgroup have .to operate enables also closer spacing of the playback heads of each subgroup, or a larger number of playback heads for producing the same overall time delay eiect than could be possible otherwise, and thus makes it possible to ren duce vthe time delays between the outputs of the successive heads of the sequence and give better blending of their outputs. With the greater spacing between the successive playback heads thus made possible, the problem of mechanical mounting of the playback heads and their crowding, is eliminated.

The foregoing advantages apply to the distributions of the several subgroups of playbacks, either on dierent sections of a single endless magnetic tape, or on several distinct endless magnetic recording tapes moving at the. desired speed, or different speeds. In addition, such vdistribution of the magnetic playback heads makes it possible to decrease the disturbing effect of the soldering joint discontinuity on the reverberation supplied by the equipment. Each of the array of time-delay playback heads forming part of such reverberation equipment, is associated with a suitable attenuator so that the output component derived from each playback head has the proper exponential in decay relation with respect to the output component supplied by the preceding playback head, suitable decoupling means, such as decoupling resistors, being combined with such attenuator so as to prevent the output of one playback head from becoming impressed on another playback head and bringing about a recording of the impressed signal.

In the practical form of reverberation equipment illustrated in the drawings, it was found sumcient to provide a single attenuator unit for the playback heads which operate with a smaller time delay. Thus, as shown, the pairs of playback heads Pi--P2, 'P3-P4, P5-P, P'IPB, Pil-Pit, which operate with shorter time delay intervals, are connected in series to the attenuator units Al, A34, A56, A18, ASU, respectively, while the playback heads Pl i to PM. which operate with larger time delay intervals, are each connected to a separate attenuator unit Ai, All, A2 i, A25, respectively.

Each attenuator unit is provided with an ad-z justable rheostat element 3? connected in series with a decoupling resistor 38, which are so proportioned that the fraction or' the output voltage of the playback heads applied to the rheostat elements 3l of the successive playback heads A42 is attenuated to a greater degree with generally exponential characteristics in the way indicated by the increased length of decoupling resistor` elements 38 of each attenuator unit.

Various ways may be used for combining the outputs of the successive time delay playback heads, for instance, by decoupling networks inteconnecting the various heads. If all the playback heads reproduce the signal recorded by a single recording head, the attenuators of all the playback heads may be interconnected so as to add fractions of their outputs in series and combine them in the way indicated in the drawings, the so-combined or mixed outputs of all the playback heads cooperating with the same recording head being applied to an input stage of the playback amplifier PMA.

In an arrangement, such as shown in Fig. l, in which the array of playback heads required for producing the overall reverberation effect is segu regated into a plurality of groups, cooperating with different tape sections, each associated with a separate recording head, the combined attenuated output of the playback heads associated with the individual diierent recording heads may ndividually be impressed on a dierent input amplier stage 40A, lll-B, dll-C, so as to form separate channels in the way shown. The outputs of the three input amplifier stages dll-A, dll-B, 40C of the three channels, are suitably mixed and further amplied, for instance, by the playback and mixing amplifier PMA.

In combining the outputs of the three tubes 4I, it is important to make it impossible for the out put of one channel to modify the operation of the other channel. Any arrangement which combines the output of the three channels with suitable decoupling means may be used for this purpose. In the arrangement shown, the output circuits of the three tubes are coupled through independent transformers, the Seoondaries of which are connected in series and are feeding into the mixing amplifier PMA. In order to eliminate any interference between the microphone ampliiler IA on one hand and the pickup mixing amplifier on the other hand, decoupling attenuator networks are provided in the direct channel and in the reverberation channel. The pickup mixing amplifier PMA is equipped with a frequency discriminating network or tone control so as to give the reverberated signal any desired frequency response, thus being able to simulate sound effects in a reverberated hall having walls of different sound absorbing and sound reflecting characteristics, such as a large church with a dome, or a large auditorium. i

As shown in Fig. l, the input stage of such ampliiler may comprise, for instance, an amplifier tube 4I having a cathode 42, an anode 4I and a control grid '44, the cathode being shown, for instance, connected througha self-biasing resistor to a ground lead G, and the control grid M being connected by way of a conventionalA coupling condenser to the input lead 48 which impresses thereon the combined properly attenuated outputs of the several time-delay playback heads.

Since in the arrangement shown in Fig. 1, the array oi' playback heads PI to PII is divided into subgroups, each subgroup cooperating with a separate recording head RA, RB,'RC, respectively, three similar input amplifier stages are provided, and the control grid 4l of each tube is similarly connected to the serially connected output taps of the attenuator units associated with the respective groups of playback heads, the low potential side of the rheostat element 3l of the attenuator unit Al, A, AIB associated with the playback heads of each subgroup supplying the greatest output component being shown grounded to complete the input circuit for each input amplifier stage.

As explained hereinabove, the discontinuity, such as the soldering Junction Il, which has to be provided in each endless magnetic tape, produees a succession of high voltage impulses as it passes through the relatively large number of successive magnetic heads required to supply articial reverberation, thereby impressing on the playback channel a prolonged disturbance that has heretofore made it impossible to use an endless magnetic recording tape as a time delay medium for artificial reverberation or the like.

By using a long endless magnetic tape so that, during each complete endless tape cycle, a long interval will pass before the soldering point passes through the magnetic head, its disturbing effect is rendered less objectionable.' In addition, according to the invention, the disturbing effect impressed by the soldering junction of an endless magnetic recording tape on the playback channel is suppressed by providing automatic means for momentarily cutting oil the outputs of the playback heads without disturbing the overall operating results desired from the equipment.

In the arrangement shown, at a point of the endless magnetic tape 3| which passes the playback heads somewhat later than the soldering joint, is provided a depression 39 cut, for instance, along the edge of the tape, and a switch 5I placed along the path of the motion of the tape is arranged to be actuated by the depression to a position in which it momentarily cuts off the output of the playback heads and prevents the electric disturbance induced in the playback heads by the soldering joint 35 from being impressed on the playback amplifier.

Various arrangements may be utilized for carrying out such control operation in response to the operation of such cutoff unit. In the arrangement shown, a point between two series grid resistors l2, I3 is connected through a resistor M to one contact of the switch Il the other contact of which is connected to a source of negative blocking voltage, indicated by a minus terminal of the playback ampliner PMA, so that when the cutoff switch II, which is normally in the open position, is momentarily released by the tape depression Il to its closed position, it instantaneously applies a negative blocking potential to the control grid 44 of the amplifier stage and instantaneously cuts off the amplifying action of the amplifier tube 4 I.

The network through which the negative blocking potential is applied to the control grid M includes also a condenser 5l, and the elements of the network so formed are so proportioned that when switch 5I, after momentarily closing and applying the blocking potential and cutting off the gain of the amplifier stage I I, is immediately reopened, the blocking potential is not immediately removed from the control grid M of the amplifier tube I I, but is only gradually decreased as the .charge on the condenser I5 leaks .through the resistors 52, M, so as to gradually restore the effectiveness of the input amplifier stage, while the soldering point 35, having already moved past the flrst group of playback heads, is still passing through the remaining playback heads.

However, since the playback heads which are farther away from the recording head contribute only a minor fraction of the output level compared to the contribution of the first few mag- -netic heads through which the soldering point has moved while the amplifier stage was completely cutoif, the gradual increase in the gain of the amplifier stage-will not be sufficient to cause any disturbing effects while the soldering point is passing through the last few heads, because the amplified disturbing signal outputs of 4these last few heads has negligible effect compared to the amplified signal level derived from the first few heads, which operate with the least amount of attenuation.

In other words, while the soldering joint passes the nrst few reproducing heads which contribute most of the channel output required for creating the artificial reverberation effect, the amplifier is cutoff, and as soon as the soldering joint reaches the heads which contribute only a smaller portion of the channel output, the normal gain of the amplifier is gradually restored.

In the arrangement described above, a depression along the edge of the tape located back of the soldering joint is arranged to operate a control element located along the edge of the tape so that the depression actuates the control element to operate for a split second and by so doing apply a blocking potential to an amplifier which instantaneously stops amplification and reduces the gain to zero, in combination with a time delay means, which becomes effective immediately after gain was cutoff to gradually restore the gain at a rate which assures that the synthetic reverberation effect is cutoff only for a negligibly short time and is restored so quickly that the momentary absence of the reverberation effect is not noticeable at all.

In the arrangement shown, the individual rheostat 31 of the attenuator units AI! to A25 associated with the individual playback heads may be provided either with a slider tap or with individual switch taps so that they may be controlled all as a unit, and that the attenuation of all the attenuator umts associated with the playbacks may be simultaneously increased or decreased by operating a single actuating element, such as a handle or push button.

All the rheostat taps of all the playback heads are arranged to be controlled in common so as to permit simultaneous varying of the attenuation tension of the outputs of the different playback heads, but nevertheless keep their different attenuation levels in the proper exponential relation. By moving them all at the same time in one direction or in the other direction, the reverberation time may thus be increasedor decreased. Instead of operating them by a continuous movement, they may be operated by push buttons, marked, for instance, 1,41 second, 1/2 second, 3/5 second, l second and 2 seconds. v

Alternatively, suiilcient illusions of a different reverberation time can be obtained without actually changing the attenuation of the outputs of the individual playback heads, but by adjusting v the reverberation equipment as a whole to give a long reverberation time and changing the level ratio of the direct channel output through lead 22, with relation to the reverberated channel output through lead 26. While this procedure is not in accordance with more strict theoretical con siderations, it has been found that the illusion is generally satisfactory for broadcasting applications.

In the arrangement'of Fig. l, such attenuation mixers are shown at 23 and 2l by combining' in each a constant impedance attenuator connected in the direct channel with a constant impedance attenuator in the'reverberation channel, and operating them manually so that While the attenuation is increased in one channel, it is decreased in the other channel. v

The endless magnetic tape recording arrangement of the type shown in Fig. l may be also utilized for providing a plurality of independent reverberation channels, for instance, one reverberation channel for supplying one type of reverberation to a singer, another type of reverberation to the orchestra music and another type of reverberation to a choir. It is merely necessary to provide each tape section, such as shown in Fig. l, with a full complement of magnetic playback heads, so that each of the three tape sections having its own recording head RA, RB, RC, respectively, operates with a full array of playback heads Pi, P2, P3 Pl3, PH, as an independent complete reverberation channel, the combined output of all the playback heads of each independent channel being impressed upon the initial amplification stage, such as shown at lit-A, for instance, of its own individual playback amplifier equipped with its tone control and other accessories. The outputs of the amplifiers of each playback channel so formed are then properly combined through a suitable mixer and impressed in the desired proportions on the output lead 24, as described above.

The acoustic characteristic of a room are determined by its reverberation time. This reverberation time depends upon the acoustical treatment of the walls, iloors, ceiling, furniture, and other object in the room. If more sound absorbing material is used on the walls, ceiling, etc. of the room, its reverberation time will be shorter, and the acoustic liveliness of the room is considerably reduced.

In many cases, it is desirable to have flexible room acoustics, particularly if performances of different character take place in the same room, for instance, public meetings, concerts, theatrical performances, since music requires different reverberation time than intelligible speech. In order to modify the reverberation time of a room, some of the radio broadcasting studios have to be built with movable walls and special curtains having high sound absorbing qualities. To change the acoustic liveliness of such studio, the physical dimensions of the room are changed, and the curtains are raised or dropped. This is not only complicated, but requires time. In ad dition, such studios are expensive to build and their layout is complicated, and their adaptability to diierent conditions is limited.

According to the invention, an endless magnetic tape time delay recording arrangement of the type shown in Fig. 1 is utilized for varying the acoustics of a room, such as radio broadcasting studios, without in any way changing its physical dimensions or resorting to furniture treatment, curtains or the like. The invention is based on the following considerations: The

acoustic liveliness of a room depends on the sound reected from the walls, ceiling, floor and other objects in the room. and other objects of the room absorb of the sound energy, the room is completely sound dead.

In accordance with the principles of the invention, a room which is substantially sound dead is provided with different sources of sound, such as loudspeakers, distributed at diiierent points of the room and operated by a time delay sound recording arrangement so as to reproduce sound from a source, such as microphones suitably located in the room, an effect that would be produced in such a room if the room would be of different dimensions or `have different Wall treatment. Sound from a sound source placed in a room equipped with such artificial acoustic system will travel in all directions until it reaches the boundaries of the room where it will be completely absorbed, but at the same moment the sound delay recording system to which the loudspeakers, distributed along the walls of the room, are connected, will reproduce sound of the desired reilected character, such as would be produced by the source in a room of different dimensions having a special acoustic treatment of its interior.

The loudspeakers for producing the illusion of an acoustically alive room are distributed around the walls and the ceiling, and at least one loudspeaker is placed in the neighborhood of the source of sound where, for instance, the microphone setup is placed. The microphone or microphone setup is connected through suitable ampliers to a number of time delay sound recording channels, and these sound delay channels are connected to the individual loudspeakers, the individual channels being provided with frequency discriminating and attenuating networks so that the individual loudspeakers reproduce the sound so that the overall sound delivered from the loudspeakers has the same character of reverberated acoustic energy.

In other words, the reverberation channels connected to the different loudspeakers are so designed and arranged as to deliver through the loudspeakers sound which, when blended, will create in the room the illusion of the selected desired character of liveliness. Thus, a room provided with an artificial acoustic treatment system of the invention may be given acoustic characteristics of a -big church or a very small intimate room. It is merely necessary to know If the walls, ceiling the reverberation time of the particular church and the particular small room, and adjust the electrical control elements of the reverberation channels to operate with the same reverberation time, and supply the loudspeakers with an output having the same frequency characteristics.

Fig. 2 illustrates diagrammatically and in simplified form one exemplication of an artificial room acoustic system. A room 60, having sound dead interior walls, is provided with a loudspeaker RO located near the source of sound, for instance, a speaker, next to whom a microphone MC is placed, and several groups of loudf' speakers Ri, R2, R3, etc. are distributed along the walls of the room.

In the arrangement shown, it is assumed that the microphone MC is located at the source of sound. The loudspeakers R2 are shown located along points of the wall which will be reached by sound from the source in two time intervals required for the sound to reach the loudspeakers Ri, and three such time intervals will be required for the sound to reach the location of the loudspeakers R3. There is also provided a magnetic recording time delay arrangement comprising an endless magnetic tape 32, such as shown in Fig. 1, driven at a uniform speed in clockwise direction, for instance, past an obliterating head OA, a recording head RA, and a succession of playback heads PI to P14, for instance, so that the playback heads reproduce with a series of time delay intervals the sound recorded by the recording head, which is shown connected to an output lead from an amplifier IA, the input lead 2i of which is connected to the microphone MC.

The magnetic recording arrangement operates similar to that described in Fig. 1, and is provided with a cutoff switch 5l for momentarily cutting ofi? the output of the playback heads at the moment the soldering junction reaches the first playback head. Since the several sets of loudspeakers Ri, R2, R3 are shown symmetrically located in the room, the reverberation effect of the room will be symmetrical, and it is suiilcient to operate each of the groups of loudspeakers from the same energy source.

The loudspeaker R0 which is located adjacent the sound source, should be supplied directly with unmodified amplified energy representing, in effect', the usual public address system loudspeaker. The three groups of loudspeakers RI, R2, R3, are supplied through three dierent reverberation channels which may be designed to have the same' reverberation time, but they should distinguish from each other as follows: The group of loudspeakers Rl which are closest to the sound source, should reproduce the sound with a time delay corresponding to the time interval required for the sound of the sound source to reach them. The loudspeakers R2, which are spaced from the source by a time interval that is twice greater, should reproduce the sound with a twice greater time delay interval, and the loudspeakers R3 with a three times greater time delay interval. Assuming, for instance, that the sound has to travel from the sound source to the speakers Rl l/nth of a second, to the speakers R2 fnth of a second and to the speakers R3 1,4th of a second, the speakers RI will bevsupplied through different reverberation channels corresponding to these requirements.

According to the invention, the output of the individual playback heads of an endless magnetic tape time delay recording device designed for supplying artiilcial reverberation, for instance, is utilized for supplying diierent reverberation channels with time delayed playbacks of the recorded sound, each channel being independently controlled to operate with different reverberation characteristics.

In order to produce the eil'ect of the three diiierent reverberation channels, it is merely necessary to combine fractions of the output of the successive playback heads PI to PM which reproduce the signal recorded by the recording head so as to operate in the same way as if three separate sets oi playback heads were used for providing three different reverberation channels, in the way described in connection with Fig. l, for instance. Thus, assuming that the time delay interval between the successive playback heads PI to PH is 1,-/20th of a second, then for supplying the reverberation eifect required for the loudspeakers Rl, a component of the output of all the playback heads should be combined; for supplying the reverberation eiiect required for the loudspeakers R2, a portion of all the playback heads, except the iirst one, should be combined; and for supplying the reverberation effect required for the loudspeakers R3, a portion of the output of all the playback heads, except the ilrst three playback heads, should be combined.

Various known ways may be used for supplying a portion of the output of the same playback head to diierent ampliiier or reverberation channels, while assuring that no energy is fed back to the playback head. In the arrangement shown, the outputs of the various playback heads are combined without using special decoupling networks by providing the pole piece of each head With a plurality of distinct windings and connecting them into different channels with the properly gradated attenuation to produce the desired reverberation effect,

In the arrangement shown in Fig. 2, the three sets of loudspeakers RI, R2, R3 are connected to three output leads El, 62, 63 of the three playback amplifiers PB-L PB-2, PB-3, on

the inside of which is impressed the input oi three different reverberation channel leads 46-A, 46-B, 46C, respectively As shown in Fig. 2, each playback head Pl to PH has a pole piece interlinked with three output coils ll, l2, 13, each connected to an attenuator unit formed of an adjustable rheostat element 31 and a decoupling resistor 38 designed so that the output applied to the resistance element of .the successive heads is exponentially attenuated relatively to the preceding playback head.

In order to supply the reverberation channel lead A with the required reverberation effect output, the rheostat elements 3l connected to the playback coil ll of all the playback heads are interconnected in series tothe playback amplifier channel lead 46-A. Similarly, in order to supply the required reverberation eifect to the loudspeaker R2, the rheostat elements 3l connected to the coil l2 of all the playback heads, except the playback head PI, are connected to serially combine their output and impress it on playback channel lead AG-B; and the rheostat elements 3l connected to the coll 13 of all playback heads, except the playback heads PI, P2, P3, are connected to additively impress their output on the playback channel lead 48-C leading to the playback amplier PB-3. Each playback amplifier PB-I to PIB- 3 may be provided with a suitable tone control arrangement TC and an attenuation control arrangement AC, so as to permit amplification and adjustment of the characteristics of the amplified output impressed on the loudspeakers so as to produce the desired acoustic effect.

Instead of using an arrangement of the type described in Fig. 2 for operating a plurality of diiierent reverberation channels for supplying artificial acoustic treatment to a room, the different reverberation channels may 'be used, for instance, for supplying one type of reverberation characteristic to the voice of a singer, the other to the music of the orchestra, another to the choir, in lieu of three separate reverberation channels, as described in Fig. 1.

Fig. 3 illustrates diagrammatically an arrangement using different reverberation channels REA, REB, REC operating with a single endless magnetic tape l32 for supplying dilerent reverberation effects to different groups of loudspeakers Rl, R2, R3 used, for instance, for giving to a room, such as shown in Fig. 2, the desired acoustic treatment, the arrangement of Fig. 3 operating in the same Way as that in Fig. 2, except that each reverberation equipment REA, REB, REC is equipped with a recording head and a full complement of playback heads, such as described in connection with Fig, 1, for independently producing any desired reverberation effect or other time delay effects with a single endless magnetic tape recording system.

When using arrangements, such as shown in Figs. 2 and 3, for supplying artificial acoustic treatment to a room, the distances between the playback heads are so arranged as to operate the different loudspeakers or sound reproducers with a time delay relatively to the source corresponding to the time required for the sound to travel from the source to the location of the loudspeakers. However, in many cases, for instance, if special sound effects are desired, the sound delay supplied by the magnetic recording arrangement may be made greater than the actual time required for the sound to travel from the source to the particular part of the room. It is thus possible to give a room artificial acoustical enlargement.

Although for ideal conditions, the room should be practically sound dead, it will not be necessary in practice. Thus, if the room has a relatively small reverberation time, the acoustic effects added by time delay recording arrangements described in connection with Figs. 2 and 3 may be only used to add .acoustic liveliness or other desired acoustic characteristics to a room.

An endless magnetic tape recording device of the type designed in accordance with the principles of the invention to operate as a time delay medium, in the way described in connection with Figs. 1 to 3, is also ideal for supervising the transmission of messages, for instance, broadcasts, to prevent dissemination of improper information, such as of war secrets during news broadcasts in wartime, An arrangement of such type exemplifying one form of the invention is shown in Fig. 4.

A source of signal energy, such as a microphone MC, delivers its output to an amplifier IA, the output of which, instead of being impressed directly on a modulator and radio broadcast transmitter TR, is supplied to a recordin-g head 4-R of an endless magnetic recording arrangement shown in the for-m of an endless magnetic tape 3| driven at uniform speed in clockwise direction, for instance, in the Away described in connection with Fig. 1.

'I'he signal recorded on each element of the moving endless tape remains thereon until, after a full operating cycleof the endless tape, it reaches a playback head d-P, the output coil of which is connected to the modulator and radio broadcast transmitter TR, so as to bring about the broadcast of the message from the microphone in the same way as ii' it had been directly supplied to the radio broadcast transmitter. An obliterating head 4 0, shown in the form of a permanent magnet, is placed between the recording head 4-R and the adjacent main pickup head l-P, so that each element reaching the recording head is ready to receive a new recording.

The equipment for supervising or censorng the transmitted message is shown as consisting of two censorng stations CA, CB, although more stations may be provided. Each censorng station is provided with a set of four push button switches K, LI, L2, L3, and with headphones 8|, a signaling lamp 82 and auxiliary switching elements for performing the various operations as explained hereinafter.

Upon actuating the push button LI of each station, the headphones 8| of the station are connected to an output lead 83 from the amplier iA, so as to enable the censor to listen to the transmitted message as it is being recorded on the endless magnetic tape 3| for subsequent transmission, after the time delay determined by the recording length of the magnetic tape, for instance, one; minute, to the radio broadcast transmitter TR. Upon releasing the push button Ll, and actuating the push button L2, the censor at each station may connect his headphones to the output lead of a playback amplifier AM-l upon which is impressed the output of a playback head located on an intermediate portion of the tape so as to enable the censor, if,

AM-Z which amplifies the output of a third playback head 4-P3 placed a distance ahead of the main playback head 4-P, and as soon as he hears the initial phrase of the objectionable message, he has merely to push button K which completes a circuit sending an obliterating current to the obliterating head 4-03 placed ahead of the playback head 4-P, thereby cancelling the message recorded on the tape and preventing its transmission when the elements of the tape on which it was recorded pass through the main playback head 4-P.

In order to make such censorng system eiective, at least two censorng stations, such as-gindicated at CA, CB, must be provided so that while one censor traces an objectionable message and attends to its being cancelled, the other censor may attend to the censorng of the further part of the lmatter that is being broadcasted. Under normal transmission of a broadcast, only one censor has to watch the transmissionof the broadcast.

In order to indicate to the censor that it is his duty to listen in, the signaling lamp of the proper station, for instance, station A in the case shown, is lighted by a circuit completed through the contact of a stepping switch 88 having an operating coil B1, which, upon energization, advances the contact of the stepping switch one step, thereby cutting out thel signal lamp of station A and lighting up the signal lamp of station B, indicating to the censor oi the latter station that it is his duty to watch the transmitted message by closing his push button switch Li.

With this arrangement, censor A, whose signaling lamp 82 is lighted and has his push button Ll operated to listen to the transmitted message. on noticing that the message contains objectionable matter, merely releases the push button Li and actuates the push button L2, which immediately connects his headphones to the intermediate playback head 4P2 for enabling him to pick up the starting phrase of the objectionable message. On closing his push button switch L2, the censor at station A also completes a circuit including the normally closed contacts of the two auxiliary relays 88, 89 to the operating coil of the stepping switch, thereby advancing it to the next step in which his own signal lamp 82 is extinguished and the signal lamp 82 of the other censor is lighted up, whereupon the censor B actuates his push button switch connecting his own headphones to the amplifier IA for watching the transmitted message.

In the meantime, the censor at station A, upon having picked up' the beginning phase of the objectionable message, is releasing his push button L2 and actuating his push button L3 so as to connect his head phone to the playback head 4-P3 and be ready to cancel the message as soon as he hears the initial phrase of the message in question, whereupon he actuates his cancelling button K for sending an obliterating current to the obliterating head 4-O3.

In order to prevent accidental obliteration of a message that is being transmitted, the obliterating push button switches K are normally locked by a locking relay 9| which is actuated to release the obliterating switch only while the cancelling push' button L3 of one or the other of the two stations is actuated to cancel the message, thereby completing a circuit to the operating coil of the locking relay 9|. A foolproof censoring system, which prevents interference with the transmitted message, but enables tracing of an objectionable part of the message while its transmission is delayed, and its obliteration before it is transmitted, is thus made possible.

Obviously, a larger number of message tracing playback heads, such as l--P2 may be provided for enabling the censor to pick up dierent parts of the message which are stored in the endless tape, while they are being advanced from the recording head 4-R, during the time interval of the full endless tape recording cycle, to the point where they are picked up by the playback head l-P for delivery to the radio broadcast transmitter TR.

The endless magnetic recording devices of the type described in my copending application Serial No. 197,851, led March 24, 1938, which will issue as Patent 2,224,854, and described therein in connection with Figs. 1, 5, 10 and 19, and which have proved successful in actual operation, lend themselves for utilization as a time delay medium for supplying artincial reverberation and for similar applications constituting the subject matter of the present application. In Figs. 5 and 6 is shown one form of arrangement for utilizing a magnetic recording device of the type described in connection with Figs. 8 to 10 of my copending application Serial No. 197,851 as a time delay recording equipment supplying artiiicial reverberation, in the way described in connection with Fig. 1, for instance.

Inside a suitable casing, not shown, is mounted a rectangular frame structure 5-0 consisting of a front frame l-I and a similar rear frame 5 2 formed of metallic angle bars reinforced by corner braces and suitably held together by cross bars or struts. In the comers of the rectangular frame are mounted four cylindrical guide rollers 5-6 having bearings journaled on stationary shafts supported in the comer braces of the frame structure.

A long endless magnetic carrier in the form of a magnetic tape 5-13 is wound over the four guide rollers in the form of a helix, a suitably guided cross loop completing the endless tape. The frame with the four rollers forms thus a spool-like structure 0n which the endless tape may be readily wound. One of the guide rollers is driven through a belt by a motor, not shown, suitably supported within the frame structure so as to impart through the frictional driving engagement between the motor-driven roller with the portions of the tape helix engaging its surface a uniform speed to the helix along the guide rollers. Suitable comb bars maintain the helices in their proper position along the rollers.

In order to make it possible to place a large number of magnetic recording and playback heads along a portion of the endless tape, the spool-like frame structure has mounted on one side thereof, on arms 5-2l suitably secured to one side of the frame structure, an elongated supporting bar 5-25, shown as having a T- shaped cross section, and an elongated slot 5-28 is formed in the forwardly facing portion of the bar 5-25 so as to enable adJustable mounting thereon of a series of magnetic heads 5-21.

The magnetic heads are preferably of the type described in connection with Figs. 34 to 42 of my copending application Serial No. 340,030, led June 12. 1940, and as indicated in the drawing. each magnetic head consists of two halves placed on opposite sides of the magnetic tape S-II and held in position by a bracket 5 2! which is adjustably clamped to the front face of the supporting bar 5-25 by a bolt extending through the elongated slot 5-28 thereof. The location of the individual magnetic heads and their relative spacing may be readily adjusted along the supporting bar 5-25.

On the upper and lower ends of the supporting bar 5-25 are journaled two aligning rollers l-JI for guiding one loop brought outwardly from the loop helix guided on the rollers 5-6 so as to pass through the succession of magnetic heads l-ZI mounted on the bar B-25, an auxiliary crossover roller H2, suitably mounted on a bracket A extending from the upright mounting bar -2l,

leading the helix loop passing through the magnetic head from its position on the frame rollers 5-26 to its path of motion between the aligning Irollers B-Sl.

By using magneticheads of the type shown in Figs. 41 and 42 of the above application Serial No.

340,030. so that the coils in the pole pieces of vsuccessively arrayed heads placed in adjacent rows overlap each other, using one set of heads for cooperating with one portion of the tape and the other set of heads for cooperating with an adjacent portion of the tape, so as to 'utilize a single tape with two recording tracks, it is possible to playback a recorded signal with shorter time delay intervals between successive playbacks, such arrangement requiring, of course, a record ing head for each playback track. The foregoing, of course, applies to an endless tape driving arrangement, while the arrangement shown in Figs. 41 and 42 is intended to illustrate an oscillatory tape drive. The foregoing principles are also ap plicable for an arrangement in which more than two'adjacent recording tracks are 'provided on a single endless tape.

For the sake of illustration, Fig, illustrates a series. of playback heads PI, P2, etc. to PH, which are utilized for supplying artificial reverberation in the arrangement of Fig. 1, mounted successively adjacent to each other on the supporting bar 5-25 and enclosed in a magnetic shield 5-35, a recording head R and an obliterating head O being mounted ahead of the first playback head Pl outside the shield enclosing the playback heads. An additional playback head is also shown mounted ahead of the obliterating head so as to permit utilization of the device shown as a censoring equipment. There is also secured to the mounting bar 5-25 ahead of the magnetic recording and playback heads a switch structure 5-5I which is utilized for automatically cutting out the operation of the playback heads each time the soldering junction passes them, in the way explained hereinabove.

As shown in Fig. 6, any required number ci guide bars 5-25 with sets of magnetic heads mounted thereon in the way shown in Fig. 5, may be suitably mounted along one side of the spoollike guide roller structure S-l, 5-2 for guiding dierent loops of the endless tape helix 5-l3 over different sets of magnetic heads, thus enabling the operation of a device, such as shown in Figs. 5 and 6, with different reverberation channels or in the way described in connection with the various applications described in connection with Figs. 1 to 4.

In order to support the individual guide bars 5--25 of the magnetic heads in the different locations along one side of the frame structure, two angle bars 5-4|, for instance, may be detachably secured to the side surfaces of the front and rear frames 5-l, 5-2 of the spool-like roller structures, and the mounting arms 5 2! for carrying the guide bars 5-25 are suitably secured, as by bolts, to the two cross arms 5-4I.

In Figs. 7 and 8 is shown the construction of the switch mechanism 5-5I, of the type shown at 5| in Figs. l to 4, for performing the controlling operation which automatically cuts oil the output of the playback heads when the soldering point passes through them, or for performing any other desired signaling operation in conjunction with the operation of an endless magnetic tape recording arrangement.

On forwardly projecting wall portions 5-52 of a supporting block 5-50, the back side of which is suitably clamped to the front face of the guide bar 5-25, are revolvably mounted two grooved guide rollers 5--53 aligned so as to provide a guiding support for the tape 5-l3 as it moves upwardly towards the magnetic heads in the way shown in Fig. 5, two spring arms 5-54 CII serving to yieldably retain the tape 5-I3 within the groove of the rollers as it is being impelled at a uniform speed toward the magnetic head. An additional biasing spring 5-55 urges the tape 5|3 to engage with its opposite edge the side walls of the two rollers 5-53 along which it is guided.

The control switch 5l for performing the control operations described in connection with Fig. l. is shown in the form of 'a commercially available unit, such as sold under the name Micro switch, and designed, for instance, in the way described in Patent 1,780,758, to release the contact from its open position shown to the closed position, if its operating arm 5-56 pivotally mounted on a bracket extending from the switch casing releases the operating pin 5--51 of the switch to the outward position under the biaslng action of the flexed spring contact of the switch 5i.

The free end of the operating arm 5-56 is provided with a bracket on which is pivotally mounted a grooved roller 5-58 engaging the facing edge of the tape so as to ride thereon as the tape is being impelled, and keep the switch operating pin 5--37 in its inward position in which it holds the switch contact 5i open, and to release the switch 5| so as to close during a short instant while the depression 5-39 formed along the edge of the tape moves past the roller .tl-58.

Various other switching arrangements may be used to perform in a similar or analogous manner for automatically performing control operations in response to an automatic actuation by a signal element, such as a depression formed on the tape, so as to synchronize a. control operation with a predetermined selected position of the endless tape as it is being continuously impelled.

In the guide and driving arrangement for an endless magnetic recording tape described above, the magnetic tape is guided and driven at a uni-1 form speed along a helix coiled around the outer surfaces of a plurality of spaced guide rollers, one oi' the guide rollers having frictional driving engagement with loop portions of the helix being driven by external forces for impelling the tape helix past the magnetic transducing means, such as the recording and playback heads, for recording and reproducing sound, at least one guide roller being frictionally driven by the moving tape helix and having sufficient mass so as to exert on the tape helix inertia reaction forces which suppress fluttering of the tape helix and maintain it at substantially uniform speed.

' In addition, the spacing of the guide rollers is designed in the way described in my aforesaid application Serial No. 197,851, filed March 24, 1938, which will issue as Patent 2,224,854, so that the spacing between the guide rrollers is suiliciently large and the effective guide roller surfaces engaged by the tape helix are sufficiently small to permit lateral slippage of the tape portions of the helix over the roller surfaces while they are impelled along the path of the helix.

In addition, the effective length of the helix portions frictionally engaged with the guide roller surfaces is made sufilciently small and the effective length of the free helix portions extending between the spaced guide rollers is made sufficiently large as to permit limited slippage of the tape helix portions over the guide roller roller surfaces to assure uniform speed along its helical path.

Furthermore, the effective length of the helix portions frictionaliy engaged with the guide roller surfaces are so proportioned in relation to the effective length of the free helix portions, and the inertia of the guide roller masses frictionally driven by the tape helix are designed to be sufciently large as to permit limited slippage of the tape portions over the guide roller surfaces while being kept in sufficiently effective frictional engagement therewith for maintaining uniform speed of the tape along the helix.

Furthermore, the frame for supporting the guide rollers is arranged to form a spool-like structure permitting winding of the endless tape helix around the outer roller surfaces when it is being joined into an endless tape, and the interior space of the Spool juncture may be readily designed as a support for the driving motor mechanism and the amplifier means associated with the operation of the endless magnetic tape recording system.

In Figs. 9 to 14 is shown an endless magnetic tape recording device designed in accordance with the principles disclosed in connection with Figs, l, 5, 13 and 27 to 29 of my copending application Serial No. 197,851, but embodying various desirable improvements thereover, exemplifying various other features of the invention. Inside a casing, not shown, for the sake of clarity, is mounted a rectangular frame structure consisting of a front frame 9 5, and a rear frame 9 2 of metallic angle bars reinforced at the corners by brace plates 9 3, 9 4, 9 5, 9 6 and held together by cross bars or plates 9 1, 9 8, 9 9, the latter being clamped to the intermediate angle bars 9 Il secured between the side members of the front frame 9 l and the rear frame 9 2, in the way shown in Figs. 9 and 10.

In the form shown, two cylindrical guide rollers 9 2I are revolvably mounted on shafts 9 22, the ends of which are supported by the corner braces 9 3, 9 4 shown in the lower part of the frame structure, and two pairs of additional rollers 9 23, 9 2# are similarly revovably mounted on shafts suitably held by the corner brace plates 9 5, 9 6 on the opposite part of the frame structure. A long endless magnetic tape 9 3i is would in the form of a helix, first as an inner helix layer, for instance, by starting on the left sides of the rollers, as shown in Fig. 9, and winding it from the left to the right around the two pairs of rollers 9 2I, 9 23, and then back from the right side of the rollers, as shown in Fifz. 9, in the same direction, but on the two pairs of rollers 9 2I and 9 24, back to the left end of the rollers, shown in Fig. 9, to a point where the end of the outer helix joins the starting point of the inner helix, so as to form a continuous endless tape. Two layers of tape are thus would on the lo'wer pair of rollers 9 2l, and one layer of tape is would on each of the other two pair of rollers,9 23, 9 24.

In order to guide the successive loops of the endless tape 9 3I along the helices so formed, comb-like bars 9 32 are mounted in front of a few of the guide rollers, for instance, in front of the guide roller 9 2I held by the brace plate 9 3, and the guide roller 9 23 held by the brace plate 9 5 in a diametrically opposite corner of the frame structure, so that if the tape is irnpelled in clockwise direction, each loop of the two tape helices will be retained in its proper operating position relatively to the other loops forming part of the endless tape.

An electric driving motor 9-34 has its vibration absorbing bracket structure 9 35 mounted on the cross plate 9 1 of the spool-like frame structure so formed, and a driving pulley mounted on one end of the motor shaft drives through a rope belt 9 36 one of the guide rollers 9 23 so as to impart to the tape a uniform motion, in clockwise direction, for instance, as indicated by the arrow.

In the design of such endless magnetic recording tape guide and driving arrangement, the following considerations are important: If the drivin'g arrangement is to be used for synthetic reverberation, it is of utmost importance to eliminate even traces of flutter due to irregularities of the motor speed or any other causes, because if even slight changes of the momentary tape speed occur, the signal will be picked up by the different playback heads with a varying phase shift giving uncontrollable differences in pitch. Since the signal reproduced by the reverberation playback heads is combined with the original signal, such differenecs in pitch would bring about a modulation effect causing undesirable disturbance of the artificial reverberation supplied by such recording arrangement.

By using an endless rope belt 9 36, of the type employed in dental drives, and winding the belt in the form of a plurality of loops so as to be guided in suitable grooves formed on the motor pulley and corresponding grooves formed in the periphery of the motor driven guide roller 9 23, such belt may be operated with sufficiently small tension per belt loop as to take up in the belt any fluctuations of the motor speed without transmitting them to the motor driven roller 9 23. Such driving arrangement is also of great advantage because it reduces the belt tension and wear and also reduces the friction losses. As a result, a smaller motor may be used and the remanent mechanical noises inherently present in any mechanical drive is brought down to a very low level.

As shown in Figs. 10 and 14, the correct tension of the -belt 9 36 is maintained by a cross-over pulley 9 31 journaled on the end of an arm 9 38 which is pivotally mounted on a bracket extending from the motor, a suitable biasing spring 9 36 serving to supply the pressure with which the cross-over roller 9 31 maintains the crossover loop of the endless belt 9 36 at the desired tension. The end grooves of the motor pulley of the motor driven guide roller 9 23 are sufciently wider than the grooves over which the other loops of the endless belt 9 3; are wound so as to enable the cross-over loop to be guided from the point where it leaves the motor pulley to the point where it enters the starting point of the belt helix on the motor driven guide roller 9 23. One of the loops of the rope belt 9 36, which is not used for transmitting the driving force to the guide roller 9 23, is utilized as the cross-over loop.

In order to secure the full effectiveness of such drive in suppressing even traces of flutter, the magnetic heads are arranged to engage with their pole pieces one or more loops of the inner tape helix layer guided over the guide rollers 9 23 so as to impart to the inner tape helix the tension required for maintaining its loop portions that pass over the guide rollers 9 2I, 9 23 int 1good frictional driving engagement therewi In order to assure long foolproof operation without requiring servicing of the moving parts, the guide rollers are revolvably mounted on their shafts 9-22 by ball bearings 9-42 in the way shown in Fig. 12. Compared to sleeve bearings, ball bearings have the great advantage that they operate with less noise than sleeve bearings, a factor which is very important in such magnetic tape drives, for instance, when used as a reverberation medium in the studio where the sound to which the reverberation is applied originates. The ball bearings are interposed between the shaft and the rollers so that the shaft remains stationary while the roller revolves in the way shown in Fig. 12.

The ends of the shaft 9-22 are supported by a resilient member, for instance, Neoprene, in the way shown in Fig. 12, in order to assure that any remanent mechanical vibration imparted to the shaft is not transmitted to the frame structure 9-|, 9-2 having a large sound radiating u are ,from which noise should be kept away. In ord r to reduce the noise, the surface of the guide rollers 9-2I, 9-23, 9--24 is made of non-metallic sound-absorbing material. Since such nonmetallic soundabsorbing material is usually an electrical insulator, it will accumulate in the course of operation electric charges due to friction with the tape, and such charges,r if accumulated until they reach a potential sufficient to spark over to the frame structure, will induce electric noise effects in the playback circuits associated with the recording and playback equipment. This disturbing effect is eliminated by providing, as shown in Fig. l2, a sliding metallic contact member 9-45, supported by the metallic frame structure and pressing against the metallic portion associated with the shaft, such as the inner ball race of the ball bearings 9-42.

It has been found that the guide bars, such as shown at 9--32 for guiding the loops of the helices in th'eir positions along the rollers, may cause a disturbing high pitched noise caused by the mechanical vibrations of the comb teeth, as the tape loops which they keep apart move past them.

Fig. ll illustrates a comb guide bar arrangement 9--3I for the tape helices which' was found to be highly efficient in properly guiding the loops of the tape helices, without causing any noise. As shown, a bar 9--3I of suitable nonmagnetic material, such as brass, for instance, holds in suitable grooves or holes formed therein tooth members 9-5l in the form of pins or tooth-shaped sheet metal portions projecting between and separating adjacent loops of the helix, two strips of felt 9--52, 9--53 being held in position on the teeth so as to confine the loops of the tape and the projecting teeth portions between them, and suppress any vibrations of the same.

The two felt strips 9-52, 9-53 are held suitably clamped, for instance, by an overlying bar 9-54, likewise'of non-magnetic material, which is suitably attached to the comb bar 9-3l, for instance, by screws, as shown in Fig. 11. This arrangement of .the guide bars not only suppresses vibrations of the tape and of the guide bar teeth, but also automatically cleans the tape.

In addition, it distributes the tension of the tape uniformly over its individual loops.

Obviously, an endless magnetic tape guide and drive arrangement of the type shown in Fig. l

`may be, of course, made to operate with a larger or smaller number of such rollers. For instance, the endless tape may be wound in a similar way in the form of two helix layers over only one roller 9-2I and one roller 9--23 and one roller 9--24, the three rollers being supported on two frame` members, such' as two side members of the frame structure 9 4, 9-2 interconnected by a cross plate 9 8, in the way shown. Such endless magnetic tape driving arrangement may be used in the various applications outlined in my vprior application Serial No. 197,851, filed March 24, 1938, which will issue as Patent 2,224,854, and it is desired that the specification of said applications, together with its drawings, should be considered as a part ofthe present application.

The endless tape guide and drive arrangement shown in Figs. 9 to 14 is particularly suited for use as a time delay medium for supplying artificial reverberation or other acoustic effects requiring the operation of the magnetic tape with several sets of magnetic recording and playback heads arranged in different magnetic tape portions.

As shown in Figs. 9, 10 and i3, the frame structure 9|, 9--2 also serves as a support for three additional rollers 9--6I, 9-62 mounted in a way similar to the other guide rollers in three corners of the frame structure. Any number of loops of a continuous endless tape guided and driven on the inner rollers 9-2i, 9-23, 9-24 may be led from their position on the inner rol 1ers so as to move over the outer rollers 9SI, 9-62,ifor instance, the lst, the 11th, the 31st loop of the inner helix or similar loops of the outer helix may be guided out of their inner posi tion to the position along the outer rollers S-SI, 9-62.

In the arrangement shown in Figs. 9 and l0, two loops 9-63A, 9-63B are shown guided from their position along the inner helix moving over the two pairs of rollers 9-2I, 9-23 so as to move over the outer rollers S-l, 9-62 and return to their positions in the inner helix, suitable gaps being provided when winding the loops of the outer helix around the rollers 9--24 for permitting the loops 9-63A, 9-63B to be led from their position on the inner helix to their position on the outer rollers 9--6|, 9-62.

As shown in Figs. 9 and l0, angle metal cross bars 9-'ll are suitably secured to the brace plates of the two frames 9-i, 9-2 so as to provide mounting supports for guide bars 9-l2, 9-13 on which any desired array of magnetic heads may be mounted for cooperation with the tape loops 9-63A, 9-63B passing thereover.

As shown in Figs. 9 and l0, channel members are used for the guide bars 9--l2, 9-13 and an elongated slit 9-14 is provided in each guide bar permitting adjustable mounting thereon of the bracketsr 5-28 of the magnetic heads, as by screws 9-'I5, and nuts, in the way shown. A magnetic shield 5-35 for enclosing the magnetic heads, as in the arrangement of Fig. 5, is likewise arranged for detachable mounting on the guide bars 9`H, 9-13.

To secure maximum effectiveness in suppressing flutter, in a tape driving arrangement of the type shown in Figs. 9 and 10, the magnetic heads should be placed on a tape loop portion which has to traverse at least one of the tape driven guide rollers, such as 9-2|, 9-24 before it reaches the motor driven roller. Thus, in the arrangement shown i-n Figs. 1 and 2, the portion of each tape portion 9-3I leaving the magnetic heads located on an outer loop guided over the outer rollers S-SL 9-82, has to pass over at least four tape driven rollers 9-2i, 8-24 along the outer helix and the two tape driven rollers 9-'2l along the inner helix before coming into driving engagement with the surface of the motor driven roller 9-23. One of the driving belt loops which does not transmit the driving iorce is used as a cross-over loop, and the belt idler roller 9-31 urges this cross-over loop in outward direction so as to enable the cross-over roller to guide the cross-over loop from the point of the motor pulley where it leaves the end of the belt helix to the point on the roller where the belt helix starts, without interfering with the other loops of the belt 9-I8.

The use of the roller arrangement of the type shown in Figs. 9 and 10 makes it possible to separate the convolutions of one or more outer tape helix layers guided over the outer guide rollersV 9--24 from the inner tape helix guided over the rollers 9-23 over the major portion of its length without requiring much more room for guiding' a very long endless tape, since only a relatively short portion of the loops of the inner tape helix has superimposed thereon portions of the tape helix wound around it. This is not objectionable because the overall length of the tape is made sufficiently long and so related to the frictional engagement o! the tape loops of the inner tape helix llayer with the driving rollers as to leave in the outer tape helix sulcient slack to assure that no portion of the outer tape helix is under excessive tension and moves loosely over the inner tape helix.

A larger number of additional helix layers may be wound in pairs over the outer helix layer engaging the rollers 3-24, one layer of each pair of outer helices being wound around the spoollike structure formed by the rollers 9-24, 9-2I, in the same direction as the inner helix layer shown in Fig. 13, and then back in the direction or the outer helix layer, the end of the outermost layer of the last pair being Joined to the end of the inner helix layer wound around the rollers 9;-23, 9--2 I. By employing the construction of the type shown in the drawing, a great length of endless tape can be guided and driven with a small spool-like tape guiding and driving structure. The size of the guide rollers s determined by the tape thickness. It is important that the curvature of the rollers shall be large enough to assure that the bending to which the tape is subjected in passing thereover does not impose on it stresses that eilect its magnetic properties. Thus, a magnetic tape of tungsten steel having a thickness of three thousandths of tape roller shaft 9-22 on which the tape roller the guide roller in the adjusted position, in the way shown in detail in Fig. l2.

It two layers of magnetic recording tape are permitted to remain in engagement with each other and are subjected to excessive pressure, a fraction of the magnetic energy of a recorded signal stored in one tape will be impressed on the adjacent tape layer resulting in undesirable echo eiects when reproducing the, original record. By adjusting the tape in the way described above, such undesirable eilects may be eliminated.

As pointed out, hereinafter, when a magnetic tape machine of the type described herein is used as a time delay device, for instance, for producing reverberation effects, the undesirable echo eiects described above are of no significance because only the portion of the tape which is engaged by the magnetic heads mounted immediately followingthe recording head is utilized for performing the operation.

In special applications where it is essential to suppress even the slightest traces oi undesirable echo effects, the outer tape layers extending over the rollers -9--2I on which the inner as well as the outer helix layers are guided, may be separated by the teeth of the guide combs over adjacent guide roller surface portions so that one tape layer does not engage another. However, if provisions are made to assure that the loops of the innermost tape helix wound around the inner rollers '9-3l are held suillciently tensioned to maintain the driving engagement of its loops with the motor driven rollers, sufticlent slack may be provided as to assure that at all times the outer tape helix layer is suiiiciently loose to positively eliminate any traces of undesirable echo effects. With such arrangement, any required number of outer helix layers may be wound and operated over the first outer helix provided the outer tapes have suiilcient slack as to enable them to slide freely along their path without being subjected to excessive pressure.

.According to the invention, the pole pieces of the magnetic heads, which are used for recording and reproducing the signal, are utilized to exert sufiicient friction on the inner tape helix as to maintain it in the required operating tension. The magnetic heads lmay be supported in the space of the inner helix as described in my prior applications. However, in many cases, such as in time delay machines used for reverberation, where a large number of magnetic heads have to be employed, it is desirable to mount the heads so that they are accessible on the exterior of the supporting structure. According to the invention, one or more loops of the inner tape helix layer are brought out and guided in the space beyond the outer tape helix so as to make it possible to guide such outwardly brought tape helix through magnetic heads mounted in such space.

As shown in the arrangement of Figs. 9, 10 and 13, a suitable number of loops of the inner helix are brought out through gaps lett in the outer helix layer and guided over the outer guide rollers so as to guide two sides of each loop along the periphery of the frame structure through a plurality of magnetic heads mounted along them, and thus maintain the inner helix at the proper tension, while leaving the outer helix layers sufilciently loose as to permit them to slide past each other.

Since a large number of magnetic heads placed along these helixes automatically subject the inner helix to a substantial friction, the loops

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