US2096760A - Tone control system - Google Patents

Description

Oct. 26, 1937.

E. S. PURINGTON TONE CONTROL SYSTEM 3 Sheets-Sheet l dim.. 1:6 0302 4 OJ l 'u 'l mozruarulh Filed April 3, 1936 n w/ M, F

V & WH/.M w w .mo m .m w n Em Oct. 26, 1937; E. s. PURINGTON TONE CONTROL SYSTEM I Filed April 3, 1936 I: Sheets-Sheet 2 o dIQDT L INVENTOR Ellison S. Pur'ingfon. BY

v ATTO NEY Oct. 26, 1937.

E.- S. PURINGTON TONE coNT RoL SYSTEM Filed April 3, 1956 K i l 0 cy c l as 3 Sheets-Sheet 3 KHocycles .l'as .25

' INVENTOR EH ison S Puvi n gTOh RNEY' Patented on. a e, i937 on rreosr 2,096,760 TONE ooN'raoL srs'rsM Ellison S. Purington, Gloucester, Mass, assignor to John Hays Hammond, Jr., Gloucester, Mass.

Application April 3, 1936, Serial No. 72,520

21 Claims. (Cl. 179-1001) This invention relates to the reproduction of sound from a sound record and has for its purpose to provide automatic means for controlling the tonal characteristics of the reproduced sound.

More specifically the invention provides an automatic means for discriminating against the reproduction of sound of a given frequency range, unless there are suitable signals in that frequency range.

The invention further .provides for the masking of background noises, by suppressing the transmission of noise frequency signals unless the desired signals are of suitable nature to mask the presence of the noises. The invention also provides for increasing the tonal contrasts of the reproduced sound.

In accordance with the present invention, the tone control is effected by providing a transmission channel having a variable filter network which includes the primary of a transformer,

the secondary of which is variably loaded by a thermionic impedance. The thermionic loading is controlled by voltages obtained by rectifying suitable energy derived from the tonal energy output of the sound record.

' The control action depends upon the principle that the effective primary impedance of a transformer, which inthis case determines the impedance properties as far as the transmission network is concerned, .is dependent upon and controllable by the impedance loading of the secondary circuit. The general effect of lower;

ing the value of a resistance impedance shunted across the secondary-winding of a transformer, is to increase the secondary loading and to reduce the equivalent primary impedance, especially for the higher frequencies. The relative transmission of currents of different frequencies through the network in which the primary of the transformer is connected is therefore modified by secondary thermionic impedance control.

A feature of the invention accordingly consists in replacing an inductance element of a conventional filter, such as a low pass, band pass or high pass filter, by the primary of a transformer having a variable secondary thermionic loading, whereby the properties of the filter may be controlled, as to cut-off frequency,'sharpness of cut-off etc., inaccordance with the energy by which the thermionic impedance is modified. 'Another feature, consists in connecting the secondary to a pair of thermionic impedances, each carrying direct current, with a balanced secondary winding arrangement such that the as equivalent primary impedance is controlled solely by the alternating current thermionic impedance values, and is not modified by the changes of direct current through those impedances. A wide range of transmission values is thus achievedby one transformer, the balanced operation minimizing any distortions that may result from non-linearity of tube characteristics. Furthermore a rapid tonal control is obtained due to the fact that no change of the direct current flux of the core occurs during operation.

The invention also consists in the various new and original features of construction and combinations of parts hereinafter set forth and claimed.

Although the novel features which are be lieved to be characteristic of this invention will be particularly pointed out inthe claims. ap-- pended hereto, the invention itself, as to its objects and advantages, the mode'of itsoperation and the manner of its organization may be better understood by referring to the'following de' scription taken in connection with the accompanying drawings forming a part thereof, inf which Fig. 1 illustrates diagrammatically the system as applied to the reproduction of sound from the sound record of a talkingmoving picture projector and phonograph.

Fig. 2 is a series of curves illustrating the relative frequency transmission characteristics of certain portions of the circuit shown in Fig.1.

Fig. 3 is a schematic diagram of a modified form of the filter circuit shown in Fig. 1;

Fig. 4 is a series of curves illustrating the relation between the frequency and the transmission characteristics of the modified circuit shown in Fig. 3. V

Fig. 5 shows a circuit to illustrate the insertion loss which may be produced in accordance with this invention. 40

Fig. 6 is a series of curves illustrating the ing sertion losses of the circuit depicted in Fig. 5.

Fig. 7 shows a circuit to illustrate the insertion loss which may be produced with a different circuit arrangement. 45

Fig. 8 is a series of curves illustrating the insertion losses of the circuit depicted in Fig. '7.

Like reference characters denote like parts in the several figures of the drawings.

In the following description and in the claims 50 parts will be identified by specific names for convenience, but they are intended to be as generic in their application .to' similar parts as the art will permit.

Referring to the accompanying drawings, and

I more than 25 to50 percent as the space current more particularly to Fig. 1, a phonograph pick-up In is shown which is connected to two contacts of a double-pole, double-throw, transfer switch II, the other contacts of which are connected to the circuit of a photo-electric cell l3 including bat tery l5. Light from a lamp I5 is focused by means of a lens l1 through an aperture la in an aperture plate l9 upon the sound record of a moving photographic film 23. The light which passes through this film then impinges upon the photoelectric cell l3.

The blades of the transfer switch II are connected to the primary winding of input transformer 2|, one side of the secondary of which is connected to ground and the other to the grid of a separate heater type triode amplifier tube 22.

The cathode of this tube is connected to ground through bias resistor 33, in parallel with by-pass condenser 3|. The plate supply for tube 22 is taken from battery 23, the negative side of which is grounded, and the positive side of which is connected through feed resistor 28a and coupling resistor 29 to the plate of tube 22. A by-pass condenser 23b is connected from the junctions of feed resistor 28a. and coupling resistor 29 to ground. Battery 28 serves also as plate supply to other tubes to be mentioned later.

A blocking condenser 23, the primary of the tone control transformer 25 and one resistance winding 25 of a manual volume control rheostat are connected in series between the plate of the amplifier tube 22 and ground. The ends of the secondary windings of transformer '25 are connected to the plates of two space discharge tubes 32, the alternating currentimpedances of which control the tonal transmission in a manner to be described. Space current for the tubes 32 fiows through feed resistor 35, one terminal of which is able contacts to resistance winding 21, one end of 5 which is connected to primary of coupling transformer 31, of which the other end is connected to connected to the positive side of battery 28* and v the other terminal to the center tap of secondary of transformer 25. The secondary winding is so arranged that the D. C. space currents'to the two plates produce opposite magnetizing effects upon the core of the transformer. The cathodes of tubes 32 are joined together, and to the junction of bleeder resistors 33 and 34, which in turn are connected in series between center tap of the secondary of transformer 25 and ground. Resistor 34 serves also as a cathode bias resistor, and the circuit is so designed that the value of the voltage across this resistor 34 does not increase of tubes 32 changes from zero to maximum value. The grids of tubes 32 are connected together and between them and ground are two resistors 54 and 53, the functions of which will be described later. The cathodes of tubes 32 are connected to one side of filtering and timing condenser 35, the other side ot which is connected to the grids. The impedances of tubes 32 are controlled by the voltage across the condenser 35, which varies in accordance with the nature of the signal output of amplifier tube 22 in the manner to be described.

The winding of the transformer 25 which is connected to the plates of tubes 32 is referred to as a secondary winding because the tubes 32 serve as power consuming devices, and not as an amplifier of tonal frequencies. The balanced nature of the windings prevents variations of voltage' across grid-cathode condenser 35 from producing similar variations, of current through the primary winding of the transformer. The impedance loading upon the secondary of transformer ground. The secondary terminals of transformer v 31 are connected in a be anced fashion to the grids of two driver amplifier triode tubes 38 of 10 the separate heater type, and the center tap of the secondary of the transformer is connected to ground. The cathodes of tubes 38 are connected together and connected to one side of bias resistor 45, the other side of which is connected to ground. In parallel with resistor 45 is a bias by-pass condenser 46. The plates of tubes 33 are connected to the primary winding of coupling transformer 39, the center tap of which is connected to the positive side of battery 23. The 39 secondary winding of transformer is connected in a balanced, fashion to the grids of two power amplifier triode tubes 40, and the center tap of the secondary winding is connected to the' negative side of bias battery 41, the which is connected to ground. The filaments of tubes 40 are connected in parallel, and are bridged by center tapped filament resistor 48, the center tap being connected to ground. The

plates of tubes 40 are connected to the primary 30 of output transformer 4|. The center tap of the primary of the transformer 4| is connected to the positive side of power supply battery 44, the negative side of which is grounded. The secondary of transformer 4| is connected to the voice coil 35 winding of loud speaker 42. The field winding 43 of loudspeaker 42 is connected across battery 44.

For producing variation in the control voltage across condenser 35, some of the energy amplified 40 by tube 22 and fed through condenser '23 is diverted to the grid of amplifier tube 5| of the separate heater type. For this purpose, a filter coupling network 49 is connected between the junction of condenser 23 and primary of transformer 25, the grid of tube 5|, and ground, this filter being of the high pass type and terminating in resistor 53 connected between grid of tube 5| and ground. This resistor 50 serves to terminate the filter, and to make the operating or 50 average grid potential substantially that of ground. The cathode of the amplifier tube is connected to one end of bias resistor 52 the other endof which is connected toground. In parallel with resistor 52 is by-pass condenser 53. Plate-m current for tube 5| is supplied from the positive side of battery 28, and fiows to the plate through feed resistor 58, and coupling resistor 55. The

connection between resistors 55 and 53 is connected to one terminal of by-pass condenser 51, 00

the other terminal of which is connected to ground. The plate of amplifier tube 5| is connected to one terminal of the blocking and filtering condenser 55a, the other terminal of which is connected to one terminal of poten- (E3 tiome'ter 56, the other terminal of which is connected to ground. The variable contact of the potentiometer 55 is connected to the grid of triode amplifier tube 54 of the separate heater minal of the primary, of coupling transformer 75 positive side of 5:5

The cathode of tube 54 is connected to 70 6| the other terminal of which is connected to an end of feed resistor 65, the other end of which is connected to the positive side of battery 28. The Junction of resistor 65 and primary of transformer 6| is connected to one terminal of by-pass condenser 6511, the other terminal of which is connected to ground. One terminal of the secondary of transformer 6| is connected to the cathode of the triode type rectifier tube 62 of the.

separate heater type. The other terminal of the secondary is connected to one end of rectifier load resistor 63 and also to one end of resistor- 64 which serves as a coupling, timing and limiting resistor. The other end of resistor 63 is connected to ground, and the other end of resistor 64 is connected to the grids of tubes 32. The

up ill or the photo-electric cell 3 passes to the amplifier 22, where it is amplified and fed through the blocking condenser 23 and primary of the tone control transformer 25 to the volume control adjustable rheostat windings 25 and 21. From the winding 21 theenergy passes through the transformer 31 to the input of the driver amplifier tubes 38. Here it is further amplified and then passes through the transformer 39 to the input of power amplifier tubes 40. The output of the power amplifier tubes 40 then passes through the transformer II to the loud speaker 42, where it is reproduced as speech or music in the usual manner.

For controlling the operation of the automatic variable filter, some of the energy from the output circuit of the amplifier 22 passes through the high pass filter network 49 to the amplifier 5|. Here the energy is amplified and fed through the potentiometer 56 to the second amplifier 54. Here the high frequency energy is further amplified and is fed through the transformer 6| to the rectifier 62, where it is rectified to produce a current fiowing unidirectionally through resistor 63 in the direction of the arrow.

' The pulsations of rectified current are smoothed by resistor 64 and condenser 35 to produce a control potential on the grids of tubes 32. The amount of potential change produced is dependent upon the strength and frequency distribution of electrical energy in the output of amplifier tube 22, and the grids become more positive as the energy is increased without change of distribution, or as the energy distribution is shifted toward the high frequencies without change of energy. In the absence of signals, the tubes 32 are biased sufliciently by resistor 34 to reduce the space currents substantially to zero so that the secondary of transformer 25 is not loaded, and the effect of the presence of the transformer primary is principally the choking effect due to the actual inductance of the primary winding. This discriminates against the passage of higher frequency signals, including surface noise, for example in the range of 1000 to 4000 cycles.

The discrimination against the higher tones is shown by curve III in Fig. 2 which shows the lowered transmission of the higher frequencies from tube 22 to the volume control resistance 25 caused by the filter including the primary of transformer 25, when the rectifier current through resistor 83 is small and the control tube impedances are high. This corresponds to the transmission when the needle is in the record groove in the absence of any appreciable recorded signal capable of causing rectification by the tube 62. 1

When there are recorded signals of sumcient strength or of suificiently high tone to mask the presence of the background noise discriminated against by the choking action, the rectifier circuit is operated to reduce the bias on tubes 32. This decrease is in accordance with the characteristics of the control channel as for example shown by curve 1|, which shows the relative transbe produced by a small amount of high frequency energy or. a larger amount of lower frequency energy. When the rectified energy is large so that the impedances of the control tubes 32 are low, and the equivalent primary inductance very small, the transmission of the main channel is according to curve 12, with greatly improved transmission for the higher frequencies over that shown by curve 10. While the condition illustrated in curve 12 permitsthe passage of background noises, this occurs only when desirable signals sufficiently strong in the background noise range, or much stronger in the lower tonal ranges, exist; hence the presence of the background noises will be completely masked. The constants of the rectifier-circuit are so chosen, as by making capacity of condenser 35 small, that control action is rapid and changes of transmission characteristics occur change of the nature of the signals.

This speed of action is not hindered by any delay of the plate circuit operation, since the change of plate current does not change the magnetic energy content of the iron core, and therefore no inductive voltages are produced across either half of the secondary windings of transformer 25.

In conventional amplifiers, fixed scratch or noise filters are used, with condensers and inimmediately upon ductors which are adjusted for optimum transmission under conditions of weak or no signals, corresponding for example to curve 10 of Fig. 2. By this invention, with a rapidly acting automatic tone control system, the freqeuncy range of any type of record may be effectively increased, since the filtering action applied against tones in the higher frequency range is automatically reduced so that the filtering action is never greater than necessary for a given signal energy distribution. The circuit therefore, for a given type record, operates to reproduce on the average much higher frequency tones than would be permissible if background or surface noises were suppressed by-co-nventional type scratch filters.

In the modified form. of the invention shown in Fig. 3 the phonograph and photo-electric pickwith those of Fig. 1 and also have been omitted. In Fig. 3 the secondary terminals of the input transformer 2! are connected to the grids of two space discharge tubes BI and 82 of a pushpuil amplifier 30. These tubes are of the separate .heater type. The cathodes are connected together and to one terminal of bias-resistor a and of bias resistor by-pass condenser 80b, the other terminals of which are grounded.

Plate supply for the plates of tubes 8| and 82 is taken from battery H8, the negative side of which is grounded. This battery serves as supply also for other tubes of Fig. 3. Filaments of tubes 8i and 82 are heated by a source not shown, preferably a secondary winding of a transformer, the center tap of which is grounded or connected to the cathodes of the tubes which are heated thereby. It will be understood that the filament and plate supply may be from any suitable source. Bridged across from. plate of tube 6! to plate of tube 82 is a variable low frequency absorption circuit and plate feed circuit 38 comprising two resistors 89, two chokes 9t and the primary of a tone control transformer 92, across whichis shunted a tuning condenser The center tap of the primary of transformer $52 is connected to the positive terminal or" battery M8.

Also bridged from plate of tube iii to plate oi tube it? is a variable high frequency power ab sorption circuit and intertube coupling circuit comprising in series one primary of coupling transformer 83, the primary of the tone control transformer 9'? and the other primary of coupling transformer 38.

The secondary terminals of tone control transformers t2 and ii? are connected in a balanced fashion to the plates of two sets of tone control space discharge triode tubes 35 and 328 of the separate heater type. Center taps of sec ondaries of these transformers are connected to one terminal or a feed resistor lit, other ter minal of which is connected to the positive terminal of the battery li'ii. Between the center tap of the'secondary of transformer and ground are connected in series the bleeder resistor iZ l and the tapped cathode bias and limiter resistor i ll. Similarly connected between the center tap of the secondary of transformer and ground are bleeder resistor M5 and cathode bias re sistor M6. The cathodes of the tubes Q5 are joined together and to the junction of resistors i2 5 and ill, and the cathodes of the tubes 98 are joined together and to the junction of resisters E25 and 528. The grids of tubes 95 and. 98 are connected together, and between them and ground are connected in series, the resistors H6, H5, and M9, the functions of which will be described later. Filtering, timing and bypass-= ing condensers 95a, 98a, 95b, and 932) are connected between the grids and cathodes of tubes 95 and 98, respectively, and between the cathodes of tubes 95 and 98 respectively and ground.

The secondary terminals of the coupling transformer 83 are connected to two terminals of manual volume control potentiometers 85 and 86, the other ends of which are connected together, to ground and to the center tap of the secondary of transformer 83. The adjustable contacts of potentiometers 85 and 86 are connected to the grids of a pair of tubes 81a and 81b of a push-pull driver amplifier 81. These tubes are of the separate heater type. The cathodes are connected together, and between them and ground are connected in parallel a. cathode bias resistor 81c and a cathode bias resistor by-pa s's condenser 81d.

\ The plates of tubes 81a and 81b are'connected to the primary terminals of transformer 39, the center tap of which is connected to the positive end of battery 8.

As shown in Fig. ;1, and explained above, the secondary windings 39 are connected to the grids of a power amplifier 40 which in turn operates a. loud speaker 42.

For controlling the potentials of the grids of tubes and 98 there is provided between plate of tube 82 and grid of a triode amplifier tube N3 of the separate heater type, and ground, a high pass filter ill! of the condenser-resistor type with condenser elements in series and resistor elements in parallel. Connected between grid of tube 503 and ground is a filter-terminating and bias-determining resistor I02. Between the oathode of tube ")3 and ground are in parallel a cathode bias resistor I030 and by-pass condenser with.

Between the positive terminal of battery H8 and plate of the tube 33 are connected in series 2. feed resistor iii ands. coupling resistor W4. Between the junction of the resistors i2l and lil i and ground is connected a by-pass condenser iilt Etween the plate oi tube I63 and ground is connected an output circuit comprising blocking condenser iota and potentiometer we in serice, the variable contact of potentiometer being connected to the grid of the triode ampliiier tube till. Between the cathode of the tube to? and ground are connected in paraliel a bias resistor and by-pass condenser sou). Between the positive endof the battery l lii and the plate of tube 5%? are connected in series a feed resistor 52?. and. a coupling resistor 66%, the junction of which connected to one terminal of bypass condenser M2, the other terminal of which is grounded.

Between the ground and plate of tube llil is connected an amplifier output circuit including a first plate-cathode branch of a double unidi rectionally conductive tube H3, in parallel with a rectifier circuit resistor ms. A blocking condenser ill is connected from the junction of resistor hi9 and the first cathode of tum H3. From this junction to the second plate of tube H2; is a smoothing and limiting resistor H5, and from the second plate of tube H3 to the grids of tubes and lit is a smoothing and limiting rcsistor i it. The cathode of the second plate-cathode branch of tube i i3 is connected to the tap on resistor ill. I

It will be understood that while condensers soc, H2,-lfl3b, Niki-and Ill have been described as by-pass and blocking condensers, the values of capacitance may be chosen less than if the amplifier circuits of tubes I03 and I01 were desired to amplify uniformly at all frequencies. Lowering the capacitance of any of these condensers decreases the ability of the amplifier circuits to amplify low frequency tones. Accordingly the entire filtering action from plate of tube 82 to the first cathode-plate branch of tube I I3 is not concentrated in filter H, but involves further discriminatory action of the amplifier circuits in favor of the higher tones.

In the operation of the modified form of the invention shown in Fig. 3, the energy from the pick-up device passes through the transformer 2| to the push-pull amplifier 80, where it is amplified and then passes through the transformer 83 to the second push-pull amplifier 81. Here it is further amplified and passes through the transformer 3% to the power amplifier 40 shown in Fig. 1,

" thence through the transformer 4| to'the loudspeaker 42 where it is reproduced as speech or music in the usual manner, The transmission from tubes 8| and 82 to tubes are and 81b is dependent upon the internal impedances of the tone control tubes 95 and 98, which in turn are dependent upon the potential of the grids of these tubes with respect to cathodes. In general lowered impedances of tubes 98 decrease the choking action of the primary winding of transformer 83 in a manner similar to that of similar elements of Fig. 1. This results in increased transmission of higher frequencies. On the other hand, the circuit 88 shunted from plate of tube 8| to plate of tube 82 is designed so that chokes 98 prevent the absorption of higher frequency energy, and this circuit modifies the transmission chiefly for low frequencies. The lowering of the internal impedances of tubes 95 results in a lowered pri-- mary impedance of transformer 92, and more loss of transmissionforthe low frequencies.

For producing control voltage, some of the energy from the output of tube 82 is fed through filter IM to the grid of amplifier tube I83. The filter elements of I8I are of high impedance and the amount of energy diverted to amplifier I83 is relatively small. The filtered energy is selectively amplified by tube I83 and associated circuits, and impressed upon the grid of tube I8'I. It is again selectively amplified and impressed upon the first plate-cathode branch ,of the tube II3. This tube rectifies due to its undirectional characteristics, since it passes current only when the cathode is negative with respect to the grounded plate. The rectifier resistor I89 carries A. C. tonal current and also rectified D. C. current, the direction of which is indicated by the arrow. As a result the D. 0. potential of the first cathode becomes positive with respect to ground although the potential goes negative instantaneously when the rectifier is operating. The pulsations of potential of this cathode are smoothed out by resistors H and H6 in series, and condensers 95a, 95b, 98a, and 981 so that a potential between grids and cathodes of the tubes 95 and .98 exists substantially devoid of the high tonal frequencies present in resistor I89. This potential changes with change of the amount of energy impressed upon the rectifier circuit. The second plate-cathode branch serves as a spillway for rectified energy whenever the potential of the plate exceeds the potential of the cathode which is the same as that of the tap on resistor IN. This makes the potential of the grids of tubes 95 and'98 change more on a proportionate basis when the tonal signal energy is low than when it is high. It is understood, however, that even if the limiting action of the second plate-cathode branch is not used, limiting actions will occur when the grids of tubes 95 and 98 become positive with respect to the cathodes, due to the resultant voltage drop through resistors H5 and 5 against low tones when -high tones are especially present.

' The transmission cha acteristics may be better understood with reference to Fig. 4, in whichtransmission of various parts of the system is shown as dependent upon the frequency of the energy being transmitted. Curve I28 illustrates thetransmission of energy through the control channel, showing the relative ability of energies of different frequencies at the secondary of transformer 2| to produce control potential on the grids of tubes 95 and 98. Curve I2I shows the relative transmission of energies of different fre-' quencies from the transformer 2| to transformer 39 under the condition of the control circuit passing negligible energy, and curve I22 shows the same under the condition of passage of maximum potential change due to transmissionof' a sufflciently large amount of energy through the control circuit.

It will be understood that musical selections utilize volume contrasts and tonal contrasts. In addition to the use of the circuits of .Fig. 1 and Fig. 3 to minimize surface noise effects, they may be used to emphasize the tonal contrasts. Thus at a given acoustical volume level, when the emphasis is intended to be upon the low tones, the circuits automatically increase the emphasis on the low tone, and when the emphasis. is intended to be upon the high tones, the circuits automatically increasethe emphasis on the high tones.

It will be understood, however, that the use of a transformer with an automatically variable loading is not limited to the arrangements specifbe achieved by use of certain transformer arrangements in combination with condensers and resistors. The curves of Fig. 6 show the insertion loss produced by the circuit of Fig. 5, when transformer I38 and primary condenser I3I, with different secondary loadings due to resistor I31 and thermionic impedance in parallel, are cut in between resistors I32 and I33. The insertion loss is measured by the decibel decrease of energy to resistor I33 when switch I35 is transferred icallyshown. For example, in Fig. 5 to Fig. 8 are showntypical insertion loss effects which can from the up position to the down position. Sim

ilarly the curves of. Fig. 8 show the insertion loss producible by the arrangement of Fig. '7, for

which the network consists of a transformer I42- with variable secondary loading, and with a condenser I48 and resistor MI shunting the primary, the transfer switch connecting the transformer primary in parallel with the receiving resistor.

While in the circuit of Fig. 5 the variable impedance element especially controls'the passage of high frequency tones, in the circuit of Fig. 6 it especially controls in an inverse sense the passage of low frequency tones.

- This control may be in accordance with any desired tonal stimulus, with increased signal causing either increased or decreased loading. It will be understood that the invention is not limited to the arrangement of Fig. 1 or Fig. 3. For example it is within the scope of the invention to use the control arrangements of Fig. '7

and Fig. 8, in an automatic tonal compression" device, for use in producing records. For this purpose the low secondary impedance R.=10,000 ohms would correspond to strong signals, and the amount of volume compression would be especially great for the lowest tones. Increased effects may be produced by using a plurality of tone control elements preferably in different stages of the 6 phonograph or talking picture recorder or reproducing amplifier.

' While certain specific embodiments of the lnvention have been set forth for purposes of 11 lustration, it is to be understood that various changes and modifications may be made therein as will appear to a person skilled in the art. The scope of the invention is only to be limited in accordance with the following claims when interpreted in view of the priorart.

What is claimed is:

1. In a system for recording or reproducing sound, a transmission channel having a filter network therein including a choke coil, said choke coil constituting a primary of a transformer, means applying a balanced load to the secondary of said transformer whereby variations in said load are prevented from reacting on the flux in the core of said transformer and means responsive to the signal energy for automatically varying said load so as to change the characteristics of said filter in accordance with the characteristics of said signal energy.

2. In a system for recording or reproducing sound, a transmission channel havinga filter network therein including a choke coil, sai choke coil constituting a primary of a transformer, a pair oi variable impedance devices connected across the secondary of said transformer so as to provide balanced, load thereon, whereby the variations in load are prevented from reacting on the'flnx in the transformer core and means responsive to the signal energy for automatically varying the impedance oi said devices soas to change the load on said. transformer in accordance with the characteristics of the signal energy.

S. In a system for recording or reproducing sound, a transmission channel having a filter network therein, a transformer having a primary and a balanced secondary, said primary being disposed in said filter network, means applying a variable load to said secondary, said load being adapted to vary the effective primary impedance without substantially changing the flux density of the'core, the primary impedance when the secondary is unloaded serving to prevent passage of the high frequency components of the signal and, when the secondary is loaded, serving to progressively pass more of said high frequency components, and means responsive to the certain frequency components of. the signal for controlling said load.

4. In a system for recording or reproducing sound, a transmission channel having a filter network therein, a transformer having a primary and a balanced secondary, said primary being connected in said filter network so as to control the transmission characteristics thereof in accordance with the primary impedance, a pair of space discharge devices connected to the balanced secondary and adapted to variably load the same for altering the effective primary impedance of said transformer, a control channel responsive to certain components of the signal,

means in said control channel to produce a control voltage proportional to the strength of said components and means applying said control voltage to said space discharge device so as to vary the impedance thereof in accordance with the strength of said components.

5. In a system for recording or reproducing sound, a transmission channel having a filter network therein, a choke coil in said network, means to vary the impedance of said choke coil without changing the flux density thereof and means responsive to the signal energy for controlling said last means so as to vary the transmission characteristics oiLthe filter in accordance with applied signal.

6. In a system for recording or reproducing sound, a transmission channel having a filter network therein, an inductive impedance in said network, means varying the value of said impedance in accordance with the applied signal and means to prevent such changes in impedance from causing a change in the direct current flux of said impedance whereby the timing is not infiuenced by the reluctance of the flux path.

7. In a system for recording or reproducim sound from a, sound record, a transmission channel having a filter network therein including an inductive impedance device, and means applying a variable balanced load to said device which.

and a balanced secondary, meansapplying a var-=- iable load to said secondary, means responsive to the signal energy for automatically varying said lead in proportion to the change in signal energy, said primary being so disposed in said filter network that the impedance thereof, when the secondary is unloaded, prevents passageoi high frequency components and an increase in the secondary load serves to lower the primary impedance so as to permit passage of the high frequency components.

9. In a system for recording or reproducing sound, a transmission channel having a filter network therein, a transiormer having aprimary and a balanced secondary, said primary being disposed in said filter network, means epplying a variable load to said secondary, said load being adapted to vary the eilfective primary impedance without substantially changing the fins: density of the core, the primary impedance, when the secondary is unloaded, serving to prevent passage oi. the high. frequency components of the signal and, when the secondary is loaded,

serving to progressively pass more of said high frequency components, and means responsive to the high frequency components of said signal for varying said load in proportion to the strength and frequency distribution of said components,

wherebysaid components are passed by said filter network only when they are of a predetermined minimum strength.

10. In a system for recording or reproducing sound, aftransmission channel having a filter. network therein, a transformer having a primary and a balanced secondary, said primary being connected in said filter network so as to control the transmission characteristics thereof in accordance with the primary impedance, 9. pair of space discharge devices connected to the balanced secondary and adapted to variably load the same for altering the eflective primary impedance of said transformer, a control channel responsive to the high frequency components of the signal, means in said control channel to produce a control voltage proportional to the strength of said space discharge devices so as to vary the impedance thereof in accordance with the strength of said components.

11. In a system for recording or reproducing sound, a transmission channel having a filter network therein, a transformer having a primary and a balanced secondary, said primary being connected in said filter network so as to control the transmission characteristics thereof in accordance with the primary impedance, a pair of space discharge devices connected to the balanced secondary and adapted to variably load the same for altering the effective primary impedance of said transformer, a control channel responsive to the high frequency components of the signal, means in said control channel to produce a control voltage proportional to the strength of said components and means applying said control voltage to said space discharge devices so as to change the impedance thereof inversely as the strength of said components whereby a balanced load is applied to said secondary which varies directly as the strength of said components.

12. In a system for recording or reproducing sound, a transmission channel having a filter network therein, a transformer having a primary connected in said filter network and having a balanced secondary, means variably loading said secondary in accordance with the characteristics of the impressed signal for altering the effective primary impedance, the primary impedance. when the secondary is unloaded, being such as to discriminate against the higher frequency components of the signal, the discrimination be-,

ing progressively reduced as the secondary is loaded and, under conditions of maximum load, discriminating somewhat in record.

' 13. In a system for recording or reproducingY sound, a transmission channel having-a lfilter network therein, a variable impedance in said network comprising the primary ofa transformer, means to variably load and means responsive to the signal energy-for controlling said load so that signals in'the background noise frequency range are only trans-' network therein, a transformer having a primary connected in said filter network to control, by its impedance, the transmission characteristics thereof, a pair of variable impedance devices connected in balanced relation to said secondary for applying a balanced load thereto dependent upon their impedance, and means varying the impedance of said devices in accordance with the components of the signal which are'normally suppressed by said filter.

15. In a system for recording or reproducing sound, a transmission channel having a filter network therein including a shunt choke coil, said choke coil constituting a primary of a transformer, means applying a load to the secondary of said transformer and means responsive to the signal energy for automatically varying said load so as to change the characteristics of said filter in accordance with the characteristics of said signal energy.

16. In a system for recording or reproducing sound, a transmission channel having a filter network therein including a shunt choke coil, said choke coil constituting a primary of a transfavor of the lower frequencies so as to compensate for the under cutting of the lower frequencies on the sound, v st {nae-is aD the secondary of saidtransformer without changing the fiux density former, a variable impedance device connected to variably load the secondary of said transformer and means responsive to the signal energy for automatically varying the impedance of said device so as to change the load on said transformer in accordance with the characteristics of the signal energy.

17. In a system for recording or reproducing sound, a transmission channel having a filter network therein, a pair of transformers having primary windings connected respectively in shunt and in series in said filter network, means applying a variable load to the secondary windings for varying the effective primary impedances, and

cordance with the primary impedances, a pair of.

space discharge devices connected to each balanced secondary and adapted to variably load the same for valteringthe effective primary impedanceoffth "transformers, a control channel responsivejto; ertain components of the signal,

me s aid "ii -"trol .vO t-agefoliortion'al-tol'the strength of said components and: mfeans japplying said control :voltage to said- .spac e 'discharge devices so'as to ntrolchannel to produce a con-.

h'a-rige-f the impedance; thereof inversely as the reng'th' of-saidicomponents whereby a balanced pliedfto' the secondaries which varies directly 'thej streng-thiof,saidcomponents.

'- 9 'In acsystem for recording or reproducing sound, having a"-transmissio'nchannel including ;-a-varia-blelimpedance device constituting the sec- 'o'nda'ry. of: a transformer, the method of varying .-'the' iinpedance of said device which comprises appilyingga variable balanced load to the second'aryawhich-fvariesinproportion to the characteristicsof the. signal, and causing the changes in the secondary load to alter the effective pri- .ma'ry impedance of the transformer in such manner that certain frequencies are suppressed by the filter except when they are of a predetermined minimum strength.

20. In a system for recording or reproducing sound having a transmission channel including a filter network and a variable impedance device in said filter network comprising the primary of the transformer, the method of varying the transmission characteristics of said network which comprises variably loading the secondary of said transformer in accordance with the signal while maintaining the flux density of said transformer substantially independent of said load.

21. In a system of the class described having an audio frequency transmission channel including a variable filter, the method of producing t nal contrast which comprises utilizing the average volume of the high frequency tones applied to said channel for varying the frequency transmission characteristics of said filter in such manner that the filter discriminates-in favor of low F (ill

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