USRE20422E - Sound translating system - Google Patents

Description

June 22, 1937. A. PFISTER SOUND TRANSLATING SYSTEM Original Filed July 14. 1954 V 2 Sheets-Sheet 1 IAAIAAAA OIUIOIIO Reiaued June 22, 1931 'PATENT orrica SOUND TRANSLATING SYSTEM i Arthur Pflster, Queens Newtone Engineering Inc., New York, a corporation of New York Village, N. Y., as sign'or to N. Y.,

Original No. 2,052,110, dated August 25, 1936, Se-

rial No. 735,190, July reissue April 8, 1937,

29 Claims.

The system may likewise include suitable tone I control devices arranged to produce different ltransrnission characteristics for different frequency registers. The apparent quality of the sound, on reproduction, may thus be improved.

According to a preferred form of the invention as applied to a sound reproducing system a portion'of the sound energy 'may be passed through an amplifier and a frequency selective circuit and fed to a sound output device. Another portion of the sound energy may be passed through a control circuit where it may be averaged and utilized to control the gain ratio of the'amplifier. Control is eifected preferably by varying the potential applied to a separate control electrode situated in the space discharge path of said amplifier.

Limiting devices may be included in the control circuit to determine the limits to'which the control potential may be varied without causing distortion of the selection. The control channel may likewise preferably include sound frequency suppression elements to prevent sound frequencies from reaching the control electrodes and causing distortion of the selection.

Adjustable delay means may also be included in the control channel whereby the rate of change in gain ratio, on crescendo and decrescendo, may be adjusted according to the characteristics of the selection, whether it be speech or music, to render its reproduction'more pleasing.

Other refinements of the present system, whereby its operating characteristics are improved and the translation of the sound selection is rendered more perfect, are more fully brought out in the following detailed description.

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

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended hereto, the invention itself, as to its objects and advantages, and the manner in which it may be car- 14, 1934. Application for Serial No. 135,702

ried out, may be better understood by referring.

to the following description taken in connection with the accompanying drawings forming a part thereof, in which Fig. 1 shows an embodiment of the invention in a sound reproducing system;

Fig. 2 is a diagram illustrating action;

Fig. 3 is a diagram illustrating the controllable delay on crescendo and decrescendo;

Fig. 4 is a diagram illustrating tone compensating action; and I Fig. 5 is a diagram of the variation in volume range of a selection on recording and reproducing.

In the following description and in the claims, various details will be identified by specific names for convenience, but they are intended to be as limiting control generic in their application as the art will permit.

In the drawings accompanying and forming part of this'specification, certain specific disclosure of the invention is made for purposes of explanation, but it will be understood that the details may be modified in various respects without departure from the broad aspect of the invention.

Referring now to the drawings, and more particularly to Fig. l, a sound reproducing system is shown having an electro-mechanical pick-up device I arranged to pick up sound energy from a mechanical sound record and a photoelectric cell 2 arranged to receive sound energy variations from a photographic sound record.

These pick-up devices opposite poles of double-pole, double-throw switch 3 adapted to connect either device to the input circuit of potentiometer 4L Potentiometer 4 is provided with sliding contacts 5 arranged to vary the input of energy to primary winding 5 of transformer 1.-

Secondary winding 8 of transformer I is connected in the input circuit of vacuum tube amplifier 9. The output circuit of amplifier 9 comprises the primary winding 10 of audio-frequency transformer II and also the input circuit l3 of the control channel. I

Secondary winding I2 of transformer l I is connected in the input circuit of amplifier l4 comprising tetrode vacuum tubes l5 and I6. Tube l5 contains cathode l1, anode l9 and two grid or control electrod es 2| and 23. Tube l6 contains cathode l8, anode 29 and two grid or control electrodes 22 and 24.

.Control electrodes 2| and 22 are located nearest the cathodes and are connected to the output l and 2 are connected to transformer 29.

' is determined by thebiasing of the control elec-' I circuit 2| of the control channel, as will be more fully described later.

Control electrodes 23 and 24 are located nearest the anodes and may, ifdesired, comprise r shield grids for the anodes. These control electrodes are connected to the ends of secondary winding l2 of transformer H. with biasing battery 26 connected to the midpoint thereof, thus provides part of the :sound energy input circuit for necting the input circuit amplifier II. By conto the shield electrodes distortion will be kept at a minimum. It is quite feasible, however, and within the scope of the present invention, to con-' nect the signal input circuit to grids 2| and 22 and the control circuit to grids 23 and 24.

The anode or output circuit of amplifier I 4 com-' prises primary winding 28 of audio-frequency I The mid-point of winding 28 is connected to plate biasing battery 27.

Transformer 29 is designed with certain desired irequency characteristics whereby it wiil act as a band-suppression filterat low signal volumes to reduce the transmission of the intermediate frequency registers to a greater extent than the high and low registers. however, the transformerhas substantially uniform efficiency throughout the audible frequency range.

The correct design for transformer 28*may be determined by well known and standard methods and will depend upon the characteristics of the amplifier tubes II and It with which it is to be connected. the transformer windings are tuned to discriminate against intermediate frequencies. With a tuned circuit of this type it is well known that the frequency discriminating effect will be a direct function of the circuit impedance. In the present instance the impedance will be determined by the amplifier resistance which, in turn,

trodes.

When the tube impedance is high the input load for the transformer primary 28 is negligible. Under these conditions the frequency discrimination is most effective and the transmissionof high and low frequencies is accentuated due to the tuning of the transformer. When the tube impedance is lowered the input load for primary 23 will be raised and the frequency discrimination will decrease until substantially uniform response over the entire frequency range will be obtained for high signal volumes. A

Secondary winding 30 of transformer 29 is connected in the input circuit of push-pull amplifier 34 comprising vacuum tubes 35 and. In the Present embodiment the nected to the inner grids which are adjacent to the cathodes. The shield grids (nearer tothe anodes) are connected through biasing battery 3| to the cathodes.

The outputcircuit of amplifier 34 includes the primary 33 of output transformer 33. The midpoint of winding 33 is connected to the cathode through pla e biasing battery 32 and battery 3|. Thesecondary ll of output transformer 33 is connected through potentiometer 4| to the moving coil of loud-speaker 43. The fixed coil of the speaker is supplied with current by means of battery I. Potentiometer; II is provided with input circuit is consliding contacts 42 for varying the volume of the anodes of a pair oi' two element At high signal intensities, envelope of the rectified In any event, certain sections ofnected to sliding contacts "to the anode or plate of i3 is connected to the input of amplifier tube I in the control circuit by a condenser-resistance coupling comprising condenser Ii and resistance 32.- The output of amplifier. Ellis connected through transformer 53 to push-pull ampliiler 54 comprising vacuum tubes "and 58.

The output of push-pull a the primary I! of transformer 53. Secondary winding '3 of transformer 53 is connected to the thermionic tubes BI and 82, together comprising a full-wave rectifler Bil.

The center of secondary winding 59 and the cathodes of tubes BI and 62- are connected to opposite ends of resistance 63, which will hereafter he called the control resistance. Current from rectifier 60 will pass through control resistance 63 rendering the upper end of the resistance (as seen in Figure 1) positive with respect to the lower end.

Connected in parallel with resistance 63-is a condenser 64 which serves as a filter for the audio-frequency fluctuations in the rectified cur- Circuit amplifier il includes rent from rectifier 60. The current through resistance 63 will thus'be averaged to represent-the currents or, in other words, the intensity of the audio-frequency signal.

Control resistance 63 comprises part of the output circuit 25 of the control channel.- The entire output circuit'includes the following elements in series: control grids 2| and 22 of amplifier l4, resistance 65, control resistance 33, biasing potentiometer 3'! -andcathodes i1 and 13 of amplifier ll. Biasing battery 66 is connected in parallel with potentiometer 6". to furnish a potential gradient across the potentiometer.- The normal or initial bias on control grids 2| and 22 may be adjusted by moving sliding, contact Bl along potentiometer 61.

Discharge devices 69 and III are connected across control resistance 63 and serve to limit the potential which can be developed across this resistance. The cathodes of these tubes are conand 14 adapted to slide on potentiometer resistances l2 and II, respectively, both of which are connected across biasing battery 66 in the same manner as potentiometer 61.

The anodes of tubes 68 nected to the 63.

and III are both conpositive end of control resistance The grid of tube 33 is connected directly the tube. Thefgrid. of tube Hi, however, is-connected to contact '13 arranged to slide on control resistance 63.

. A variable delay arrangement is also associated with output circuit 25 of the 'controlchannel. Thisdelay arrangement comprises resistance 65, connected in output circuit 25 and variable condenser 13 connected across both resistances 63 and 35. These elements form a condenser-resistance discharge device the inertial characteristics of which may be vatied by adjusting variable condenser 16.

Connected in parallel with resistance 85 are two three-element vacuum tubes 11 and II. These tubes are connected in parallel opposing relation, that is, the anode of tube I1 and the cathode of tube 18 are both connected to one end of resistance 65 and-the cathode of 11 and anode of II to theother end. '-'I'he grid elements of tubes l1 and 13 are connected, respec tively, to sliding contacts 13 and 80 arranged to slide on resistance 65. Thus the grid biases of the tubes may be varied independently to regu- 1m the individual internal impedance of these tubes; The inertial or delay action may thus be separately regulated ior build-up and decay of shown) dynamic expansion will take place. If

the switch is thrown to the up position the control resistance is shunted and the sound amplifier connected to operate without dynamic expansion at a gain ratio which will be determined by the setting of biasing contact ll assoclated with potentiometer 12. If the switch 8| is thrown to the "down" position, the control resistance is shunted to eliminate dynamic expansion, and the gain ratio oi the amplifier will be determined by the setting of biasing contact [4 associated with potentiometer 15.

Operation In the operation of the system in reproducing a selection from a mechanical sound record, pickup device i is placed in operative relation to the. moving sound track of the record, or if a. photographic recording is used, cell 2 is arranged to receive light variations from the moving photo.- graphic sound track. The chosen pick-up deviceis connected by switch 3 to potentiometer 4. Contacts 5 are adjusted to allow a desired volume of input energy to reach transformer I.

The sound energy passes through transformer I and is amplified by amplifier 8. Part of the amplified energy passes through transformer II and is further amplified by push-pull amplifier l4 whose gain ratio is simultaneously varied by the control grids 2i and 22.

The control of the gain ratio is brought about as follows: Part of the signal energy which has been amplified by vacuum tube 9 is fed through control input circuit l3 and the condenser-resistance coupling 5|, 52 to amplifier 50. The output energy from amplifier 50 passes through transformer 53 and is further amplified by push-. pull amplifier 54. The output of 54 passes through transformer 58.

The resulting alternating current is rectified by full wave rectifier 60 to yield a direct current which passes through control resistance 63 from the cathodes of rectifier 60- to the center of sec ondary winding 59 of the transformer. It will be obvious that the average strength of this direct current over any limited interval will be directly proportional to the strength of the input signal. Condenser 64 serves to filter out the audio-frequency fluctuations of the rectified current. The current through resistance 63 will then represent an envelope of the rectified cur-' rent impulses.

The potential across resistance 53 will accordingly be varied-along with the strength of the rectified current,'the upper end of the resistance (as shown in Fig.1) being made positive with frequency signals. By such control action the gain ratio of amplifier l4. may in some instances be varied over a range of 30 decibels between low and high signal volumes. By adjusting contact 68 on potentiometer I! the initial bias on the control electrodes and hence the initial amplification setting of the amplifier tubes l5 and I6 may be adjusted to any desired value.

Discharge tubes 69 and Ill serve to limit the change in bias which the control circuit may produceon the control electrodes. Tube 8.9 prevents the control channel from causing more than a predetermined maximum change in the bias oi control electrodes 2| and 22. controls the percentage change in bias which the control channel may bring about for any signal volume. r

The action of the limiter tubes may be illustrated by referring to Fig. 2 in conjunction with Fig. 1. 'With the settingof the potentiometer.

contacts 68, H and 14, as shown (Fig. 1) the anodes of limiting devices 69 and ill will be negatively biased with respect to their cathodes as long as no current is passing through resistance 63. When rectified signal current passes through resistance 53, thereby causing a potential drop in this resistance, the anodes of devices 69 and 10 will be made less negative and will occasionally become positive with respect to their cathodes when the higher signal volumes are reached. The anode of device 10, for example, will become positive when the potential drop across resistance 63, determined by the volume of input signal, has reached a value sufiicient to overcome the initial'bias on the anode. This is repre- 'sented by point I08 in Fig. 2. When this potential value is reached a discharge will begin between the anode and cathode of device 10, this device thereby serving to shunt a portion of the current received from rectifier 50.

While the anode or plate bias of tube 10 is varied by the change in total potential drop across control resistance 63 the grid bias of this tube may be varied as any fraction of the potential drop across the control resistance. This fraction is determined by setting sliding contact 13 to'any desired point on the control resistance. Accordingly, device ill will have a variable impedance the value of which is determined by the potential drop across control resistance 53.

Thus by setting contacts 13 and 14 the potential drop across control resistance 63, and hence the expansion characteristics of the amplifier.

' may be adjusted to correspond to any one of sev- Tube ll.

that the potential drop across resistance 63 has the values indicated by curve ill] for various values of input signal. In the reproduction of music the potential drop may preferably have the values shown by curve lil.

Should the input energy approach very high values, there is danger that the bias on control electrodes 2! and 22 may be reduced to an unduly small negative value thereby causing .distortion of the signal; Such excessive changes in the bins are prevented by discharge tube 69. When the potential drop across control resistance 63 increases to a point where it is suflicient to overcome the initial bias of tube 69 (indicated by point Hi9, Figure 2) this tube will begin to discharge. Since the grid element of tube 59 is connected directly to the anode, the imped- 7 contact Il tential drop across control resistance 83 will thus be held to a substantially constant value (indicated by horizontal line H2 in Figure 2) and the gain ratio of the amplifier will, of course, also be constant.

The efiect of the limiter tubes in shaping the gain characteristics will be more evident by comparing the above mentioned curves with curve 3 (Figure 2) which represents the variation in control potential for various values of input signal volume when no limiting devices are used.

During reproduction sudden changes in signal volume will often occur, as on rapid crescendo and rapid decrescendo. By variously delaying the change in gainratio of the amplifier when these changes in signal occur a more pleasing and natural efi'ect will be obtained. The variable delay circuit permits separate adjustment of the delay for crescendo and decrescendo.

If a sudden increase in signal volume'occurs during reproduction, as on rapid crescendo, the potential drop across control resistance 63 will increase immediately. The change in potential applied to the control electrodes will be delayed, however, due to the action of the variable delay circuit. The length of delay will depend on the capacity of condenser 18 and the impedance of tube Il. Likewise, onsudden decrease of signal volume, as on rapid decrescendo, the potential drop across resistance 63 will decrease with the decrease in volume but the change in potential 40 on the control electrodes will be delayed. The length of delay will depend on the capacity of condenser 16 and the impedance of tube 18. Adjustment of the capacity oi condenser 18 will accordingly vary the delay for both crescendo and decrescendo.

' dered..

Fig. 3 illustrates diagrammatically the delay on crescendo and decrescendo. the curves represent-- ing the change in potential across condenser 16 on build up and decay of signal. It is obvious that the gain ratio of amplifier II can be represented bysimilar curves.

. Curve I" shows the charging rate of the condenser for rapid attack or build up of amplification. In this instance, contact 19 will be set near the lower end of resistance I! (as shown in Fig. 1) and the impedance of tube 11 will be low. Curve 7 III shows the charging rate of the condenser for slow attack or-build up of amplification. The timelndicated on the graph by the distance between points I]! and I" represents the variation in delay which may be achieved by adjusting In addition, separate adjustment of delay on Curves I04 and I05 represent the ratesof dis-- charge of condenser 16 on signal volumedecay or decrescendo. Curve I shows the discharging rate of the condenser when contact "is set'fcr rapid decay of the signal (1. e. set near the upper end of resistance 65, as shown in Fig.-l)-."-1"'"Curve Hi5 shows the discharging rate of the condenser forslow decay or falling oil of amplification. In

this case, the contact 80 would beset near the lower end of resistance *as shown inaFig. 1. The time represented on the graph by the distance between points 106 and ill-1 represents the variation in delay on descrescendo which may be achieved by'adiusting contact 80.

It will usually be found desirable with high quality-recordings to adjust the variable delay circuit to cause rapid attack and slow .decay of the gain ratio with changes in signal volume. In some instances other settings may be found usefuland can readily be obtained by adjusting condenser 16 and contacts 19 and B0.

In case it is desired to operate the amplifier without dynamic amplification, i. e. without change of the gain ratio of the amplifier during the rendering of the selection, switch Cl is moved to the "up or down" position (as shown on Fig. 1). If the switch is moved to the fup"position the control electrodes are given a steady bias the value of which is determined by the position of contact H on potentiometer l2. 1 If switch II .is thrown to the down position, the control electrodes are given a steady bias determined by the position of contact 14 on potentiometer II. It will thus be seen that the amplifier may be set to operate at any one of a plurality of steady gain ratios by simply operating switch 8|.

Switch 8|, in combination with potentiometer contacts H and 14, provides a convenient means for testing and adjusting the bias on the limiting devices 69 and 10. Thus by moving the switch 8| to the up position and then adjusting con-' tact H to give amplifier H the maximum gain ratio at which it is desired to operate, and then returning the switch to the middle position, tube 69 will be given a bias which will cause it to shunt the control resistance when the said maximum value of gain ratio is reached. If switch Ii is thrown to the down position, contact I4 may be manipulated. In this case the resulting gain ratio of amplifier M will indicate the point at which tube ill will start to shunt the control resistance when the switch is again set for dynamic expansion.

With the switch 8| set for dynamic expansion the audios-frequency signal passing through transformer II will be variably amplified by amplifier ll under control of the control channel and will then be fed-through transformer 29 to amplifier 34. speaker 43 throughtransformer 39 and potentiometer II. The volume 01' output may be adjusted by moving contacts 42 associated with potentiometer l I.

The separate control electrodes in amplifier ll,

by isolating the control channel from the audiofrequency channel, prevent distortion of the signal by any audio-frequencies which are not filtered out of the control current bycondenser 84. Likewise, any even harmonics of the audio-frequency signal introduced into the output circuit of amplifier ll by the control circuit, will be suppressed due to the push-pull arrangement of tubes l5 and I 8.

Transformer II will exert a frequency selective eflcct on the signalias brought out'above). i. e.

Here it is further amplified and fedto loud I for low. signal values the high and low frequency registers willbe accentuated. For high volumes all frequencies will be transmitted with substantially equal efilciency. It is well known that when the volume of a sound is decreased the sensitivity of the human ear to the low and high frequency components falls of! morerapidly than the sensitivity to the intermediate frequencies. The present frequency selective arrangement compensates for this peculiarity or defect of the ear, whereby selections reproduced at lower volume levels than the original rendition will pro-- duce proportionally the same effect on the ear of the listener for all frequency registers. This effect is illustrated diagrammatically in Fig. 4.

If a highly intensity sound were produced having all frequencies present in equal intensities throughout the audible frequency range, the intensity at all frequencies would be represented on-the diagram (Fig. 4) by a horizontal line A, which gives the actual intensity of the sound in decibels for any frequency. The effect on the ear of a listener, or the apparent intensity at different'frequencies for this sound, will then be represented by curve B. If the sound were reproduced with the volume uniformly lowered through the frequency range, the output intensity of such a perfect" reproducer would be represented by horizontal line C.

For this decreased intensity, however, the high er will be affected to a lesser extent by the low volume high andlow frequency tones than by the low volume intermediate frequency tones, as indicated in curve D.

I The present reproducing circuit is arranged to compensate for this peculiarity of the human ear by reproducing the high and low frequencies with less reduction from the original intensities than the intermediate frequencies. This is illustrated in curve E, which represents the actual intensity on a'eproduction by the present circuit, of a sound having an original frequency distribution represented by curve A. j

The effect on the ear of the low volume sound represented in curve E, would then be equal for all frequencies, as indicated by horizontal line F, which coincides with horizontal line C on the frequency distribution diagram. The original high intensity sound is thus reproduced at a lower intensity while preserving substantially the same effect of all frequencies on the car as would be 0 be made nearly independent of signal volume.

produced by the original sound.

By adjusting input potentiometer 4, the frequency control action may be reduced or increased. By reducing the input, for example, the frequency characteristics of transformer 29 may The output volume,rof course, may be kept the same (when the input is reduced) by adjusting" ao,4aa I pensatingmeans are provided whereby sounds may be reproduced in a normal manner'for all signal volumes. Various other features have been provided whereby the operation of'the system in controlling sound energy has been improved.

It will be recognized that in the recording of selections on sound records of either mechanical or photographic type there are certain upper and .lower limits of volume above or below which tra. For purposes of comparison the flducial or v reference point, marked 0 decibels on the scale,

' has been taken to represent a volume level equal in electrical units to the power dissipated in a 500 ohm resistance having 2.5 volts across its terminals. The loudest sound reached by the orchestra may then have a value of 35 decibels above the reference level and the softest sound may be substantially decibels below that reference level,--

as shown. This minus 80 decibel point will correspond substantially to the threshold of hearing.

Even with modern high quality recording, the soft notes must be amplified before recording in order to bring them above the volume level of the background noises, such as scratch introduced by irregularities in the record. The loudest sounds on the other hand, must be attenuated in intensity in order to avoid overcutting of the sound track. For satisfactory high quality recording, i

the sounds might preferably be contracted to a volume range between plus 20 and minus 35 decibels as indicated in the area between A and B (Fig. 5). The sound may then be satisfactorily recorded.

On reproduction it is desirable to simulate the volumes of sound produced by the original sound source and hence the volume range may preferably be expanded as indicated in the area between C and D of Fig. 5. It will be noted that the softer sounds are actually decreased in volume on reproduction and thus needle scratch and background noises are suppressed or eliminated.

The present circuit presents an ideal means for reproducing and expanding the sound selection. It will be noted, however, that applicant's circuit is likewise adaptable for recording ,the sounds in which case the circuit will be arranged to compress the volume range as indicated in the area between A and B (Fig. 5).

While certain novel features, of the invention have been disclosed and are pointed'out in the annexed claims, it will be understood that various omissions, substitutions and changes may be made by those skilled in the art without departing from the spirit of the invention.

What is 'claimedis:

1. In an energy translating system for use with a sound record a first translating device,

7 a second translating device fed thereby and means between said devices for discriminating against intermediate sound frequencies when the signal energy is low and for passingall sound frequencies with substantially equal efficiency when said energy is high. I

2. In a sound record system, a pick-up device, a filter network'fed thereby, said network being adapted to reduce the transmission of intermedihigh signal intensities and a translating device ted by said network.

3. In a sound translating system for use with a sound record, a first translating device, an

' amplifier fed thereby, a transformerTed by said amplifier, said transformer discriminating against intermediate sound frequencies when said amplifier impedance is high and passing all sound frequencies with substantially equal efliciency when said amplifier impedance is low, and a second translating device fed by said transformer.

4. In a sound reproducing system for use with a sound record, a pick-up device, an output device ted thereby, means between said devices for discriminating against intermediate frequencies when the signal volume is low and for passing all frequencies with substantially equal efilciency when the signal volume is high.

5. The method of improving the characteristics of sound on reproduction from a sound record which comprises reproducing the high and low sound frequency registers in higher ratio 01' volume to the intermediate frequencies for low sound volumes than 101' high sound volumes.

- 6. In a sound translating system for use with a sound record, a first translating device, an amplifier ted thereby, a second translating device ted by said amplifier, a control circuit for varying the gain ratio of said amplifier and a governing device for varying the effectiveness of the control exercised by said control circuit over a substantial range of signal volumes.

7. In a sound translating system for use with a sound record, a first translating device, an amplifier ted thereby and a second translating device ied by said amplifier, a control circuit fed by said first translating device for changing the gain ratio 01' said amplifier according to a function of the energy received by said control circuit and a device ,ior governing the effect 01 said control circuit whereby said iunctlon may be varied. I j

8. In a sound translating system for use with a sound record, a first translating device, an amplifier ted thereby and having a control electrode, a second translating device fed by said amplifier,

means for varying the gain ratio of said amplifier ratio or said amplifier, a. first limiting device for determining the efi'ectiveness oi. said control circuit overa continuous ,range of values, and a secondlimitingdevice for determining the maximum' eflectiveness of said control circuit.

10. The method of predeterminin'g the maximum gain ratio of a dynamic amplifier used in. a sound translating system for a sound record and having a limiting device associated therewith which comprises adjusting the setting of said amplifier to operate at said maximum gain ratio and causing said setting to determine the point above which said limiting device will become cn'ective.

- 11. In a sound translating system for use with a sound record. a first translating device, an ammeans ior. delaying the action or said control plifler ted thereby, a second translating device fed by said amplifier and a control circuit for varying the gain ratio of said amplifier, said control circuit having a timing arrangement comprising a variable impedance and a variable capacitance for determining its rate of action.

12. In a sound translating system for use with a sound record, a first translating device, an amplifier fed thereby and having a control electrade, a second translating device fed by said amplifier, means for varying the gain ratio of said amplifier comprising a control circuit having a biasing impedance for determining the bias on said control electrode, said control circuit having means for delaying the control action, said means comprising a second impedance and a I variable condenser.

control action and comprising a second impedance in series with said biasing impedance'and means for varying the value of said impedance.

14. In a sound translating system for use with a sound record, a first translating device, an amplifier fed thereby and having a control electrode,

a second translating device fed by said amplifier, f

a control circuit for varying the gain ratio of said amplifier fed by said first translating device and having a biasing resistance in the circuit of said control electrode, a second resistance in series with said biasing resistance and said electrode, a capacitance in shunt with said two resistances, an impedance in shunt with said second resistance and means for varying the value thereof.

, I 15. In a sound translating system for use with a sound record, a first translating device, a second translating device fed thereby, a control electrode for varying the eificiency with which audio-frequency energy is transmitted from said first to said second device according to the potential applied thereto, means ior varying the potential applied to said electrode comprising a circuit element having a variable potential gradient across its terminals, an impedance in series with said circuit element, a capacitance in shunt with said element and said impedance and means for ad-- lusting the eflective'value of said impedance.

16. In a system for reproducing sound from a sound record, a pick-up device, an amplifier ted thereby and having a control electrode, a translating device fed by said amplifier, a control circuit fed by said pick-up device for varying the -.'gain ratio of said amplifier, a timing circuit connected between said control circuit and said control electrode, said timing circuit comprising 'a.

capacitance and a resistance in the charging and discharging circuit of said capacitance, a pair of unidirectional discharge devices oppositely connected in shunt with said resistance and means for separately adjusting the impedance of 89.1 discharge devices- 1 17. In a system for reproducing sound from a sound record, a pick-up device, an amplifier fed thereby and an output device red by said amplifier, a control circuit ted by said pick-up device for varying the gain ratio of said amplifier according to a moving average of the signalenergy.

circuit and means separately varying the length of delay for crescendo and for decrescendo.

18.; In an energy translating system .for use with a sound record, a first translating device, a transformer fed thereby, means rendering said transformer normally selective to predetermined frequency bands, means rendering said transformer non-selectively responsive to high signal volumes and a second translating device fed by said transformer. x

19. In a sound reproducing system for use with a sound record, a pick-up device, an output device i'ed thereby, and means between said devices forselectively discriminating against a predetermined band of recorded sound frequencies when the sound volume is low and for passing all recorded sound frequencies with substantially equal efficiency when said volume is high.

20. In a sound translating system for use with a sound record, a first translating device, an amplifier fed thereby and a second translating device fed by said amplifier, a control circuit fed by said first translating device for varying the gain ratio of said amplifier responsive to the energy received-from said device, and a variable limiter for varying the degree of control exercised bysaid control circuit responsive to the energy received by said control circuit.

21. In a sound translating system foruse with a sound record, a first translating device, an amplifier ted thereby and having a control electrode, a second translating device fed by said amplifier, means to vary the gain ratio of said amplifier comprising a control circuit including a biasing resistance for determining the bias on said control electrode, a discharge device connected in .shunt with said resistance, a grid electrode for determining the impedance'of said discharge device, and means for controlling the bias on said grid electrode responsive to the potential drop across said biasing resistance.

.22. In a' sound translating system for use with a sound record, a first translating device, a secr ond translating device fed thereby, a control elecacteristics or the amp trode for vafi'lng the efiiciency with which audioi'requency energy is'transmitted from said first to said second device according to the potential applied thereto, means for varying the potential applied to said electrode comprising a circuit element having a variable potential gradient across its-terminals, an impedance in series with said circuit element, a capacitancein shunt with said element and said impedance, and means for adlusting said impedance 'tohave difl'erent values for charge and for discharge of said capacitance. 23. In a sound record system, a pick-up device, an amplifier red thereby and a translating device fed by said amplifier, said amplifier having a signal input "grid and having a negatively biased control grid for varyin the amplification charer accordance with-the signal strength.

24. In an energy "tran lating system fdr' use with a :sound record. a translating device; a

space discharge device havinga grid connected to receive audio-frequency variations from said first translating device and a separate, negatively biased grid connected tovary the transmission characteristics of said discharge device in accordance with the energy value of said audio-irequency variations and a second translating device fed by said discharge device.

25. Inc. sound reproducing system for use with a sound record, a pick-up device, an amplifier fed thereby and a translating device fed by said amplifiensaid amplifier comprising a space discharge device having an anode, a first control electrode fed by audio-frequency variations from said pick-up device and -a second negatively biased control electrode fed by sub-audio-frequency variations from said pick-up device for controlling the gain characteristics of said amplifier.

26. In an audio-frequency amplifier system, a gain control stage comprising a vacuum tube having an anode, a cathode, a first control electrode and a second negatively biased control electrode, means for applying audio-frequency variations to said first control electrode and means for applying sub-audio control variations to said second negatively biased control electrode.

27. In an audio amplifier of the type including a space discharge device having a cathode, a, signal input electrode, a signal output anode and at least one auxiliary negatively biased control electrode, automatic means actuated in accordance with an increase in average audio input intensity grid, an output electrode and at least one negatively'biased control electrode functioning as a gain control electrode, said signal grid and cathode being connected to said input circuit, an

audio range expansion network comprising an audio signal amplifier having an input circuit arranged to'have audio signals impressed thereon, means for developing a direct current potential irom rectified audio signals and means impressing said direct potential on said negatively biased electrode in a polarity sense such that the amplifier gain increases with increase in audio signal intensity. a

29. The method of obtaining gain variations in a signal amplifier forming part of a sound energy translating system and having an,anode and a plurality of control electrodes, at least one of which is negatively bi'asedj which comprises introducing sub-audio control variations into said system through said negatively biased control electrode and introducing audio-frequency variations into said system through the other of said control electrodes.

ARTHUR Plasma.

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