US3286042A

US3286042A - Synthetic reverberation systems

Info

Publication number: US3286042A
Application number: US203252A
Authority: US
Inventors: Richard H Dorf
Original assignee: SCHOBER ORGAN CORP
Current assignee: SCHOBER ORGAN CORP
Priority date: 1962-06-18
Filing date: 1962-06-18
Publication date: 1966-11-15
Anticipated expiration: 1983-11-15

Description

Nov. 15, 1966 R. H. DORF I SYNTHETIC REVERBERATION SYSTEMS Filed June 18, 1962 A 5 Sheets-Sheet 1 PLAYBACK PLAYBACK PLAYBACK ERASE RECORD No N02 N03 5 42 25 3'6 512 4b i l 2 f; 5 BIAS ERASE OSCILLATOR 4 RECORD PB PB PB AMP. AMP. AMP. AMR g No I No 2 No 3 5 FEEDBACK ISOLATION CONTROL 5 AMP. AMP AME AMP.

ISOLATION OUTPUT AMP. AMP. I6 5 RICHARD H. DORF FIG.|

Nov. 15, 1966 R. H. DORF 3,286,042

SYNTHETIC REVERBERATION SYSTEMS Filed June 18, 1962 5 Sheets-Sheet 5 A on RICHARD H. DORF HPHI #3 zwmm W Zn JmaDm KmBOm 9m: F l cmooum IT ATT RNEY United States Patent C 3,286,042 SYNTHETIC REVERBERATION SYSTEMS Richard H. Dorf, New York, N.Y., assignor to Schober Organ Corporation, a corporation of New York Filed June 18, 1962, Ser. No. 203,252 5 Claims. (Cl. 179-100.2)

This invention relates to synthetic reverberation systems and particularly to improvements in electrical reverberation systems of the type having record means and selectively delayed playback means for continuously recording and playing back a signal representative of sound energy to generate reverberation components, which are combined with an unrecorded portion of said signal to form a composite reverberated signal.

Heretofore, record-playback reverberation systems have generally comprised means for recording a signal to be reverberated on an endless record material such as magnetic tape. A plurality of playback devices are disposed about the path of travel of the recording medium in a manner such that the output signal component from each playback device have selected time spacings and amplitudes to form corresponding points on a selected reverberation time curve for the system. A portion of the signal output from one of the playback devices is usually fed back to the recording device so that it may be re-recorded and the whole cycle repeated. The system thus provides a number of repetitions of the original sound which successively decrease in magnitude to conform to the desired reverberation curve.

It is customary in synthetic reverberation systems of this type to initially align and adjust the system for a single, predetermined reverberation time. Since the output signal from the record-playback device is combined with a direct signal, control of reverberation time is then effected by adjusting the magnitude of the signal which passes through the reverberation channel with respect to that which passes through the direct channel. When other than the predetermined reverberation times is desired, the effect of controlling reverberation in this manner has been generally unsatisfactory a the effect created is unnatural and artificial.

For such predetermined reverberation time adjustments, each successive repetition decreases in amplitude a substantially uniform amount and thus usually conforms to the reverberation curve which closely simulates a desirable natural curve produced acoustically, since the original sound ceases at its maximum amplitude and fades away at a uniform rate. However, when control of reverberation time is attempted by merely reducing the signal level at the input or output of the channel which produces the repetitions, an abrupt change in sound level of considerable magnitude is heard by a listener to the system after cessation of the direct signal. Furthermore, the slope of the decay curve, though lower in absolute level, remains the same as at the initially selected reverberation time, since the differences in magnitude between the repetitive signal components have not been changed by such a type of control. No parallel in nature can be found for this condition and the ear merely interprets it as the result of an abrupt cessation of sound followed by an echo.

In order to overcome this problem, it has been proposed to control the reverberation for time periods less than that initially predetermined as described above, by adjusting the amplitude of the feedback signal so that the second and all successive sets of repetitive signals initiate-d by the feedback control are commenced at an amplitude set selectively by the magnitude of the feedback signal. ment over merely reducing the gain of the reverberation Although this type of control is an improve- 3,286,042 Patented Nov. 15, 1966 channel, yet it is still artificial in quality since the abrupt changes in magnitude between each set of repetitive signals initiated by the recording of the feedback signal produce sudden changes in signal level which are heard by the ear as a series of echoes with results similar to those of the system previously mentioned.

It is therefore a primary requirement for a realistic synthetic reverberation system, if it is to have means for varying the reverberation time and yet simulate the effect occurring in the physical creation of sounds in rooms having different reverberation times, that the continuation from each signal repetition to the next be substantially uniform for all reverberation times. Thus, the decay curve must remain smooth, but its slope must change with shorter reverberation times, so that the signal decrement rate is constant commencing at the instant when the original, direct signal ceases.

It is therefore an object of the invention to provide a synthetic reverberation system in which the slope of the reverberation decay curve is changed in response to selected changes in the reverberation time of the system.

It is another object to provide a synthetic reverberation system having a variable reverberation signal decay time in which the difference in magnitude between successive repetitive signals is substantially constant for each selected decay time.

It is still another object of the invention to provide means for smoothly varying the reverberation time in a synthetic reverberation system by changing the slope of the reverberation decay curve without introducing abrupt changes in magnitude into the reverberated signal output.

In accordance with the broader aspects of the present invention, means are provided for recording on an endless record material such as magnetic tape, a signal to be reverberated. A direct signal channel is also provided. A plurality of playback heads are positioned along the path of travel of the recording medium in a manner such that the output signal components from each playback head have a selected time spacing and amplitude. The signal from the last playback head is fed back to the recording device at a selected amplitude. Means are further provided for adjusting, with respect to the direct channel, the respective amplitudes of the signals from each of the playback heads, and from the feedback channel, such that the difference in amplitude between successive signals is uniform. A reverberation time control for varying the reverberation time of the system is provided which varies the magnitude of each reverberation signal component accordingly while maintaining a uniform difference magnitudes of successive time spaced signals. Thus, for each change in the setting of the reverberation time control, the magnitudes of all reverberation signal components are changed an equal amount in order to maintain uniform the attenuation between successive signals.

An additional feature of the present invention is the provision of a novel input and output gain control arrangement for the system which maintains constant the gain and signal-tO-noise ratio of the system over wide range of signal input magnitude variations.

For a better understanding of the invention, together with other and further objects thereof, reference is made to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a block diagram of a synthetic reverberation system constructed according to the invention.

FIGS. 2a, 2b are graphs illustrating reverberation characteristics obtainable with prior art apparatus.

FIG. 3 is a graph illustrating reverberation characteristics obtainable with the system of FIG. 1.

FIG. 4 is a schematic diagram of a synthetic reverberation system constructed according to the present invention.

Referring now to the drawings, there is shown in FIG. 1 in block schematic form, a reverberation system in accordance with the present invention which has a direct channel and a reverberation channel 12. The direct channel 10 has a pair of input terminals 11 to which may be connected any suitable source of sound to be reverberated such as a radio receiver, or an electronic musical instrument. Channel 10 also has a pair of output terminals 16 which may be connected to other amplifiers or incorporated into an audio system or may be connected directly to loudspeakers, if desired. Terminals 11 are connected to the input of an isolation amplifier 18, the output of which is connected to a conventional amplifier 20 which may be a cathode follower amplifier to provide a low impedance output, if desired. As the signal applied to input terminals 11 passes through direct channel 10 without alteration, then, if the remainder of the system becomes inoperative, the original signal will be heard without change.

Also connected to the input of isolation amplifier 18 is the input of a second isolation amplifier 22 which may be a conventional voltage amplifier and which serves to isolate the input to the reverberation channel from the direct signal channel. The output of isolation amplifier 22 is connected through a variable attenuator 24 to the input of a conventional recording amplifier 26, the output of which is connected to a recording head 28. Recording head 28 is suitably positioned along the path of travel of endless recordable material 30, which for example, may be magnetic tape as shown in FIG. 1, or a rotatable drum with magnetic material on its outer perephery. However, any type of recording apparatus having a continuous recording characteristic and on which an original signal may be impressed at one point and recovered at one or more later points, may be employed. For example, an electrostatic recording system or a p-hotoelectric-phosphorescent system may be used, if desired.

Endless magnetic tape 30 is supported by idler rollers 32 and is adapted to be driven by a constant speed motor 34 in the direction indicated by the arrows. Also mounted adjacent the path of travel of tape 30 are a plurality of

playback heads

36, 38, and an erase head 42 which may be located at any selected position along the path of tape travel between playback head 40 and record head 28. Erase head 42 is adapted to continuously erase any signal recorded on tape 30 immediately prior to the recording of a new signal by recording head 28 and is accordingly connected to a convention biaserase oscillator 44. This oscillator may operate at a frequency of 25 kc. or above, and also furnishes a bias signal to record head 28.

Since by definition, reverberation comprises multiple repetitions of sound, physically caused by multiple reflections which the ear interprets as a prolonged decay of the sound after the source has ceased, then an artificial system must furnish a number of repetitions of the original sound; and the period of the repetitions and decrement thereof must be carefully selected. Thus, the intervals between repetitions should be substantially constant while the periods between repetitions should be sufliciently long so that the unnatural effect of a small room with excessive reverberation is not produced. On the other hand, if the period between repetitions is too long, then pulse-like sounds such as speech, or music with staccato notes, tend to produce discrete repetition pulses which are heardas discrete echoes.

. However, it has been found by experiment that repetitions in the range of 100 to 150 milliseconds are a satisfactory compromise. By adjusting the spacings between record head 28, and

playback heads

36, 38 and 40 respectively, in cooperation with a selected speed of tape 30, the desired repetition period may be achieved. For example, if the speed of tape 30 were 7 /2 inches per second, then for a selected reverberation period of milliseconds the spacing between

heads

28, 36, 38, 40 respectively, would be approximately .82 of an inch.

Playback head 36 is connected to the input of playback amplifier 46, the output of which, in turn, is connected through resistor 48 to the input of control amplifier 50 to form playback channel #1. The output of control amplifier 50 is connected through resistor 52 to the output of isolation amplifier 18. Likewise, playback head 38 is connected to the input of a playback amplifier 54, the output of which, in turn, is serially connected with vari able attenuator 56 and resistor 58 to the input of control amplifier 5t) to form playback channel #2. Playback head 40 is, in a similar manner, connected to the input of playback amplifier 60 the output of which, in turn, is connected through a variable attenuator 62 to the input of a control amplifier 64 to form playback channel #3. The output of control amplifier 64 is connected through resistor 66 to the output of isolation amplifier 18. Also connected in parallel with the input to control amplifier 64 is the input of a feedback amplifier 68, the output of which in turn is connected through a resistor 70 to the output of isolation amplifier 22.

In operation the signal recorded on tape 30 by record head 28 is detected after a selected time interval has occurred, such interval corresponding to the chosen repetition period of the reverberation system, amplified by amplifier 46, control amplifier 50, and is mixed with the direct signal appearing at the output of isolation amplifier 18. In a like manner, after an additional time interval has occurred, the signal is detected by playback head 38, amplified by

amplifiers

54 and 50, and is also mixed with the direct signal appearing at the output of isolation amplifier 18.

Similarly, the signal detected by playback head 40 is amplified by

amplifiers

60 and 64, and mixed with the direct signal appearing at the output of isolation amplifier 18. In addition, the signal appearing at the input of control amplifier 64 is transmitted to feedback amplifier 68 where it is amplified and mixed with the direct signal appearing at the output of isolation amplifier 22. The feedback signal is thus re-recorded to establish cyclic repetitions of the original signal with each repetition occurring at successively .lower amplitudes.

An important feature of the invention is the arrangement of the attenuating devices in the record, playback and feedback channels so that (1) the magnitude of each repetition signal is less than the magnitude of the next preceding repetition signal by a substantially uniform amount regardless of the number of complete cycles of repetition, and (2) the uniformity of difference between signal levels is maintained, and only the slope of the reverberation curve is changed, when the variable attenuation controls 24, 56, 62 are manipulated to secure reverberation times less than maximum. These objectives are graphically shown in FIGS. 2a; 2b and 3.

In FIG. 2a, the effect of attempting to control reverberation time by merely attenuating the amount of reverberated signal mixed with the direct signal, a well known prior art technique, is graphically illustrated by

curves

202, 204. Assuming that the direct signal is at a zero db reference level, that three playback heads are provided, and that the attenuation between each playback head is fixed at 1 db, then for maximum reverberation time only, the reverberation curve 202 will be a substantially smooth, straight line having a slope of 1 db per .11 second reverberation time. This is only true for maxim-um reverberation time.

If the output of the reverberation channel is merely reduced in an attempt to reduce the reverberation time, then an attenuation curve such as 204 results. An excessively large and unnatural drop in output level occurs immediately after the direct signal ceases, for example, the illustrated 8 db drop, and then the slope of the repetitive signal curve remains the same as the curve 202 obtained for maximum reverberation time. This sudden drop in volume without change in attenuation slope is wholly unnatural and unobtainable under natural reverberation conditions.

Another prior art system is shown .in FIG. 2b wherein there is graphically shown an attempt to control reverberation time by increasing feedback attenuation. Such attempts produce distinctly audible drops in signal level for each cycle of repetition. Examples of curves obtained by two different adjustments of controls in such a system are shown in curves 206, 208. It will be noted that while both curves 206, 208 have a relatively smooth initial attenuation from the level of the direct signal for the first cycle of repetition signals provided by the three heads, yet upon the commencement of the second cycle at point 210, there is an audible drop in signal level provided by the increased attenuation of the feedback control. Then the sec-0nd repetition cycle commences with the same attenuation slope as for the first cycle. In the case of curve 208 shown in dotted outline, the audible drop at point 210 is even greater, since, in the example illustrated, the feedback signal is attenuated 4 db. Controlling reverberation time by this system produces an unnatural reverberation signal having audible discontinuities due to the nonuniform attenuation curve.

In FIG. 3 is shown a plurality of

curves

212, 214, 216 respectively, obtained with the reverberation system of the present invention. Regardless of the length of the reverberation time, the attenuation curves are smooth and free of bumps, since only the slope is change-d, a natural occurrence and analogous to physical reverberation changes. These curves are obtainable, as pointed out above, by changing uniformly the attenuation between the several playback heads and feedback head so that the difference therebetween is always the same. In curve 212, the attenuation bet-ween each reverberation time signal, spaced .11 second, is 1 db. In curve 214, the attenuation between signals is 2 db, thus providing a different slope than curve 212, Whereas in curve 216 the attenuation shown in 4 db, again providing a shorter reverberation time than either

curve

212 or 214.

Referring again to FIG. 1 the novel attentuation arrangements of the several signal channels in the reverberation channel to achieve the results shown by the curves of FIG. 3, will now be explained in detail.

It will be noted that

attenuators

24, 56 and 62 are variable, and may be conventional potentiometers with selected tapers. When these controls are set in an off position and thus do not attenuate the signals passing through the associated circuits, the reverberation channel 12 will then be adjusted for maximum reverberation. The slope of the maximum reverberation signal attenuation curve will then be determined by the permanent attenuating devices still operative in the system when controls 24, 56, 62 are off. These permanent attenuating devices are all adjusted so that each recorded signal differs in magnitude from an adjacent, time-spaced signal by the same amount.

For example, assume that each signal repetition ditfers in magnitude from the next preceding repetition signal by 1 db. Then the gain of

amplifiers

46, 50 and the proportionin-g of

voltage dividers

48, 58 and 52, 66 are such that the signal picked up by the playback head 36, 110 milliseconds after it is recorded by record head 28, appears at the output of

isolation amplifier

18, 1 db lower than the direct signal output of this amplifier. Likewise, the signal picked up by playback head 38, 220 milliseconds after it has been recorded by record head 28, is adjusted in magnitude by adjustment of the gain of playback amplifier 54, the proportioning of

voltage divider

48, 58, the gain of control amplifier 50 and proportioning of voltage divider 52-66, so that the signal appearing at the output of isolation amplifier 18 is 1 db less than the signal provided by

playback head

36 and 2 db less than that 6 of the direct signal applied to input terminal 11. It is assumed that attenuator control 56 is in the straight through or off position.

Three hundred and thirty milliseconds after it has been recorded by record head 28, the signal picked up by playback head 40, is adjusted in magnitude by adjusting the gain of playback amplifier 60, control amplifier 64, and the proportioning of

voltage divider resistors

52, 66 so that the signal appearing at the output of isolation amplifier 18 is 1 db less than the signal originating from

playback head

38 or 3 db less than the direct signal applied to input terminals 11 and appearing at the output of isolation amplifier 18. In this instance, variable attenuator control 62 is in the straight through or off position.

The signal from playback head 40 is also amplified by feedback amplifier 68 and transmitted through resistor 70 to join the direct signal at the output of isolation amplifier 22, but the gain of feedback amplifier 68 and the value of resistor 70 are so designed that the feedback signal appearing at the output of isolation amplifier 22 is 3 db less than the direct signal at the same point, obtained from the input of amplifier 22. The signal from feedback amplifier 68 is accordingly amplified by record amplifier 26 and re-recorded by record head 28 to thus start a second cycle of repetitive signals.

As in the case for the first cycle, the re-recorded feedback signal is picked up by the playback head 36 amplified by playback channel #1 and again appears at the output of isolation amplifier 18. As channel #1 provides 1 db attenuation with respect to the magnitude of the direct signal, and as the feedback signal has been recorded at a level 3 db less than the direct signal, the resulting magnitude of the re-recorded signal provided by playback channel #1 to the input of amplifier 20 is therefore minus 4 db with respect to the direct signal arriving at the same point. It will be seen that in a like manner, the repetition provided by playback channel #2 on the sec-0nd cycle will be attenuated 5 db, and likewise the signal provided by playback channel #3 will be attenuated 6 db.

Accordingly, every repetition of every cycle differs in magnitude from adjacent time spaced signals by the same amount. However, as reverberation time is definition is the time required for a repetition signal to be 60 db less than the direct signal from which it originated, then in the example just given, as each repetition is 1 db less than the immediate preceding repetition, 60 repetitions are required to fulfill the definition. As the time delays between repetitions are arbitrarily chosen at .11 second, then for this example the maximum reverberation time is 60 .11 or 6.6 seconds.

The operation of the system to maintain uniform the attenuation differences between adjacent signal repetitions for reverberation times shorter than the maximum of the system will now be described.

The reverberation time is easily shortened from the maximum reverberation time set for the system to 0 by simply varying

attenuators

24, 56 and 62, which preferably are ganged together for tandem operation. These attenuators may be a plurality of series connected resistors with switchable output taps between each resistor, or potentiometers with selected tapers if a continuous adjustment from maximum to 0 is desired.

Although the maximum resistance values of the attenuator controls may be different due to different impedance requirements of the circuits in which they are connected, they have definite relationships to each other with respect .to the amount of attenuation for given r0- tational positions, or switch positions. As will be explained in detail below, for a system having three playback channels, controls 24 and 56 have the same rate of attenuation in db per degree of shaft rotation or for each switch position, whereas control 62 has twice the rate of attenuation of

controls

24 and 56. In the case of four playback channels, the attenuation control therein corresponding to control 62 would have a rate of attenuation triple the attenuating rate for

control

24 and 56. Thus, for example, in a three playback channel system, if rotation of the ganged

attenuators

24, 56, 62 introduces an attenuation of db into the direct channel and playback channel #2 respectively, then attenuator control 62 will have an attenuation of db.

To better understand this operation, assume that a reverberation time of 3.3 seconds is desired, which in the case of three playback channels requires that each signal repetition be 2 db less than the immediately preceding signal. To fulfill the reverberation definition, repetitions are required at .11 second time spacing therebetween to provide a reverberation time of 3.3 seconds. It will be remembered that the permanent attenuation with respect to the magnitude of the direct signal for playback channels #1, #2 and #3 as 1 db, '2 db, -3 db respectively. Of course, any other absolute value of attenuation per channel may be selected as desired, pro- 8 level provided by each playback'head being 2 db less than the immediately preceding signal. The relative output levels in db attenuation provided by the

controls

24, 56, 62 and the db decrement between repetitions is summarized in the following table for several selected Summarized below are the attenuations obtainable if a system having 5 playback heads is employed and a reverberation time of 3.3 seconds is desired.

vided the uniformity of difference in attenuation between adjacent channels is maintained.

Then, if the signal provided by playback channel #1 is to appear at the output of isolation amplifier 18 with a magnitude of 2 db less than the direct signal appearing at this point, control 24 must introduce 1 db loss into the record channel. The 1 db so provided, in addition to the 1 db permanent attenuation provided by playback channel #1 will give the desired 2 db attenuation to the output signal of playback channel #1. As the output signal from playback channel #2 must be 2 db less than the output signal of

channel #

1, or 4 db less than the direct signal, control 56 must introduce 1 db to playback channel #2. This is due to the fact that the signal level in playback channel #2 is reduced 1 db due to the record level of the signal provided by the record channel and 2 db by the permanent attenuation of playback channel 2 so that control 56 must provide 1 db for the required 4 db total attenuation.

In a like manner the signal level of playback channel #3 must be reduced 6 db with respect to the direct signal appearing at the output of

isolation amplifier

18, or 2 db less than the signal provided by playback channel #2. As the record signal is reduced 1 db by control 24, and the permanent attenuation of playback channel #3 is 3 db, then control 62 must contribute 2 db to fulfill the -6 db requirement. It will thus be seen that the attenuation value of control 62 is twice that of

controls

24 and 56.

As the attenuation of the feedback signal is the same as that of the signal level of playback channel #3, as mentioned above, the signal furnished by feedback amplifier 68 to be rerecorded is therefore 6 db less than the direct signal at the output of amplifier 22. This -6 db signal is additionally attenuated by the 1 db provided by control 24, recorded by record head 28, picked up by playback head 36, and fed through playback channel #1 where it is further attenuated 1 db by the permanent attenuation of this channel and appears at the output of isolation amplifier 18 at a level 8 db less than the direct signal.

A second repetition cycle has thus commenced with the uniformity of difference in magnitude between adjacent repetition signals still being preserved. Accordingly, curve 214 of FIG. 2c is produced with the signal Based on the foregoing, the attenuation requirements for the several channels forming the overall reverberation channel 12 may now be summarized and expressed in general mathematical notations as follows:

(1) The variable attenuation control of any channel,

VA (db)'= (N-1)xA where N=the playback channel number, and A =the variable attenuation of the first playback channel (with respect to the direct signal). (2) The permanent attenuation of any selected playback channel PA NXPA (3) The selected total attenuation for playback channel #1, with respect to that of the direct channel, determines the minimum permanent attenuation for all playback channels.

Referring now to FIG. 4, a schematic circuit diagram of a reverberation system in accordance with the present invention having 3 playback channels, is shown. Where units are identical, the reference numerals of FIG. 1 are used. A signal from a source of audio signals such as an electronic musical instrument is fed to input terminals 11 through potentiometer 302 by means of movable arm 304 to the input grid of an electronic valve 306. Potentiometer 302 is connected in series with a resistor 308 to ground, and the control electrode of valve 306 is connected to the junction point thereof. Valve 306 is included in the circuitry of isolation amplifier 22, which serves to separate electrically, reverberation channel 12 from direct channel 10.

The control electrode of valve 310 forming part of isolation amplifier 18, is also connected to the control electrode of valve 306 to enable the input signal appearing across resistor 308 to be fed to isolation amplifier 18 of direct channel 10. The direct signal is amplified by valve 310 and fed through capacitor 312 to an output level adjusting potentiometer 314 which has one terminal connected in series with a resist-or 316 and ground. The movable arm 317 of potentiometer 314 transmits the signal developed t'hereacross through capacitor 318 to the control electrode of a valve 320 included in output amplifier 20, which is connected in a cathode follower configuration. The output of amplifier 20 is fed through a capacitor 322 to a pair of output terminals 16.

An important feature of the present invention is the arrangement of the input and output gain controls in order to minimize the system noise Wherever possible. It will be noted that the movable arms of

potentiometers

304, 317 of

potentiometers

302, 314 respectively, are ganged to each other. These potentiometers are preferably, s-ubstantially identical in taper although they need not be necessarily identical in absolute resistance value. However, the ratio of the resistance of resistor 308 to the resistance of control potentiometer 302 is preferably substantially identical to the ratio of the resistance of resistor 316 to its associated potentiometer 314. For example,

potentiometers

302, 314 may be one hundred thousand ohms wherea

resistors

308, 316 may be ten thousands ohms. Thus, the input and output voltage may be adjusted over a range of approximately 11-1.

In operation, it is preferable to adjust

control

302, 314 to as low a point with reference to ground as postible, which accordingly will provide, due to voltage divider action, maximum expected signal to reverberation channel 12. When it is desired to apply maximum input signal to terminals 11,

movable arms

304, 317 are at maximum resistance positions of

potentiometers

302, 314, with respect to ground. Under this condition, the input signal will be attenuated to the level required for the maximum recording signal in reverberation channel 12, whereas the output control 314 is set to provide maximum signal output to terminal 16. The gains of

amplifiers

18, 20 are such that under this maximum output condition, the output signal is at the same level as the input signal for direct channel 10.

If only a minimum signal is available for application to input terminals 11, then

movable arms

304, 317 are set to a minimum resistance position for

potentiometers

302, 314, with respect to ground. Because of the voltage divider action of these controls, the full input voltage is applied to both the reverberation and directs channels. The maximum voltage is then still delivered to reverberation channel 12. However, since the gain of this system remains unchanged, the location of movable arm 317 at low resistance point of its ossociated control thereby causes the direct signal to be attenuated so that the signal output from channel appearing across output terminal 16 again remains the same as the input signal level. However, under these conditions, any noise or hum which may have originated in the system is also attenuated.

Thus, it will be seen that in accordance with this feature of the invention, for high level input signals, no attenuation of internally generated noise is provided, since none is needed under high signal level conditions. On the other hand, when the input signal is at a low level, maximum noise attenuation is provided. Thus, the signalto-noise ratio of the system remains substantially constant over a large range of available input signals, rather than decreasing with signals of smaller amplitude.

Continuing with the description of the circuitry of reverberation channel 12, the signal developed across resistor 308 and applied to the input electrode of valve 306 is amplified thereby and appears across series connected

resistors

324, 326 which are connected to a source of positive voltage 327. Voltage divider action is provided by

resistors

324, 326 so that the signal, at reduced output level, is transmitted through capacitor 328 to one terminal 329 of control 24.

Control 24 comprises a plurality of serially connected resistors 330, 332, 334, 336, 338 with the last terminal of the series string connected to ground. The junction points between the serially connected resistors 330-338, are connected to taps which are selectively engaged by movable arm 340. Also connected to terminal 329 is the output signal from valve 394 of feedback amplifier 68, which is developed across resistor 342 and fed through capacitor 344 and resistor 346 to "terminal 329. The combined gains of isolation amplifier 22 and feedback amplifier 68 are preferably adjusted so that the signal appearing at terminal 329 has an amplitude operative to drive recording amplifier 26 to the maximum output required by the record head when movable arm 340 engages tap 329 in the zero attenuation position of control 24.

The reverberation signal is next amplified by record amplifier 26 and fed through capacitor 343 and transformer 345 to record head 28. Valve .347 functions as a conventional bias and erase oscillator operating at a frequency suitable for the heads in use. Thus, the erase signal is fed to erase head 42 through

capacitors

348, 350 respectively, and bias signal for record head 28 is fed thereto through transformer 345. As the circuit arrangements for record amplifier 26 and bias and erase oscillator 44 are conventional and well known in the art, no further description thereof is necessary.

The recorded signal from the tape is picked up by playback heads 36, 38, 40, each connected to associated

amplifiers

46, 54 and 60 respectively, which are conventional two-stage voltage amplifiers. The output of amplifier 46 is fed through capacitor 352 and isolation resistor 48 to the input electrode of valve 356. One output terminal of amplifier 54 is connected through a capacitor 358 to a terminal 372 of attenuator control 56. This control comprises a plurality of serially connected

resistors

362, 364, 366, 368, 370 connected between terminal 372 and ground. The junction points between resistors 362-370 are connected to taps which are engaged by wiper arm 374 which in turn is serially connected through an isolation resistor 58 to the input electrode of valve 356. The output signal from amplifier 54 is thus developed across attenuator resistors 362-370, and by voltage divider action, is applied to the input electrode of valve 356 through a circuit path comprising a selected tap of attenuator 56, wiper arm 374, and resistor 58.

The signal from playback amplifier 60 is fed through capacitor 376 to terminal 378 of attenuator 62, which comprises a plurality of serially connected resistors 380, 382, 384, 386, 388 connected between terminal 378 and ground. The junction point between each resistor is con nected to an output tap which is engaged by a wiping arm 390 which is connected to the input electrode of valve 392 of control amplifier 64.

The input electrode of valve 394 of feedback amplifier 68 is also connected to the input electrode of valve 392. The signal received therefrom is amplified by valve 394 and fed to the input of record amplifier 26 in the manner described hereinbefore. The permanent attenuation of the signal is, of course, set to the point described hereinbefore, by adjustment of the impedances of amplifier 68, such as resistors 341, 346 and capacitor 344.

The outputs of

control amplifiers

50, 64 are respectively fed through capacitors 354, 360 to

resistors

52, 66 which have a common junction point connected to the higher resistance terminal of output level potentiometer control 314 Where the signal output from reverberation channel 12 is mixed with the direct signal input from isolation amplifier 18 of direct channel 10.

Resistor 58, associated with playback amplifier 54, is so adjusted in magnitude with respect to resistor 48 associated with playback amplifier 46 that, when all of the shunt impedances of the two amplifiers are considered, the signal appearing at the control electrode of valve 356 derived from amplifier 54 is 1 db less than that derived from amplifier 46, which is the permanent attenuation mentioned above and provides 1 db attenuation between playback channels if desired. Likewise, the values of

output resistors

52, 66 are so adjusted that the voltage divider action provided thereby is operative to cause a permanent attenuation of 3 db with respect to the direct or recorded signal in order that the additional 1 db attenuation between playback channel 2 and playback channel 3 is achieved. As there are three playback channels, the values of resistors 378-388 of attenuator control 62 should be selected so that for each setting of the ganged attenuator controls 24,56, 62, the attenuation in decibels provided by control 62 is twice thatof the other controls.

While step attenuators having six positions are shown for purposes of illustration, it will be understood that attenuators having more or less positions may be used, if desired, or ganged potentiometers with suitably selected tapers and having an infinite number of positions may also be used.

Although a reverberation system utilizing three playback channels has been described for purposes of illustration, it will be understood that by following the principles of the present invention, additional playback channels including heads and amplifiers, may be added if desired, or even a two-channel playback system may be employed.

While the present invention has been disclosed by means of specific illustrative embodiments thereof, it would be obvious to those skilled in the art that various changes and modifications in the means of operation described or in the apparatus, may be made Without departing from the spirit of the invention as defined in the appended claims. I

What is claimed is:

1. In a synthetic reverberation system having input terminals coupled to a' direct-signal channel and to a reverberation signal channel and having means for combining the outputs of both channels to provide output for the system, and the reverberation signal channel including delay means combined with plural means for generating from an input signal successive series of time-spaced signals of decreasing amplitude, and further including feedback means coupled to receive the last of the generated signals in each series andreintnoduce it into said delay means to generate a new series, improved variable attenuating means for controlling the amplitudes of said spaced signals comprising:

(a) separate variable attenuation means controlling the relative output amplitudes of said signals from i the plural generating means to said combining means;

(b) means at the input of the reverberation signal channel for adding the feedback and the input signal together;

(c) variable attenuation means coupling the adding means to the delay means;

(d) means for ganging together all said attenuation means for simultaneous adjustment tomultiple selectible positions; and

(e) the attenuation means coupled to the adding means having an attenuation characteristic tapered to cooperate with matched tapers of the attenuation means which .are coupled to the generating means for making the amplitudes of all delayed signals in successive series always decrease along a smooth curve with respect to time for all said positions.

2. In a system as set forth in claim 1, said variable attenuation means being adjustable to equal zero, and fixed attenuation means in series therewith and selected to provide a predetermined minimum attenuation between successive signals, and the variable attenuation associated with each of said generating means after the first thereof being substantially equal to (N1)A Where:

N=the number in succession of the generating means other than the first one, and

A =the variable attenuation of the first of said generating means beyond said predetermined attenuation.

3. In a system as set forth in claim 2, the fixed attenuation of any of said generating means other than the first thereof being substantially equal to N PA where:

PA =the fixed attenuation of the first of said generating means.

4. In a system as set forth in claim 1, the attenuation means for said adding means and for the first of said generating means having substantially identical tapers, and the other attenuation means having tapers equal to (N-l) multiplied by the taper of the adding means attenuation, where N=the number in succession of the generating means other than the first one.

5. In a system as set forth in claim 4, the variable attenuation means of the adding means being common to both the direct input to the reverberation signal channel and the feedback signal input to the reverberation signal channel and the feedback signal input to the reverberation channel.

References Cited by the Examiner UNITED STATES PATENTS 2,493,638 1/1950 Olson- 1791.6 2,674,660 4/1954 Ambrose 179-1002 2,712,040 6/1955 HeytoW et al 330144 2,757,245 7/ 1956 Pihl 330-444 3,057,969 10/1962 Stolle et al l79100.2 3,062,923 11/1962 Stolle 179-1002 3,069,956 12/1962 Bode 84-124 3,073,900 1/1963 Victoreen 330-155 3,095,482 6/ 1963 Whiteford l79100.2 3,110,771 11/1963 Logan etal 179l.6

I BERNARD KONICK, Primary Examiner.

Claims

1. IN A SYNTHETIC REVERBERATION SYSTEM HAVING INPUT TERMINALS COUPLED TO A DIRECT-SIGNAL CHANNEL AND TO A REVERBERATION SIGNALS CHANNEL AND HAVING MEANS FOR COMBINING THE OUTPUTS OF BOTH CHANNELS TO PROVIDE OUTPUT FOR THE SYSTEM, AND THE REVERBERATION SIGNAL CHANNEL INCLUDING DELAY MEANS COMBINED WITH PLURAL MEANS FOR GENERATING FROM AN INPUT SIGNAL SUCCESSIVE SERIES OF TIME-SPACED SIGNALS OF DECREASING AMPLITUDE, AND FURTHER INCLUDING FEEDBACK MEANS COUPLED TO RECEIVE THE LAST OF THE GENERATED SIGNALS IN EACH SERIES AND REINTRODUCE IT INTO SAID DELAY MEANS TO GENERATE A NEW SERIES, IMPROVED VARIABLE ATTENUATING MEANS FOR CONTROLLING THE AMPLITUDES OF SAID SPACED SIGNALS COMPRISING: (A) SEPARATE VARIABLE ATTENUATION MEANS CONTROLLING THE RELATIVE OUTPUT AMPLITUDES OF SAID SIGNALS FROM THE PLURAL GENERATING MEANS TO SAID COMBINING MEANS; (B) MEANS AT THE INPUT OF THE REVERBERATION SIGNAL CHANNEL FOR ADDING THE FEEDBACK AND THE INPUT SIGNAL TOGETHER; (C) VARIABLE ATTENUATION MEANS COUPLING THE ADDING MEANS TO THE DELAY MEANS; (D) MEANS FOR GANGING TOGETHER ALL SAID ATTENUATION MEANS FOR SIMULTANEOUS ADJUSTMENT TO MULTIPLE SELECTIBLE POSITIONS; AND (E) THE ATTENUATION MEANS COUPLED TO THE ADDING MEANS HAVING AN ATTENUATION CHARACTERISTIC TAPERED TO COOPERATE WITH MATCHED TAPERS OF THE ATTENUATION MEANS WHICH ARE COUPLED TO THE GENERATING MEANS FOR MAKING THE AMPLITUDES OF ALL DELAYED SIGNALS IN SUCCESSIVE SERIES ALWAYS DECREASE ALONG A SMOOTH CURVE WITH RESPECT TO TIME FOR ALL SAID POSITIONS.