Method of and apparatus for signal reproduction

June 25, 1963

METHOD OF

J. S. WHITEFORD 3,095,482

AND APPARATUS FOR SIGNAL REPRODUCTION

Filed May 8, 1959

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nrrontvErs

United States Patent Office

3,095,482

Patented June 25, 1963

3,095,482 METHOD OF AND APPARATUS FOR SIGNAL REPRODUCTION

Joseph S. Whiteford, Seabrook, N.H. Filed May 8, 1959, Ser. No. 811,937

Claims. (Cl. 179—100.2)

The present invention relates to methods of and apparatus for signal reproduction and, more particularly, to the reproduction of sound signals.

It has heretofore been proposed to simulate desirable sound reverberation characteristics produced in a confined space, such as a large church or the like, by locating in a room, building or other space that does not have such desired reverberation characteristics, a plurality of loudspeakers at successive positions along the walls of the room, building or other space, corresponding in positional sequence to the time sequence of operation of a corresponding plurality of successively operated sound reproducing transducers, energized successively from a source of sound signals and connected with the corresponding loudspeakers. The sound is thus caused to be fed successively to the successively positioned loudspeakers, say, from the front to the back of the room, building or other space, thereby simulating the actual travel of sound waves from the front to the back. By controlling the level of the sound energy emanating from each loudspeaker, a somewhat realistic sound-decay pattern can be obtained which, when supplemented by a signal fed back from the last of the plurality of reproducing transducers to the source, in a carefully controlled manner, can effect an over-all simulated reverberation time that corresponds to the desired reverberation time.

The reverberation produced by this kind of device consists of a series of segments, comparable to pulses or discrete echoes, the duration of which is equal to the time between successive energizations of the successive reproducing transducers. In the case of a magnetic tape source of signals, with a plurality of substantially equally spaced playback-head transducers, this duration would correspond to the time that it takes any given point on the tape to travel from one head to the next. The successful operation of the system, however, depends upon keeping this time duration short enough; and this can be accomplished by close head-spacing and high tape-speed. Unfortunately, there are several very severe limitations imposed upon the application of these techniques. In a particular installation, for example, a time duration of a tenth of a second has been found to be sufficient for organ music and choral singing. When more explosive sounds with sharp transients are involved, such as, for example, cymbal clashes in orchestral music, a much shorter time would be required in order to mask the discrete-pulse effect inherent in the system. Since whatever natural reverberation that may exist in the room, building or other space assists in this masking, the maximum permissible pulse-time length has to be, in the last analysis, decided by test for any given room, building or other space, taking into account the types of sounds that are to be reverberated.

Another audible, regularly recurring, undulating pulsation inherent in a system of this kind is what may be termed “pumping.” It occurs each time any given signal re-enters the recording head via the feedback-path from the last reproducing playback head, and is, therefore, in time, equal to the short pulse time described above multiplied by the number of playback heads. In the case of six playback heads, for example, and the onetenth second time duration, this would be six-tenths of a second. This disturbance can be minimized by very careful adjustment of all level controls, so that the over10 all signal decay is kept as linear as possible. The only way completely to eliminate this difficulty, however, would be to use no feedback at all and, consequently, a sufficiently large number of playback heads to obtain the required reverberation time. This is, in most instances, impractical cost-wise, but can be approached by the use of as many playback channels as possible with consequently as little feedback as possible. Such design, furthermore, assists in obtaining good linearity from the system, a prime requirement for long reverberation time.

In simulated reverberation systems of the prior art, the principal disadvantages are at least two-fold.

First, since the pulses or echoes are being reproduced by pairs of loudspeakers operative in numerical sequence with the reproducing playback channels, from the front toward the rear, as before described, the feeding-back of the signal from the last playback head to the recording head, causes the sound suddenly to jump toward the front, again, and then to travel the same path from front to back, once more. Because the sound does this several times over before it dies out, it can readily be seen that the only acoustical dimension that the room, building or other space thus acquires is that of length. It appears to have no breadth and very little height, in spite of the fact that the loudspeakers may be placed well above earlevel. All the echoes seem to converge upon the rear and, every time they get there, one completely loses track of them because they suddenly jump toward the front again. The result is an illusion of a sound-pile created by an immense reflecting rear wall. One has the impression that all the reverberation comes from there and almost from nowhere else.

Secondly, the “pumping” effect, before described, demands very linear adjustment of the overall reverberation decay, frequency-response-wise as well as level-wise. Unfortunately, this is not the way reverberation occurs in an actual pleasingly reverberant building, where the low frequencies are usually absorbed far less rapidly than the high frequencies, and where this unevenness of absorption is even more pronounced as the distance from the sound source increases.

An object of the present invention, accordingly, is to provide a new and improved method of and apparatus for sound reproduction of the character described that shall not be subject to any of the above-mentioned disadvantages; but that shall, to the contrary, add breadth and height to the reverberation illusion and substantially eliminate the pumping and piling effects.

A further object is to provide a novel sound-reproducing apparatus of more general utility, as well.

Other and further objects will be explained hereinafter and will be more particularly pointed out in the appended claims.

The invention will now be described in connection with the accompanying drawing, the single FIGURE of which is a combined block-and-schematic circuit diagram illustrating the invention in preferred form.

For purposes of illustration, the invention is shown applied to a church or similar auditorium 1 having organ pipes at the front, operated from a console 3, and in which it is desired to simulate the 'acoustic response of a much larger building, or one with less acoustically absorptive interior surface, or both. It is to be distinctly understood, however, that the invention is by no means limited to churches, auditoriums or similar structures, but is applicable wherever the novel results attainable with the invention may be desired; including, but not limited to, amphitheaters or other open areas, homes, and other partially or completely enclosed spaces. For the purposes of this example, five pairs of loudspeakers 2—2', 4—4', 6—6', 8—8' and 10—10'

3,095,483 are disposed at successively positioned intervals, from front to ’back, along the oppositely disposed side walls of the church or auditorium 1, as previously discussed, and a further pah' of speakers 12—12' is provided at the back. Again, however, it is to be understood that more or less speakers may be employed, as desired. Each of the pairs of loudspeakers is energized to produce an audible indication of the signal from a corresponding sound playback-head or pick-up transducer, as of the magnetic type, associated with a magnetic tape 5 passed between a take-off reel 7 and a take-up reel 9. Conventional erase heads 11, as of the permanent magnet 'and attenuating-current types, may insure that the tape 5 is free of residual signals before reaching the recording head 13. The recording head 13 is energized through a mixer and recording amplifier system 15, connected by conductor paths 25 and 25' to the respective pair of 'microphones 23 and 23', in turn disposed to receive the music from the organ pipes. The recorded tape 5 then proceeds successively past the successively disposed play-back heads 20, 40, 60, 80, 100, 120, each positioned therealong at substantially equal intervals, preferably substantially the same as the distance between the recording head 13 and the first playback head 20, as shown.

The successively energized playback heads 20, 40, 60, 80, 100 'and 120, will thus transmit signals through corresponding pre-amplifier channels 22, 24, 26, 28, 30 and 32, having conventional level, bass and treble controls, by way of corresponding conductor paths 42, 44, 46, 48, 50 and 52, to corresponding further amplifier stages 22', 24', 26', 28', 30' and 32', thereby successively to energize the pairs of loudspeakers 2—2', 4—4', 5—6', 8—8', 10—10' and 12—12', respectively connected thereto.

While creating the illusion of space, when adding reverberation to music, is natural and therefore desirable, this space-illusion should not all occur in one direction only, as in the lengthwise direction before discussed. It should also add breadth and height. In a real building, of course, an infinity of reflections strikes the listener’s ears from all directions so that the reverberation is not confined to sound waves shuttling back and forth between front and back. There is, however, some feeling of sound-travel from front to back in a building which is spacious and, more specifically, oblong. For this reason, for a given building of this basic general shape, one is well 'advised to retain, therefore, the fundamental concept of connecting the channels sequentially.

In accordance with the present invention, accordingly, simulated secondary, random reflections are created, emanating from the loud-speakers of 'properly chosen channels at proper times, in addition to the primary scheme of sequential echoes, thereby creating the illusion of a three-dimensional reverberant space.

To achieve this effect, it has been found necessary to observe the following requirements.

First, there must be sufficient randomness in the simulated secondary reflections. It is advisable, for instance, to avoid insofar as possible, simulating a secondary reflection of a signal from one Channel in an adjacent channel. Secondly, the levels of these simulated reflections must be carefully determined and, more often than not, 'their overall level should be kept slightly below that of the primary scheme. If thus properly adjusted, these simulated secondary reflections have been found to add such a feeling of realism to the overall reverberation, that frequency response corrections may be introduced to a large extent 'without inducing any consciousness of “pumping” or “piling” in the listener.

To attain this end, the pre-amplifier 22 associated with the first-channel playback head 20 and the first pair of loudspeakers 2—2' is shown feeding part of its primary signal output through a level control, schematically illustrated as a variable resistor R₂, to the pre-amplifier stage 28 of the fourth channel associated with the fourth pair of loudspeakers 8—8'. Similarly, the second-channel preamplifier 24 is connected by R₄ to ‘the fifth-channel preamplifier 30 associated with the fifth pair of 5 loudspeakers 1®—10'. Other randomly selected interconnections of channels are 'effected by Ri₂, between 'the sixth channel associated with loudspeakers 12—12' and the first channel; by R_lo, between the fifth channel associated with the fifth pair of loudspeakers 10—10' and 10 the third channel associated with the third pair of loudspeakers 6—6'; by R_s between the fourth and sixth channels; and by R₆ between the third and second channels.

A part of the primary signals reproduced at the successively operated pairs of speakers will thus be repro15 duced as secondary signals at other randomly selected speakers, simulating random reflections thereof and giving the illusion of three-dimensional reverberation, as above discussed. In order to produce the desired effect, moreover, it has been found that the level of the part 20 of the primary signal thus fed from one speaker channel to another speaker channel should be adjusted to the same level as the primary signal of the said one speaker channel from which it originates. In some cases, this level of the secondary signal may be reduced by any 25 required reduction constant representative of the amount, if any, by which the secondary or simulated reflection signal, should be lower in level than the primary signals of the loudspeakers.

As an illustration, in a small church about eighty 30 feet in length, the successive pairs of speakers were spaced at ten-foot intervals. The actual sound-wave travel time for this spacing was approximately 0.01 second. By employing a tape speed and playback head spacing such as to produce a simulated travel time (between energiza35 tion of successive pairs of speakers) of about 0.1 second, the apparent length of the church was increased by a factor of ten. The following chart illustrates the selected primary signal levels for the six channels, corresponding to the successive pairs of speakers 2—2', 4—4', 6—6', 40 8—8', 10—10' 12—12'; the random interconnection of channels as previously described; and the adjusted levels of the parts of the primary signals fed by such interconnections to serve as secondary signals at other channel speakers, thus to simulate random reflections. The iever46 beration time desired was about two and a half seconds.

<td> Channel 50</td><td> Primary signal level (decibels)</td><td> Secondary signal originates from channel</td><td> Secondary signal level unreduced (decibels)</td><td> Secondary signal, using reductionconstant of 3 decibels down (decibels)</td>

<td> 1„...............</td><td rowspan="7"> 1 1 i W Q M ta to O bi\</td><td rowspan="7"> 6 3 1 2 4</td><td rowspan="7"> -12¼ -5 -10 0 -2¼ -7¼</td><td rowspan="7"> -15¼ —8 -13 -3 • -5¼ -10¼</td>

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<td> 4..... ..........</td>

<td> 5_____________________</td>

<td> 6_____________________</td>

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₆₀ In accordance with an additional feature of the present invention, compensation is effected for the previously mentioned phenomenon that the decrease in level at the speakers, successively more distant from the front, is less for the low sound frequencies than for the high _6g sound frequencies. By adjusting the frequency response of the successive pre-amplifiers so that the bass is boosted and the treble attenuated more and more in the successive channels from front to back, the required less reverberation of the high frequencies can be attained. Feedback from the last channel pre-amplifier 32 to the recording amplifier 15 will then be adjusted by frequencyresponse and level controls so that a greater level of lows than highs is fed back, and a smooth reverberation decay is obtained.

- It will be clear that any other type of storage or other signal-reproducing system than the magnetic type may be

3,095,482 employed, and that, indeed, the techniques of the invention may be used with other kinds of signals than audible sound signals, wherever the results attainable by the invention are desired.

Instead of recording live sounds, moreover, one could inject recorded or stored sound signals into a first playback head replacing the recording head 13, preferably employing a relatively large number of successive playback heads 20, 40, 60, etc. to avoid the necessity for the before-described feedback. The recorded or stored sounds need not be stored on magnetic tape, since phonograph records, optically or photographically recorded films, or other kinds of record media may also be employed.

Further modifications will also occur to those skilled in the art and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.