US2838608A - Artificial stereophonic sound system - Google Patents

Description

June 10, 1958 s. w. ATHEY ETAL ARTIFICIAL STEREOPHON IC SOUND SYSTEM Filed Jan. 13, 1955 2 Sheets-Sheet 1 HMPL.

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June 10, 1958 -s. w. ATHEY ETAL 2,838,608

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United States I atent ARTIFICIAL STEREOPHONIC SOUND SYSTEM Skipwith W. Athey, Mount Kisco, and Weld S. Carter, Jr., Peekskill, N. Y., assignors to General Precision Laboratory Incorporated, a corporation of New York Application January 13, 1955, Serial No. 481,556.

5 Claims. (Cl. 179-1) This invention pertains to sound reproduction systems which relate the apparent sound source location to the character of the sound. This invention particularly concerns the generation of stereophonic sound by the use of only a single electrical sound channel.

The reproduction of moving visual images on a screen,

such as in :motion pictures and in television, is conventionally accompanied by sound reproduction. Thus the speech of actors seen on the screen, or music or sound effects, accompany the action on the screen. When the screen is large, as in theatre motion pictures and theatre television, the naturalness of the reproduction is heightened if the sound accompanying an action appears to originate at the location of the actor or other source on the screen. Several loudspeakers are concealed behind the screen and the sound accompanying an action is caused to be. emitted only bythe loudspeaker concealed behind the appropriate screen location.

Inconventional motion picture stereophonic systems a plurality of microphones are placed for reception of sound from selected parts of the stage; The microphone outputs generate separate sound tracks on the motion picture film beside the pictures thereon, and in projection the separate sound tracks operate individual loudspeakers located at different positions behind the motion picturescreen. Thissystem requires special, film carrying a, plurality of sound tracks.

The instant invention provides a stereophonic effect but employs conventional motion picture film in the sense that. the film carries only a single sound track. This system therefore, requires but a single sound channel. The film employed is unique in that it carries on the single sound track, in addition to the sound to be reproduced with the picture, a plurality of tones which are rendered inaudible to the listener but which serve as control signals. In making this special film a plurality of alternating signal currents are added to the audio currents of the pick-upmicrophone circuit. These signal currents have frequencies outside of the audio current frequency range,

. and may be either higher or lower in frequency, although as:a practical matter it is convenient to position the signal currents in the range below 50 C. P. S. These control signals are manuallyadded, without intermodulation, at such times and in such intensities as to represent at 7 several stage locations the instant sound distributions.

For example, a 30 C. P.1S. signal may represent sound emanating from the right of the stage, a 35 C. P. S.

, signal may represent a center stage source and a 40 C. P. S.- signal may represent a left stage source. Now

'if an actor at the right speaks, only the 30 C. P. S. signal is applied, at full amplitude. If anactor speaks at left center on the stage perhaps both the 35 C. P. S. and

. the 40 C. P. S.- signals will be introduced, but each at half i amplitude, representing the location of the source between centertstage and left stage.

2,838,608 Patented June 10, 1958 "ice amplified, and is transmitted to a control unit, where the audio, signal is separated from the control signal. The former is transmitted through three sets of amplifiers to three loudspeakers positioned behind the screen, one. at the left, one at the right and one in the center. The control signal containing only 30, 35 and 40 C. P. S. frequencies is separated into its three frequency components, and these are made respectively to control the gains of the three amplifiers. Thus each of the loudspeakers is individually controlled by a control signal as to the amplitude of its output in accordance with the location of the sound source in the scene originally photographed. Each loudspeaker is additionally varied in amplitude in the usual manner in accordance with the varying amplitudes of the source.

The instant invention also provides several refinements necessary in connection with the use of existing projection equipment and present-day motion picture projection practices. For example, when conventional film having soundtrack carrying no control signal is utilized, the device of the invention automatically changes to single-output-channel operation, and when film having a single sound track adapted to the uses of this invention is employed the device automatically changes to the virtual stereophonic loudspeaker operation of this invention. This invention alsoprovides automatic reversion to single-channel operation upon failure of almost any part of the equipment of the invention. This singlechannel or non-stereophonic mode of operation may of course employ any number of the described loudspeakers operated as a single unit, but for clarity the detailed description describes a circuit in which but one speaker is employed. Since in single-channel operation the electronicv portions of the entire control unit are bypassed and the signal channel contains only passive equipment, such operation is not subject to electronic failure and thus is highly reliable. The equipment also contains level measuring equipment to facilitate proper settings of gains to insure stability of operation.

The general purpose of this invention is to provide a device for synthesizing stereophonic sound signals from a single audio channel composite signal.

Another purpose of this invention, in theatre screen reproduction of images andaccompanying sound and employing a single electrical soundchannel, is to generate sounds seeming to emanate from a selected one or ones of a plurality of positions on the screen.

Still another purpose of this invention is to provide an artificial stereophonic sound system employing only one inputaudio channel with automatic change-over from single-channel to stereophonic operation and vice versa.

Still another purpose of this invention is to provide an artificial stereophonic sound system employing only a singlev audio input channel, the system automatically reverting to single-output-channel operation upon failure of any part thereof. 7

Still another purpose of this invention is, in motion picture reproduction of images with accompanying sound reproduced from a.single electrical sound channel, to generate sounds seeming to emanate from a selected one or ones of a plurality of positions on the motion picture screen in consonance with the visual action reproduced thereon.

A further understanding of this invention may be I secured from the detailed description together with the Inprojecting the film, the composite sound signal is derived from the film sound track in the sound head, is

accompanying drawing in which Figures 1 and 2 when taken together constitute a schematic diagram of an in strument embodying the invention.

Referring now to Fig. 1, a motion picture projector attenuator 22 connected in shunt thereto.

I I 3 sound head 11 is energized of a motion picture film. The sound track contains a composite sound record composed of the audio signal record which is to be reproduced and heard accompanyingthe picture reproduction, and a control signal record. The control signal record is an additive combination or sum of three component control signal records and this sum amplitude is designed to be constant. It is actually held in practice as near a constant value as the art of the operator mixing the signals during filming can make it. The constant sum of the three control signals is ten percent of the total allowable maximum amplitude of the sound track record. The audio signal is conventional, and includes all frequencies between 50 C. P. S. and 12,000, with attenuated signal below 50 C. P. S. The control signal is composed of three constant-frequency control signals having frequencies of 30,35 and 40 C. P. S. The amplitudes of these three signals have been adjusted in making the film in 's uch manner that each amplitude at any instant represents the presence or absence of sound origination at a selected section of the stage being filmed. The 30 C. P. S. signal is chosen to represent the origination of sounds from the right side of the stage, the 35 C. P. S. signal represents the center, and the 40 C. P. S. signal represents the left side. These three control signals and the audio signals have been simply added substantially without intermodulation to make the composite sound track record.'

The electrical output of the projector sound head 11 is conducted through a single electrical channel 12 to a preamplifier 13 and through a single electrical channel comprising conductors 14 and 14 to terminals 16 and 16' where the channel is split into two parallel channels.

' These terminals 16 and 16 are connected through conductors 17 and 17' to a primary winding 18 of a transformer 19, and through conductors 21 and 21' to an These two parallel channels constitute the respective control signal channel and audio signal channel. The audio signal channel 21, 21 is made more non-receptive to the control signals by the interposition of a high-pass filter 23, and the control signal channel is made non-receptive to the audio signal by the interposition of a low-pass filter. The attenuator 22 is provided so that the audio signal amplitude can be adjusted Without affecting the control signal amplitude, and takes the place of a ganged fader conventionally used at the power amplifier inputs. The attenuator 22 is employed, rather than control devices before the splitting terminals 16-16, because of the low level of the control signal and'because the control signal amplitude must remain under the exclusive control of the input control signal records.

Three loudspeakers 26, 27 and 28, Fig. 2, are positioned behind the right, center and left portions of the motion picture screen respectively, and each is energized by its power amplifiers 29, 31 and 32. Power amplifiers 29 and 32 are switched in and out of circuit by their respective relay contact armatures 33 and 34, and power amplifier 31 is switched between two circuits by its relay contact armature 36. In the drawing all relay contact armatures are shown in the normal'position with the relay coil unenergized. In this position of the relay the loudspeakers 26 and 28 are out of the circuit because their power amplifier inputs are short circuited and the center loudspeaker 27 is energized through normal contact'37, conductor 38, warping circuit 39, conductor 41, normal or back relay. contact 4-2, (Fig. 1) relay contact armature 43, and conductor 44 connected to the high pass filter 23. The function of the warping circuit 39 is to modify the high end of the frequency characteristic in accordance with requirements. The center loudspeaker in this condition of operation is the only loudspeaker in the circuit, receives all of the audio signal and provides single output-channel operation. It is to be from the single sound track noted that this single-output-channel operation is provided through a path containing no stereophonic equipment, so that such operation is not interrupted by any failure of stereophonic components, an essential feature for motion picture theatre service continuity.

When the relay armatures 33, 34 and 36, Fig. 2, are

operated to their forward or'upward positions and relay armature 43, Fig. 1, of the same relay is operated to its forward or down position, the three loudspeakers are connected for virtual stereophonic operation. The audio signal is applied from the high pass filter 23 through conductor 44, armature 43, forward contact 46 and conductor 47 to a warping circuit 48 which modifies the frequency characteristic in accordance with the possibly diiferent requirements of stereophonic operation. From the warping circuit 48 the audio signal is applied through an attenuator 49 and transformer 51 to three control amplifiers. Attenuator 49 permits adjustment of the virtual stereophonic channel for approximately unity gain, so that this channel will have the same gain as the single output channel, which contains no amplifiers. The transformer 51 matches the attenuator 49 output impedance to that of the three control amplifier input circuits in parallel. The grounded center-tapped secondary winding 52 of transformer 51 is connected through conductors 53 and 53', Fig. 2, to three pairs of coupling capacitors 54, 54', 56, 56 and 57, 57' connected to the control grids of the three control amplifiers. These amplifiers comprise push- pull pentodes 58, 58', 59, 59', 60 and 60'. Their outputs are connected to the three power amplifiers through output transformers 61,. 62 and 63 and forward relay contacts 64, 65 and 66.

The'control signal, consisting of a mixture of three signals having frequencies of 30, 35 and 40 C. P. S., is applied to the primary winding 18 of transformer 19. The secondary winding 67 thereof is half-tapped and its output is controlled by the switch arm 68. Switch arm 68 is connected through conductor 69 to a voltage divider 70. The slider 71 thereof is connected to a twostage RC low-pass filter comprising the first-stage resistor 72 and capacitor 73, and the second-stage resistor 74 and capacitor 76. The output terminal 77 is con- 1 which is much sharper because of the negative feedback than the cutoff of a two-stage RC filter alone.

The control signal, after amplification by triode 79 is applied through isolating resistors 86, 87 and 88 to three band-pass filters 89, 91 and 92. These filters are constructed to pass 30, 35 and 40 C. P. 8., respectively. Their outputs are coupled through voltage dividers 93, 94 and 96 to amplifying triodes 97, 98 and 99. V The alternating potential outputs of these amplifying tubes are rectified by means of capacitors 101, 102 and 103 and diodes 104, 106 and 107. The resulting constant-direction potentials are integrated or smoothed in the RC integrators consisting of resistors 108, 109 and 111, and capacitors 112, 113 and 114. The resulting constant-direction smoothed potentials at terminals 116, 117 and 118 represent the magnitudes of the original 30, 35 and 40 C. P. S. control signals. These smoothed potentials are coupled through resistors 119, 121 and 122 and applied through test switches 123, 124 and 126 and conductors 127, 128 and 129, Fig. 2,

' to the control grid circuits of the control amplifiers at junctions 131, 132 and 133, where these direct current voltages serve as grid biases controlling the gain of the respective amplifiers. These direct current grid bias voltages are applied to the control amplifier control applied to capacitor 144 grids through isolating resistors 134, 134, 136, 136', 137 and 137. These resistors also perform a second function in cooperation with capacitors 54, 54', 56, 56', 57 and 57' of further smoothing the control grid directcurrent biasing voltages.

The three control amplifiers are similar. Each consists of a pair of sharp cut-off pentodes, for example, pentodes 58 and 58'. The screen grid circuits are stiff; that is, each screened grid voltage is taken from the midpoint of a relatively low resistance voltage divider, such as resistances 138 and 139, with the result that the screen grid voltage is substantially independent of screen grid current. The screen grid voltage is slightly less than one-half of the anode voltage. The grid bias versus gain characteristic is approximately linear from minimum to maximum gain. This circuit has the peculiar and valuable property of having a sharp knee or bend at the top of the grid bias/ gain characteristic, so that maximum gain occurs at a selected grid bias and further increase of bias produces no further gain increase. Moreover, and most importantly, this limitation of gain occurs at the knee without any increased distortion of the output tone.

The three direct current control signals, in addition to controlling the three control amplifiers, are added to form a sum signal for the automatic control of switching from single-output-channel operation to virtual stereophonic operation and vice versa. The three signals are taken from terminals 116, 117 and 118 (Fig. 1) and are added in resistor 140, 141 and 142 to form in the common conductor 143 a signal proportional to the sum of the direct-current potentials at terminals 116, 117 and 118. The selection of these points for the summation, rather than an earlier point such as at the output of amplifier 79 has several advantages. It is far easier to sum three direct currents accurately than to sum three alternating currents of ditterent frequencies; and failure of any of the three rectifiers 104, 186 and 197 causes the automatic switch to switch to singleoutput-channel operation, inclusion of these rectifiers further protecting against disrupting failure. I

The sum potential in conductor 143 is applied to charge a capacitor 144. Since, as has been stated previously, the sum of the potentials of all three control signals is intended to be constant, the sum potential should be approximately constant during virtual stereophonic operation. The potential of capacitor 144 is applied to the control grid 146 of a triode 147 having a direct current plate supply indicated by the terminal marked +B. A 60 C. P. S. source represented by terminal 148 is applied through capacitor 149 and resistor 151 to the cathode 152. Thus the 60 C. P. S. signal appearing between grid and cathode'is amplified in accordance with the tube gain as controlled by the capacitor 144 potential and appears at terminal 153 with an R. M. S. amplitude change representing about five times the amplitude change of the potential of capacitor 144. This alternating signal is connected through conductor 154 to a quadrupler rectifier comprising diodes 156, 157, 158 and 159, Fig. 2,

and capacitors 161, 162, 163 and 164. This quadrupler rectifier provides a direct potential output having a gain of about fifteen times referred to changes in the potential of "capacitor 144, Fig. 1. This D.-C./A.-C./D.-C. amplifier system is employed rather than a conventional direct-coupled amplifier principally because it fails safe. That is, if any component should fail the output signal will fall to zero, causing the entire control unit to revert to single-output-channel operation.

The direct voltage output of the voltage quadrupler is applied to an integrator consisting of resistor 166,

Fig. 2, and capacitor 167. The latter is grounded through a normally closed relay contact 168 and relay contact armature 169. Operation of the armature 169 removes the ground from capacitor 167 and applies ground to a signal lamp 171, lighting it. Thus the introduction of the sum control signal switches the control unit to the virtual stereophonic condition with a delay of about 7 seconds. However, relay 178 delays only two seconds in returning to normal position when the control signal is lost or reduced because capacitor 167 is out of circuit. This short delay is due to the integrating action of components preceding capacitor 167 in the circuit.

The terminal 172 is connected to the control grid of a triode 173, where the signal is further amplified and applied from anode 174 through switch contact 176 and switch arm 177 to a relay coil 178, which is connected in series with the source of anode positive potential represented byterminal 179.

Switch arm 177 is connected mechanically to switch arms 181 and 182, comprising a three-deck four-position manually operated switch. In the upper position as shown the relay coil circuit is open. The entire power supply is also turned ofif, as connection is broken by main switch arm 182 between the llS-volt mains represented by terminal 183 and the power supply unit 184. In the next lower switch position the power supply unit is energized and anode potential is supplied to tube 173, but coil 178 is open; its relay contacts are in their normal positions, and the control unit is in the single-outputchannel condition of operation. In the next or on switch position the circuit is ready for automatic virtual stereophonic operation, and in the lowest or emergency on position power is applied to relay coil 178, operating the relay and putting the control unit manually in the virtual stereophonic condition of operation.

In the operation of the control unit with any standard motion picture projector, the fact that both electronic and manual gain controls in the control unit are in tandem with gain controls normally provided in conjunction with the projector constitutes a condition of inherent operating instability. In order to secure stable operation a milliammeter 186 has therefore been provided to measure the cathode/ anode current of triode 173. This current is a measure of the gains and losses inserted at the projector as, for example,- by the preamplifier 13 and by any sound head output matching networks, and will change with changes of these gains as well as of photocell sensitivity and of exciter lamp current. The gain ahead of the control unit can thus be maintained constant during operation by the use of milliammeter 186.

It is obviously necessary to set the gains of all three control amplifiers to be equal with equal control signal voltages. Although this can be approximated by equalizing the gain control bias voltages, it is more accurate, in that it corrects for differences among tubes, to adjust the grid control bias voltages for equal control amplifier cathode currents under the condition of maximum control signals. This is done with the aid of taps on a four-position switch 187 to connect the milliammeter 186 to read the cathode currents of the three control tube pairs. With maximum control signals applied, voltage dividers 93, 94 and 96 (Fig. 1) are adjusted to give equal cathode currents.

In adjusting the control unit, use is made of a test record containing several pure frequency signals alone and in combination. A four-section, three-position test switch is provided having the three contact armatures 123, 124 and 126 (Fig. l) in the three control signal is in its midposition the control amplifier grids are returned to ground so that tubes 58, 58, 59, 59', 60 and 60, Fig. 2, can be tested. In the third test switch position the control amplifiers are connected for normal operation and the transformer 19, Fig. 1, half-voltage tap is connected. This half voltage connection is required because of the design of test records now available.

In employing the test record the test switch is put in its third position as described. The first portion of the test record contains a 1000 C. P. S. tone. Since no control signal is present, relay 178, Fig. 2, remains normal for single-output-channel operation and the several controls are adjusted for the desired level at loudspeaker 27. The second section of the record contains only a 35 C. P. S. tone. This should operate relay 178 and the control signal should pass through the 35 C. P. S. channel including tubes 98, Fig. 1, 59, Fig. 2, and 59', providing opportunity to adjust this channel. The third section of the test record contains both 1000 and 35 C. P. S. signals together at full amplitude for the check of loudspeaker 27 loudness when actuated through the channel including tubes 59 and 59'. By the use of other sections of the test record the 30 and 40 C. P. S. chan nels are similarly adjusted.

In the operation of the control unit in connection with motion picture film, if the film sound track contains no control signal the relay 178 remains normal. The audio signal then passes through the channel including relay contact armature 43, Fig. 1, normal contact 42, conductor 41, warping unit 39, Fig. 2, conductor 38, normal contact 37, contact armature 36 and power amplifier 31 to the center loudspeaker 27. If the sound track contains the 30, 3S and 40 C. P. S. control signal, relay coil 178 is energized by one or more of these signals, closing its contacts 64, 65, 66, 46 and 168' after a delay of about'seven seconds. This delay precludes false operation of the relay during the running of single-sound-channel reels by loud sounds containing frequencies in the 30 to 40 C. P. S. region, since such sounds are not likely to persist for as long as seven seconds. Armature 4-3 applies the audio signal to the three control amplifiers; and three armatures 33, 36 and 34 connect the three power amplifiers to their control amplifiers for virtual stereophonic operation, and armature 169 removes large capacitor 167 from the circuit, so that when relay 178 is deenergized it will release after'only two seconds delay. This armature 169 also energizes lamp 171 as a signal that the unit is in the stereophonic condition of operation.

Thus a system is provided which will produce stereophonic sound effects or single-output-sound channel effects, and in the event any element of the stereophonic control circuit fails will automatically switch the system to the single output condition.

What is claimed is:

1. An artificial stereophonic sound system for producing stereophonically related sound efiects from a single composite signal consisting of an audio signal and a plurality of control signals having discretely different frequencies comprising, an input circuit having said composite signal impressed thereon, an audio signal channel connected thereto, means in said audio signal channel for transmitting only said audio signal, a plurality of control amplifiers equal in number to the number of control signals each of which includes at least one sharp cutofi pentode and each of which has its output coupled to a loudspeaker individual thereto, a first audio branch circuit having its output connected to the control grids of all of said pentodes, a second audio branch circuit having its output coupled to one of said loudspeakers at a point subsequent to the associated pentode, a control signal channel, means in said control signal channel for transmitting only said control signals, a plurality of control branch circuits equal in number to the number of control signals connected to said control signal channel, a bandpass filter in each of said control branch circuits each of which passes an individual and different one of said control signals, a rectifier in each of said control branch circuits for converting the individual control signal impressed thereon into direct current potentials of corresponding amplitudes, means for impressing individual ones of said direct current potentials on the control grids of said pentodes to adjust the gain thereof, and switching means for alternatively connecting the input of said first and second audio branch circuits to the output of said audio signal channel, said switching means including means for introducing a first selected time delay in connecting said first audio branch circuit to the output of said audio signal channel and for introducing a second selected time delay of less duration than said first time delay in connecting the input of said second branch circuit to the output of said audio signal channel.

2. An artificial stereophonic sound system in accordance with claim 1 including means for adding said direct current potentials to produce a sum potential, and means for operating said switching means in accordance with the amplitude of said sum potential.

3. An artificial stereophonic sound system in accordance with claim 1 including means for adding said direct current potentials to produce a sum potential, means for modulating said sum potential by an alternating current to produce a modulated sum potential, means for rectifying said modulated sum potential to produce a second direct current sum potential whose amplitude decreases to a minimum on the failure of any component in the path between said control signal channel and said last entioned rectifying means, and means for operating said switching means to connect the input of said first audio branch circuit to theoutput of said audio signal channel in response to amplitudes of said second direct current sum potential above a selected level and to connect the input of said second audio branch circuit to the output of said audio signal channel at amplitudes of said second direct current potential below said selected level. 4. An artificial stereophonic sound system in accord ance with claim 3 including means for measuring said second direct current sum potential.

' 5. An artificial'stereophonic sound system for producing stereophonically related sound effects from a single composite signal consisting of an audio signal and a plurality of control signals having discretely difierent frequencies comprising, an input circuit having said composite signal impressed thereon, an audio signal channel connected thereto, a plurality of control amplifiers equal in number to the number of control signals each of which has its output coupled to loudspeaker means individual thereto, a first audio branch circuit having its output connected in parallel to the inputs of all of said control amplifiers, a second audio branch circuit having its output coupled to one of said loudspeaker means at a point subsequent to the control amplifier associated therewith, a plurality of control branch circuits equal in numher to the number of control signals each having an individual one of said control signals impressed thereon, a rectifier in each of said control branch circuits for converting the individual control signal impressed thereon into a direct current potential of corresponding amplitude, means for controlling the gain of individual ones of said control amplifiers by individual ones of said direct current potentials, means for adding said direct current potentials to produce a sum potential, means for modulating said sum potential by an alternating current to produce a modulated sum potential, means for rectifying said modulated sum potential to produce a second direct current potential whose amplitude decreases to a minimum on failure of any component in the path of said control signal channel up to and including said last mentioned rectifying means, switching means for selecl y t finfg the inputs of said first and second audio 10 branch circuits to the output of said audio signal channel, channel at amplitudes of said second direct current sum and means for operating said switching means to conpotential below said selected level. nect the input of said first audio branch circuit to the 7 output of said audio signal channel in response to ampli- References Cited in the file of this Patent tudes of said second direct current sum potential above 5 UNITED STATES PATENTS a selected level and to connect the input of said second 2,714,633 Fine Aug. 2, 1955 audio branch circuit to the output of said audio signal

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