O
`
`United States Patent (19)
`Cowan
`
`IIIHIIIHIIIHIIII
`11
`Patent Number:
`5,204,909
`45) Date of Patent:
`Apr. 20, 1993
`
`US005204909A
`
`54 AUDIO PROCESSING SYSTEM USING
`DELAYED AUDO
`76 Inventor: John A. Cowan, 962 Harris Pkwy.
`Cartersville, Ga. 30120
`(21) Appl. No.: 758,578
`22 Filed
`Sep. 12, 1991
`SC:
`9. 4
`51
`int. Cli............................................... H03G 7/00
`52) U.S.C. .................................... 381/106; 381/110;
`333/14; 341/132,328/173
`58) Field of Search .................. 381/110, 106; 333/14;
`341/132; 328/173
`(56)
`References Cited
`U.S. PATENT DOCUMENTS
`3,924,070 12/1975 Seaver ................................. 38/110
`3,995,224 li/1976 Sutphin, Jr. ......................... 328/173
`
`4,071,695 1/1978 Flanagan et al. ................... 381/106
`4,192,979 3/1980 Jankowski, Jr. ..
`... 381/10
`4,357,488 11/1982 Knighton et al. ................... 38/10
`Primary Examiner-Jin F. Ng
`Assistant Examiner-Nina Tong
`57)
`AB
`CT
`An audio processing method is described wherein an
`audio input signal is delayed and then submitted for
`signal processing. The signal is sampled prior to the
`delay and control parameters for speech compression,
`voice gating, and voice activation of ancillary equip
`ment generated. The delayed audio is then processed
`according to the parameters determined prior to the
`delay to eliminate poor transient response and signal
`clipping.
`
`6 Claims, 4 Drawing Sheets
`
`26
`
`28
`
`32
`
`38
`
`42
`
`
`
`
`
`
`
`
`
`AUDIO
`AMPI FIER
`
`
`
`
`
`THRESHOL)
`DETECTOR
`
`
`
`ANCILLARY
`EQUIPMENT
`
`48
`
`

`

`U.S. Patent
`
`Apr. 20, 1993
`
`Sheet 1 of 4
`
`5,204,909
`
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`U.S. Patent
`
`Apr. 20, 1993
`
`Sheet 2 of 4
`
`5,204,909
`
`
`
`DETECTOR
`
`DETECTOR
`
`

`

`U.S. Patent
`
`Apr. 20, 1993
`
`Sheet 3 of 4
`
`5,204,909
`
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`
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`Page 004
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`
`

`

`U.S. Patent
`U.S. Patent
`
`Apr. 20, 1993
`Apr. 20, 1993
`
`Sheet 4 of 4
`Sheet 4 of 4
`
`5,204,909
`5,204,909
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`
`
`

`

`1.
`
`AUDIO PROCESSING SYSTEM USING DELAYED
`AUDO
`
`REFERENCES
`
`U.S. Pat. Documents
`Seaver
`Schiffman
`Anderson et al.
`Gillig et al.
`Gagnon et al.
`Dolby
`
`3,924,070
`3,936,610
`4,489,442
`4,550,426
`4,140,876
`3,631,365
`
`5,204,909
`2
`and the audio gate, or the turn on time of any ancillary
`equipment in the case of voice activation. When the
`delayed audio signal emerges from the delay line, the
`proper amplification parameters are in place, allowing
`the signal to be smoothly compressed without the tran
`sient distortion characteristic of the old technology. In
`a like manner, the voice gate and any voice activated
`accessories have time to fully activate prior to the ar
`rival of the audio signal and therefore no loss of signal
`or front end clipping occurs.
`O
`Delay lines have been used in the prior art primarily
`to effect changes in the frequency spectrum or to create
`a reverberation effect. In addition, techniques for band
`width compression have been implemented, wherein
`the signal is reduced in bandwidth for transmission or
`storage and then later re-expanded.
`This new technology finds a unique and straightfor
`ward way to incorporate delay lines in conjunction
`with techniques used in the prior art to achieve a signifi
`cant improvement in the art of audio processing.
`It is possible to implement this new technology either
`with purely analog components or as a hybrid device
`using both analog and digital signal processing tech
`niques. The audio delay is achieved by the use of either
`an analog delay line (so called "Bucket Brigades”) or by
`the use of analog to digital conversion and digital mem
`ory storage.
`BRIEF DESCRIPTION OF THE DRAWINGS
`In FIG. 1, the important signal levels are shown
`which distinguish the delayed audio signal compression
`system,
`In FIG. 2, the important signal levels are shown
`which distinguish the delayed audio voice activation
`systern.
`In FIG. 3, the important signal levels are shown
`which distinguish the delayed audio voice gate system.
`In FIG. 4, a block diagram of the necessary compo
`nents to implement delayed audio signal compression is
`shown.
`In FIG. 5, a block diagram of the necessary compo
`nents to implement delayed audio voice activation is
`shown.
`In FIG. 6, a block diagram of the necessary compo
`nents to implement the delayed audio voice gate func
`tion is shown.
`In FIG. 7, a block diagram of the necessary compo
`nents to implement the delayed audio signal compres
`sion, voice gate, and voice activation processes in com
`bination is shown.
`In FIG. 8, a circuit diagram illustrating one of the
`many anticipated methods of implementing the delayed
`audio signal compression, voice gate, and voice activa
`tion process is shown.
`DETAILED DESCRIPTION OF THE
`INVENTION
`The important features that distinguish the practice
`of this new technology from what is old, can best be
`understood by observing the signal waveforms resultant
`from the practice of this technology in each of the sev
`eral embodiments.
`In FIG. 1, the signal waveforms that distinguish the
`practice of this invention as used for signal compression
`is depicted. An audio signal 10 undergoes peak detec
`tion and a peak voltage level 11 is determined. Using the
`old technology, the application of this voltage in a feed
`
`15
`
`35
`
`BACKGROUND OF THE INVENTION
`This disclosure relates to an improved audio signal
`processing system for use in audio amplification and
`recording systems, and in the voice controlled activa
`tion of radio transmitters, recording devices, and other
`20
`ancillary equipment.
`Since the early days of electronic audio amplification,
`methods have been devised to automatically control the
`gain of electronic amplifiers so that a wide range of
`input levels can be accommodated while maintaining a
`25
`fixed output level. This technology is useful in Public
`Address applications to prevent speaker overload and
`excess amplification of the audio input, and in the re
`cording industry to prevent overdriving the recording
`equipment. Over the years, various modifications of this
`30
`technology have been introduced, all of which use some
`form of negative feedback to alter performance charac
`teristics.
`In addition to audio compression and level control,
`voice activation techniques have emerged whereby the
`presence of an audio signal is detected and a control
`signal generated so that ancillary equipment, such as
`recording devices or radio transmitters, can be acti
`vated.
`As a variation of voice activation, the presence of an
`audio signal has been used to activate a switch in the
`audio signal path in a manner that interrupts the signal
`path when the audio input levels fall below a given
`threshold. This “Voice Gate" technology is frequently
`used to eliminate background noise and undesired
`45
`breath sounds between words, and to mute unused in
`puts in a multiple channel audio circuit.
`All of this technology suffers because the processes
`are applied in real time on the incoming audio signal.
`Since a finite time is required to analyze the signal and
`50
`calculate the appropriate response parameters, transient
`control is inaccurate. In the case of speech compression,
`the initial amplification will be inappropriately high,
`distorting the output signal. Likewise, activation of the
`voice gate and ancillary voice operated equipment is
`55
`delayed, so that the initial audio information is improp
`erly processed or lost altogether.
`SUMMARY OF THE NEW TECHNOLOGY
`The many shortcomings of the prior art have been
`60
`overcome by introducing a delay interval between the
`time the input signal is analyzed and the subsequent
`processing stages. The input audio signal is immediately
`analyzed by peak level detection circuitry and the cal
`culation of the appropriate parameters begun. The input
`65
`signal is then conveyed to an audio delay line, whose
`time delay interval is set to equal the response time of
`the feedback circuitry in the case of speech compression
`
`

`

`10
`
`5,204,909
`4.
`3
`will equal the reference output. If the instantaneous
`back scheme results in the waveforms at 12 and 13. The
`input signal level is less than the peak value, the output
`detection delay would cause an undelayed audio signal
`at 33 will be proportionally less. Thus, the peak audio
`to be initially processed with excess gain as seen at 12
`output of the multiplier will be limited to the level of the
`and only later does the level control become appropri
`reference voltage over a wide range of input conditions.
`ate 13. In the new technology, the input audio, delayed
`The actual input 29 to the multiplier is derived from
`by time interval 15, is shown at 14. The improved con
`the analog delay line 28, whose delay interval is set to
`trol of the output level resulting from the application of
`equal the sum of the response time of the peak detector
`feedback control to the delayed signal is shown at 16.
`and all arithmetic components. In this manner, the
`In FIG. 2, the signal waveforms that distinguish the
`audio signal arriving at the multiplier input is identical
`practice of this invention as used in voice activation
`to the original audio input only displaced in time by an
`applications is depicted. An audio signal 10 undergoes
`amount equal to the delay interval. Therefore, transient
`peak detection and a peak voltage level 11 is deter
`gain distortion is eliminated because the proper multi
`mined. When the peak voltage reaches the threshold
`plier gain coefficient is in place prior to the actual ar
`point 17, the activation control line goes active 18. At
`rival of the signal. The level stabilized output of the
`20 and 21, the undelayed audio output signal as seen in
`15
`multiplier at 33 is buffered by 34 and conveyed to the
`the old technology is depicted. Because of detection
`output at terminal 35.
`delays and the time needed for ancillary equipment to
`In FIG. 5, a block diagram showing the functional
`become operational, the front end of the signal21 is lost.
`implementation of a delayed audio voice activation
`Using the new technology, when the audio is delayed
`device is shown. This embodiment of the invention can
`by time 15, and used as the output 14, no loss of signal
`20
`be easily constructed using a digital or analog approach.
`information occurs. In order to maintain the proper
`An audio signal emanating from a microphone or
`length of activation time, the activation signal must be
`other audio signal source 25 is applied to an audio am
`extended by a time equal to the audio delay interval 19.
`plifier 26 whereby the input signal is brought up to a
`In FIG. 3, the signal waveforms that distinguish the
`satisfactory level. This amplified signal at terminal 27 is
`practice of this invention as used in a voice gated appli
`25
`delivered to an audio delay line 28 and a peak voltage
`cation is depicted. An audio signal 10 undergoes peak
`detector 30. The peak detector determines the maxi
`detection and a peak voltage level 11 is determined.
`mum voltage level present in the audio signal and has its
`When the peak voltage reaches the threshold point 17,
`time constant set so its output quickly rises to the maxi
`the activation control line goes active 18. The audio
`mum level and slowly decays to baseline. The resultant
`gate immediately goes active allowing the audio to pass
`30
`peak level control signal at terminal 31 is delivered to a
`on to the output. At 22 and 23, the result of using unde
`threshold detector 46, which functions to trigger an
`layed audio is depicted. Due to detection delays, the
`output control signal 47 when an input signal of suffi
`front end of the audio signal is clipped 23. Using the
`cient level is present. The control output 47 is available
`new technology, the audio signal, delayed by time 15
`to operate ancillary equipment 48.
`and then gated, is cleanly passed 24. In order to main
`35
`The amplified audio signal at terminal 27 is also pres
`tain the proper length of gate activation, the activation
`ented to the delay line 28 and delayed by a time interval
`signal must be extended by a time equal to the audio
`sufficient to allow detection of the input signal and to
`delay seen at 19.
`allow the ancillary controlled equipment time to fully
`In FIGS. 4 through 7, block diagrams showing the
`activate. The audio output of the delay line at terminal
`electronic and mathematical functions necessary for the
`29 is delivered to an audio output buffer 36 and made
`practice of this improved technology are depicted. It is
`available as output 37. The threshold detector 46 is set
`obvious and anticipated in this declaration that the
`to hold the "on' condition for a time interval beyond
`method of this technology can be implemented using
`further input so the audio delay line will have time to
`either analog or digital signal processing techniques.
`clear stored signals before allowing the ancillary equip
`In FIG. 4, a block diagram showing the functional
`45
`implementation of a delayed audio signal compression
`ment to turn off.
`In FIG. 6, a block diagram showing the functional
`device is shown. This embodiment of the invention can
`implementation of a delayed audio voice gate device is
`be easily constructed using digital or analog compo
`shown. This embodiment of the invention can be easily
`ents.
`constructed using a digital or analog approach. An
`An audio signal emanating from a microphone or
`audio signal emanating from a microphone or other
`other audio signal source 25 is applied to an audio am
`audio signal source 25 is applied to an audio amplifier 26
`plifier 26 whereby the signal level is brought up to a
`whereby the signal is brought up to a satisfactory level.
`satisfactory level. This amplified signal is delivered by
`This amplified signal at terminal 27 is delivered to an
`terminal to 27 an audio delay line 28 and a peak voltage
`audio delay line 28 and a peak voltage detector 30. The
`detector 30. The peak detector determines the maxi
`55
`peak detector determines the maximum voltage level
`mum voltage level present in the audio signal and has its
`present in the audio signal and has its time constant set
`time constant set so its output quickly rises to the maxi
`so its output quickly rises to the maximum level and
`mum level and slowly decays to baseline. The resultant
`slowly decays to baseline. The resultant peak level con
`peak level control signal at terminal 31 is delivered to a
`trol signal at terminal 31 is delivered to a threshold
`divider module 42. A voltage reference source 44 is set
`detector 46, which functions to trigger an output con
`to the maximum desired fixed output level and this
`trol signal 47 when an input signal of sufficient level is
`control voltage is also delivered to the divider 42 by
`present.
`signal line 45. The division of the reference voltage by
`The amplified input signal at terminal 27 is also pres
`the peak voltage yields an amplification coefficient
`ented to the delay line 28 and delayed by a time interval
`which is delivered by signal line 43 to a multiplier mod
`65
`sufficient to compensate for the detection delay of the
`ule 32. The gain of the multiplier 32 is thereby con
`threshold detector. The output of the delay line atter
`trolled, so that if a signal equal to the peak level arrives
`minal 29 is delivered to the audio gate 38 which serves
`at the multiplier input, the output level at terminal 33
`
`

`

`10
`
`15
`
`5,204,909
`6
`5
`passes to the output unimpeded. Since the gate control
`to interrupt the signal path under the control of the
`signal is derived from the audio input prior to the delay,
`threshold detector's output via terminal 47. The thresh
`the gate has time to become fully operational before the
`old detector 46 is set to hold the "on' condition for a
`input audio actually arrives for processing. The thresh
`time interval beyond further audio input so the audio
`old detector 46 is set to hold the "on' condition for a
`delay line will have time to clear before the gate closes.
`time interval beyond further input so the audio delay
`The gate function is such that when the control signal is
`line will have time to clear before closing the gate and
`low, no audio signals can pass. When the control signal
`so that the ancillary equipment will be held on until all
`is active, audio passes to the output unimpeded. Since
`audio has been processed. The output of the audio gate
`the gate control signal is derived from the audio input
`at terminal 39 is delivered to an output buffer 40 and
`prior to the delay, the gate has time to become fully
`made available as output 41.
`operational before the input audio actually arrives for
`In FIG. 8, a circuit diagram is depicted which illus
`processing. The audio output of the gate at terminal 39
`trates one of the many anticipated methods to imple
`is delivered to an audio output buffer 40 and made avail
`ment this new technology. Wherever possible, numeri
`able as output 41.
`cal references on this figure correlate to functional
`In FIG. 7, a block diagram showing the functional
`implementation of a delayed audio signal processing
`blocks and signal paths illustrated on FIG. 7.
`The input audio signal from a microphone or other
`device that includes all the embodiments of this inven
`input source is applied to terminal 25, R1, R2, R3, R4,
`tion is shown. This embodiment of the invention can be
`R5, C1, C2, C3, and IC1 form an audio amplifier corre
`easily constructed using a digital or analog approach
`lating with block 26 on FIG. 7. The amplifier gain is
`and incorporates delayed audio speech compression,
`20
`adjustable by R1. This amplified signal at 27 is con
`voice activation, and voice gating.
`veyed to a peak detector formed by C9, R27, R28, R29,
`An audio signal emanating from a microphone or
`D1, C7, and IC7. This correlates with block 30 on FIG.
`other audio signal source 25 is applied to an audio am
`7. The time constant characteristics of the peak detector
`plifier 26 whereby the signal is brought up to a satisfac
`are established by C7, R28, and R29. The output of the
`tory level. This amplified signal is delivered by terminal
`peak detector at terminal 31 is conveyed to a threshold
`to 27 an audio delay line 28 and a peak voltage detector
`detector-extender element and an analog divider. The
`30. The peak detector determines the maximum voltage
`threshold detector-extender is formed by R30, R31,
`level present in the audio signal and has its time constant
`R32, R33, D2, C8, C8, IC9, and IC10. This correlates
`set so its output quickly rises to the maximum level and
`with block 46 on FIG. 7. The threshold level is set by
`slowly decays to baseline. The resultant peak level con
`30
`the voltage divider formed by R30 and R31. The time
`trol signal at terminal 31 is delivered to the threshold
`extension interval is set by R32 to equal the audio delay
`detector 46 and a divider module 42. A voltage refer
`line interval. The output of this element at terminal 47 is
`ence source 44 is set to the maximum desired output
`made available as an output to control ancillary equip
`level and this control voltage is delivered to the divider
`ment and also to control the audio gate 38. The output
`42 by signal line 45. The division of the reference volt
`of the peak detector at terminal 31 is also conveyed as
`age by the peak voltage yields an amplification coeffici
`an input to the divider. The divider is composed of R18,
`ent which is delivered by signal line 43 to a multiplier
`R19, R20, R21, R22, R23, R24, R25, and IC5 which in
`module 32. The gain of the multiplier 32 is thereby
`this case is an XR-2208 integrated circuit. This corre
`controlled, so that if a signal equal to the peak level
`arrives at the multiplier input, the peak output level at
`lates with block 42 on FIG. 7. This divider also takes as
`an input, a reference voltage level generated and ad
`terminal 33 will equal the reference output. If the instan
`justed by R26 and a 10 volt power source. This corre
`taneous input signal level is less than the peak value, the
`lates with block 44 on FIG. 7. The output of this refer
`output at 33 will be proportionally less. Thus the peak
`ence is conveyed to the divider input by signal path 45.
`audio output the multiplier will be limited to the level of
`The output of the division is conveyed by signal path 43
`the reference voltage over a wide range of input condi- 45
`to one input of a multiplier stage. The multiplier is
`tions. The input to the multiplier 32 from terminal 29 is
`formed by R10, R11, R12, R13, R14, R15, and IC4
`derived from the analog delay line 28, whose delay
`which is an XR-2208 integrated circuit. This correlates
`interval is set to equal the sum of the response time of
`the peak detector and all arithmetic components, and
`with block 32 on FIG. 7.
`The amplified audio input at terminal 27 is also con
`the turn on time of any ancillary equipment. In this
`veyed to an audio delay line. This delay line is formed
`manner, the audio signal arriving at the multiplier input
`by R6, R7, R8, R9, C4, C5, IC2, and IC3. IC2 is a
`is identical to the original audio input, only displaced in
`MN3008 bucket brigade integrated circuit and IC3 is a
`time by an amount equal to the delay interval. There
`MN 3101 clock driver integrated circuit. R7, R8, and
`fore, no transient gain shifts occur because the proper
`C4 determine the clock frequency and therefore the
`amplifier gain parameters are in place prior to the ar
`55
`delay interval of the delay line and C5 serves to filter
`rival of the signal.
`out unwanted high frequency sampling noise. This cor
`The output of the peak detector 30 is also delivered
`relates with block 28 on FIG. 7. The output of the delay
`by signal line 31 to a threshold detector 46, which func
`line is conveyed by signal path 29 to become the second
`tions to trigger an output control signal 47 when an
`input to the multiplier.
`input signal of sufficient level is detected. This output is
`60
`The output of the multiplier stage is conveyed by
`made available to control ancillary equipment 48 and
`signal path 33 to a field effect transistor 38 which serves
`also as a control signal for the audio gate 38.
`as an audio gate under the control of the threshold
`The compressed output of the multiplier at terminal
`detector-extender via signal path 47. Since the block
`33 is delivered to the audio gate 38 which serves to
`function is produced by a single component, the transis
`interrupt the signal path under the control of the thresh
`65
`tor carries the same number as the block on FIG. 7. The
`old detector's output via terminal 47. The gate function
`output of the transistor switch is conveyed via signal
`is such that when the control signal is low, no audio
`path 39 to an output buffer formed by R16, R17, and
`signals can pass. When the control signal is active audio
`
`40
`
`25
`
`35
`
`50
`
`

`

`25
`
`5,204,909
`7
`3. A method of generating a voice gate in an audio
`IC6. This correlates with block 40 on FIG. 7. The buff
`channel whereby the output of the channel is muted
`ered audio output is available to ancillary equipment via
`when no audio is present and switched to full amplifica
`signal path 41.
`tion when input audio above a given threshold is de
`I claim as my invention:
`tected, using delayed audio so that the gate becomes
`1. A method of limiting the peak audio signal level 5
`fully operational before being presented with the audio
`from a signal source of varying levels to a fixed value,
`signal, comprising the following steps of:
`using delayed audio to improve transient response,
`amplifying the input signal to a suitable level,
`comprising the following steps of:
`generating a reference level,
`amplifying the input signal to a suitable level.
`sampling the amplified input signal by means of an
`sampling the amplified input signal by means of an 10
`analog or digital peak level detector wherein the
`analog or digital peak level detector wherein the
`output of said detector rapidly rises to the level of
`output of said detector rapidly rises to the level of
`the amplified input signal and slowly decreases as
`the amplified input signal and slowly decreases as
`the input level falls,
`the input level falls,
`comparing the output of the peak level detector to
`generating a reference level for signal comparison, 15
`the reference level by means of an analog or digital
`dividing the output of the peak level detector into the
`threshold detector so that a control signal is gener
`reference level by means of an analog or digital
`ated whenever signals above the reference level are
`divider and thereby calculate a multiplication coef.
`present in the amplified input signal,
`ficient suitable for achieving compression of the
`using an analog or digital delay line to delay the am
`amplified input signal,
`20
`plified audio signal after the sampling stage, by a
`delaying the amplified input signal after the sampling
`time equal to the time required for threshold detec
`stage by means of an analog or digital delay line for
`a time interval equal to the cumulative delay intro
`tion,
`extending the control signal from the threshold detec
`duced by the sampling stage, the divider, and the
`tor after cessation of audio input by a time equal to
`subsequent multiplier,
`multiplying the delayed audio signal by the multipli
`the time delay of the audio delay line so that the
`control signal is held active until all audio signals
`cation coefficient by means of an analog or digital
`have cleared the delay line and making this signal
`multiplier in a manner so that the peak output of
`the multiplier will be equal to or less than the refer
`available for the control of an audio gate,
`inserting an analog or digital audio gate in the audio
`ence level,
`30
`buffering the output of the multiplier for use by tape
`signal path after the delay line, controlled by the
`extended output of the threshold detector in such a
`recorders, radio transmitters, public address ampli
`manner that the audio path is interrupted when the
`fiers or any other equipment requiring an audio
`input signal.
`threshold detector is inactive,
`buffering the audio output of the audio gate for subse
`2. An improved method of detecting the presence of 35
`quent use by public address amplifiers or any other
`audio signals from a signal source and using the pres
`equipment requiring a voice gated audio input.
`ence of the audio signals from said source to activate
`4. A method of limiting the peak audio signal level
`tape recorders, radio transmitters, or other ancillary
`from a signal source of varying levels, and using the
`equipment requiring an audio input signal, that uses a
`presence of said audio signal to activate tape recorders,
`delayed audio signal so that said equipment becomes 40
`radio transmitters, public address amplifiers, or any
`completely operational before the arrival of the audio
`ancillary equipment requiring an audio input signal, and
`signal, comprising the following steps:
`further using the level of said audio signal to abruptly
`amplifying the input audio signal to a suitable level,
`mute the audio channel when the input level falls below
`generating a reference value,
`a predetermined threshold, using delayed audio to in
`sampling the amplified input signal by means of an 45
`prove transient response and eliminate signal loss com
`analog or digital peak level detector wherein the
`prising the following steps of:
`output of said detector rapidly rises to the level of
`amplifying the input signal to a suitable level,
`the amplified input signal and slowly decreases as
`generating a reference level,
`the input level falls,
`sampling the amplified input signal by means of an
`comparing the output of the peak level detector to 50
`analog or digital peak level detector wherein the
`the reference value by means of an analog or digital
`output of said detector rapidly rises to the level of
`threshold detector so that a control signal is gener
`the amplified input signal and slowly decreases as
`ated that immediately activates the ancillary equip
`the input level falls,
`ment whenever signals above the reference level
`dividing the output of the peak level detector into the
`are present in the amplified input signal,
`55
`reference level voltage by means of an analog or
`using an analog or digital delay line to delay the am
`digital divider to calculate a multiplication coeffici
`plified audio signal after the sampling stage, by a
`ent satisfactory for compression of the input signal,
`time equal to the sum of the time required for
`using an analog or digital delay line to delay the am
`threshold detection and the turn on time of the
`plified audio signal after the sampling stage, by a
`60
`ancillary equipment,
`extending the control signal from the threshold detec
`time equal to the cumulative delay of all arithmetic
`steps, and the turn on time of any ancillary equip
`tor after cessation of audio input by a time equal to
`the time delay of the audio delay line and making
`ment,
`multiplying the delayed audio signal by the multipli
`this control signal available so that the ancillary
`cation coefficient by means of an analog or digital
`equipment will continue operation until all audio 65
`multiplier in a manner so that the peak output of
`signals have cleared the delay line,
`the multiplier will be equal to or less than the refer
`buffering the audio output of the delay line for use as
`the audio input of the ancillary equipment.
`ence level,
`
`

`

`5,204,909
`10
`9
`an audio amplifier stage,
`comparing the output of the peak level detector to
`a peak voltage level detector,
`the reference level by means of an analog or digital
`a variable voltage reference source,
`threshold detector so that a control signal is gener
`a voltage threshold detector having the means to
`ated whenever signals above the reference level are
`compare the peak voltage to the reference and
`present in the amplified input signal and extending 5
`generate a control signal,
`the activation time by an additional time interval
`a pulse extender having the means to lengthen the
`equal to the audio delay intervalso that this control
`control signal by a variable amount so that this
`signal is held on until all signals have cleared the
`control signal is held on until all signals have
`delay line,
`cleared the delay line, the thusly lengthened signal
`making this extended control signal available for the
`being additionally available to operate external
`control of ancillary equipment and the audio gate,
`equipment,
`inserting an analog or digital audio gate in the audio
`an analog divider whereby the voltage reference is
`signal path after the delay line, controlled by the
`divided by the peak voltage and an output signal
`extended output of the threshold detector in such a 15
`made available,
`manner that the audio path is interrupted when the
`an analog delay line and associated clocking circuitry
`for the delay of the input audio signal, said delay
`threshold detector is inactive,
`buffering the audio output of the audio gate for subse
`interval being adjustable,
`quent use by ancillary equipment.
`an analog multiplier operating on the delayed audio
`5. A circuit for limiting the peak audio signal level 20
`signal and the output of the divider, acting as a
`from an audio signal source of varying levels, and using
`variable gain stage for the purpose of audio com
`pression,
`the presence of said audio signal to activate tape record
`an audio gate having the means to interrupt the audio
`ers, radio transmitters, public address amplifiers, or any
`signal path between the multiplier and the output,
`ancillary equipment requiring an audio input signal, and
`being controlled by the extended control signal,
`further using the level of said audio signal to abruptly 25
`an output buffer stage.
`mute the audio channel when the input level falls below
`6. A circuit according to claim 5, where the audio
`a predetermined threshold, using delayed audio to im
`gate is a field effect tra