United States Patent [191
`Mai et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,484,344
`Nov. 20, 1984
`
`[54] VOICE OPERATED SWITCH
`[75]
`Inventors: Don L. Mai, Garland; Bruce W.
`Campbell, Richardson, both of Tex.
`[73] Assignee: Rockwell International Corporation,
`El Segundo, Calif.
`[21] Appl. No.: 353,670
`[22] Filed:
`Mar. 1, 1982
`[51]
`Int. Cl.3 ................................................ GlOL 1/00
`[52] U.S. Cl . ....................................... 381/46; 381/110
`[58] Field of Search .................................. 381/46, 110
`[56]
`References Cited
`U.S. PATENT DOCUMENTS
`3,555,192 1/1971 Hymer ................................ 381/110
`4,052,568 10/1977 Jankowski ............................. 381/46
`4,187,396 2/1980 Luhowy ................................ 381/46
`Primary Examiner-E. S. Matt Kemeny
`
`Attorney, Agent, or Firm-Roger N. Chauza; V.
`Lawrence Sewell; H. Fredrick Hamann
`ABSTRACT
`[57)
`A voice operated switch having minimum signal ampli(cid:173)
`fication in the stages which separate the voice signals
`from the noise signals. The majority of amplification is
`then applied only to the syllabic rate voice component
`detected by an offset threshold in the syllabic rectifier(cid:173)
`amplifier circuit to produce a two-state signal, one state
`representative of the presence of voice energy and the
`other state representative of the absence of voice en(cid:173)
`ergy. The two states are compared with a threshold
`voltage adjustable between a maximum and minimum
`voltage. A maximum and minimum setting allow a re(cid:173)
`spective nontransmission and transmission of the voice
`signals irrespective of the presence or absence of such
`signals.
`
`8 Claims, 2 Drawing Figures
`
`+V
`
`RI
`A
`
`SYLLABIC
`RATE
`FILTER
`
`18
`
`D4
`
`+V
`
`-v
`
`-=
`
`10
`-v
`VQ=\~--<>--------------~----+-CT
`
`SWITCHED
`o-;,-------COMPOSITE
`VOICE/NOISE
`
`Page 1 of 6
`
`GOOGLE EXHIBIT 1021
`
`

`

`.i::..
`.i::..
`"' vl
`.i::..
`00
`"' .i::..
`.i::..
`
`N
`
`::r
`tf.l
`
`..... -0 .....,
`
`(1)
`(1)
`
`12
`
`VOICE/NOISE
`COMPOSITE
`
`I ~~SWITCHED
`
`SWITCH
`ANALOG
`
`FIG. 1
`
`(
`1.0
`
`~
`
`VOICE/NOISE
`COMPOSITE
`
`9 -\0
`~
`~ a (1) a
`
`~
`
`N
`
`•
`Vl
`~ •
`
`I Hrn n J---..--28
`
`l--24
`
`COMPARATOR
`
`!ANALOG
`
`l---22
`
`AMPLIFIER
`CLAMPED
`
`GAIN= 40
`
`26~ THRESHOLD I
`
`REFERENCE
`
`POTENTIAL
`
`20
`
`18
`
`16
`
`I I~ \.A.A.)
`RECTIFIER
`I
`FULL WAVE
`
`FILTER-5HZ
`!SYLLABIC RATE
`
`DETECTOR
`
`ENVELOPE
`
`1-4
`
`\
`
`LPF
`
`GAIN= 1
`
`GAIN= 4.5
`
`GAIN= 1
`
`GAIN= 1
`
`Page 2 of 6
`
`

`

`~
`~
`w
`~ ,..
`00
`~
`~ ,..
`
`N
`0 ....,
`N
`.....
`CD
`CD
`::r
`Cll
`
`~
`00
`
`9 -1,0
`z 0
`~ a 0 ::s
`c:: .
`
`N
`~
`
`f""t-
`
`•
`t,J
`
`VOICE/NOISE
`o-+--------COMPOSITE
`
`SWITCHED
`
`VOICE/NOISE_,.__ ______________ ____._ _____ --+----<Y
`
`COMPOSITE
`
`1.0
`
`-v
`
`FIG. 2
`
`20
`
`04
`
`+V
`
`12
`
`28
`
`26 ~
`
`+V
`
`22 05
`
`18
`
`SYLLABIC
`
`FILTER
`RATE
`
`~
`
`1~
`
`IC1
`
`02
`
`t:t..,.
`
`1
`
`LOW PASS 1
`
`FILTER
`
`Page 3 of 6
`
`

`

`1
`
`VOICE OPERATED SWITCH
`
`4,484,344
`
`2
`assure a respective permanent disabling or enabling of
`the transmission channel.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a block diagram according to the preferred
`embodiment of the present invention.
`FIG. 2 is a combined block diagram and circuit sche(cid:173)
`matic of the various stages of the voice operated switch.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`FIG, 1 depicts, in block diagram form, the voice
`operated switch according to the preferred embodiment
`of the present invention. A broad overview of the in(cid:173)
`vention will be given first, followed by a detailed disclo(cid:173)
`sure.
`A transmission channel 10 couples composite voice
`and noise signals from, for instance, a communication
`voice line or a radio received IF, to other circuitry such
`as an audio amplifier, not shown. The transmission
`channel 10 is enabled and disabled by an .analog switch
`12 in series with such channel. The voice detection
`circuits are responsive to the presence or absence of the
`voice signal component to control the analog switch 12.
`More particularly, the low pass filter 14, the envelope
`detector 16 and the syllabic rate filter 18 comprise the
`circuitry for separating the voice signals from the noise
`signals to generate other signals representative of the
`presence of the voice signal component. It should be
`noted that the gain of each such stage is made as close
`to unity as possible. In this manner the composite voice
`and noise signals appearing at the input of the VOX are
`subject to minimum amplification so that the signal to
`35 noise ratio of the processed signal, and thus its sensitiv(cid:173)
`ity, is preserved. It will be discussed in connection with
`FIG. 2 why the gain of the syllabic rate filter 18 is
`greater than unity.
`The amplifier 22 provides the requisite amplification
`to produce a two-state signal of sufficient amplitude to
`drive an analog comparator 24. The output of amplifier
`22 is clamped such that its output low level state is an
`indication of the absence of voice signals, and the out(cid:173)
`put high level state represents an indication of the pres-
`ence of voice signals.
`The two-state output of the clamped amplifier 22 is
`then compared with an adjustable reference threshold
`potential 26 to determine if the analog switch 12 should
`enable or disable the transmission channel 10. The maxi-
`mum reference potential is greater than the amplifier
`output high state, and the minimum reference potential
`is less than the amplifier output low state. This feature
`of the invention allows an adjustment of the reference
`potential to a maximum voltage to permanently disable
`the transmission channel. A reference potential mini(cid:173)
`mum adjustment comparably assures a permanent en-
`abling of the transmission channel. A hold circuit 28
`prevents the analog switch 12 from operating at a syl(cid:173)
`labic rate and "chopping" the voice signal at a syllabic
`rate.
`In FIG. 2, for clarity of understanding, some of the
`functional blocks of FIG. 1 are shown in circuit sche(cid:173)
`matic form. The preferred embodiment of the present
`invention is utilized in a radio receiver. In this environ(cid:173)
`ment the low,pass filter 14 is used to limit the frequency
`band to those frequencies below 750 Hz. In other appli-
`cations, such as for instance a telephone subscriber line,
`a low-pass filter may not be required because the electri-
`
`BACKGROUND OF THE INVENTION
`The invention disclosed herein pertains generally to 5
`voice detection circuits, and more particularly to voice
`operated switches employing syllabic rate detection
`circuits.
`Voice operated switches (VOX's) find a variety of
`applications in communication radio receivers. Used in 10
`a squelch circuit, the VOX can enable audio output
`from a receiver only upon the reception of voice signals
`so that the listener is not burdened with listening to a
`constant level of background noise. Voice operated
`switches may also have a particular utility in controlling l5
`the application of power to a transmitter, or the like,
`such that the transmitter is powered up only during the
`reception of voice signals. It is apparent that the appli(cid:173)
`cation of power only during the useful period of a trans-
`mitter can result in substantial economical benefits.
`It is well known in the art that a transmission channel
`can be controlled by the type of voice operated
`switches which detect the presence or absence of voice
`energy vis a vis noise energy. While this method of
`voice detection is simple, it is subject to false triggering 25
`due to the inability to discriminate between the pres(cid:173)
`ence of voice and non-voice energy components.
`Another voice detection method divides the voice
`band into two frequency bands such that the majority of
`voice energy falls into a lower band. The voice signals 30
`plus noise in this lower band are then compared with
`the noise energy in the upper band to determine the
`presence or absence of a· voice signal. This method of
`voice detection is commonly known as the two-band
`energy detection method.
`A third method, the syllabic rate detection method,
`overcomes the noted discrimination problem by first
`detecting the composite voice and noise envelope, then
`passing the envelope through a syllabic rate band pass
`· filter to define the presence or absence of syllabic rate 40
`energy.
`
`20
`
`SUMMARY OF THE INVENTION
`The voice operated switch according to the present
`invention employs a conventional low-pass filter, enve- 45
`lope detector and syllabic rate filter to separate the
`voice signals from the noise signals. Such stages process
`the voice and noise signals with minimum amplification
`so as to preserve the signal to noise ratio. The syllabic
`rate filter provides an indication of the presence of the 50
`voice signal, separate from the noise. This syllabic rate
`energy is then amplified by the majority of the circuit
`..
`gain.
`The major amplification is of sufficient magnitude to
`produce a two-state signal. This two-state signal is then 55
`compared with a reference potential to derive another
`signal for enabling or disabling the transmission channel
`switch. Each state of the two-state signal determina(cid:173)
`tively defines the presence or absence of a voice signal
`and thereby alleviates the need to make adjustments for 60
`compensating changes in the voice signal level. The
`invention therefore represents an advance in the art of
`discerning voice signals from noise signals.
`In the preferred embodiment, the maximum voltage
`of the reference potential is greater than the high state, 65
`and the minimum reference potential is less than the low
`state. This allows an adjustment of the reference poten(cid:173)
`tial to a maximum voltage or to a minimum voltage to
`
`Page 4 of 6
`
`

`

`4,484,344
`
`3
`cal characteristics of such line inherently limit transmis(cid:173)
`sion to these lower frequencies.
`As noted previously, one feature of the invention is to
`produce an indication of the presence of voice signals
`without disturbing the signal to noise ratio. To that end, 5
`the voice operated switch stages up to and including the
`envelope detector 16 include a gain as close to unity as
`possible. A conventional ideal diode detection 16 is
`provided with unity gain to detect low level signals.
`Such a detector eliminates diode offset voltage and 10
`permits small amplitude signals to be processed without
`amplification. The diodes D1 and D2 are poled to pro(cid:173)
`duce positive polarity output signals. Other configura(cid:173)
`tions providing for ideal diode characteristics may of
`course be used.
`The positive signals of the detector 16 are tracked by
`capacitor Cl to form an envelope. The value of the
`capacitor Cl is chosen such that the voltage developed
`thereacross is representative of the envelope of the
`inband composite voice and noise signals.
`The syllabic rate filter 18 is also of conventional de(cid:173)
`sign having a center frequency of 5 Hz and 3 db points
`at 3 Hz and 9 Hz. Such a filter processes the detected
`envelope to further separate the voice component from
`the noise component. The syllabic rate filter eliminates 25
`the higher frequency noise component and produces an
`output signal which varies in time according to the
`syllabic content of the voice component. It should be
`noted that the presence of the syllabic rate signal is
`therefore a direct indication of the presence of the voice 30
`signals on the transmission channel 10.
`It should also be noted that the syllabic rate filter 18
`is of the type which processes the signals without the
`insertion of offset or bias voltages. In other words, the
`syllabic rate signal coupled to the full-wave rectifier 35
`stage 20 is referenced around the ground potential. The
`absence of an offset voltage is significant when consid(cid:173)
`ering the operation of the full-wave rectifier 20.
`In brief summary, it is seen that the circuit stages up
`to and including the syllabic rate filter greatly enhance 40
`and distinguish the syllabic rate energy components of
`the detected envelope, relative to other frequency com(cid:173)
`ponents.
`A full-wave rectifier 20 is employed chiefly to de(cid:173)
`velop a unipolar signal so that subsequent stages can 45
`compare the amplitude of such signal with a single
`reference voltage. In this manner, a single threshold
`level can be used rather than comparing a bipolar signal
`with a high and low threshold level.
`The full wave rectifier requires two inputs, one 180 50
`degrees out of phase with respect to the other. Ampli(cid:173)
`fier 30 provides this phase inversion. Schottky diodes
`D3 and D4 are poled so that the combination produces
`a negative full-wave rectified representation of the sig(cid:173)
`nal appearing at the output of the syllabic rate filter 18. 55
`Diodes D3 and D4 are forward biased by resistor Rl
`current. Upon reactification of input signals, diodes D3
`and D4 introduce an offset of 0.3 volts at node A. The
`introduction of offset at this point prevents syllabic rate
`signal with an amplitude of less than 0.3 volts from 60
`appearing at node A and thus at the input of amplifier
`22. It should now be apparent that some amplification
`must precede the full-wave rectifier in order that low
`level composite voice signals can be processed with
`sufficient amplification to overcome the 0.3 volt offset. 65
`The 0.3 volt offset threshold essentially performs a peak
`detector function which discriminates against voice
`signal component amplitudes to pass acceptable syllabic
`
`4
`rate voice signal components and reject unacceptable
`components. In the preferred embodiment, this offset
`threshold is fixed as contrasted to the comparator stage
`variable threshold which performs a different function
`to be discussed later.
`The gain represented by amplifier 19 in FIG. 2 is for
`the purpose of producing the proper scaling between
`the input to the voice operated switch, and the 0.3 volt
`offset at node A. If syllabic rate filter 18 is an active
`filter, then this gain can be incorporated in the construc(cid:173)
`tion of filter 18. If the syllabic rate filter is a passive
`device, then the gain can be provided by separate ampli(cid:173)
`fier as illustrated. Assuming a nominal composite voice
`signal level of zero VU, the appropriate gain corre-
`15 sponding to amplifier 19 is 4.5. By way of example, if
`the nominal signal level were -20 VU, then the gain of
`amplifier 19 should be 45. If conventional silicon diodes
`with a 0.6 volt threshold are used instead of the
`Schottky type diodes illustrated, then amplifier 19
`20 should have a gain of about 9, rather than 4.5.
`It is important to note that the gain of amplifier 19 is
`only applied to the syllabic rate energy (5 Hz) and not
`to the noise.
`The signal voltage appearing at node A appears as an
`input to the clamped amplifier 22. The clamping ampli(cid:173)
`fier 22 amplifies the node A signals, again with respect
`to ground, by a factor of about 40. It is evident that the
`majority of amplification within the VOX stages occurs
`after the syllabic rate signal has been separated from the
`noise.
`It can be seen from FIG. 2 that amplifier 32 of stage
`22 operates between the + V and - V supply. It is thus
`evident that a rectifed signal peak extending below
`ground by 0.25 volts or more will drive the output of
`amplifier 32 upward to + V. However, Zener diode D5
`prevents the output voltage of the amplifier 32 from
`being driven to the + V, -V limits. Zener diode D5 is
`a silicon diode having a 3.9 volt breakdown voltage.
`Therefore, the amplifier output voltage is maintained at
`-0.6 volts for the absence of voice signals, and limited
`to +3.9 volts for the presence of voice signals. The 3.9
`volt level is the high state and the -0.6 volt level is the
`low state.
`In brief review, the full-wave rectifier stage 20 pro(cid:173)
`vides an offset so that small signals, which cannot be
`denomin·ated as either voice or low frequency noise, are
`not thereafter processed. This aspect of the invention
`enhances the overall discriminatory sensitivity of the
`VOX circuit. The amplifier stage 22 generates a digital
`output voltage having a high state representative of the
`presence of voice signals, and a low state representative
`of the absence of voice signals. The digital high state
`and low state voltage levels correspond respectively to
`the reverse and forward voltage drops of the Zener
`diode D5. The significance of the high and low states as
`applied to the comparator stage 24 will be described
`next.
`The comparator stage 24 essentially compares the
`amplifier high and low states with a threshold potential
`to produce an output indicative of the presence or ab(cid:173)
`sence of voice signals to thereby enable or disable the
`transmission channel 10.
`In achieving one feature of the present invention, the
`reference threshold potential 26 is adjustable to a maxi(cid:173)
`mum value + V, and a minimum value - V, where such
`values are greater and less than the respective voltage
`levels of the amplifier high and low states. A maximum
`threshold voltage adjustment ( + V) allows the compar-
`
`Page 5 of 6
`
`

`

`5
`ator to override any amplifier output indication to
`thereby assure the nontransmission of signals irrespec(cid:173)
`tive of the presence or absence of voice signals,, Corre(cid:173)
`spondingly, a minimum threshold voltage adjustment
`( - V) allows the comparator to again override any 5
`amplifier output indication to thereby assure the trans(cid:173)
`mission of signals whether or not voice signals are pres(cid:173)
`ent.
`Since the comparator 34 responds to these two-state
`signals appearing at its input, there is no need to contin- 10
`ually adjust the threshold potential 26 to accommodate
`changes in the voice signal input level appearing on the
`transmission channel. In essence, the determination of
`the presence or absence of a voice signal is made before
`the comparator stage. Therefore, the comparator does 15
`not function as a variable peak detector but rather deter(cid:173)
`mines the digital state to either open or close the analog
`switch 12.
`While the comparator amplifier inverting input could
`be connected directly to the wiper arm of the threshold
`potentiometer, a switch 36 can be added to take advan- 20
`tage of the aforementioned feature. The threshold
`switch 36 can be switched to position 1 to assure that
`the transmission channel switch 12 is open. With a refer(cid:173)
`ence threshold potentiometer wiper arm setting gener(cid:173)
`ally midway between its extreme positions, a switch 25
`setting at 2 allows the comparator to enable the trans(cid:173)
`mission channel analog switch 12 in the presence of
`voice signals and disable the analog switch 12 in the
`absence of voice signals. A switch setting at 3 assures
`that the transmission channel analog switch 12 remains 30
`closed irrespective of the presence or absence of voice
`signals.
`It should be noted that the comparator amplifier 34
`can drive the analog switch 12 through a hold circuit
`28. This hold circuit keeps the analog switch 12 closed 35
`for a minimum period of time after the comparator
`output changes from the high state to the low state.
`Since the abovedescribed VOX circuit responds to
`voice signals on a syllable-by-syllable basis, the hold
`circuit 28 provides a means by which the composite 40
`voice and noise signals appearing at the output of the
`transmission channel are not chopped or switched at a
`syllabic rate.
`In summary, the present invention provides a voice
`operated switch having a high degree of resolution for 45
`distinguishing between the presence or absence of voice
`signals, and a threshold control circuit with a feature
`which enables the transmission channel to be enabled or
`disabled irrespective of the presence or absence of such
`voice signals.
`.
`The specific embodiment disclosed herein is intended 50
`to be exemplary of the principles of the invention and
`are not restrictive thereof since various modifications,
`readily apparent to those familiar with the art, may be
`made without departing from the spirit and scope of the
`invention as claimed herein below:
`What we claim is:
`1. A circuit for detecting the presence of voice en(cid:173)
`ergy in a composite voice and noise signal, comprising:
`means for separating the voice signals from the noise
`signals to produce an indication of the presence of 60
`said voice signals, including:
`means for producing an envelope waveform of the
`composite voice and noise signal
`a syllabic rate filter for extracting the syllabic rate
`content of the voice signal from said envelope 65
`waveform, and
`means, including an amplifier, for amplifying only
`acceptable syllabic rate content exceeding an offset
`
`55
`
`4,484,344
`
`6
`threshold and thereby discriminating against unac(cid:173)
`ceptable syllabic rate components, said amplifier
`having the majority of the circuit gain such that the
`output thereof is driven to one state during the
`presence of said acceptable syllabic rate content,
`and said output is driven to another state during the
`absence of said acceptable syllabic rate content;
`whereby said one state is representative of the pres(cid:173)
`ence of voice signals and said other state is repre(cid:173)
`sentative of the absence of voice signals.
`2. The circuit for detecting the presence of voice
`energy as set forth in claim 1 wherein said amplifier is a
`unipolar amplifier, and wherein said circuit further
`includes a full-wave rectifier interposed between said
`syllabic rate filter and said amplifier, said full-wave
`rectifier having a gain essentially equal to unity.
`3. The circuit for detecting the presence of voice
`energy as set forth in claim 2 wherein said full-wave
`rectifier includes a pair of Schottky diodes.
`4. The voice operated switch of claim 1 wherein said
`means for producing an envelope waveform includes
`zero offset so that voice signals are detected irrespec(cid:173)
`tive of the amplitudes thereof.
`5. The voice operated switch of claim 1 further in(cid:173)
`cluding a comparator for comparing the output states of
`said amplifier with an adjustable reference potential to
`drive said switch, the maximum voltage of said refer(cid:173)
`ence potential being greater than one output state and
`the minimum voltage thereof being less than the other
`output state, whereby an adjustment of said reference
`potential to said maximum voltage or to said minimum
`voltage assures a respective permanent disabling or
`enabling of said transmission channel.
`6. The voice operated switch of claim 5 wherein said
`maximum voltage and said minimum voltage are essen(cid:173)
`tially equal to the respective + V and - V supplies of
`said amplifier, and wherein said amplifier includes
`means for clamping the voltage levels of said output
`states to levels intermediate said + V and - V supplies.
`7. The voice operated switch of claim 5 wherein the
`threshold potential appearing at the input of said com(cid:173)
`parator is selected from the group consisting of the
`three values:
`(a) said maximum voltage
`(b) said minimum voltage, and
`(c) a voltage intermediate (a) and (b) voltages.
`8. A voice operated switch for enabling or disabling a
`transmission channel in response to the respective pres(cid:173)
`ence or absence of voice signals, comprising:
`envelope detector means for detecting the envelope
`of composite voice and noise signals;
`syllabic filter means for generating syllabic rate sig(cid:173)
`nals corresponding to the syllabic content of said
`envelope;
`amplifier means for amplifying said syllabic rate sig(cid:173)
`nals an amount sufficient to produce a first output
`state, and a second output state in response to the
`absence of said syllabic rate signals;
`offset threshold means for preventing said amplifier
`· means from amplifying syllabic rate signals falling
`below a predetermined amplitude; and
`a comparator for comparing the output state of said
`amplifier with an adjustable reference potential, the
`maximum potential thereof being greater than said
`first output state, wherein an adjustment thereto
`disables said transmission channel, and the mini(cid:173)
`mum potential thereof being less than said second
`output state, wherein an adjustment thereto enables
`said transmission channel.
`* * * * *
`
`Page 6 of 6
`
`