Drop-out compensator for sound reproducing apparatus during tape reversal

The invention comprises a circuit arrangement for a sound-reproducing apparatus, particularly a video tape recorder having an audio channel, wherein upon the occurence of a sound signal drop-out interval within a continuous audio signal, for example within the track-switching period in multi-track sound recordings, an audio frequency signal is inserted in the drop-out interval so that the interval is rendered unnoticeable to the listener. The apparatus can be advantageously employed for recorders in which sound signals are recorded on the tracks of a magnetic tape or disc, or on grooved tracks in a disc. In one preferred embodiment of the invention the arrangement comprises circuitry for attenuating the noise burst which occurs during the track-switching period of a multi-track recorder using a frequency modulated audio signal, the degree to which the noise is attenuated being correlated to the level of the immediately preceding audio signal. Further embodiments of the invention comprise circuitry for inserting audio frequency noise signals of a predetermined level during the drop-out interval.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to drop-out compensation circuit arrangements for 
sound-reproducing apparatus in which a sound signal carried on a recording 
medium is played back, the medium being scanned by a pickup device and 
subsequently reproduced acoustically. 
2. The Prior Art 
U.S. Pat. No. 3,356,794 to Felix concernes reproducing circuits for 
wide-band magnetic recorders utilizing frequency modulation of a carrier 
with a video or other wide-band signal. As described in detail in the 
3,356,794 patent, objectionable bursts of noise are produced by the 
frequency demodulator when the signal level from the magnetic medium falls 
off to a low, substantially non-existent level such, as, for example, 
during signal dropouts. In order to avoid such objectionable noise bursts 
the variable gain amplifier-limiter associated with the frequency 
demodulator circuit receives the signal from the transducer as well as a 
supplementary signal from an oscillator. The oscillator provides a 
constant signal which is at approximately one tenth the amplitude of the 
normal video signal from the transducer. The amplifier-limiter has a 
relatively low gain when the normal video signal is present, but changes 
to a relatively higher gain when only the oscillator signal is present. 
The oscillator signal prevents system noise from being demodulated and 
causes the output of the frequency demodulator to be maintained at an 
acceptable level such that drop-outs of the signal will not result in 
noise bursts which are noticeable as visible streaks and flashes to the 
viewer. It is to be noted that the Felix disclosure does not concern 
narrow-band signals, such as audio signals, nor does it attempt to remove 
completely the background noise from oscillator 20 during those periods 
when no drop-out is present. 
U.S. Pat. No. 3,407,266 to Araki et al is concerned with the video drop-out 
which occurs in a helical-scan recorder when track-switching takes place. 
The Araki et al device produces two signal transducing heads which are 
spaced apart by an integral number of horizontal line lengths. When 
track-switching takes place, the output portion of the reproduction 
electronics is switched from one head to the other so that a few adjacent 
lines of the video signal are inserted into the track-switching drop-out 
interval. In this manner the synchronization pulses are maintained over 
the drop-out interval. A disc having an opening is rotated along with the 
heads so that a photocell will sense light passing through the opening and 
provide a switching signal at a predetermined time relative to 
track-switching. The signal from the photocell is used to momentarily 
switch the playback electronics coming from one head to the other. Those 
skilled in the art will recognize from the discussion below that the 
track-switching signal from the photocell of the Araki et al patent may be 
advantageously used in the circuitry of the present invention, as well. As 
will be described in greater detail below, the present invention is 
concerned with providing a suitable audio signal level during 
track-switching drop-outs of the type concerned in the Araki et al 
disclosure. 
U.S. Pat. No. 3,699,246 to Hodge teaches means for detecting drop-outs in 
video signals and for switching in a delayed version of the signal during 
the drop-outs. Such technique is well known in the art and performs at 
least two useful functions. First, synchronization pulses are maintained 
during the drop-out interval and, secondly, streaking and flashes in the 
video signal are eliminated by substituting an acceptable signal level for 
objectionable noise levels. The delayed video lines which are substituted 
for objectionable noise during the drop-out intervals are not particularly 
noticeable to the viewer since there is considerable similarity between 
adjacent video lines. 
In addition to the teachings of the foregoing patents which are concerned 
only with drop-outs in wide-band, or video, signals, several prior art 
patents are known which concern problems of noise bursts or drop-outs in 
narrow-band, or audio, signals. For example, German Patentschrift No. 
865,068 relates to the problem of bridging brief sound gaps which occur in 
the audio signal reproducing channel of a movie film projector at splice 
points in the film. Such sound gaps are brief, but noticeable to the 
listener, if no steps are taken to mask or bridge the gap. German 
Patentschrift No. 865,068 points this out and provides for insertion in 
the sound gap of a noise signal which is approximately the same in level 
as the audio signals which precede and succeed the gap or splice points. 
This is accomplished by adding to the film sound track a segment at the 
desired noise level. 
The present inventors have found that drop-outs in the signal from the 
recording medium transducer in a frequency modulated audio signal channel 
results in objectionable audio-frequency noise bursts at the output of the 
frequency demodulator. While such noise bursts may be blanked out entirely 
as taught, for example, in U.S. Pat. No. 2,288,000 to Kelly, the present 
inventors have found that total blanking out of such noise bursts results 
in sound gaps similar to thosd discussed in German Patentschrift No. 
865,068, and such sound gaps are noticeable to the listener. Brief signal 
gaps or objectionable noise bursts occur with such regularity in recording 
in recording systems taught, for example. In U.S. Pat. Nos. 3,958,272 and 
3,407,266, among others, that steps must be taken to render these 
occurrences innocuous to the listener if the recording systems are to be 
commercially acceptable. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide means in an 
audio-reproducing signal channel for maintaining the output 
audio-frequency signal at an appropriate level during periodically 
recurring signal drop-outs from the recording medium, so that the listener 
will not perceive such drop-outs as disagreeable disturbances. 
It is yet a further object of the present invention to provide, in a 
channel for audible reproduction of frequency-modulated audio signals, 
means for controlling noise bursts which would be objectionable to the 
listener and for assuring that an acceptable-level audio signal is 
substituted for such noise bursts. 
It is yet a further object of the present invention to provide means for 
attenuating periodically recurring noise bursts in a frequency-modulated 
audio signal reproduction channel to an acceptable level. 
Yet a further object of the present invention is to provide, during signal 
drop-outs from tape turnaround, track-switching, or the like, means for 
inserting during periodic signal drop-outs an audio-frequency signal of a 
level which is correlated to the preceding audio signal level. The term 
"correlated to" as used herein includes the case of "being the same as" or 
"being substantially the same as". 
The preferred embodiments of the present invention are intended for use, 
for example, in an audio reproduction channel of a video tape recorder of 
the reversible-drive type described in U.S. Pat. No. 3,958,272, wherein 
the audio signal is used to frequency modulate a carrier. During the tape 
turnaround interval of such a recorder the output of the playback signal 
transducer drops substantially to zero for a brief period of time, such as 
30-40 milliseconds (msecs). The output of the frequency demodulator of the 
audio channel is, during such signal drop-out intervals, an objectionably 
high-level audio frequency noise signal. This is an inherent result of the 
variable-gain amplifier/limiter which is conventionally used for 
demodulation of FM signals. One embodiment of the present invention 
comprises a squelch circuit which reduces such high-level noise bursts to 
a level which is correlated to the preceding audio signal level, the 
attenuated noise burst signal preferably being approximately 0 dB to 6 dB 
down from the preceding audio level. 
Another embodiment of the present invention, the embodiment taken up first 
in the detailed description given hereinbelow, utilizes, in a fashion 
similar to the known video signal drop-out compensation systems, a delay 
line which provides at its output the audio signal delayed by a 
predetermined time. The signal which reaches the ears of the listener is 
then the non-delayed audio signal, except during turnaround when a segment 
of the delayed audio signal is substituted for the otherwise objectionable 
noist bursts. Further embodiments of the invention provide a supplemental 
signal source such as a noise generator or an oscillator, for substitution 
during the audio drop-out noise burst interval. 
It will be seen that the arrangements of the present invention are useful 
not only with the video tape system described in U.S. Pat. No. 3,958,272, 
but also with helical scan tape systems, magnetic disc drives and other 
types of signal reproducing devices in which signal drop-outs periodically 
recur. Furthermore, the arrangements of the present invention are suitable 
for use with systems which utilize direct audio recording or amplitude 
modulated audio recording rather than frequency modulated audio recording. 
Those skilled in the art will recognize that drop-outs which occur during 
turnaround or track-switching or the like in amplitude modulation or 
direct recording system will not ordinarily result in objectionable noise 
bursts but will instead produce noticeable audio signal gaps. Such gaps 
may be compensated by insertion of a delayed audio signal or of an 
independently generated audio-frequency signal which has a level 
correlated to the preceding audio signal level from the recording medium. 
The above described objects, as well as further objects of the present 
invention, will become apparent to those skilled in the art in view of the 
following detailed description of the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows in block diagram form a reversible-drive tape recorder unit 
100 having a motor 102 and tape rolls 104 which carry both video and audio 
information in frequency-modulated form. Tape recorder 100 may be, for 
example, of the type described in U.S. Pat. No. 3,985,272, in which the 
audio information is sequentially recorded on multiple tracks of the tape. 
As described in the last mentioned patent, particularly with respect to 
FIGS. 5 and 6 of the patent, electronic circuitry is provided for 
periodically reversing the direction of tape travel when an end of the 
tape is reached, in response to detection of the end of a track of 
information. Transducer 106 picks up signals from the tape, and the video 
and audio components are separated by means of Band Pass Filter 108 and 
Low Pass Filter 110. The video signal is then demodulated by FM 
demodulator 112 and dropout detector 114 senses the end of the tape track, 
triggering a time delay/threshold device 116 for providing a control 
signal to turnaround control circuit 118 when reversal of the tape drive 
is to take place. The manner in which the drop-out detector, the time 
delay/threshold and the turnaround control circuit operate are not the 
subject of the present invention and such devices are given as merely 
illustrative of circuits which provide a control signal indicative of a 
signal drop-out which may be used in operation of the present invention. 
The above-noted U.S. Pat. No. 3,958,272 describes in somewhat greater 
detail what these circuits comprise. 
The frequency modulated audio signal off tape is received by FM demodulator 
120 from Low Pass Filter 120, and transmitted, demodulated, to delay line 
122 and terminal X of switch stage 124. The delay line may be a bucket 
brigade device or charge-coupled device (CCD), for example. The control 
signal from circuit 116 is transmitted both to turnaround control circuit 
118 which reverses the polarity of power to motor 102 and causes the 
direction of tape travel to be reversed, and to terminal Y of switch stage 
124 which normally transmits the demodulated audio signal input X to 
switch stage output Z'. In response to the control signal switch stage 124 
opens the circuit between terminals X amd Z' and closes the circuit 
between terminals W and Z. 
The output of delay line 122 is connected to terminals W of the switch 
stage. When a drop-out is to take place, the control signal activates 
switch stage 124 to insert a delayed segment of the audio signal in the 
audio channel. The outputs Z and Z' of switch stage 124 are supplied to a 
low pass filter 126 and drive a loudspeaker 128. Filter 126 removes audio 
frequency interference due to the FM demodulator, switching transients and 
the like. 
It will be understood by those skilled in the art that while the circuit of 
FIG. 1 illustrates the principal of one embodiment of the present 
invention, numerous modifications in this embodiment may be made without 
departing from the spirit and scope thereof. For example, a 
unidirectional-drive helical scan recorder such as disclosed in the 
above-mentioned U.S. Pat. No. 3,407,266 to Araki et al could be 
substituted for the reversible drive system shown here in FIG. 1. This 
last-mentioned patent teaches a helical scan video recorder in which 
track-switching signal dropouts recur at periodic intervals. A disc is 
provided which rotates with the helically scanning head, the disc having 
an opening for allowing transmission of light to signal each drop-out 
interval. The control signal for activating switch stage 124 of the 
present invention could be derived from this photocell. 
Alternatively, the drive system could include a moveable reproduction head 
mounted on a positioning device such as shown in U.S. Pat. Nos. 3,641,279 
and 3,705,270 to Ganske and Huber, respectively. In this case the signal 
used to activate the head positioning device could also be used as a 
control signal for the switch stage 124 of the present invention. 
It is well known in the art that when the input signal to the 
amplifier/limiter portion of an FM demodulator drops below a predetermined 
level, the demodulator will have at its output objectionable bursts of 
noise, as shown for example in FIG. 4. In this regard attention is 
directed to U.S. Pat. No. 3,356,794 to Felix which is similarly concerned 
with the noise bursts appearing at the output of an FM demodulator. The 
present invention accordingly provides, in the FIG. 1 embodiment, means 
for replacing the noise bursts occurring during the turnaround interval 
with a portion of the preceding audio signal so as to avoid having the 
objectionable noise bursts transmitted to the audio output loudspeaker 
128. 
FIG. 5 illustrates the relation between tape speed reversal for a drive 
system such as that described in U.S. Pat. No. 3,958,272 and the noise 
bursts of FIG. 4. The present inventors have found that the tape speed 
reversal time from full forward speed to full reverse speed can be reduced 
to around 80-100 msec., but that the audio signal output from the FM 
demodulator is acceptable during all but 30-40 msec. of that interval. The 
control signal from circuit 114 could, if desired, be modified before 
application to terminal Y of switch stage 120 so as to cause the insertion 
of the delayed audio signal to take place only over the critical 30-40 
msec. interval, rather than the entire 80-100 msec. interval. Since pulse 
shaping circuits and the like which would be suitable for such purpose are 
well known in the art, no further discussion thereof is deemed necessary 
here. 
FIG. 2 shows a circuit diagram of a switch stage 124 which is suitable for 
use in the circuit of FIG. 1. When the control signal supplied to terminal 
Y is at a low level, transistor T2 is non-conductive and diodes D11 and 
D13 are forward biased, so that the signal appearing at terminal L will be 
transmitted to terminal Z'. Inverter I assures that transistor T1 is 
conductive when transistor T2 is non-conductive, and vice versa. 
Therefore, when diodes D12 and D13 are forward biased, diodes D10 and D11 
are reverse biased and no signal passes from terminal W to terminal Z. The 
reversal of polarity of the control signal during the turnaround interval 
causes the circuit from terminal W to terminal Z to be closed and the 
circuit from terminal X to terminal Z' to be opened. The circuit of FIG. 2 
is merely exemplary and those skilled in the art will recognize that 
further switch circuits or devices could be substituted therefor without 
departing from the spirit and scope of the present invention. 
FIG. 3 shows a second embodiment of the present invention which includes a 
circuit for avoiding the objectionable noise bursts which would otherwise 
result from the frequency demodulator in the audio channel of a reversible 
drive video recorder such as that taught in U.S. Pat. No. 3,958,272. As in 
FIG. 1 above, the tape drive 100 comprises essentially a reversible motor 
102 and tape rolls 104. Transducer 106 is mounted for reproducing signals 
from the tape and provides such signals to both a low pass filter 110 in 
the audio channel, and a band pass filter 108 in the video and control 
signal channel. The components used to obtain the control signal, in 
particular band pass filter 108, FM demodulator 112, drop out detector 
114, time delay/threshold 116 and turnaround control circuitry 118, may be 
of the type described with reference to FIG. 5 of the last mentioned U.S. 
Pat. No. 3,958,272. The control signal obtained from circuit 116 is used 
for activating reversal of the tape drive by turnaround control circuit 
118, and is also used to trigger attenuating circuit 130. 
The operation of circuit 130 is as follows. The audio signal output from FM 
demodulator 120 is fed to circuit 130 and the signal passes through 
resistor R11 and amplifier AMP 1 to low pass filter 126, which comprises a 
resistor R16 and a capacitor C3, for example. The output of low pass 
filter 126 is connected to loud speaker 128. Field effect transistor FET 1 
and resistor R11 form a voltage divider circuit which serves to attenuate 
the sound signal going to amplifier AMP 1 in dependence upon the gate 
voltage on FET 1. Field effect transistor FET 2 normally inhibits the 
activation of FET 1. When the control signal is applied to the base of FET 
2 through resistor R15, FET 2 becomes a low impedance and the voltage on 
capacitor C1 is fed to the gate of FET 1. If the voltage on capacitor C1, 
is large, FET 1 represents a large resistance value connecting the input 
of amplifier AMP 1 to ground. When the voltage on capacitor C1 is small, 
the impedance of FET 1 is also small and will result in attenuation of the 
noise burst from frequency demodulator 120 during the drop-out interval. 
The voltage stored in capacitor C1 and applied to the base of FET 1 during 
turnaround in proportional to the average audio signal amplitude before 
turnaround. This is a result of diodes D14 and D15, which permit capacitor 
C1 to integrate the negative half cycles of the audio signal. The time 
constant of resistance R12 and capacitor C1 must of course be large in 
comparison with the known drop-out interval. Those skilled in the art will 
recognize that the components of attenuating circuit 130 may be chosen so 
as to attenuate the signal from FM demodulator 120 during the turnaround 
drop-out to any desired level. It is to be noted, however, that the 
inventors have found attenuation of the noise bursts to a level which is 
approximately 3-6 dB below the average level of the immediately preceding 
audio signal renders the drop-outs of signal to the FM demodulator least 
noticeable to the listener. 
FIG. 4 shows the output of the FM demodulator in the audio channel of a 
reversible-type recorder as described in U.S. Pat. No. 3,958,272. Normal 
audio signal levels 132 obtain during full-speed tape operation, but large 
amplitude bursts of audio-frequency noise 134 occur during a significant 
portion of each turnaround interval. Tests conducted by the inventors have 
indicated that a periodic noise burst 134 is noticeable to the untrained 
listener and that substitution of complete sound drop-out for the noise 
burst is likewise noticeable. The attenuating circuit of FIG. 3 serves to 
attenuate the noise bursts to an intermediate level 136 which is 
unnoticeable to the listener. This intermediate level is preferably at 
about 3-6 dB down from the preceding audio signal level. 
FIG. 6 illustrates yet another circuit for maintaining a suitable audio 
signal level during intervals of signal drop-out to the FM demodulator 
during turnaround. A control signal is obtained for indicating the 
drop-out interval in the same fashion as described above with respect to 
FIG. 1 and is used to operate a switch stage 124 such as shown in FIG. 2. 
The FIG. 6 arrangement is very similar to the arrangement of FIG. 1, with 
noise source 140 substituted for delay line 118. This noise source 
produces an audio-frequency "colored" noise signal which is inserted by 
means of switch stage 124 into the audio channel during the drop-out 
interval. 
FIG. 7 shows in greater detail the noise source 140 for use in the circuit 
of FIG. 6. The noise source comprises a "colored" noise generator 142, an 
integrating circuit 144 and a proportional gate 146. The "colored" noise 
generator is essentially a resistor R20 connected in series with a zener 
diode ZD1 between a positive dc voltage source and ground, with a 
capacitor C10 connected to the junction of resistor R20 and rectifier SCR, 
and a filter 148 for passing only selected audio frequency components of 
the noise generated. 
Integrating circuit 144 receives the demodulated audio signal from FM 
Demodulator 120 and passes the signal through a buffer amplifier AMP 3 and 
capacitor C11. Diode D20 passes the positive components of audio signal to 
capacitor C12, while diode D21 passes the negative components of the audio 
signal to ground. Capacitor C12 thereby integrates the positive 
half-cycles of the audio signal and stores a voltage proportional to the 
average audio signal level. Amplifier AMP 4 detects the voltage on 
capacitor C12 and opens transistor T10 of gate 146 in proportion to the 
average audio signal voltage. The values of capacitor C12 and resistor R21 
are selected so that the RC time constant will be somewhat greater than 
the known length of the drop-out interval, and a further controlled switch 
responsive to the control signal may be installed between FM demodulator 
120 and AMP 3 so that the voltage stored on C12 will be unaffected by 
noise bursts from the FM demodulator during the drop-out interval. Those 
skilled in the art will recognize how this may be accomplished using, for 
example, a switch stage as shown in FIG. 2. 
The output signal of noise source 140 which is connected to terminal W of 
switch stage 124 in FIG. 6 therefore comprises audio-frequency noise which 
is of a level related to the preceding segment of demodulated audio from 
tape. When the control signal operates switch stage 124 to connect 
terminal W to output Z during turnaround, the signal from the noise source 
passes through filter 126 to loudspeaker 128. The noise signal level at 
the loudspeaker is, of course, related to the immediately preceding 
program level. As noted above, the inventors have found that the drop out 
interval is least disturbing to the listener when the noise signal 
inserted during the turnaround interval is approximately 3-6 dB below the 
preceding average audio signal level. The signal could, however, be 
inserted at an amplitude roughly equal to the preceding average audio 
signal level without being substantially noticeable. 
FIGS. 8 and 9 show yet another embodiment of the present invention in which 
an oscillator stage is switched into the audio channel by switch stage 124 
in response to the control signal. The oscillator stage is shown in detail 
in FIG. 9. The demodulated audio signal from FM demodulator 120 is 
received at terminal X of switch stage 152, which may be of the type shown 
in FIG. 2. During normal tape operation the signal from terminal X is 
passed to terminal Z' and buffer amplifier AMP 5 passes the signal through 
capacitor C13 to diodes D22 and D23. Diode D23 shunts the positive 
portions of the audio signal to ground while diode D22 passes the negative 
portions of the audio to capacitor C14. Capacitor C14 integrates the 
negative half-cycles of the audio signal and stores a charge which is 
proportional to the prevailing average audio signal level. It is to be 
noted that the RC time constant of capacitor C14 and resistor R22 is 
necessarily greater than the length of the audio signal drop-out interval. 
At turnaround the control signal is supplied to control terminal Y of 
switch stage 152 and the charge on capacitor C14 is applied to an 
oscillator stage through capacitor C15 connected to terminal Z of the 
switch stage. The oscillator circuit consists chiefly of a resonant 
circuit comprising inductance L1 and capacitors C16 and C17, as well as 
transistor T11. The feedback of the circuit and the damping coefficient of 
the oscillation obtained at the output are adjustable by means of trimming 
resistor R26 inserted between capacitors C16 and C17 and the emitter of 
transistor T11. The oscillator circuit is set to a state of unstable 
equilibrium; that is, at the point where it is just ready to begin 
oscillation. This state is achieved by selecting a feedback factor less 
than 1. 
When a signal pulse from capacitor C14 is applied via coupling capacitor 
C15 to the oscillator circuit in response to the control signal, a damped 
oscillation is set up at the emitter of transistor T11, which is the 
output of the oscillator stage 150. The amplitude of the signal output 
from the oscillator stage depends upon the voltage on the storage 
capacitor C14 and the duration of oscillation depends upon the time 
constant of the oscillator circuit. Those skilled in the art will 
recognize that the oscillator circuit time constant is preferably made 
larger than the turnaround drop-out interval. Resistors 23-25 and 
capacitor C18 are used, as is conventional, for voltage supply and 
blocking purposes. 
FIG. 10 shows yet another embodiment of the present invention herein an AM 
demodulator 160, low pass filter 162, AM modulator 164 and noise source 
166 are connected for providing an audio-frequency signal to be inserted 
during the drop-out interval. A noise spectrum of suitable audio-frequency 
signals from 166 is amplitude-modulated with the low-frequency signal 
components of the envelope of the audio signal produced by AM demodulator 
160 from the output of FM demodulator 120. The low pass filter 162 is 
preferably dimensioned so that its time constant is larger than, or equal 
to, the duration of the turnaround drop-out. 
In the circuit arrangements described above it is desirable that the 
frequency range and frequency composition of the signal used for bridging 
the signal drop-out are preferably chosen so that interuptions in the 
audio information are perceived by the listener not as a loud audio noise 
burst or drop-out, but at worst as a slight change in timbre and/or 
amplitude. Tests by the inventors have shown that audio nose spectra in 
the region of 1 kHz fulfill these requirements if the audio signal 
transmitted during the turnaround interval is of an amplitude 
corresponding to approximately 1/2 the amplitude of the immediately 
preceding signal. In apparatus where switching from one or more tracks to 
adjacent tracks occurs periodically, for example in video tape recorders 
of the helical scan or reversible drive type, or in automatic video disc 
players, the length of the bridging signal will depend on the duration of 
the given switching interval. In known reversible-drive tape recorders 
wherein tape speed reversal and track switching occur simultaneously, such 
as described in U.S. Pat. No. 3,958,272, the drop-out interval is about 
30-40 msecs. 
Although the foregoing embodiments have been described with reference to 
audio reproduction channels in which the audio signal is retrieved from 
tape in frequency-modulated form, those skilled in the art will recognize 
that amplitude modulated audio signals or unmodulated baseband audio 
signals having drop-outs occurring during switching or tape reversal 
intervals or the like may be easily adapted for use with the audio noise 
insert circuits shown, for example, in the foregoing FIGS. 1, 6, 8 and 10.