Constant amplitude interference squelch system

A squelch circuit for muting constant amplitude interference when the signal is absent. The input wave's envelope characteristic is examined so as to determine if the envelope is constant and, if it is, a gate is controlled so that the input wave is muted. Time delay circuitry is provided in one embodiment in order to provide sufficient time to make a determination that constant amplitude interference is present rather than variable amplitude speech before muting the input wave.

CROSS REFERENCE TO RELATED APPLICATIONS 
My "Signal Presence Determination Method", Ser. No. 693,716, filed June 7, 
1976 and "Squelch and Message Storage System and Method", Ser. No. 
708,203, filed July 23, 1976 applications are related, in general, to 
squelch systems. 
BACKGROUND OF THE INVENTION 
There are a number of methods for sensing the presence of speech. Most of 
these methods are based upon the measurement of strength of the incoming 
wave. However, strong interference can confuse such systems and falsely 
indicate that signal is present. 
A number of alternative methods for sensing the presence of signals are 
described in patent application, Ser. No. 693,716. One of the systems 
disclosed is based upon the determination that the input wave resembles an 
expected noise or interference waveshape, and if it does resemble the 
waveshape the information is used to operate a circuit which squelches or 
attenuates the output wave. FIG. 3 of patent application Ser. No. 693,716 
shows one circuit for accomplishing the waveshape comparison operation. 
Other arrangements have been disclosed for improving squelch performance 
such as reissued patent, Re. 27,202 where the communications channel is 
split into two or more segments and means are provided for comparing the 
energy in each segment. The gain for each segment is adjusted so that for 
normal types of noise the energy in each segment is equal. Generally, the 
adjustment is made for white noise, but the adjustment can also be made 
for a constant amplitude tone. However, if the tone varies in frequency, 
the system requires constant readjustment. 
SUMMARY OF THE INVENTION 
A general object of the instant invention is to provide means for 
squelching or muting constant amplitude interfering waves. A further 
object is to produce a squelch or muting system which can provide 
protection from constant amplitude interference and, at the same time, 
provide protection from white or impulse noise. 
An additional object is to provide a system that provides reliable 
protection against constant amplitude interference and where the circuitry 
required to sense the constant amplitude wave is relatively inexpensive. 
These and other objects of this invention will become apparent to those 
skilled in the art upon consideration of the accompanying specification, 
drawing and claims. 
As mentioned in the patent application Ser. No. 693,716 there are three 
basic forms of squelch in wide use today: 
(1) The total amplitude measurement method. 
(2) The sensing of some special characteristic of the signal which 
distinguishes it from noise and the indication that signal is present when 
such conditions occur. 
(3) The comparison of the input wave with the characteristic of noise or 
interference and the indication that signal is present whenever the 
incoming wave does not match the noise or interference characteristic. 
The invention, as disclosed in patent application Ser. No. 693,716 is based 
upon the last type of signal detection. The present invention is also 
based upon this last type of arrangement and, more specifically, is based 
upon the waveshape characteristic of the interference. 
A method is described herein for accurately and reliably testing whether 
the input wave is a constant amplitude wave. Use is made of an envelope 
detector which is ac coupled to a second detector circuit. If only the 
first detector produces a voltage output, it is concluded that a constant 
amplitude interfering wave is present. This system can be used at audio 
frequencies and IF frequencies, but if it is used at IF, this system is 
limited to those types of modulation that incorporate an envelope 
modulation component such as amplitude modulation and single-sideband 
modulation. It is not applicable to IF detection of constant amplitude 
signals such as FM or phase modulation signals and, in fact, a system 
utilizing the constant amplitude of FM has been used to detect the 
presence of signal; i.e., if the input wave had a constant amplitude it 
was determined that signal was present. Such systems had been used in 
squelch systems in FM receivers and are to be distinguished from the 
present invention. In the present case, the detection of a constant 
amplitude wave indicates the presence of interference not signal. 
In the present invention, it is highly desirable to use opposite polarity 
detectors in the constant envelope determination circuitry. The reason for 
this is that when the first detector detects the leading edge of a pulse 
of interference, a transient voltage component passes through the coupling 
circuit to the second detector but it is blocked because of the polarity 
of the second detector thus avoiding a false indication of signal 
presence. 
If the constant envelope determining system is used at audio frequencies it 
is difficult to provide proper filtering of the audio tone frequency, 
especially for low frequency interfering tones. However, if a full wave 
rectifier or other form of polyphase detection is used, the ripple 
frequency is increased and the amplitude of the ripple is decreased 
reducing the requirements for ripple filtering. By reducing the 
requirements for the filtering, the time delay of operation of the circuit 
can be minimized. 
In the preferred embodiment of the invention, a fixed time delay circuit is 
incorporated having a time delay of approximately 100 to 200 milliseconds. 
The use of a time delay circuit is advantageous as it allows sufficient 
time for a determination that a constant amplitude interfering wave is 
present, rather than voice. Since voice waves have slow syllabic envelope 
modulation components it is difficult to determine that the envelope is 
actually varying in a very short period. However, if 100 or 200 
milliseconds are available for the examination a reliable determination 
may be made.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1, the sole FIGURE, shows one embodiment of the instant invention. The 
input wave is amplified in amplifier 102 which in turn feeds a time delay 
circuit 104. The delay time provided may be in the order of 150 ms. Time 
delay circuit 104 may be a endless loop magnetic delay circuit such as 
used in U.S. Pat. No. 2,880,275 and patent application Ser. No. 708,203, 
or an electronic delay circuit using, for example, charge coupled devices. 
In this specification and the attached claims, time delay circuit is meant 
to include all forms of time delay devices, including mechanical, 
electrical, acoustical, etc. 
The output of delay unit 104 feeds Gate 1, 106 which is normally off or 
open. Thus, if signal is absent, noise will not pass through Gate 1. 
Amplifier 114 samples the signal ahead of the time delay circuit 104 and 
feeds BPF1, 116 and BPF2, 118 which compare spectrum energy at two 
different spectrum points, and the levels are compared in equality 
comparison circuit 124 after being suitably weighted by attenuators 120 
and 122. The output of block 124 feeds threshold and hang circuit 126 
which in turn controls Gate 1, 106. Shunted variable resistor 108 provides 
a muting control, the smaller the resistance the higher the level of the 
muted sound. 
The portion of the block using blocks 106, 114, 116, 118, 120, 122, 124 and 
126 follow the teaching of U.S. Pat. No. Re. 27,202. For more details 
regarding the operation of the noise squelch portion of FIG. 1, consult 
Patents Re. 27,202 and patent application Ser. No. 693,716. While such a 
squelch system has important advantages in muting noise and certain types 
of interference, it is not essential to utilize the method in order to 
practice the present invention. Actually, if one is only concerned with 
constant amplitude tone type interference, it is unnecessary to provide 
any form of conventional noise squelch circuit. 
The output of Gate 1, 106 feeds Gate 2, 110 and its muting control 112. 
Gate 2, 110 is normally "on", passing signal at full strength, but, when a 
steady tone of sufficient level present at the input circuit, Gate 2, 110 
opens, attenuating the output wave. 
The circuit which detects the presence of tones and causes Gate 2, 110 to 
open, operates as follows: Amplifier 128 amplifies a sample of the 
undelayed input wave. The amplified wave is detected in the full wave 
detector circuit which consists of components 130, 132, 134, 136 and 138. 
This circuit is conventional and produces a negative voltage which is 
approximately a linear function of the input signal level. The time 
constant of RC circuit 136 and 138 is such as to produce filtering for at 
least the lowest audio frequency tone interference expected. 
The output of envelope detector 134 is ac coupled through capacitor 140 to 
the detector circuit using components 142, 144, 146 and 148. This detector 
produces a positive voltage when an ac component is produced by the 
envelope detector feeding it. However, if a constant amplitude interfering 
tone has sufficient level to overcome the signal and noise, and has a 
frequency above 300 Hz, no voltage is produced by the second detector. 
This is because a constant tone is free of envelope modulation, and 
therefore no ac voltage is coupled through capacitor 140. 
However, when a voice wave is present, envelope variations are present, and 
the resulting variations at the output of the envelope detector using 
diode 134 are coupled through capacitor 140 to the second detector using 
diode 144. Thus, when speech is present a positive voltage appears across 
resistor 146. 
The positive voltage produced across resistor 146 is fed to one end of 
potentiometer 150 and the other end of the potentiometer is connected to 
resistor 136 where a negative voltage appears. The arm of potentiometer 
150 is connected to resistor 152 and diode 154. Diode 154 is connected so 
that whenever the voltage at the arm of 150 is more positive than the 
voltage across capacitor 156 by at least the contact voltage of diode 154, 
the diode conducts. This diode circuit speeds up the turn on action of 
gate 2, thus helping to avoid clipping of initial speech sounds. If a 
negative voltage is present at the arm of 150, as would be the case when 
steady tones are present, resistor 152 provides a slower means for 
charging capacitor 156. When the top lead of capacitor 156 becomes 
sufficiently negative it causes threshold circuit 160 to open gate 2, 
muting the signal. Muting adjust 112 can be varied to adjust the amount of 
muting. 
In operation, the voltage produced across resistor 136 is a relatively 
constant negative voltage when only steady tones are received. The 
negative voltage will open the gate prior to the instant when the tone 
reaches the gate because the control circuit operates before time delay 
circuit 104 passes the signal. Thus, when the circuit is properly 
designed, the time delay introduced by time delay circuit 104 is larger 
than the time delay of the detection circuitry. For example, if the time 
delay network 104 introduces a 150 millisecond time delay, the time 
constants in the turnoff circuit should introduce an overall time delay 
of, say, 140 milliseconds, and gate 2 will open before the interfering 
tone reaches the output of time delay 104. 
Similarly, the time constants in the system controlling gate 1, 106 should 
also approximate the time delay of 104. The larger the time delay used in 
such a circuit, the more reliable the system and the less prone it will be 
to false activation by noise; i.e., false alarms. It is preferable to make 
the operating time of that circuit slightly less than the time delay of 
104 so that the initial speech sounds are not clipped, although a slight 
clipping, say 10 to 20 milliseconds, is acceptable for most services. 
Thus, it is seen that the use of time delay 104 can both improve the 
operating characteristics of the squelch circuit and provide time for the 
system to protect against annoying tone bursts. U.S. Pat. No. 3,397,401 
and application Ser. No. 708,203 discloses the use of time delay circuits 
to improve conventional squelch performance. In some applications where 
cost must be severely limited, the time delay unit may be eliminated but, 
in this case, short tone bursts would be heard and the turn on time of the 
normal squelch would have to be shortened in order to avoid clipping of 
sizeable segments of the speech. 
It should be noted that a full wave detector using diodes 132 and 134 is 
connected so as to produce negative voltage whereas diode detector 144 is 
connected to produce positive voltage. That is, the two rectifiers produce 
opposite polarity voltages. This is the preferred arrangement because, 
when a tone is received, the initial transient will only produce a 
negative voltage, and the 144 diode is connected so that the transient 
voltage cannot be passed along. Thus, by using the diodes in opposite 
polarities, the system is appreciably less sensitive to false operation. 
Gate 1 and gate 2 may be replaced by a single gate with the appropriate 
logic control circuitry, and a single muting adjust circuit may replace 
variable resistors 108 and 112. Also, the operation of gate 2 can be 
changed to "normal OFF" by changing the bias arrangement; but, while 
normal off operation will provide better protection against tone bursts, 
it may cause occasional clipping of speech. All of these alternative forms 
of operation would be recognized as readily achievable by those skilled in 
the art. 
In all cases, it is understood that the above described arrangements are 
merely illustrative of the many possible specific embodiments which 
represent applications of the present invention. Numerous and other varied 
arrangements can be readily devised in accordance with the principles of 
the present invention without departing from the spirit and scope of the 
invention.