Telecommunications – Receiver or analog modulated signal frequency converter – Squelch
Patent
1985-02-15
1986-12-02
Bookbinder, Marc E.
Telecommunications
Receiver or analog modulated signal frequency converter
Squelch
455212, 455220, 381 94, H04B 110
Patent
active
046271023
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the processing of audio signals in relation to deriving a squelch function for use with radio receivers. The term `squelch` refers to the muting of or significant reduction of the level of audio output of a receiver when there is no intelligence being received, thereby relieving the aural strain of an operator why may have to maintain a listening watch for long periods.
A variety of solutions to this requirement have been proposed. Australian Pat. Nos. 500,961 and 509,953 each deal with a squelch facility to reduce background noise by comparison of energy levels in the frequency range outside the audio range. Other techniques which have been used are:
1. Comparative assessment of energy levels present in narrow sub-bands of the received audio band;
2. Real-time analysis of the audio signal;
3. The use of coded preambles and postambles on the transmitted signal;
4. The use of a tone transmitted continuously with the signal; and
5. Real-time analysis of the zero-crossings present in the audio signal.
The technique described in the U.S. Pat. No. 3,939,425 takes its input from the intermediate frequency (IF) signal of the receiver, to derive the muting signal. It also demodulates the IF signal to recover the audio output for the receiver.
It appears, from the description, that the technique is applicable only to amplitude modulated (AM) signals. Thus, the technique requires access to a signal (the IF signal) which is not normally available as a receiver output, and is applicable only to AM signals.
The above approaches have varying degrees of complexity and perform their function with varying degrees of success. In contrast the technique which is the subject of this invention has the object of being simple to implement, while providing excellent performance, and uses as its input only the audio signal from the receiver, thus not requiring access to a receiver IF signal.
To this end the present invention provides a squelch circuit for muting audio output of a receiver except when a desired audio signal is received, comprising: an input terminal connected to a limiting amplifier and a muting gate; a phase locked loop which receives the limiting amplifier output and which contains an oscillator and a phase comparator for comparing the frequency of said oscillator and said limiting amplifier output to produce an error voltage which is fed back to control said oscillator through a network to enable the oscillator to match its frequency to that of the limiting amplifier output, the match occurring faster when the limiting amplifier output frequency drops than when the limiting amplifier output frequency rises; said muting gate being connected to the output of said phase locked loop to mute the output of the receiver when the limiting amplifier output frequency is higher than that desired.
The device of this invention performs an analysis of a receiver audio output in terms of the zero-crossings of the signal and provides an output signal which can be used to control the muting of the system.
Examination of the spectra of audio signals produced by a radio receiver shows that for speech, the frequency of the dominant spectral components lies in the range from about 100 Hz to 300 Hz for male voices and from about 200 Hz to 700 Hz for female voices. These figures hold true for the majority of english speech, the exception being with sibilants and fricatives. With noise, however, the frequency of the dominant spectral components covers a much wider range, limited by the band-width of the receiver.
Preferably the oscillator in the phase locked loop is a voltage controlled oscillator (V.C.O.). The muting gate output is preferably derived from the V.C.O. control voltage of the phase locked loop to mute the output of the receiver when the V.C.O. output frequency is higher than a pre-set frequency for more than a pre-set time.
A preferred embodiment of this invention will now be described with reference to the circuit diagram shown in the FIGURE.
The audio input is shown at 1 a muting gate at 2 and t
REFERENCES:
patent: 3939425 (1976-02-01), Toyoshima
patent: 4061979 (1977-12-01), Walker et al.
patent: 4093824 (1978-06-01), Grosjean
patent: 4117410 (1978-09-01), Bender
patent: 4228320 (1980-10-01), Celli et al.
patent: 4323393 (1980-11-01), Kumaoka et al.
patent: 4342120 (1982-07-01), Settlemire et al.
patent: 4355417 (1982-10-01), Kozak, Jr.
patent: 4388730 (1983-06-01), Nash et al.
patent: 4492233 (1985-01-01), Petrofsky et al.
Bookbinder Marc E.
Telesz Jr. Andrew J.
The Commonwealth of Australia
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