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Digital click removal and squelch control circuit for an FM receiver
| Details |
Inventors: Mattei, Anthony;
Assignee: Ford Motor Company (Dearborn, MI)
Primary Examiner: Safourek; Benedict V.
Assistant Examiner: Ng; Jin F.
Attorney, Agent or Firm: Brown; Robert W., Zerschling; Keith L.
A digital click removal and squelch control circuit for a digital FM receiver. The digital receiver generates a series of binary number electrical signals each of which is representative of the instantaneous phase angle of a received frequency-modulated electrical signal or signal derived therefrom. The phase angle signals are differentiated to produce binary number electrical signals that represent the rate of change of phase angle, or, the instantaneous frequency of the received signal. During a given time interval, the phase change caused by the modulation of the received signal must be within a range determined by the bandwidth of the received frequency-modulated signal. Phase changes outside of this range represent electrical noise or clicks and are removed by the click removal circuit of the invention. If a predetermined number of clicks occur during a predetermined time interval, the squelch control circuit of the invention provides a squelch control signal which may be used to disable the output of the FM receiver. When the click rate decreases, the output of the FM receiver is restored. |
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DETAILED DESCRIPTION It is an object of the invention to detect and remove or discard electrical noise, in the form of clicks or the like, from a frequency-modulated electrical signal received by an FM receiver or from a signal derived from such received signal.
The time derivative of the instantaneous phase angle of the signal from which it is desired to remove the electrical noise is utilized to determine whether such noise is present.
If the time derivative is above or below a predetermined threshold level, a click is determined to be present and is removed.
The phase angles of the signal are generated in the FM receiver as a series of binary number electrical signals, representative of the instantaneous value of the phase angle and occurring at equally-spaced intervals of time.
For each binary number electrical signal representative of the phase angle .
phi.
, the time derivative d.
phi.
/dt is closely approximated by calculation of the function .
DELTA.
.
phi.
.
sub.
i = .
phi.
.
sub.
i - .
phi.
.
sub.
i-1 where .
phi.
.
sub.
i designates a phase angle binary number electrical signal newly presented to the circuitry of the invention and .
phi.
.
sub.
i-1 represents a preceding, and preferably immediately preceding, binary number electrical signal representative of phase angle.
The instantaneous frequency or phase change .
DELTA.
.
phi.
.
sub.
i is a good approximation of d.
phi.
/dt where the sampling rate, that is the rate at which phase angle binary number electrical signals are supplied to the click removal and squelch control circuit, is much larger than the highest frequency component in the modulating signal.
According to the invention, where a click removal circuit is supplied with a series of binary number electrical signals as described above, a click removal circuit is provided which comprises first circuit means, supplied with the binary number electrical signals, for generating a difference binary number electrical signal each time one of the binary number electrical signals is supplied to the first circuit means, thereby, to generate a series of difference binary number electrical signals
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Telecommunications 
