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Circuit and method of suppressing inter-station radio tuning noise
| Details |
Inventors: Wieschhoff, Reinhard; Seibold, Peter;
Assignee: Blaupunkt Werke GmbH (Hildesheim, DE)
Primary Examiner: Griffin; Robert L.
Assistant Examiner: Kuntz; Curtis
Attorney, Agent or Firm: Frishauf, Holtz, Goodman & Woodward
To suppress tuning noise interferences, and particularly transients, such as whistles and the like, especially in FM receivers during returning of the receiver, an inter-station retuning muting circuit is, in accordance with the invention, modified by including therein a switchable filter (2) having a low-frequency cut-off (f.sub.g2) which is higher than the frequency of noise interference due to retuning of the receiver, the filter being switched to the higher frequency cut-off level for a period of time (T.sub.E) which exceeds the normal muting time (T.sub.E) of FM receivers. |
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DETAILED DESCRIPTION The audio stage of an FM radio is partially schematically shown in FIG.
1 as a low-frequency source, shown merely as a generator coupling resistor 1.
A signal transmission path extends for example from the output of an FM discriminator circuit from the FM receiver, coupled to an audio output terminal 5 thereof, providing a signal voltage U.
sub.
S1.
The signal voltage U.
sub.
S1 at the output 5 of the source 1 is connected, in accordance with a feature of the invention, to a switchable high-pass filter 2.
The output of the switchable high-pass filter 2 is connected to a gate circuit 3 which, in turn, is connected to the output 6 of the circuit, at which an output voltage U.
sub.
S2 will be available.
The terminal 6 may be connected, as known, for example to a power amplifier and then to a loudspeaker.
The output voltage U.
sub.
S2, thus can be further amplified and then reproduced.
The filtering characteristics of the switchable filter 2 can be controlled from a control input 8.
The gate circuit 3 is controlled from a control input 9.
A control circuit 4 provides control signals u.
sub.
2 at terminal 8, and signals u.
sub.
1 at terminal 3.
The control signals are generated by the control circuit 4 functioning as a sequential muting control based on signal u.
sub.
0 on signals applied thereto at an input terminal 7.
The signals applied to the input terminal 7 initiate the muting of the receiver.
The sequential muting control circuit 4, essentially, includes square-wave output wave-shaping circuits which convert the signals received at terminal 7 into suitable control signals at the respective outputs 8 and 9 to effect switching of the respective elements 2 and 3.
OPERATION, WITH REFERENCE TO FIG.
2 Graph a of FIG.
2 shows the signal u.
sub.
0 applied to terminal 7 of the control circuit 4.
This signal, during the time when the receiver is being tuned to a transmitter, has the digital value 1; during retuning, the signal u.
sub.
0 changes to the value 0.
The beginning of the retuning is shown at time t
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Telecommunications 
