Stereo demodulating circuit triggered by a minimum input signal level

Claim 2

An FM stereo receiver for a stereophonic composite signal including a pilot signal, comprising: generating means for generating a D.C. voltage in response to the pilot signal; a single positive feedback amplifier connected to said generating means and triggered into one of operative and inoperative states by the generated D.C. voltage; demodulating means for synchronously demodulating the stereophonic composite signal comprising two pairs of differential amplifier type switch Circuits, each pair was connected in series with first and second composite signal amplifying transistors, respectively, at least one of which is supplied with the stereophonic composite signal; a third transistor connected in series with either of said first and second transistors, further switching means for switching said third transistor into an ON state in response to a first output of said positive feedback amplifier in the presence of a generated D.C. voltage higher than a predetermined reference level and for switching said third transistor into an OFF state in response to a second output of said positive feedback amplifier in the presence of a generated D.C. voltage lower than said predetermined reference level in order to effect monaural muting operation and to squelch a received stereophonic composite signal having less than a predetermined input level; and audio output means connected to said demodulating means for deriving an audio output from said demodulating means when said D.C. voltage is higher than said predetermined reference level.

Claim 3

An FM stereo receiver according to claim 1, further comprising separating means for separating the stereophonic composite signal to apply separated composite signals having an opposite phase relationship to each other to the bases of said first and second transistors, respectively, and level changing means for changing the input level of one of said first and second transistors to adjust the separation of the separated signals.

Claim 4

An FM stereo receiver according to claim 1, wherein said demodulating means comprises a 38 KHz switching signal generating circuit including a fourth transistor for amplifying a 38 KHz switching carrier and a fifth shorting transistor connected to the base circuit of said fourth transistor for controlling generation of the switching carrier by controlling said fifth transistor in response to said feedback amplifier means.

Claim 5

An FM stereo receiver according to claim 1, further comprising a capacitor or capacitor-resistor timing circuit connected to the base of said third transistor for slow operation thereof.

Claim 6

An FM stereo receiver according to claim 3, further comprising a capacitor or capacitor-resistor timing circuit connected to the base of said fifth transistor for slow operation thereof.

Claim 7

An FM stereo receiver according to claim 1, further comprising a fourth transistor parallelly connected with the input of said feedback amplifier, and detecting means for detecting the absence and presence of a tuned broadcast signal from an IF circuit to urge said fourth transistor on and off, respectively, whereby said feedback amplifier is switched into an operative state by the D.C. voltage of said generating means only when a stereophonic broadcast signal is received and said amplifier means is switched into an inoperative state by the absence of a tuned signal detected by said detecting means.

Claim 8

An FM stereo receiver according to claim 1, further comprising: a fourth transistor parallelly connected with the input of said feedback amplifier a second positive feedback amplifier connected through a pilot signal passing narrow band filter to a pilot signal detector of said generating means, said second amplifier being switched into one of operative and inoperative states in response to the presence and absence, respectively, of a detected pilot signal in order to urge said fourth transistor off and on, respectively; a stereo indicator lamp means which is switched into an operative state by the presence of a detected pilot signal and which is switched into an inoperative state in a detuned state of the receiver where large interstation noises occur.

Claim 9

An FM stereo receiver according to claim 1, further comprising a shorting transistor parallelly connected with the input of said feedback amplifier; a second positive feedback amplifier connected through a 38KHz passing narrow band filter to a 38KHz subcarrier generating circuit of said demodulating means, said second amplifier being switched into one of operative and inoperative states in response to the presence and absence, respectively, of a 38KHz subcarrier from said generating means in order to urge said shorting transistor off and on, respectively; and stereo indicator lamp means which is switched into an operative state when a stereophonic broadcast is received and which is switched into an inoperative state in a detuned state of the receiver where large interstation noises occur.

Claim 10

An FM stereo receiver according to claim 1, further comprising a fixed bias circuit and third switching means coupled between said bias circuit and the base of said third transistor, said third switching means having first and second positions in which said bias circuit is connected to and disconnected from, respectively, the base of said third transistor, wherein in the first position of said third switching means said demodulating means is maintained in an operative state for producing a monaural output signal from said audio output means when a monaural audio signal is received and for producing separated stereo output signals from said audio output means when a stereophonic composite signal is received and wherein in said second position of said third switching means no monaural output signal is derived from said audio output means when a monaural audio signal is received.

Claim 11

An FM stereo receiver according to claim 1, wherein said demodulating means comprises a switching signal producing circuit for producing switching signals for synchronous demodulation, including a fourth transistor for amplifying a 38KHz carrier; and further comprising a fifth shorting transistor connected to and driven by said positive feedback amplifier means and connected in parallel with said fourth transistor, said fifth transistor being switched into an ON state to short out said fourth transistor in response to said second output of said positive feedback amplifier and being switched into an OFF state for deriving said 38KHz carrier from said fourth transistor in response to said first output of said positive feedback amplifier.

Claim 12

An FM stereo receiver according to claim 9, further comprising a 38KHz switching signal producing circuit and a stereo indicator driving circuit for driving means to indicate reception of a stereo signal, said 38KHz switching signal producing circuit, said stereo indicator driving circuit and said third transistor all being controlled by the output of said positive feedback amplifier, wherein when said third switching means is in its second position and said positive feedback amplifier is switched to an OFF state upon reception of a monaural signal or a stereo signal having a signal level below a predetermined input level, said switching signal producing circuit, said indicator driving circuit and said third transistor are switched to OFF states, wherein when said third switching means is in said second position and said positive feedback amplifier is switched to an ON state in response to reception of a stereo signal having a signal level above said predetermined input level, said switching signal producing circuit, said driving circuit and said third transistor are switched into ON states, and wherein when said third switching means is in its first position and said positive feedback amplifier is switched into its OFF state in response to reception of monaural signals and stereo signals having a signal level below said predetermined input level, said switching signal producing circuit and indicator driving circuit are held in said OFF states and a monaural output is provided by said audio output means in response to a monaural input, and wherein when said third switching means is in its first position and said positive feedback amplifier is switched into its ON state in response to reception of a stereo input signal having a signal level above said predetermined input level, said swItching signal producing circuit and indicator driving circuit are switched into their ON states and a separated stereo output is provided from said audio output means.

Claim 13

An FM stereo receiver according to claim 1, comprising a transistor circuit for half-wave rectification of the 19KHz pilot signal for producing 38KHz switching carrier, and means for imparting a base bias to said transistor circuit less than a potential difference between emitter and base electrodes of said transistor circuit, including diode means having a similar temperature-voltage characteristic to that of the base-emitter voltage and having a nearly constant voltage thereacross irrespective of variations in current flowing therethrough, or including a combination of a metal and a semiconductor or a combination of different semiconductors.

Claim 14

An FM stereo receiver according to claim 10, further comprising a capacitor or capacitor-resistor timing circuit connected to said fifth transistor for slow operation thereof.

Claim 15

An FM stereo receiver for receiving a monaural signal and a stereophonic signal which includes a 19KHz pilot signal, comprising: a first amplifying stage for amplifying a composite signal; a second amplifying stage connected to said first amplifying stage for amplifying and half-wave rectifying said pilot signal, including first transistor means for amplifying said pilot signal and second transistor means connected to said first transistor means for half-wave rectifying said pilot signal; constant bias means for supplying a constant bias voltage to said second stage; generating means connected to the output of said second amplifying stage for generating a 38KHz switching carrier signal when a predetermined D.C. voltage level component of the rectified pilot signal appears at the output of said second stage; a positive feedback amplifying stage, including a positive feedback amplifier connected to an output of said generating means, which is switched into one of on and off states in accordance with the presence and absence, respectively, of a minimum D.C. voltage level at the output of said generating means; demodulation means for synchronously demodulating said stereo signal, including two pairs of differential amplifier on-off means, respectively connected in series with first and second composite signal amplifiers; switching means connected in series with both of said first and second composite signal amplifiers, said switching means being switched between on and off states in response to said positive feedback amplifier means in order to demodulate a received stereophonic composite signal above a predetermined level by said demodulation means, and effect monaural muting operation and to squelch a received stereophonic composite signal having less than said predetermined input level respectively; constant level means for maintaining a substantially constant stereo-on level irrespective of variations in power supply current through the collector of said first transistor means and variations in the contact potential difference across a p-n junction of said second transistor means; indicating means for indicating reception of a stereo signal by the receiver; energizing means connected to said feedback amplifying means for energizing said indicating means and said generating means; and signal supplying means connected between said first amplifying stage and said demodulation means for applying a first composite signal to said first composite signal amplifier and a second composite signal of opposite phase from said first signal to said second composite signal amplifying means, said signal applying means including level varying means for varying the level of the signal applied to said second composite signal amplifying means for adjusting stereo separation.

Claim 16

An FM stereo receiver for receiving a monaural signal and a stereophonic signal which includes a 19KHz pilot signal, comprising: a first amplifying stage for amplifying a composite signal; a second amplifying stage connected to said first amplifying stage for amplifying and half-wave rectifying said pilot signal, including first transistor means for amplifying said pilot signal and second transistor means connected to said first transistor means for half-wave rectifying said pilot signal; constant bias means for supplying a constant bias voltage to said second stage; generating means connected to the output of said second amplifying stage for generating a 38KHz switching carrier signal when a predetermined D.C. voltage level component of the rectified pilot signal appears at the output of said second stage; a positive feedback amplifying stage, including a positive feedback amplifier connected to an output of said generating means, which is switched into one of on and off states in accordance with the presence and absence, respectively, of a minimum D.C. voltage level at the output of said generating means; demodulation means for synchronously demodulating said stereo signal, including two pairs of differential amplifier on-off means, respectively connected in series with first and second composite signal amplifiers; switching means connected in series with both of said first and second composite signal amplifiers, said switching means being switched between on and off states in response to said positive feedback amplifier means in order to demodulate a received stereophonic composite signal above a predetermined level by said demodulation means, and effect monaural muting operation and to squelch a received stereophonic composite signal having less than said predetermined input level respectively; constant level means for maintaining a substantially constant stereo-on level irrespective of variations in power supply current through the collector of said first transistor means and variations in the contact potential difference across a p-n junction of said second transistor means; indicating means for indicating reception of a stereo signal by the receiver; energizing means connected to said feedback amplifying means for energizing said indicating means and said generating means; and signal supplying means connected between said first amplifying stage and said demodulation means for applying a first composite signal to said first composite signal amplifier and a second composite signal of opposite phase from said first signal to said second composite signal amplifying means, said signal applying means including level varying means for varying the level of the signal applied to said second composite signal amplifying means for adjusting stereo separation.

Claim 17

An FM stereo receiver according to claim 14, wherein: said generating means comprises third transistor meAns for amplifying said 38 KHz switching carrier signal, said third transistor means having a first output from which said 38 KHz signal is obtained and a second output from which said D.C. voltage level component of said pilot signal is obtained; said indicating means comprises an indicating lamp connected to a lamp driver transistor; and said energizing means comprises fourth transistor means connected to said third transistor means for shorting said third transistor means; wherein the output of said positive feedback amplifying means is applied to said fourth transistor means and said driver transistor to energize said indicating lamp when a stereo signal is received by said receiver and to de-energize said indicating lamp when no stereo signal is received.

Claim 18

An FM stereo receiver according to claim 14, wherein: said indicating means comprises an indicating lamp connected to a lamp driver transistor; and said generating means comprises third transistor means for amplifying said 38 KHz switching carrier signal and fourth transistor means connected to the base of said third transistor means for shorting said third transistor means, wherein the output of said positive feedback amplifying means is applied to said fourth transistor means and to said lamp driver transistor to energize said lamp when a stereo signal is received by said receiver.

Claim 19

An FM stereo receiver as claimed in claim 16, in which a capacitor or a CR time constant circuit is disposed in the base circuit of said fourth transistor so as to slowly turn on and off said fourth transistor.

Claim 20

An FM stereo receiver as claimed in claim 17, in which a capacitor or a CR time constant circuit is disposed in the base circuit of said fourth transistor so as to slowly turn on and off said fourth transistor.