FM stereophonic broadcasting system utilizing simultaneous companding and modulation/demodulation

In a transmitter for a biphonic FM radio transmission system, apparatus comprising:

• first means for combining stereophonically related audio frequency signals L and R to obtain sum and difference signals M and S respectively comprising (L+R) and (L-R),
• second means for compressing said difference signal S and simultaneously amplitude-modulating a first sub-carrier signal with said compressed difference signal, said second means including
• a unitary multiple-input port multiplier having a first input port connected to receive said difference signal S, a second input port connected to receive said first sub-carrier signal, a third input port and an output port, and
• control signal generating means responsive to a signal produced at the output port of said multiplier for generating and applying to the third input port of said multiplier a gain control signal which has an amplitude related to the amplitude of said difference signal S according to a predetermined compression characteristic,
• said multiplier being operative simultaneously to multiply the difference signal S, the first sub-carrier signal and said gain control signal for producing at the output port of said multiplier said first sub-carrier signal amplitude-modulated with said difference signal S compressed according to said predetermined compression characteristic, and
• third means for amplitude-modulating a second sub-carrier with said difference signal S, said third means including said multiplier and further comprising
• a source of reference potential, and
• switching means operative at a predetermined switching rate for alternately applying to respective input ports of said multiplier at said predetermined switching rate (1) a first combination of signals consisting of the first sub-carrier signal, the stereo difference signal S and said gain control signal and (2) a second combination of signals consisting of the second sub-carrier signal, the stereo difference signal S and said reference potential, for enabling said multiplier to both (a) modulate the second sub-carrier signal with the stereo difference signal S and (b) modulate the first sub-carrier signal with the stereo difference signal S compressed according to said predetermined compression characteristic.

Receiver apparatus for an incoming FM signal including equal frequency quadrature-related FM signals respectively amplitude-modulated with an audio frequency stereo difference signal S and a compressed version S' of said stereo difference signal, said apparatus comprising:

• generator means for generating a reference FM sub-carrier signal having first and second phases,
• first demodulator means connected to receive said incoming signal and said first phase of said reference FM subcarrier signal for producing the difference signal S, and
• second demodulator means connected to receive said incoming signal, said second demodulator means comprising
• a unitary multiplier device having at least first, second and third input ports and an output port,
• means for applying said incoming signal to a first port of said multiplier device,
• means for applying said second phase of said reference FM sub-carrier signal to a second input port of said multiplier device, and
• control signal generating means responsive to both said demodulated difference signal S and a signal produced at the output port of said multiplier device for producing and applying to a third input port of said multiplier device a direct current gain control signal which has an amplitude related to the amplitude of said demodulated difference signal S according to a predetermined expansion characteristic,
• said multiplier device being operative simultaneously to multiply said incoming signal, said second phase of said reference FM sub-carrier signal and said gain control signal to produce a noise-reduced difference signal S at the output port of said multiplier device,
• said first demodulator means including said multiplier device and further including
• a source of reference potential, and
• switching means operative at a predetermined switcheing rate for alternately applying to respective input ports of said multiplier device at said predetermined switching rate, (1) a first combination of signals consisting of said second phase of said reference FM sub-carrier signal, said incoming signal and said gain control signal, and (2) a second combination of signals consisting of said first phase of said reference FM sub-carrier signal, said incoming signal and said reference potential, for enabling said multiplier device to both (a) demodulate the stereo difference signal S and (b) simultaneously demodulate the compressed version S' of the stereo difference signal and expand the demodulated signal S' according to said predetermined expansion characteristic.

In a transmitter for a biphonic FM broadcasting system, apparatus comprising:

• a source of FM stereo difference signal (L-R),
• first and second generators for respectively generating first and second FM sub-carrier signals,
• first means for modulating said first FM sub-carrier signal with said difference signal (L-R), and
• second means for modulating said second FM sub-carrier signal with said difference signal (L-R) compressed according to a predetermined compression characteristic, said second means for modulating comprising a unitary three-input multiplier connected to receive at respective input ports and for simultaneously multiplying together said difference signal (L-R), said second FM sub-carrier, and a gain control signal which has an amplitude related to the amplitude of said difference signal (L-R) according to said predetermined characteristic and producing at the output of said multiplier said second FM sub-carrier signal modulated with said difference signal compressed according to said predetermined compression characteristic,
• said means for modulating said first FM sub-carrier signal including said three-input port multiplier and further comprising
• a source of reference potential, and
• switching means operative at a predetermined switching rate for alternately applying to said multiplier at said predetermined switching rate a first combination of signals consisting of the second FM sub-carrier signal, the stereo difference signal (L-R) and said gain control signal, and a second combination of signals consisting of the first FM sub-carrier signal, the stereo difference signal (L-R) and said reference potential, for enabling said multiplier to both (1) modulate the first FM sub-carrier signal with the stereo difference signal and (2) modulate the second FM sub-carrier signal with the stereo difference signal compressed according to said predetermined compression characteristic.

In an FM stereo broadcasting system for transmitting and receiving an FM multiplex signal derived from stereophonically related audio frequency signals L and R, said multiplex signal including a sum channel comprising the sum (L+R) of said L and R signals, and at least a first stereo difference channel comprising the difference (L-R) of said L and R signals compressed according to a predetermined characteristic modulated on a sub-carrier, means for developing said first stereo difference channel comprising:

• a source of said stereo difference signal (L-R),
• a generator of said sub-carrier signal,
• means for generating a gain control signal which has an amplitude related to the amplitude of said stereo difference signal according to said predetermined characteristic, and
• multiplying means consisting of a unitary three-input port multiplier connected to receive at respective input ports and for simultaneously multiplying together said stereo difference signal, said sub-carrier signal, and said gain control signal and producing at the output of said multiplier said sub-carrier signal modulated by said stereo difference signal compressed according to said predetermined characteristic.

In a stereo transmission system for generating and transmitting an FM multiplex signal derived from left and right audio frequency stereo channel signals, said multiplex signal including a sum channel containing the summed left and right stereo channel signals and first and second difference channels respectively containing the difference between said first and right stereo channel signals and the difference between said left and right stereo channel signals compressed according to a predetermined compression characteristic, means for developing said first and second difference channels comprising:

• a source of difference signal containing the difference between left and right stereo channel signals,
• generator means for producing first and second sub-carrier signals,
• first modulator means for modulating said difference signal on said first sub-carrier signal, and
• second modulator means comprising:
• multiplying means consisting of a unitary three-input port multiplier a first input port of which is connected to receive said difference signal, and a second input port of which is connected to receive said second sub-carrier signal, and
• control signal generating means responsive to a signal produced at the output of said multiplier for generating and applying to a third input port of said multiplier a gain control signal which has an amplitude related to the amplitude of said difference signal according to a predetermined compression characteristic,
• said multiplier being operative simulaneously to multiply the difference signal, the second sub-carrier signal and said gain control signal for producing at the output of said multiplier said second sub-carrier signal modulated with said difference signal compressed according to said predetermined compression characteristic.

Receiver apparatus for an incoming FM signal including equal frequency quadrature-related sub-carrier signals respectively amplitude-modulated with a stereo difference signal S and a compressed version S' of said stereo difference signal, said apparatus comprising:

• a first phase shifter for shifting the phase of said incoming FM signal by an amount proportional to the frequency of said FM signal;
• a first multiplier for multiplying said incoming FM signal and the output of said first phase shifter thereby producing a sum signal (L+R);
• a generator of a reference FM sub-carrier signal;
• a second multiplier connected in parallel with said first multiplier for multiplying said incoming FM signal, the output of said first phase shifter, and the output of said generator thereby producing a difference signal (L-R);
• a second phase shifter for shifting the phase of the output of said generator by 45.degree.;
• and
• a third multiplier connected in parallel with said first and second multipliers, for multiplying said incoming FM signal, the output of said first phase shifter, the output of said second phase shifter, and a gain control signal which has an amplitude related to the amplitude of said incoming FM signal according to a predetermined expansion characteristic thereby producing a noise-reduced difference signal (L-R).