Blend-on-noise stereo decoder

1. An FM stereo decoder of the kind comprising a composite signal conductor adapted for carrying a standard stereo composite signal containing a pilot signal, and a stereo decoding circuit means connected to said composite signal conductor for demodulating the composite signal and generating separate left and right stereo audio signals and including a blend-control means with a blend-control signal input for changing the degree of separation of the left and right audio signals as a function of the level of a signal applied to said blend-control input, said decoder additionally comprising:

a first generating means for generating at least a first signal having a dominant frequency component equal to the frequency of the pilot signal component in the composite signal; and

a noise monitor means connected to said composite signal conductor and to said first generating means for producing at a noise-monitor-means output a noise-indication signal of amplitude that is related to the magnitude of the noise in the composite stereo signal within a narrow frequency band near the pilot signal component in the composite signal, the output of said noise monitor means connected to said blend-control means input so that blending and the changing of the separation of the left and right signal is a function of the magnitude of the noise near the pilot signal component in the composite signal,

said noise monitor means comprises a noise demodulator means connected to said composite signal conductor and to said generating means for mixing said composite and first signal to transform said narrow frequency band of the composite signal to a demodulated noise-containing band the lower end of which is at zero frequency, a multiplier having one input connected to the output of said noise demodulator means; and a low pass filter having about half the band width of said noise containing frequency band, the input of said filter connected to the output of said multiplier; a rectifier connected between the output of said low pass filter and said blend-control input; the other input of said multiplier being feed-back connected to the output of said noise demodulator means for stabilizing and controlling the gain of said noise monitor means.

2. The FM stereo decoder of claim 1 wherein said generating means is additionally for generating the at least one signal with a phase of 90.degree. with respect to the pilot signal component of the composite signal for eliminating the pilot signal from the output of said monitor mixing means.

3. The FM stereo decoder of claim 2 wherein said at least one signal is a primary W1/90.degree. Walsh function signal with reference to the pilot frequency and said generating means is additionally for generating a second signal that is a W7/90.degree. F.sub.p Walsh function with reference to the pilot frequency F.sub.p, and wherein said monitor mixing means is a Walsh type mixer for combining said W1/90.degree. F.sub.p and W7/90.degree. F.sub.p signals with said composite signal to obtain the demodulated noise-containing band.

4. The FM stereo decoder of claim 2 wherein said standard stereo composite signal includes an (L+R) band of frequency spaced apart from an (L-R) band known as the S-band of higher frequencies, a pilot signal of frequency F.sub.p centered in the space between the (L+R) and (L-R) bands, the (L-R) band comprised of two AM side bands about a suppressed sub-carrier frequency F.sub.c twice that of the pilot signal frequency F.sub.p ; and wherein said signal generating means is additionally for generating the in phase Walsh function signals W1/0.degree. F.sub.p and W7/0.degree. F.sub.p with reference to the pilot frequency F.sub.p and for generating the additional Walsh function signals W1/90.degree. F.sub.c, W7/90.degree. F.sub.c, W1/0.degree. F.sub.c and W7/0.degree. F.sub.c ; said stereo decoding circuit means comprising a first dual channel Walsh type mixer with the primary input coincident with said stereo-decoding-circuit-means input and with the two ancillary inputs connected to points in said signal generating means at which respectively the W1/0.degree. F.sub.c and W7/0.degree. F.sub.c are generated.

5. The FM stereo decoder of claim 4 wherein said blend-control means comprises a voltage controlled attenuator having two signal channels connected interposed between said two ancillary inputs of said first Walsh type mixer and said signal-generating-means points at which said W1/0.degree. F.sub.p and W7/0.degree. F.sub.p and are generated, said voltage controlled attenuator having an attenuation-control input that is coincident with said blend-control input of said blend-control means.

6. The FM stereo decoder of claim 4 wherein said standard stereo composite signal is a standard FMX signal that further includes another (L-R) band of frequencies known as the S'-band that is shifted 90.degree. from the S-band relative to the sub-carrier, said stereo decoding circuit means further comprising a second dual channel Walsh type mixer with the primary inputs connected to said input conductor with the two ancillary inputs connected to points in said signal generating means at which the Walsh signals W1/90.degree. F.sub.c and W7/90.degree. F.sub.c are generated, said stereo decoding circuit means further comprising a signal summing means for adding the signals at the two outputs of said first dual channel Walsh type mixer, for adding the signals at the two outputs of said second dual channel Walsh type mixer and for adding the added outputs of said first and second Walsh type mixers at a summing means output so that a blendable composite demodulated L-R signal is generated there.

7. The FM stereo decoder of claim 6 wherein said stereo decoding circuit means further includes an FMX expander circuit to which said summing means output is connected.

8. The FM stereo decoder of claim 1 being adapted for incorporation in an FM radio that generates a composite signal wherein the pilot has a frequency F.sub.p of 19 KHz and said narrow frequency band extends over about a 4 kilohertz band centered about the FM-radio-19 KHz-pilot signal.