Abstract: A sub-audible in-band tone system is disclosed for identifying an FM stereophonic radio broadcast which is specially encoded, as with dynamic range improvement encoding or quadraphonic encoding, for example, A constant frequency pilot tone is added at a low level (e.g. at -70dB) to the transmitted signal at or near the upper frequency limit of the audio frequency spectrum (15 kHz). The audio spectrum is not notched or otherwise altered to accommodate the tone. FM stereophonic receivers detect the pilot tone by heterodyning the received tone with a stable mixing signal at 15.2 kHz derived from the 19 kHz FM stereo pilot tone. The shifted frequency tone is passed to a low frequency narrow band detector which can control a visual display and switch in appropriate signal decoding circuitry when the tone is detected.

Abstract: A variable area sound track is scanned laterally to derive width-modulated pulses which can be demodulated to audio. In order to eliminate the effect of noise on the clear area of the track, each pulse is initiated by a black-to-clear transition sensed in the scanning but is not terminated until it is certain that the clear-to-black transition has been reached, e.g. at reference instants of time.

Abstract: A variable area sound track is scanned laterally to derive width-modulated pulses which can be demodulated to audio. In order to eliminate the effect of noise on the clear area of the track, each pulse is initiated by a black-to-clear transition sensed in the scanning but is not terminated until it is certain that the clear-to-black transition has been reached, e.g. at reference instants of time.

Abstract: The invention concerns a dynamic range compressor type encoder or expander type decoder, in which a main signal component in a main path is boosted or bucked by a further signal component derived from a point in the main path by a further path having the characteristics of so restricting the further signal component that the boosting or bucking action is only appreciable below a low level threshold. In the present invention the further signal component is a difference signal formed between a direct signal derived from a point in the main path and a delayed version of either the same signal or of another signal derived from another point in the main path. At the frequency equal to the reciprocal of the delay, and at harmonics of this frequency, the direct and delayed signals cancel. The compressor or expander action, and hence noise reduction action, takes place only at intervening frequencies.