H - Electricity – 04 – M
Patent
H - Electricity
04
M
325/73
H04M 5/00 (2006.01) H04B 1/16 (2006.01) H04H 1/00 (2006.01) H04H 5/00 (2006.01)
Patent
CA 1122278
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 other- wise 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.
324247
Dolby Ray M.
Fetherstonhaugh & Co.
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