Apparatus and method for the identification of specially encoded FM stereophonic broadcasts
US4281217A · kind A · utility
Inventor
Key dates
| Filing date | Mar 12, 1979 |
| Grant date | Jul 28, 1981 |
| Priority date | — |
| Expiry date | Mar 12, 1999 |
Classification
- Technology area (CPC H)Electricity
- CPC primaryH04H40/45
- WIPO fieldTelecommunications
- WIPO sectorElectrical engineering
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.
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.