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Zweikanalton ("two channel sound", A2 Stereo) is a television sound transmission system used in Germany and other countries. It relies on two separate FM carriers. This offers relatively high separation between the channels (compared to a subcarrier-based multiplex system) and can thus be used for bilingual broadcasts as well as stereo. Unlike the competing NICAM standard, Zweikanalton is an analog system.

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How it works

The second FM carrier is transmitted on a frequency 242 kHz higher than the main FM carrier. (This frequency is 15.5 (15.5 x 15625Hz = 242187,5 Hz) times the line frequency which, being an odd multiple of half line frequency, reduces the visibility of intermodulation products between the two carriers.) It can carry either a completely separate audio program, or be used for stereo sound transmission. In the latter case, the first FM carrier carries (L+R)/2 for compatibility, while the second carrier carries R (not L-R.)

After combining the two channels, this method improves Signal-to-noise ratio by reducing the correlated noise between the channels. The second carrier also contains a control tone to indicate whether the transmission is stereo or dual sound. This control tone is a 50% AM modulated tone. The carrier frequency is 3.5 times line frequency (54687.5 Hz) and the modulated tone frequency is 117.5 Hz for stereo transmission and 274.1 Hz for dual sound. Absence of this tone is interpreted as a monaural transmission.

System M variant

There is a version of A2 used in Korea, compatible with the M standard of TV transmission. In this case the second FM carrier is 14.25 times the line frequency, or about 224 kHz, above the first carrier; pre-emphasis is 75 microseconds; the stereo control frequency is 149.9 Hz; the dual sound control frequency is 276 Hz; and the second channel carries L-R (not R).

Other names

Zweikanalton is known by a variety of names worldwide. Most commonly used names are German Stereo, A2 Stereo, West German Stereo and IGR Stereo.

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