Zweikanalton ("two-channel sound") or A2 Stereo, is an analog television sound transmission system used in Germany, Austria, Australia, Switzerland, Netherlands and other countries that use or used PAL-B or PAL-G. South Korea formerly utilised its variant of this format in analogue television system until 31 December 2012. It relies on two separate FM carriers.
This offers a relatively high separation between the channels (compared to a subcarrier-based multiplexing system) and can thus be used for bilingual broadcasts as well as stereo. Unlike the competing NICAM standard, Zweikanalton is an analog system.
A 2nd FM sound carrier containing the right channel for stereo is transmitted at a frequency 242 kHz higher than the existing mono FM sound carrier, and channel mixing is used in the receiver to derive the left channel.
The second sound carrier also contains a 54.6875 kHz pilot tone to indicate whether the transmission is stereo or bilingual.
The pilot tone is 50% amplitude-modulated with 117.5 Hz for stereo or 274.1 Hz for bilingual.
|mode||existing OIRT 6.5 MHz or BG 5.5 MHz sound carrier contains:||2nd OIRT 6.258 MHz or BG 5.742 MHz carrier contains:||pilot tone in 2nd carrier:|
|mono||mono||carrier is absent||none|
|stereo||mix of left & right (L+R)||right audio channel (R)||54.6875 kHz 50% AM with 117.5 Hz|
|bilingual||1st language||2nd language||54.6875 kHz 50% AM with 274.1 Hz|
Zweikanalton can be adapted to any existing analogue television system, and modern PAL or SECAM television receivers generally include a sound detector IC that can decode both Zweikanalton and NICAM.
Zweikanalton can carry either a completely separate audio program, or can be used for stereo sound transmission. In the latter case, the first FM carrier carries (L+R) for compatibility, while the second carrier carries R (not L-R.) After combining the two channels, this method improves the signal-to-noise ratio by reducing the correlated noise between the channels.
The frequencies are chosen such that they cause minimal interference to the picture. The difference between the two sound carriers is 15.5 times the line frequency (15.5 x 15625 Hz = 242187.5 Hz) which, being an odd multiple of half line frequency, reduces the visibility of intermodulation products between the two carriers. The pilot tone frequency is 3.5 times line frequency (54687.5 Hz). The modulated tone frequency is 117.50 Hz for stereo transmission and 274.1 Hz for bilingual transmission. Absence of this tone is interpreted as a monaural transmission.
There is a version of A2 used in South Korea, compatible with the System 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 pilot tone frequency is 149.9 Hz; the bilingual pilot tone frequency is 276 Hz; and the second channel carries L-R (not R).
Zweikanalton is known by a variety of names worldwide. Most commonly used names are Zweiton, German Stereo, A2 Stereo, West German Stereo and IGR Stereo.