A transmission channel carrying information that can augment that being transmitted in an ordinary transmission channel such that a special television set that can receive both channels can get a better picture than those available from the main channel alone.

Some ATV schemes require the augmentation channel to be adjacent to the main channel. Others can theoretically accept a non-adjacent augmentation channel, though, at the time of this writing, the acceptability of non-adjacent channels has not been proven to everyone’s satisfaction.