Virtually all NTSC LaserDiscs, and older PAL discs, are provided with two subcarriers in a frequency band below the video signal. On most discs these subcarriers are frequency-modulated with analog audio signals. Channel 1 is used for Left and Channel 2 for Right information when the programme is in stereo, or for Primary and Secondary audio programmes respectively in dual-channel format, and on monaural discs they are identical. A variant of the CBS Corporation's Compatible eXpansion noise-reduction technique, originally developed for phonograph records, may be applied to the audio channels in monaural or stereo mode, and is generally triggered automatically by a code on the disc.