Dolby Noise Reduction

The Dolby systems for noise reduction employ circuitry which pre-emphasizes high frequencies before they are recorded onto tape in order to make them larger than the tape hiss noise with which they compete.

The circuitry is amplitude-sensitive: only soft high-frequency sounds are emphasized. If loud high-frequency sounds were emphasized, it might drive the tape into its distortion levels. Upon playback, a matched de-emphasis circuit is employed to restore the high frequencies to their proper balance with the other part of the recorded signal.

Dolby Systems

The Dolby systems for noise reduction employ circuitry which pre-emphasizes high frequencies before they are recorded onto tape in order to make them larger than the "tape hiss" noise with which they compete.

Dolby Recording and Playback

If a tape is recorded using a Dolby noise reduction system, then it should by played back with the same system. If the recording and playback systems do not match, several types of problems arise, as summarized in the table.

The problems discussed here are most severe in common practice with Dolby-B, which is the most common for non-professional use. The more recent Dolby-C does not produce as severe a problem when played back on non-Dolby equipment.

Dolby B-system Encoding

In the Dolby-B system of noise reduction, low-level, high frequency signals are pre- emphasized upon recording as shown by these encoding curves. They are de- emphasized upon playback to discriminate against tape hiss.

Dolby-B Encode/Decode