Noise Reduction and Tape Hiss

The fidelity of a tape recording is limited by "tape hiss", high frequency random noise resulting from the fact that the magnetizations of the oxide particles of the magnetic emulsion do not add to exactly zero even in the absence of a recorded signal. Noise reduction systems like the Dolby and dbx systems help to overcome this by pre-emphasis of high frequency, low-intensity music signals upon recording so that they, along with the tape hiss, can be de-emphasized upon playback.

Tape Hiss

Once the signal is recorded by the tape head, the random variation in magnetization from the oxide granules gives a random noise signal up in the kilohertz range called "tape hiss" On the unmagnetized tape, the oxide granules in the tape emulsion are almost completely random, but still have enough magnetization to give you a high pitched "frying" sound if you play the blank tape with high gain.

A persistent random noise signal from the residual magnetization of the oxide granules limits the fidelity of magnetic tape recording. Because of the small size and close spacing of the granules, the noise is of high frequency and is descriptively called "tape hiss". Fortunately, noise reduction schemes such as Dolby and dbx can discriminate against this tape hiss to provide higher fidelity tape recordings.

dbx Noise Reduction

The dbx system is an compression-expansion scheme for increasing dynamic range and for decreasing tape hiss.

Two inherent problems with recording onto tape cassettes are the limitation on dynamic range for distortion-free recording and the presence of high-frequency tape hiss. The live music to be recorded might have a dynamic range of 70 decibels, but you will do well to get 40 dB of dynamic range recorded on the tape. If you have carefully matched electronics to squeeze or compress the signal to 40 dB before recording and expand it back to 70 dB upon playback you can not only retain more of the dynamic range of the original music but also accomplish some tape noise reduction in the process. Note that soft music signals are made larger than the tape hiss level for recording, and then they are made smaller again on playback. This playback expansion also reduces tape hiss dramatically. Note also that the compression avoids distortion from tape saturation.