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Browse Prior Art Database

Recording or Broadcasting Utilizing Automatic Gain Control Compressor

IP.com Disclosure Number: IPCOM000091814D
Original Publication Date: 1968-May-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 45K

Publishing Venue

IBM

Related People

Cannon, MR: AUTHOR

Abstract

Low-speed, narrow-track audio recordings suffer from poor signal-to-noise ratio. Equalization, to give good high-frequency response, normally amplifies noise as well as signal. While this noise may not be objectionable against a background of normal recording signal, it can be annoying during a lull or a low-signal level recording.

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Recording or Broadcasting Utilizing Automatic Gain Control Compressor

Low-speed, narrow-track audio recordings suffer from poor signal-to-noise ratio. Equalization, to give good high-frequency response, normally amplifies noise as well as signal. While this noise may not be objectionable against a background of normal recording signal, it can be annoying during a lull or a low- signal level recording.

Audio broadcasting or recording should be made at an essentially constant volume which is high enough to mask all background noise, but not high enough to cause signal distortion. For example, a musical selection with a dynamic range of 60 db can be compressed to a dynamic range of a few decibels and broadcast or recorded at the maximum undistorted level. This can be done using conventional automatic gain control ACC circuitry. In order to permit a volume expander in the playback process to exactly reproduce the original sound, the ACC control voltage is also broadcast or recorded. This control voltage causes an expander to restore the original dynamic range. The expander is not actuated by noise, so amplification is extremely low during program lulls and noise is not noticeable. Such an arrangement permits heavy postequalization without a noticeable decrease in signal-to-noise ratio.

A single channel stereo broadcasting or recording system in an operating range compatible with stereo FM broadcasting is in drawing A. It includes an ACC compressor. The details of a typical AGC compressor are shown...