Dynamics Processors -- Technology & Applications

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Appendix -- Compressor & Expander History Note

What "compression" is and does has changed significantly over the years. Reducing the dynamic range of the entire signal was the originally use of compressors, but advances in audio technology makes today's compressor use more sparing.

The need for compression arose the very first time anyone tried to record or broadcast audio: the signal exceeded the medium. For example, the sound from a live orchestra easily equals 100 dB dynamic range. Yet early recording and broadcasting medium all suffered from limited dynamic range, e.g., LP record: 65 dB, cassette tape: 60 dB (with noise reduction), analog tape recorder: 70 dB, FM broadcast: 60 dB, AM broadcast: 50 dB. Thus, "6 pounds of audio into a 4 pound bag" beget the compressor.

Early compressors did not have a "threshold" knob; instead, the user set a center ("hinge") point equivalent to the midpoint of the expected dynamic range of the incoming signal. Then a ratio was set which determined the amount of dynamic range reduction. Figure 29 shows that reducing 100 dB to 50 dB requires a 2:1 ratio.

The key to understanding compressors is always to think in terms of increasing and decreasing level changes in dB about the set-point. A compressor makes audio increases and decreases smaller.

From our example, for every input increase of 2 dB above the hinge point the output only increases 1 dB, and for every input decrease of 2 dB below the hinge point the output only decreases 1 dB. If the input increases by x-dB, the output increases by y-dB, and if the input decreases by x-dB, the output decreases by y-dB, where x/y equals the ratio setting. Simple -- but not intuitive and not obvious.

This concept of increasing above the set-point and decreasing below the set-point is where this oft-heard phrase comes from: "compressors make the loud sounds quieter and the quiet sounds louder." If the sound gets louder by 2 dB and the output only increases by 1 dB, then the loud sound is made quieter; and if the sound gets quieter by 2 dB and the output only decreases by 1 dB, then the quiet sound is made louder (it did not decrease as much).

With advances in all aspects of recording, reproduction and broadcasting of audio, the usage of compressors changed from reducing the entire program to just reducing selective portions of the program. Thus was born the threshold control. Now sound engineers set a threshold point such that all audio below this point is unaffected, and all audio above this point is compressed by the amount determined by the ratio control. Therefore the modern usage for compressors is to turn down (or reduce the dynamic range of) just the loudest signals.

Just like compression, what "expansion" is and does has evolved significantly over the years. The original use of expanders was to give the reciprocal function of a compressor, i.e., it undid compression.

Recording or broadcasting audio required compression for optimum transfer. Then it required an expander at the other end to restore the audio to its original dynamic range.

Operating about the same "hinge" point and using the same ratio setting as the compressor, an expander makes audio increases and decreases bigger as diagrammed in Figure 29.

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