2. Principles of operation

If we were to transmit a natural, untreated signal with its full dynamic range, we would often have a problem: Even in nature, sound events can cover a dynamic range of more than 100 dB; think of the difference in volume between the rustling of leaves and loud thunder during a strong storm, for example.

In radio broadcasting, these kinds of dynamics cannot be transmitted; here, the usable dynamic range lies between 40 and 50 dB. To satisfactorily transmit the wide dynamic range of a symphonic orchestra, for example, the signal must be compressed. This is achieved with broadcast compressors.
In order to achieve a satisfactory sound on many different playback systems, a compromise must be found between the dynamic range of the recording and the technical capabilities at the playback end. If you have access to a good level meter (such as the ones on a mixing console), you can see for yourself how heavily some music is compressed. Listen to a recent rock or pop album and watch the meter: does it move much? Probably not. Whether that is a good thing is subject to much debate among audio engineers, but in reality the dynamic range of most current rock and pop music is reduced to a mere 3-6 dB.

How is this achieved? The compressor makes it possible! But how? The following chart shows how compression works, physically speaking:

Compression graph

At a compression ratio of 1:1, nothing happens. At higher ratios (2:1, 4:1, 6:1, or 8:1), the output signal is progressively „squeezed“ in relation to the input, resulting in a reduced dynamic range.

The second important parameter of a compressor is its threshold. It describes the level above which the compression sets in. For example, if the threshold is set to -10 dB, then the compressor will only process those portions of the signal that are louder than -10 dB.