A compressor limiter (compressor) is a device used in most community radio station transmission systems. It is used to process the signal that comes from the broadcast studio before it is transmitted. The compressor/limiter keeps the level of your station's signal constant and at the best possible volume, both for your listeners and to compete with other stations.
In broadcast terms, we say the compressor is used to control the dynamic range of the station's sound. Dynamic range is the range between the loudest and quietest sounds that are broadcast. Reducing the dynamic range of the signal by using a compressor is called compression. The effect of compression is to make your station sound louder and clearer on air.
The limiter, on the other hand, sharply controls the audio output level at a predetermined level. This is often a much more harsh adjustment to the sound, and must be used with caution, as it can make your audio sound very bad if done improperly.
More usually, we use devices that combine both functions, allowing you to compress the signal to a certain extent, and limit it sharply after a certain point. Such devices are called compressor/limiters.
Not all compressors work exactly as shown above. For example, some compressors monitor both the input and the output audio. Also, there is computer software to carry out the role of the compressor. However the underlying principle remains the same.
Pyle PPCL-E2020 Compressor Limiter Expander/dynamic Enhancer
The compressor control panel generally has two meters:
- one meter indicates the amount of gain reduction or compression
- the other meter usually shows the signal level of the compressor's audio output.
Focusrite Red 3 Classic Dual-Channel Compressor/Limiter
The speed at which the gain is reduced in response to an increase in input signal level is called the attack time. This is usually specified in milliseconds, and is set using the attack control.
The speed at which the gain is restored to its original level after the input is removed is called the release time. This is set using the release control.
PreSonus Eureka Pro Recording Channel
You need to listen to your signal very carefully in order to set these parameters. If the attack time is set too fast, the compressor will respond to even the shortest peaks (loudest sounds), causing the level to change very quickly. For example, the beats in a dance track often cause short peaks in the signal. If the attack time of the compressor is so short that it responds to the individual beats, the compressor will reduce the signal level on each beat, and produce a very unnatural sound. On the other hand if the attack time is too slow, the compressor's output may exceed the desired maximum before the compressor acts on the signal.
Too fast a release time causes "breathing" as the gain changes rapidly. If the release time is too short, quiet sections of music will be lost as the compressor will still be reducing the gain, even though the loud input signal is no longer there.
Many compressors have an auto (automatic) switch for the attack and release time. When the auto switch is on, the attack and release times are set dynamically, based on the input. This is often the best for CR stations, as the system is self adjusting to the needs of different kinds of audio and programming.
Some compressor/limiter units have an additional gain knob that can be used to match the final output gain to any of a variety of equipment. The output level resulting from using the gain control can be monitored on the output level meter.
Some compressors also have a stereo link button, which links the two channels of the compressor together so that both channels process their signals in the same way. If your compressor doesn't have a stereo link button, you will have to set controls for each channel to exactly the same level.
Pressing the bypass' button bypasses the compression circuitry, and allows you to quickly compare the compressed and uncompressed output.
| The compressor has a variety of possible settings. Small adjustments to any of them can make a considerable difference to your station's on-air sound. It is a good idea to document the compressor settings that work best for your station and compressor. Keep this record available for reference. Also, once the compressor has been set for your station, put a guard or protective covering on the front panel to prevent anyone from accidentally changing the settings. |
The compressor settings you choose will depend very much on your preferred sound. However, many stations have been happy using:
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- a threshold level of 0 dB
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- compression ratio of between 1.5:1 and 4:1
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- Set the output gain so that the output level meter reads the ideal
- input signal level for subsequent equipment (often 0 dB).
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