Punch, apparent loudness, presence... just three of the many terms used to describe the effects of compressing and limiting on an audio signal.

The terms compression and limiting have been in the audio vocabulary for years, yet there is some confusion over their definitions. The confusion arises from the fact that both the compressor and the limiter are devices that restrict the dynamic range of a signal, and the difference between them is one of degree, with the limiter having the most effect. To simply define each:

Compressor: An amplifier, whose gain decreases as its input level is increased.

Limiter: A compressor, whose output level remains constant, regardless of its input level.

Both definitions are valid only after the signal being processed reaches a certain level. Therefore, one more definition needs to be thrown out for consideration:

Threshold: The level at or above which the compressor or limiter begins functioning.

In a situation where input and output are idealized for a combination compressor/limiter, as the input level increases from -10dB to 0dB, the output level, likewise, increases from -10dB to 0dB. Here the device is functioning as a simple unity gain amplifier, with no effect on the signal level.

Compression

Once the signal level exceeds the compression threshold of 0dB, the output level will follow the compression curve, assuming a compression ratio of 2:1, as the input increases 10dB; the output will yield only 5db more gain.

Limiting

In a limiting situation, with a limiting threshold of +20dB, once the input level reaches +20dB, there is no further increase in output level. Hence, the device is operating as a limiter,. In actual practice, compression ratios of greater than 10:1 are considered as limiting. Once the limiting threshold of +20dB has been reached, the output level remains at +10dB, despite further increases in input level. Therefore, it should be understood that the limiter threshold does not necessarily indicate the maximum allowable output level of the device. Rather, it indicates the input level at which the limiter begins working.

Variable Thresholds

It should be noted that the same compression ratio of 2:1 mentioned earlier will have different effects on the overall dynamic range depending on the point at which compression begins (threshold). Also, the positioning of the compression threshold will influence the point at which limiting must begin, if a certain maximum output level is not to be exceeded.

Variable Compression Ratios

Most state-of-the-art compressors offer audio engineers a variety of compression ratios from which to choose. Assuming that an audio signal must be kept below +10dB, using higher compression ratio settings will allow for a greater dynamics range of the signal being processed.

Pumping and Breathing

It is relatively easy to determine the compression threshold and ratio needed to prevent a wide dynamic range signal from exceeding a specified output level. However, it should be realized that - especially at high ratios - the action of the compressor might become audibly obtrusive. To understand why, remember that the compressor is a variable gain device. The higher the compression ratio the greater the change in gain. A constant high level signal, say +10dB will cause more gain reduction. When the high level is removed, the amount of gain reduction decreases as the compressor returns to unity gain. If the gain reduction fluctuates rapidly, it may be quite audible as the background noise goes up and down in time with the compressor action, i.e. attack /release times, causing a breathing like sound. This can be used sometimes as an effect producing some killer results.

Example: The use of extremely short attack times and longer release times may create a backward-like sound, especially on percussive instruments. The fast attack immediately drops the signal, and then as the signal naturally decays, the release time setting brings up the gain, working against the normal decay. This effect is particularly noticeable on a drum set, and particularly on cymbals.

Program Limiting

Often compression may be applied to the overall program rather than to an individual instrument. Known as program limiting, this practice will prevent cumulative levels of the various instruments from getting too high or falling too low. This type of gain control must be approached with care, since the adverse effects of compression are heard on the entire program.

Program limiting is often used to raise the apparent loudness of a record. Since the ear averages the sound level over a period of time, a low level program with occasional high level peaks will not seem as loud as an average level program with no high level peaks. (Confused yet?). In the quest for louder sounding recordings and broadcasts this type of loudness boosting is often overdone, much to the detriment of the finished product. Meaning, what you hear coming over the radio or television is often much different than the original producers, engineers and mastering people had in mind.

Stereo Program Limiting

When a stereo broadcast program is limited, the gain regulating sections of the left and right track compressors must be electronically interlocked, so that the compression in one track causes an equal amount of compression in the other track. This keeps the overall left-to-right stereo program in balance.

Consider a stereo program in which the right track occasionally needs some compression. During compression, a center placed solo would apparently drift to the left whenever the gain of the right track is affected by the compressor. To prevent this center channel drift, the stereo interlock function reduces the gain in both channels simultaneously whenever one exceeds the threshold of compression. This keeps the center placed information from moving from side to side with each action of one or the other compressor channel.