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Compressor / Limiter Applications
The guest speaker steps to the pulpit wearing a wireless lavalier. The sound operator adjusts the volume for a seemingly pleasant level. But, when the guest looks down to review his notes, the volume suddenly increases. (The guest's mouth moved much closer to the microphone, which means the input sound was much louder.) When the guest looks up again, the volume returns to a pleasant level. The irritating yo-yo of sound levels continues until the guest is finished. The congregation is glad he's done, too. This affect can occur with any microphone when the talker moves too much or when the talker has a very dynamic voice. Fortunately, there is a solution for this sound irritation. And, it's not a costly one. The solution is a simple compressor/limiter. The device automatically adjusts the level to smooth the volume variations. There are two primary controls on a compressor/limiter: the compression ratio and the threshold. This article will describe these two controls and how they work together. It will also provide some practical applications of the compressor/limiter. Figure 1
 Figure 2
 Compression Ratio The compression ratio control determines how much the output will be modified. The knob markings are often 1:1, 1.3:1, 2:1, 4:1, 10:1 and ¡:1. The ratio reflects the units of input needed to produce one unit of change in the output. For instance, if the ratio is 1:1, the output will be the same as the input. The compressor is actually in a bypass mode. Figure 1 shows how the output level is affected when the compression ratio is set at the 2:1 position. At this setting the input level, once it's past the threshold level, would need to increase by 2 units (dB) to produce a 1 unit (dB) output increase. Likewise, a 10:1 setting would require a 10 dB input increase to produce a 1 dB output increase (Figure 2). A compressor with a 10:1 ratio setting would not allow very much dynamics. The sound is often described as being squashed. Figure 3
 The limiter (Figure 3) part of the compressor/limiter can be a part of the compressor or is often another section of the compressor/limiter. The limiter is really a compressor with a high ratio setting of 20:1 up to ¡:1. In other words, once the input exceeds the limiter threshold, the output will not get any louder. This can be used to prevent any signal from getting too loud and damaging the speaker system. A talker could be shouting and the volume will only get as loud as the limiter setting will allow it to go. Figure 4
 Threshold The threshold control will adjust the point at which the compressor will actually start to modify the input signal (Figure 4). The operator can adjust the threshold to have the compressor start working on low volumes or to only work when the volume gets loud. When the input signal is below the threshold level, the output is not affected. However, as the input signal increases and passes the threshold, the signal is compressed. The amount of compression is dependent on the compression ratio. Applications For speech, a 2:1 to 4:1 ratio is often used to provide control yet maintain natural sound. If a compressor was used for the guest in the opening example, the speaker's volume would have sounded more pleasant to the congregation. As the guest looked down and his mouth moved closer to the microphone, the level would have passed the threshold and the volume would have been adjusted. The compressor adjusts so fast the listener does not hear the compressor working. These settings would also help in a situation in which the talker has a very large dynamic range. Again, once the level reached the threshold, the volume would be automatically adjusted. Another application for the compressor/limiter is for musical instruments. You can control the levels to provide better, more consistent musical mixes. The vocals, both solo and groups could be compressed to help keep them in better balance with the instruments. A compressor/limiter also improves tape recordings. A poorly recorded tape often has sections where the signal is at a very low level so there will be a lot of noise on the tape. On the other hand, there are sections when the level is very loud and the tape can't handle it; so the recording becomes distorted. Listening to a poorly recorded tape is not very pleasant. Summary The compressor/limiter can be a very good tool in the sound operators' toolbox. Please call us if you have any questions or if any applications are giving you a problem. We may have a simple solution. by Ron Huisinga © 1996 Internet Sound Institute (www.soundinstitute.com). This article is for personal use only. Any commercial reproduction is not permitted without permission. To obtain permission, contact ISI at hopi@soundinstitute.com
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