Compressors/limiters, Expanders/noise gates

DENOISER SNR2000

The usable dynamic range of electro-acoustic equipment is limited to both the low end and the high end. The thermal noise of the electrons in the components results in an audible basic noise floor and thus represents the bottom limit of the transmission range. The upper limit is determined by the levels of the internal operating voltages; if they are exceeded, audible signal distortion is the result. Although, in theory, the usable dynamic range is spreading from low to high end, it is considerably smaller in practice, since a certain reserve must be maintained to avoid distortion of the audio signal if sudden level peaks occur.

Technically speaking, we refer to this reserve as “headroom”—usually about 10 to 20 dB. A reduction of the operating level would allow for greater headroom, i.e. the risk of signal distortion due to level peaks would be reduced. However, at the same time, the basic noise floor of the program material would be increased considerably.

Fig. 4.2: The interactive relationship between operating level and headroom

It is therefore useful to keep the operating level as high as possible without risking signal distortion in order to achieve optimum transmission quality.

4.2Compressors/limiters

By measuring the dynamic range of instruments in live recording situations, you will experience that extreme amplitudes occur which often lead to overload on the various devices in the signal chain. Especially in broadcasting and record cutting techniques, these signal peaks can lead to heavy distortion. To avoid this kind of distortion or, for example, to avoid loudspeaker damage, compressors or limiters are used. The basic function used in these devices is dependent on automatic gain control as mentioned in the previous section, which reduces the amplitude of loud passages and therefore restricts the original dynamics.

4.3Expanders/noise gates

Audio, in general, is only as good as the source from which it was derived. The dynamic range of signals will often be restricted by noise. Synthesizers, effects devices, guitar pickups, amplifiers, etc. generally produce a high noise level, hum or other ambient background hiss, which can disturb the quality of the program material.

Normally, these noises are inaudible if the level of the usable signal lies significantly above the level of the noise. This perception by the ear is based on the “masking” effect: noise will be masked and thus becomes inaudible as soon as considerably louder sound signals in the same frequency band are added. However, the further the level of the usable signal decreases, the more the noise floor becomes a disturbing factor.

4. TECHNICAL BACKGROUND