Different Types of Dynamics Processors for Studio RecordingThere are several different types of dynamics processors. The most commonly used a compressor, gate and an expander. A compressor takes the loudest parts of an input signal and reduces their volume. A compressor only affects those portions of the signal that are louder than a certain level, known as the threshold. Portions of the signal that exceed the threshold are reduced toward the threshold by a set factor, known as the ratio. Most compressors also raise the level of the entire audio signal by a certain amount. This amount is known as the master gain. Both the threshold and the master gain are measured in decibels. The compression of the loudest parts of the audio signal helps to make sure that the average signal level can be boosted without exceeding the maximum allowable peak level. In general, the master gain should not exceed the reduction in dynamic range that results from the compression. If it is larger, clipping may result. Compressors are usually applied with compression ratios ranging from 1.5:1 to 8:1. An as example, suppose that a compressor is set with a threshold of -18dB and a ratio of 3:1. In input signal of -12dB (6dB above the threshold level) will produce an output signal of -16dB (only 2dB above the threshold). The amount by which the signal exceeds the threshold is reduced by a factor of 3. When the ratio is much higher (around 10:1 or more), a compressor effectively prevents the audio signal from exceeding the threshold value. In this case, the effect is referred to as a limiter. An expander takes the softest parts of an input signal and reduces their volume. An expander only affects those portions of the signal that are softer than a certain level, once again known as the threshold. Portions of the signal that fall below the threshold are reduced below the threshold by a set factor, known as the ratio. When the ratio is high, an expander effectively eliminates all portions of the audio signal that fall below the threshold. In the extreme case, this is known as a noise gate, or simply a gate.
When a signal exceeds the threshold level of a compressor, or falls below the threshold level of an expander, the signal is modified. The attack time is the length of time it takes the dynamics processor to respond to increases in signal level. The release time is the length of time it takes the dynamics processor to respond to decreases in signal level. In general, attack times are set to be relatively short, while release times are much longer. This means that compressors and limiters generally activate quickly and release gradually, while noise gates close slowly, and then re-open quickly. By adjusting the attack and release times, you can produce smoother changes in signal levels that might otherwise change abruptly. If the attack time is too short, you may experience unnatural or sudden changes in volume. If the attack time is too long, the dynamics processor may not have the effect you want. If the release time is too long, compression may continue through low-volume sections of audio, making them inaudible. All dynamics processors monitor the input signal to estimate the current signal level. The estimate is made using one of several common level detection method:
Compressor / Gate A compressor takes the loudest parts of an input signal and reduces their volume. This effect includes the following parameters:
When to Use a Compressor / Gate: There are a variety of reasons why you might want to use a compressor: • Compressors enable you to increase the overall signal level of a mix, without causing the distortion that would result from exceeding the maximum allowable peak level. Increasing the average signal level makes a recording sound louder, with an apparent increase in “punch.” • If you plan to reproduce your audio using a medium with limited dynamic range, such as magnetic tape, the increase in the average signal level that you achieve with a compressor helps prevent important audio material from being lost in the background noise. • Compression can even out unwanted volume changes to achieve a smoother and fuller sound. For example, if a vocalist moves closer in and further away from a microphone while making a recording, the recorded volume levels may be uneven. A compressor can smooth out theses variations. • Compressors can be used to increase an instrument’s sustain, by using a release time longer than the instruments decay. When you use a compressor, the master gain adjustment can result in increased low-level system noise. As a result, it is common to use a gate in combination with a compressor. A compressor / gate combines a compressor and a gate into a single effect, so you can increase the overall dynamic level of an audio signal, without distorting the loud parts, and without excessively boosting low-level noise. The gate threshold is normally set low enough to remove low-level system noise, but not high enough to remove important program material. Since other audio effects can produce background noise, you normally place the noise gate near the end of the effects chain. However, if you’re using delay and reverb effects, you may want to place them after the noise gate, so that the sound trails off naturally rather than being truncated suddenly by the gate. On to Part 3: Common Recording Problems that a Compressor Can Fix
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