Discussion in 'Effects [BG]' started by Funk Warts, Mar 6, 2002.

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  1. Funk Warts

    Funk Warts

    Jun 13, 2001
    London, UK
    OK, what is a compressor? How much do they cost (in £ if possible!)? Why are they a good idea?

    At the moment I have a Crybaby Bass-Wah and a Big Muff distortion, and apparently a compressor will help, but I have no idea what one is or what it does! All help much appreciated. Thanks.
  2. A compressor is basically a variable gain device, where the amount of gain used depends on the level of the input. In this case, the gain will be reduced when the signal level is high which makes louder passages softer, reducing the dynamic range. A compressor's input/output relationship is often described by a simple graph. The horizontal axis corresponds to the input signal level, and the vertical axis is the output level (both measured in decibels). A line at 45 degrees corresponds to a gain of one - any input level is mapped to exactly the same output level. The compressor changes the slope (makes it more horizontal) of that line above some value called the threshold (which is most often adjustable). The height of the line defines the dynamic range of the output, and the slope of that line is the same as the compressor's gain. The compressor weakens the input signal only when it is above the threshold value. Above that threshold, a change in the input level produces a smaller change in the output level. The compressor setting is usually stated as a ratio, such as 2:1, which means that the input level would have to increase by two decibels to create a one decibel increase in the output. With a 4:1 setting, the input would need to change by 4 dB for a 1 dB change in the output level, and so on. Limiting is simply an extreme form of compression where the input/output relationship become very flat (10:1 or higher). This places a hard limit on the signal level. the compressor makes loud signals quieter, but it does not make quiet sounds louder (although it may be perceived that way). However most compressors do have a secondary gain stage for adjusting the output level so that if you turn the compressor on while playing, the extra gain will prevent your instrument's volume level from dropping. You can make a case that this extra gain stage is or isn't really part of a compressor, but in any case, that is what makes the softer sounds louder. Now discussing exactly how the level detector in the compressor operates. It is usually some sort of time average of the input (often a root-mean-square (RMS) calculation). Alternatively, the instantaneous peak voltage or sample value can be used, in which case, the compressor becomes a hard limiter.

    When the level sensing function is a short time average, the compressor will take a little time before the gain is adjusted to meet the new input level. The amount of time the compressor takes to respond when the input level rises above the threshold is called the attack time, and is usually fairly short (under 100 ms.). When the input level is above the threshold and then drops below it, the compressor will take some time to increase the gain as well. This is the release time of the compressor, which is generally larger than the attack time (possibly up to a second or two). The effects of a compressor on a signal. Only the middle portion of the input is above the compressor's threshold. Note the overshoot when the signal level increases (it takes some time for the gain to decrease), and the attenuation when the input signal returns to the first level (and the gain increases). The release time is generally longer than the attack time.

    There are a ton of compressor threads active right now sorry. Closed
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    Sep 26, 2021

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