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compressor ?

Discussion in 'Effects [BG]' started by Godcreep, May 12, 2002.


  1. Ok, I did a search on compressors (I'm very new to this 'effect' stuff) .. but I still have some questions if you don't mind ..
    I know it equals the volume, but is it doing any other things with your sound .. like, changin' it ? Can someone explain me what it exactly does and why I should use one ... (I'm using an Ampeg SVT 3 pro)
    Thanx guys ! ;)
     
  2. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Well, it's a tad more complicated than that. A compressor actually decreases the ratio of the output gain to the input level after a certain threshold is reached. Getting the right threshold and gain structure can be tricky. Further, you've got to figure out some good attack and release times to avoid pumping and breathing. Oh, it's an existential morass, this compression thing. Here's my current situation.

    http://www.talkbass.com/forum/showthread.php?threadid=47393
     
  3. 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.
     
  4. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Bigfeet ~ Thanks for elaborating on that. I guess I'm going to have to go to the component level to flesh this out a little more ... NOT!

    I think you did drive my point about as far home as it can go. Audio compression is a complex process.
     
  5. Yeah I do that allot, I should get that checked.
     
  6. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Me too, but you're better.