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Thread: Compressors

  1. #1
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    Compressors

    As I understand it, the general use for compressors is to reduce the dynamic range of a signal...which, technically, would mean bringing up the lower ranges, and bringing down the higher ranges; in essence, "squashing" the signal from both the bottom and the top. But it seems as though compressors are strictly used for squashing only the upper portion of a signal's amplitude.
    Do I have this correct?

    This is is so, then why use a downward expander? Wouldn't that only increase noise by bringing the signal even closer to the noise floor?

  2. #2
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    ...which, technically, would mean bringing up the lower ranges, and bringing down the higher ranges; in essence, "squashing" the signal from both the bottom and the top. ?
    I think this is the part you have wrong. In limiting the higher dynamic peaks nothing is done to the low level portions of the signal. It simply cuts the dynamic peaks. Say you have a drummer with an inconsistant beat-to-beat level. His loudest snare drm hits register at 20vu. His softest at 15vu. His average is 17vu. Say u want all of the hits to be 16vu. Set the compressor to trigger at 16vu. The 15vu hits will not be affected. Nothing a compressor can do will make the 15vu hits louder, nor anything else the drummer does under 16vu.

    Downward expansion is another animal. When a peak goes above the trigger level it is compressed. As soon as it falls below the trigger level the compressor stops compressing. Often the peak is so high that it is followed by a sucking sound as the compressor stops compressing. Downward expansion helps eliminate that sucking sound, along with "soft-knee" compression. Not all compressors have this capability. What compressor do you have? I can get into much more detail if you like.
    'I think my intimate relationship with electronics started as a child when I was playing with a screwdriver and a wall plug, Doc, and...'

  3. #3
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    Re:

    Well, I'm using a Symetrix 528E, but I'm merely trying to grasp the components of signal processing. My understanding is that compressors compress the dynamic range of a signal, which I read somewhere involves bring up the low sounds and bringing down the high sounds...but obviously, compressing sounds only above a certain threshold would compress the range of the signal.

  4. #4
    When you compress above a threshhold you do indeed diminish the entire dynamic range of the signal. You are in effect attenuating the overall level of the signal. To compensate for this you would increase gain. This is where the low level portions af the signal "appear" to be increased, but it is not done by the compressor itself.

    The 528 is an excelent compressor. Good choice The downward expander and compressor sections are tied together in single processor. I believe it also does the gain compensation I mentioned above automatically. That's how I think you got the impression that a compressor also brings up the lows. But compressors by themselves do not do this. I just wanted to clarify that.

    The downward expander portion basically anticipates a compressed peaks' fall below the threshold and starts to expand it as it falls, softening the transition and eliminating the sucking sound (aka pumping and breathing). It has no effect on the noise floor since the compressor lowered it to begin with (it compresses the WHOLE signal, not just the frequencies that take the signal over the threshold, though there is a way to do this called Side-Chaining). The expander just eases it back down (hense DOWNWARD expansion) through the threshold.

    "Soft-Knee" compression is a way to soften the initial effect of compression. It slows the rate of compressing a peak when it rises above the threshold. I believe the 528 does this automatically also.

    I hope this is a better explanation.
    Last edited by penniesfromheaven; 4th Oct 2007 at 17:23.
    'I think my intimate relationship with electronics started as a child when I was playing with a screwdriver and a wall plug, Doc, and...'

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    Great, that helps a lot; thanks for the clarification. It's nice to have people to bounce my ideas off of as I learn this stuff...you can't ask manuals questions. Well you can, but they don't answer.

    Fooling around with the downward expander, it seems to act like a gate. With it bypassed, it allows background noise (like CNN news on the tellie) to infiltrate into the mic, but activated, it closes up the noise. The thing I am confused about is this...if a downward expander increases dynamic range, then it would lower the amplitude of a signal that falls below a certain threshold, right? Is the expander merely taking a chunk of the signal - all that falls below the threshold - and expanding the dynamic range of just that section of the signal (as oppose to the entire signal's dynamic range), thus increasing the volume, of everything below the threshold?

    It also says that the expander reduces the noise floor by the same ratio applied to the signal...how would it reduce the noise floor? Isn't noise "built in" to the equipment/system? I mean, isn't it something that cannot be changed, it being uncontrollable electrical noise?

    Now, as I read on, I realize that if I am correct above by saying that it increases the dynamic range of the portion of the signal below the threshold, then the noise would also be pushed down farther...so the expander would not only increase the volume, but decrease some of it...which gets increased and which gets decreased? Does the expander create its own "unity?"

    I guess I don't understand how "...an expander operating at a ratio of 1:2 will cause an input signal that falls 10dB below threshold to fall 20dB at its output" (per 528 manual) occurs with expanding dynamic range...and why that is desirable.

    Here is a separate question: do limiters attenuate the signal by a high ratio, like 10:1, or completely cut off the signal above the threshold?

    Again, thank you so much for your time and input, penniesfromheaven! Take your time, lol, I realize I ask alot...but like I said, I like to bounce this off of others so I get it right in my head.

  6. #6
    I'm going to send ya to a site that may be able to explain it better than I, cool? Go here. I'm not trying to get out of an answer, just that I don't do graphics this well.
    'I think my intimate relationship with electronics started as a child when I was playing with a screwdriver and a wall plug, Doc, and...'

  7. #7
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    I've used that site before, and looking at the expander part again, I GET IT NOW! So thanks for referring me there.

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