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Limiter/Compressor Theory

Discussion in 'Effects [BG]' started by IcedEarthWOM, Dec 29, 2005.

  1. IcedEarthWOM


    Oct 2, 2005
    From what I can gather this is what happens when you limit:
    If the signal goes over a certain volume (threshold) then the excess signal is cut by a certain amount (ratio)

    bad picture example :scowl: :

    So if the threshold is set to 1 dB and you pump a 2 dB signal into the limiter the output would be 1.5 dB. Simple, right?????

    Now for compression:
    From what I gather compression works on BOTH sides of the threshold. Above the threshold it's the same as limiting but Below the threshold it bring's signals up depending on the ratio.

    another bad picture example :scowl: :

    Hence, compressors can be used as sustainers if set with a high enough threshold. And also why they create noise when set to high ratios.

    I post this because I more or less want to know that I'm correct or wrong, and if I'm wrong then please correct me.... The other reason I post this is because I've noticed quite a few posts akin to "What the H*** DOES Compression do???" and I figure this might answer it :D

  2. fretlessrock

    fretlessrock Supporting Member

    Aug 8, 2002
    You have the basic idea. When the signal approaches the threshold level it is reduced by the compression ratio. The gain reduction is not instantaneous. In a "hard knee" scenario you get up to the compression ratio very quickly from no compression below the threshold. In a "soft knee" scenario the gain reduction rolls on more gradually.

    A limiter: Even though this can be debated on a device by device basis, think of limiting as a high ratio compressor. They typically are "soft knee" so that the device generates less artifacts. It can basically set a "brick wall" on the outgoing signal level. Very useful when you need to know that a signal will not exceed X.

    That covers the two basic functions: threshold and ratio. Some devices combine them. The ratio increases as the threshold is lowered, for instance.

    Then you get attack and release = How fast does the comp clap down, and how long does it take to let go after the signal goes back down.

    Thanks for thinking this out and making diagrams. it makes it much easier to discuss.

    BTW: my pet peeve = gain as a function of compression. A given compressor, like a RNC, may have a gain control. This is a convenience, not a compressor function. It lets you control the input and/or output levels without needing two additional preamps.
  3. IcedEarthWOM


    Oct 2, 2005
    So CAN a compressor be used to increase sustain????

    Or do I have to look to another effect.....

    You're welcome :D

    EDIT: I guess using a compressor with an output volume, compressing, then bringing the signal up with that output knob gives the artificial sustain...
  4. fretlessrock

    fretlessrock Supporting Member

    Aug 8, 2002
    Yes. The increase in sutain happens because the output signal from the compressor stays at the same level as the input level comes down. it works best with higher ratios and lower thresholds. That is how you get the "toggle" sound where the not sounds like it is on or off.

    Yes, kinda, on the output gain. That is basically "make up gain" to recover from the gain reduction.
  5. IcedEarthWOM


    Oct 2, 2005
    Now I know a heluva lot more than I did :smug:

    And at least I'll know whats going on when I finally get one :D
  6. Ben Clarke

    Ben Clarke Liquidating to fund a new business. Buy My Gear!

    Jan 6, 2005
    Western NY
    Yes and no.

    There are compressors that will boost signals below a seperate threshold, and attenuate above another threshold, but they are rare and expensive. Make up gain notwithstanding...

    Anyhow, a limiter with a 2:1 ratio is not a limiter. It's a compressor. The only difference between the two is the ratio. Knees have nothing to do with it. Generally, compression ratios above 8 or 10:1 are considering to be limiters.

    I've found that comp/lim in a bass guitar application does little for sustian. Theoretically it makes sense, but you'd be better off improve the bass itself by adding mass to the bridge and/or neck if you want that. Comp/limiters on a bass work best when used to control the dynamic range so it sits better in the mix, and to alter the tone or attack characteristics.

    Go some googling, but look for names like Daves Moulton & Fridmann for a more sophisticated view on the issue. Compression is everywhere in recording, and doing it right makes all the difference in the world. Doing it wrong just sounds like crap.
  7. fretlessrock

    fretlessrock Supporting Member

    Aug 8, 2002
    I know it is hard to convey tone in type, but my reply isn't combative.

    One example is any compressor with pre-gain. If you had say, 8db of boost, it would get your input signal to the trheshold "8db faster". It would also remove 8db of dynamic range from the bottom end, further compressing the dynamic range. In that example both thresholds are the same, and I have never seen a compresor that lets you set a threshold for pre-gain. I'd like to!

    I wouldn't say that knees have noting to do with it. The way in which the compression rate goes from 1:1 to 10:1 or 30:1 make a difference in the limiter's character. If it went from 1:1 to max compression the instant you hit the threshold it would be a prickly device indeed.
  8. fretlessrock

    fretlessrock Supporting Member

    Aug 8, 2002
    Just another comment on your graphs. The first one shows what a 2:1 compressor curve look like if you had absolutely no compression below the threshold, and 2:1 the instant the signal reached the threshold. That is a very hard "knee". it is a good basic compression curve, but not a limiter.

    The second one isn't representative of compression... at least not with respect to the threshold. It kinda looks like a gate! A gate reduces gain *until* the signal reaches a threshold, not after. Oh, the 80's! the drum sounds! The nausea....

    In a limiter you would see the curve flatten out quickly around the threshold and basically flatline. The knee is the amount of curve. It makes a right angle turn at the threshold then it is again, a very hard knee.

    I probably over emphasize the knee concept, but it is important to know that the compression is usually not instantaneous. Also, it often starts below the threshold, and that is a function of the knee.

    Most of this only look interesting if you have a good signal generator and a oscilloscope or audio analyzer. That is actually fun for some folks, me included.
  9. Ben Clarke

    Ben Clarke Liquidating to fund a new business. Buy My Gear!

    Jan 6, 2005
    Western NY
    Well, a comp with input gain is pretty common, but what I was alluding to is a unit that is non-linear at low levels. Rare indeed. I have never used nor actually seen one. They do exist, pretty much in top end mastering suites, as far as I understand it.

    I didn't mean to imply that the knee characteristic is unimportant to the sound; it's actually critical. I just meant that it's not a determinative factor in the distinction between compression and limiting.

    I quite like the controlled overshoot available from an optical device hit hard, for example.
  10. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    From a practical consumer standpoint, some compressors are better at increasing sustain than others. One reviewer lumps compressor pedals into two camps: sustainers and squashers. Even though compression and limiting can be defined as practices, the actual circuits vary quite a bit in the way they handle your sound.
  11. dunamis


    Aug 2, 2004
    Not trying to hi-jack the thread, but...

    What about leveling amplifiers? How do they differ from the limiters and compressors discussed above? Are there applications that favor an LA over a compressor?

  12. Wo, nelly!

    Speaking only of "pure" audio processing elements here - text book descriptions ok;

    compressors only attenuate gain above a threshold, they do not boost gain at all (except for output gain compensation or "make up gain.")

    expanders only boost gain below a threshold, they do not attenuate (except for output gain compensation.)

    (Put the two together and you can no longer call the resulting device one or the other. It becomes a compander, which will boost below a threshold and attenuate above a threshold.)

    Limiters are compressors, just with extreme ratio settings. A "brick wall" limiter has an infinite compression ratio, that is to say the signal theoretically never goes beyond the threshold level.

    Soft knee refers to the response of the compressor to a change in input gain.

    A normal hard knee compressor starts compressing immediately above the threshold and applies it's compression ratio absolutely linearly (ie; equally to all input signals above the threshold.)

    A soft knee compressor "eases" into it's set compression ratio around the point of the threshold. It's less noticeable or "smoother" or more natural than hard knee.

    Now, levelling amplifiers are a combination of several devices; limiters, AGCs (automatic gain correction), expanders and so on (depending on the manufacturer I guess.) They are like a compander, but more refined. Essentially you set a "target" output level, and it does whatever it can to achieve that output level within the limits placed on it by the finer controls and parameters that you can set. So, theoretically it can indefinitely maintain a constant average output level, but that doesn't necessarily sound good.

    Leveling amps are used in my job to feed the sound reinforcement chain in the Australian Parliamentary chambers. They replaced the companders that were doing the job previously. I've not heard of anyone using one with bass, but it does peak my interest!
  13. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    Careful, keep those peaks under control!
  14. ba domp tish! :rollno: