The Basics of Compression

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2006/01/17 14:40:03 (permalink)

The Basics of Compression

The Basics of compression

The ATTACK knob controls the amount of time before compression starts. The range of this control is 0.1 to 200 milliseconds. The Attack and Release controls may only function when the compressor is in Peak mode. Long attacks are useful for percussive sounds, where shorter attacks are good for melodic parts like vocals and strings. The Attack control is also useful for keeping the transients on percussive drum or bass sounds. Experiment with different short attack times on snare drums to get more or less of the "stick" attack.

Hard/Soft Knee Compression
The [HARD/SOFT] switch is used to switch between Hard and Soft knee compression styles. When a compressor is set for Hard knee, the compression ratio applies only to signals above the threshold level. If a compressor is set for Soft knee, the compression ratio gradually increases from 1:1 to the currently selected ratio over a range of approximately 5 dB, so that the transition from uncompressed to compressed is more gradual. The difference between Hard Knee and Soft Knee is more obvious at high compression ratios. Once the input signal crosses the Threshold, the unit will compress the signal at the full ratio level.

Soft knee compression is useful when performing high-ratio compression or limiting on a signal. When the compression gradually fades in, it doesn't sound as obtrusive as when it suddenly starts limiting the signal. If you're looking for a "brick wall" limiter, the switch should be set for Hard knee to stop any transients from slipping through without affecting lower level signals. Lower Ratio levels may require a hard knee setting so that the compression slope isn't too narrow and you loose some of the compressive "punch".

Output Control/Knob
The Output control is useful for making up gain which was reduced by the compression circuit or matching the input level of a mixer or recorder. If the Gain Reduction meter shows that the input signal is being attenuated by -6dB, then the Output control generally should be set around +6dB. The [OUTPUT] knob is labeled with tick marks every 6 dB (±6, 12, 18, 24dB). This control is disabled if the [BYPASS] button is pressed.

The basic role of oversampling is to move the filtering requirements of A/D or D/A converters from the analog to the digital domain, where they can be more efficiently (lower noise, flatter frequency response, no phase shift, etc.). It all comes down to the basic Nyquist theorem: When you’re sampling (A/D), you can’t have any frequencies above half of your sampling rate, or else you get aliasing (hence the term, anti-aliasing filters). In older A/D designs, an analog “brick-wall” (very steep) filter would be placed before the converter to keep all frequencies above 20KHz (in most cases) out of the A/D converter. In an oversampling A/D converter, the sample rate is much higher (64 times, for example). This means that the Nyquist frequency is now also higher, which means that the analog filter can have a much more gentle slope. If in a 48KHz sampling system the A/D is a 64 times oversampling part, then the effective Nyquist frequency is 1.536MHz. Since we don’t care about anything above 20KHz, the analog filter can start gently rolling off at 20KHz, and be cutting significantly at 1.536MHz (since it’s over six octaves away). The A/D converter then has a digital filter that removes all frequencies above 20KHz and reduces the sample rate (decimates) back down to 48KHz. This is a somewhat simplified version, but hopefully you get the idea. A similar idea is used for oversampling D/As, but in reverse. The sample rate is increased by some multiple, and a digital filter removes everything above 20KHz. The analog filter then only has to remove frequencies above the Nyquist of the new sample rate, simplifying its design.

Peak/RMS Compression/Limiting
This switch selects either the Peak or RMS compression style, which affects the detection of the signal input. When set for Peak, the compressor is looking for peaks in the input level. For example, if your tape recorder overloads every time the kick drum hits, you can use Peak limiting to keep the kick from peaking above the rest of the music.

RMS compression works by detecting a signal's average level, much like our ears adjust to loud or soft sounds. In RMS mode, your source can have more of a dynamic, transparent sound (because short peaks don't clamp down the overall level) but still be prevented from getting too loud.

Generally, if you're trying to raise the apparent volume of the track for radio or mixdown, use RMS compression. If you're trying to stop peaks from distorting your tape recorder or amplifier, use Peak mode.

The RATIO knob controls the amount of compression which will happen once the input signal crosses the Threshold level, described above. Ratio controls how much the input signal will be reduced as a ratio of the input signal level. For example, if the compression ratio is set for 6:1, the input signal will have to cross the threshold by 6 dB for the output level to increase by 1dB. The maximum setting is typically labeled _:1 (Infinity to 1), and is also called Limiting. This means that the input signal won't go above the threshold at all.

The RELEASE knob controls the amount of time the compressor takes to stop compressing after the signal crosses under the threshold. The range of this control is 50ms to 3 seconds. The Attack and Release controls may only function when the compressor is in Peak mode. Short release times are good for percussive, punchy sounds, where longer release times can make compression less obvious on vocals. Adjusting the release time may be necessary when using extreme compression and "pumping" or "breathing" is audible, or if lower level signals after peaks are getting lost. (See also the tech note on Pumping and Breathing.)

The THRESHOLD knob sets the level where compression will begin. As long as the input signal level is below the Threshold level, the NanoCompressor will do nothing to the signal. Once the input signal crosses the Threshold, the compressor will begin compressing at a ratio set by the ratio control.
In the diagram above, Figure (a.) shows the input signal to the NanoCompressor. In this example the compressor Threshold is set for -10dB and the Ratio is set for 4:1. When the third peak of the input signal crosses the Threshold, the NanoCompressor starts to reduce the signal level, as shown in Figure (b.). Figure (c.) shows the output signal level, with the original signal shown with a dotted line.

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3 Replies Related Threads

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    RE: The Basics of Compression 2006/01/17 15:51:53 (permalink)
    Kinda heady stuff. I'm starting to get some of it, but it helps if you HAVE a compressor to try out! And I don't.

    R U using a particular compressor right now with MC2 or other software? I looked at Musician's Friend and a couple or few that might work for someone in my budget range were around $100--Behringer, Alesis and maybe another one that had decent reviews..

    I know you can't really undo compression if you record with it, so I would want to do it afterwards most likely. How do you run the signal out of your software, say in MC2, if I had a compressor? All I have is a Mobile Pre unit.

    I will likely get one at some point. It's just another tool for coloring your music and achieving certain sounds--the punchiness for instance, which may be impossible otherwise.

    Do you use your compressor as a noise gate also or use a seperate processor along with the compressor? Thanks.

    Mr. Oliver
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    RE: The Basics of Compression 2006/01/17 16:49:36 (permalink)
    I use software compressors, i do not like to use hardware stuff, like you said you can not undo it. I use the classics series, and others, like grancomp, and the free ones with some other softwares. With MC you need a VST wrapper to use VST effects, like the classics, For DX effects, other than the ones in MC there are the blueline stuff Another way is to pick up a copy of Sony's Sound forge Studio, it runs around $69 and has a nice set of effects that work well in Cakewalk products, and it is a great software to have anyway.

    I'd Seize the day but i can't quite reach it!
    Music Town
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    RE: The Basics of Compression 2006/01/17 17:19:19 (permalink)
    That's a pretty good synopsis. I'd be interested to hear more about TCR (transient controlled response from the sonitus compressor) and ARC (auto release control from waves). It's kinda the new thing in compressors to have release be automatically determined by lookahead to avoid pumping. Still, that gets pretty darn specific. Thanks for posting!

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