Compression
- Thread starter FroggyG
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A compressor attenuates the frequencies in the signal that exceeds the threshold signal level. The attenuation starts at the threshold, the amount of attenuation is determined by the ratio e.g. 4:1 means attenuate the signal by -4dB for every +1 dB the signal exceeds the threshold level. The attack time determines the time it takes for the attenuation to go from 0% to 100%. The knee determines the shape of the attenuation, whether it should be in a more soft or hard curvature and the release time determines the time it takes for the attenuation to go back from 100% to 0%. Make up gain is the amount of signal gain applied at the compressor's output stage, basically the amount of signal boost on the resulting signal. Peak or RMS are settings some compressors have to determine what the threshold signal level corresponds to, whether it is the RMS (average) signal level or the peak signal level. The modes can be mono, dual mono, stereo. For dual mono you can determine whether both the channels should be linked or unlinked, meaning whether each channel (L or R) should have their own unique compression or whether they should have a common compression applied on both channels. In Link mode the compression detector will calculate the maximum level of the two channels combined and compress both channels according to the maximum level using a single set of compressor configuration settings that are dedicated to the compression of the signal on both speakers. Essentially it means the threshold on each speaker is adjusted so that an equal amount of signal attenuation is applied on both channels (L + R), the amount of attenuation on both channels (L + R) corresponding to the greater amount of attenuation between the two channels (L +R). This makes the amount of attenuation equal on both sides but because the incoming signal level on each channel might be very different on each side, this can cause some stereo unbalance. Similarly, if you switch to unlinked mode you can also get an unbalanced stereo image, for instance one of the channels might become much louder than the other by the compression, causing a shift in the stereo weight towards one of the sides. That does not happen in linked mode as long as the signature frequency and peak/rms level on each channel is roughly the same. But if you do something funky, like move the bass guitar to one side and the vocals to the other, then you'll end up with issues with the linked mode. It is not true that linked mode narrows the stereo image, because the make up gain compensates for that. But compressors in linked mode impact on the stereo image because they can act more on the content in the center or more on the content on the side depending on the content of the signal. So in linked mode it's not generally a matter of getting stereo weight unbalance between L and R, it's a matter of width impact, but depends on the signal on each channel since the attenuation on each channel is equal and the perceived signal level is level relative (the dBu/volts RMS curvature is cumulative). Many compressors can be switched into mid side mode, this gives you greater control of the stereo width impact and at the process stage rather than at the input stage. Please be aware though that this compression is applied on an M/S encoded signal in isolation on each component then decoded back to L/R. It will hence separate the mid and side more, which may or may not be desired on the particular signal it is applied on. I guess one can say that when the L R weight moves towards one side, the unlinked mode becomes more interesting since it gives you greater control of the L R weight impact. Also be aware that a mix full of sound sources that fluctuate a lot in L R movement inside of the mix, can be distracting and can help lower the perceived separation of elements in the mix. But L R fluctuations can also be creatively used to excite the stereo image. How well it works depends on how it is done and on what.
The compressor is used to control the audio's timbre, whether that audio is a particular frequency band on a particular stereo channel (L or R) of a particular sound source or whether it is the whole mix, it all depends on what signal the compressor is applied on. The reason for this control is to shape the audio within the mix and to fit the audio within the mix. (in terms of fit - to utilize the full dynamic range of the playback format as efficiently as possible, both actual and perceived)
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The tl:dr for this: Compression can reduce loud sound so the quieter sound is easier to hear
The ts:dr for this: Although to some extent true, it's a very incomplete description of what a compressor does and is used for.
A shorter explanation is better in this situation considering OP's age. Your post is good for someone who knows what it is and wants to better understand it.
crimsonhawk47
New member
No it doesn't.
A compressor doesn't attenuate frequencies, it attenuates the entire source signal. While lower frequencies peak higher, this implies a compressor is only compressing certain frequencies instead of just reacting more dramatically to different frequencies. A multiband compressor may attenuate frequencies. But when a sound exceeds a regular compressors threshold, the entire sound gets turned down. All a compressor is, is an automatic gain rider, at least most digital compressors are. Some emulations do more, but this is the gist of what a compressor does.
People have a hard time with compression because what it does and what it sounds like seem disconnected at first. What it does is not that complicated...
-When the signal hits the threshold it is reduced by some amount (usually called ratio).
-Attack is how long it takes to reduce to that ratio
-Release is how long it takes to return to zero reduction
-With a hard knee or 0 (dB) knee the signal completely unaffected unless it hits the exact threshold.
-With soft knee or greater than 0 (dB) the signal is reduced proportionately depending on how close it is to the threshold.
...that's literally it. But that's like saying "A C Major triad is when you play C, E and G together". That might be technically perfect but it gives you no insight into what a C Major triad is or when you should use one because it's subjective. Almost every question asked on these forums (with the exception of 'how do I sound like...?') can be answered with the same boring answer: "if you like it it's right".
-When the signal hits the threshold it is reduced by some amount (usually called ratio).
-Attack is how long it takes to reduce to that ratio
-Release is how long it takes to return to zero reduction
-With a hard knee or 0 (dB) knee the signal completely unaffected unless it hits the exact threshold.
-With soft knee or greater than 0 (dB) the signal is reduced proportionately depending on how close it is to the threshold.
...that's literally it. But that's like saying "A C Major triad is when you play C, E and G together". That might be technically perfect but it gives you no insight into what a C Major triad is or when you should use one because it's subjective. Almost every question asked on these forums (with the exception of 'how do I sound like...?') can be answered with the same boring answer: "if you like it it's right".
BEAT CONSTRUCTOR
New member
Compression should be the last effect added on a mix.
You are absolutely right and I am absolutely wrong, thanks for correcting me on this, it was beyond my awareness that it worked like this and not like I suggested.
VellaBeatz
New member
This threads has answered most of questions. Thank yous.
laurend
Active member
-Threshold sets the point where the output isn't proportional to the input anymore.
-Ratio sets how non-proportional is the output once the threshold is crossed.
The compressor action is reducing the gain, not the ratio.
This definition may OK, but some manufacturers count the time for a 6 dB variation, or the time for half the total gain variation. This situation is also true when measuring the attack time.
Perfect
Note that the threshold point is measured on a signal which is is ussually switch on the input. But some compressors also allow to measure the output or a mix of both input and output.
This measure can use a peak value (instant) or a pseudo-RMS value (averaged values in a floating time window) , or a mix of both.
The signal feeding the measurement section (Sidechain) for threshold can be filtred to make the process more or less sensitive to some frequencies. This sidechain signal be different than the sound to be processed.
When the Link is engaged all compressor channels use the same sidechain and all channels share most of the settings values Which is great to preserve the stereo balance.
My two cents
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crimsonhawk47
New member
I can't tell if you're being sarcastic and I'm being a pedant about your definition, because you seem to know more about mixing than I do from your posts.
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