A long while ago I scoured the net for all imformation involving recording. I organized what I found came up with a couple of notebooks. Here is one I finished on compression.

Most of this information came from a site called

www.recordingeq.com

Great site! Make sure you give them a visit!

-----
COMPRESSION


General Rules & Tips
1. With regard to attack and release parameters, the general rule is to use the fastest times that sound good.
2. When we use compression, and raise the signal’s average level, it becomes less susceptible to the Fletcher-Munson effect.
3. ! I would also advise against making any major equalization changes until the compressed mix is roughly right, as these will also affect the way the compressor 'sees' dominant signals.

To compress check to see what the track level signals are reading.
Determine the normal levels and the levels that







Compression Terms Explained

THRESHOLD - The level the input signal must exceed for the compressor to start working.

Start with the threshold set high (so that it doesn't cause any gain reduction). As you are watching the gain reduction indicator, reduce the threshold until the desired amount of gain reduction is present on the louder passages or sounds. Reducing the threshold level means that more peaks of the signal will trigger the compressor to turn down the gain.

For tracks with a consistent sound most of the time, set a high threshold, so compression starts only when the signal is too loud.
For tracks with inconsistent sound levels overall, set a lower threshold so compression will be in effect more often.








ATTACK - Sets how quickly gain reduction is applied when a signal exceeds the threshold.

Attack times very close to 0 ms and less than 5 ms tend to bring down the percussive transients of the audio.

Attack times over 5 ms (usually 10 - 30 ms) are often used to let the transients and the attack of instruments through.

Too fast of an attack time can "squash" the beginning of the sound, causing it to lose dynamics. If the sound gets too lifeless as you apply compression, try lengthening the attack time so that compression does not begin the very instant the sound exceeds the threshold.
































RELEASE - Sets the amount of time it takes for the compressor to restore normal gain when the signal drops below the threshold.

Generally the fastest release time that sounds good is best.

For general compression of mixed music a release time between 50 and 100 ms usually works well.

Release times less than 25 ms may distort bass instruments because the compressor changes gain in the middle of a cycle.

If the sound is becoming distorted (especially in the low end) this is usually a result of setting too fast of a release time. If a low frequency sound is becoming distorted while using compression, try lengthening the release time.

Release times should be adjusted for the frequency and how percussive the signal is. Compressors for bass signals must not be set to a very short release time or the gain will be changed within one cycle, causing distortion. Generally speaking, the fastest release time that sounds natural is the best.
Unwanted Breathing (compression of silent parts in dynamic music) is the sound of a compressor restoring normal gain too slowly when the signal has dropped below the threshold. If breathing is occurring, shorten the release time or another solution is to use a noise gate after the compressor which turns off the signal when the instrument isn't playing.








AUTO RELEASE - Auto Release is variable release time feature in compressors and helps make the signal's volume even to the ear. With auto release activated, the compressor's release time is longer for peaks going well above the threshold and shorter for peaks going slightly above the threshold. Auto release is often used for general compression.
RATIO - : The compression ratio is how many dB the input signal has to rise above the threshold for the compressor to allow one more dB out of the compressor.. If the input signal is 4 dB above the threshold, the gain will be turned down so that the output only rises 2 dB above the threshold (a 2 to 1 ratio). At a 4:1 ratio the input signal has to be 4 dB above the threshold for the output to increase 1 dB. When the ratio control is set to 10:1 or more, the compressor is called a limiter because the unit is effectively preventing the peak levels from increasing any significant degree above the threshold level.

Typically, I set my ratio first, and then use the threshold knob to find the point that the incoming levels are being compressed.

General Compression for more presence: 2:1 to 3:1
Level Control for audio with sudden peaks: 4:1 to 5:1
Protection from severe overload or damage: 10:1 or more.

If a high compression ratio is used with a low threshold, the loudest notes can often sound "squashed. Using a mere 2:1 compression ratio (a low ratio) with a low enough threshold setting, the sound could be made to sound twice as consistent if we read –6dB on the gain reduction meter for the loudest notes. This would be more desirable.











GAIN REDUCTION

Too much gain reduction causes the louder passages to sound restricted and "squashed."

A lifeless sound can also be the result of too much gain reduction overall, which might be caused by too high a ratio, or too low a threshold.

For general compression, 6 dB of gain reduction is a usual goal. For louder types of music or where the audio has larger fluctuations, sometimes 10 dB of gain reduction is used. Special techniques may require differing amounts of gain reduction.





KNEE - Determines how quickly the compressor approaches the ratio that has been set.

Use a soft knee to make the compression less noticeable. Use a hard knee for maximum protection.

1. Hard Knee - no compression is applied to the signal when it is below the threshold, and full compression is applied as soon as the threshold is exceeded.
2. Soft Knee - the compression ratio increases as the signal’s input level increases, and starts somewhat below the threshold with the full ratio not achieved until the level is somewhat above the set threshold.

The soft knee makes the gain reduction less obvious. Soft knee is used in applications where you are using a compressor to even out volume changes in an instrument.


SIDECHAIN - When feeding a different signal to the sidechain input, the main signal is compressed according to the other signal in the sidechain input. For this to be the case, some other signal has to feed the sidechain input.

On many compressors you can insert a different signal into the sidechain by plugging into the "sidechain" or "key" input. (on some compressors you also have to push a button labeled "sidechain" or "key"). When a different signal is put into the sidechain, the compressor operates according to the signal fed to the sidechain input rather than to the main input. In other words the main signal (going into the regular input and output) is changed by the level variations of the signal plugged into the sidechain input.

LIMITING - The term '"1imit" means that the signal will not be allowed to exceed the threshold level at all. Limiters are used to prevent distortion by setting the threshold control just below the distortion level. The limiter then prevents any peak from getting to the level where it would distort.

If you want to use a compressor as a limiter, mainly to control excessive peaks, you need to set the threshold fairly high and use a high ratio. The signal will then be unprocessed most of the time, but when a peak does occur, it will be controlled very firmly. A fast attack and release time is best, though if the sound appears to pump you'll need to lengthen the release time until the pumping is acceptable.








Common uses for Compression

Basic Compressor Setup
It is sometimes easier to set up the threshold control using a high ratio along with fast attack and release settings, as the gain-reduction meters will kick in very obviously whenever a signal peak exceeds the threshold. Simply reduce the threshold until the gain-reduction meters start to show a significant amount of gain reduction between peaks, then bring it back up until only the peaks are affected. Once you've adjusted the threshold so that only peaks are being affected, you can return the attack and release settings to more suitable values and then work on the ratio control. A practical way to set the ratio control is to watch the gain-reduction meters as you vary the ratio and aim for a maximum gain reduction of between 8dB and 10dB. However, it's still vital that you listen carefully to the processed signal to see if it sounds the way you want it to — meters can only tell you so much, and if the peaks start to sound squashed, you'll probably need to either reduce the ratio or increase the compressor attack time.
Add More Presence to a track (Exciter Compressor)
There is another technique that is sometimes used. First, the main signal is split into two channels. One of the channels is filtered and equalized to isolate the presence range of the signal (usually between 4 and 6 kHz). The other channel is usually left unprocessed. The "presence" channel is severely compressed, so that it has very little dynamic range. Use a high ratio, a low threshold, a fast attack time, and the fastest release time that is appropriate to the signal; we won’t be listening to this signal by itself, as it would sound horrible. When mixed at a low level with the unprocessed channel, the "presence" aspects of the sound remain the same regardless of the original signal’s dynamics. If used on a vocal, the high notes and low notes, loud notes and soft notes will all appear to be just as "present" in the mix.
Lawrence Horn came up with a brilliant idea. He took the vocal and split the signal so that it when to 2 console channels. Before the vocal signal went to the second channel, it went through a compressor. Now he had two channels of the vocal - one compressed and one uncompressed. On the uncompressed vocal he added very little with the equalizer and he added the reverb. On the compressed channel, he compressed the h**l out of it and added a ton of high-frequency equalization. What he would do is bring up the "natural" channel to full level to get the basic natural sound on the vocal. On the other compressed and equalized channel, he brought this up just enough to add excitement and presence to the vocal sound.
As you try this technique out you will find it works for other instruments as well. Often the frequency of EQ needs to be changed for the instrument. The vocal works well with tons of 5kHz to 8 kHz added to the "exciting compressor;" guitars work better with 3 kHz - 5 kHz and bass guitars work better with 800 hZ to 1.5 kHz.
Make Tracks and Mixes Louder
The amount of GR to a track can be turned back up in the mix and the affected track will be perceived by our ears as twice as loud. Example GR= -6DB / After Comp GR Increase using make up gain or fader by 6db.
To achieve this, engineers often seek to use very fast attack and release times with a high ratio and a hard knee. This will very quickly reduce the gain on the audio peaks, which are often not noticeable to the ear.
Increase / Mellow the Attack of Instrument without EQ
To enhance the attack, allow 5 to 10 milliseconds for the attack time parameter. When a percussive sound is compressed this way, the transient of the instrument comes through with no gain reduction (even though it is above the threshold), and gain is reduced on the sustaining portion of the sound which begins about 5 to 10 milliseconds later. If we see –6dB on the gain reduction meter, the attack transient can be made to sound twice as loud since everything that isn’t the attack transient is now 6dB lower.
To mellow the attack of a sound, essentially the opposite is done. A very fast attack time (0 ms) and release time (about 5 to 10 ms) are used. This will tend to apply gain reduction only on the transient
Extend the "sustain" of some instruments, such as lead guitar or cymbals
The basic strategy is to use a fast attack time of 0 ms and a very long release time of several seconds, estimated based on the approximate length of the sustaining sound. As the sustaining sound drops below the threshold and is fading out naturally, the compressor is slowly restoring normal gain and turning the signal up.
Help to Control Masking
Another technique can be used to automatically reduce the volume of a signal only in the presence of another. This is called ducking, and is another sidechain application. A classic example is ducking the bassline whenever the kick drum is present, which helps the kick drum to become more distinct in the mix.
Run the bass track through a compressor, and route or patch the kick drum signal through this compressor’s sidechain input. Turn on the sidechain for the compressor, and now the bass will undergo gain reduction whenever the kick is played. Make sure you set a fast enough attack and release time so that the duration of the gain reduction is only as long as the kick sound itself – we don’t want the bass to sound funny. It may only require a few dB of reduction of the bass to "unmask" the kick, so set the ratio and threshold to see an average of approximately 3 dB of gain reduction with each kick hit.

Thicken up a sound
There are times when you want to use a compressor just to thicken up a sound, and in this instance it's probably fair to say that you want to bring up the level of low sounds. To do this, set a much lower ratio -- perhaps as little as 2:1, or even less, but set the threshold quite low so that you still get between 6 and 12dB of gain reduction showing on the meters. A longer release time may give a smoother sound, but every sound is different, so let your ears decide.
Raising apparent level of percussive attack (exp. snare)
If we could cleanly sheer off the last 6 dB of the percussive attacks, we could raise the level of the signal up 6 dB to get more average level (and loudness) but still retain much of the percussive quality of the sound. The parameter settings to achieve this include a very high ratio (say 20/1 or more), hard knee, very fast attack (0 ms if possible) and very fast release (5 ms). You adjust the threshold so that you get 6 dB or so gain reduction on the peaks.
Dulling attack of percussive instruments
Sometimes sounds are "too percussive" to our ears. To dull out the attack you would want to use a low ratio (like 3/1) and a soft knee to affect the slope of the transient sound. It is often helpful to increase the release time to between 10 and 25 ms, which makes the "processing" you are doing with the compressor less obvious. Try a slightly higher amount of gain reduction, say 10 dB.
Add attack to a lacking percussive instrument
Sometimes we would like more attack than is present in the instrument or the mix. To increase the attack, use an attack time to between 5 and 10 ms. The idea is that you want the body of the sound brought down by the compressor but not the transient. My experience is that a medium ratio (6/1) and a soft knee works best in these circumstances.







Instruments

Bass

Try starting out with a ratio of 4:1, and a fast attack and medium release. I usually use the hard-knee type of compression here since bass is such an attack-oriented instrument.

Bass guitars almost always need fairly hard compression, which is best done in isolation. A ratio of up to 5:1 is typical, with the threshold adjusted so that the majority of notes lie within a 4 to 6dB window. This is a particularly important technique for dance tracks where the bass is a driving line which needs to remain very consistent. Try a slowish attack (say 10mS) to allow the transient pluck through nicely, with a long release of about 250mS or so - beware of using too fast a release, as this can introduce pronounced harmonic distortion, with the compressor tracking the low-frequency waveform of the bass signal!

Guitar

This depends on the type of sound you are using, but a good general place to start is 2:1 on acoustic, and maybe 3:1 on distorted guitar (although you may need 4:1 here.) To get a good sustain, try a 4:1 ratio, use a fast attack and slow release. Then play the note you want to sustain, and raise the ratio until the sustain is as long as you want it.

Drums

Drum signals are often compressed due to their hard-hitting attack volumes. If nothing else, compress the snare drum, because each hit will likely peak higher than other hits. Try starting out with a ratio of 3:1, and use a fast attack and release. If the signal is still peaking, try using a ratio of 4:1. This method could also be applied to the toms. As for cymbal hits, try starting with a 2:1 ratio (moving to 3:1 if needed), using a fast attack and a slow release (to preserve the natural decay time of cymbals).

Compressing drums requires a lot of care, because it is very easy to destroy the attack and crispness in the sound. This is one case where limiters actually work quite well to catch the transient peaks and allow a little increase in overall loudness. Slowing the attack time is another handy tip, as this will let the initial transient through unmolested, whilst allowing the compression to pull up the later resonance of the drums to give a fatter overall sound.








Vocals

The ratio to start at varies for each singer, since some may be very strong and loud singers, and others quieter, having a smaller dynamic range. Try starting out with a 2:1 ratio, with a fast attack, and medium to slow release. Keep increasing your ratio until you get your peaks under control.

I would almost always compress vocals fairly hard (depending on the voices and the nature of the song), because the vocal line usually needs to remain clearly audible above the rest of the orchestration. Reducing its dynamic range to something relatively small (in keeping with the performance) is a good way to achieve that, but be wary of squashing the vocal too much as pulling up the overall level with the make-up gain could emphasize breaths and lip noises disproportionately. As always, the more highly skilled the performer, the more compression can be applied. A typical starting point would be a 3:1 ratio with a moderately low threshold and fairly fast release of perhaps 160mS.


Overall Mix

The settings most often used for stereo compression of the entire mix include a low ratio (2:1 or 2.5:1), fast attack, medium release and the threshold control adjusted for about 6dB of gain reduction on the loudest peaks. Overall compression of the stereo mix is usually sounds best with a soft knee (if available).

Great stuff, keep it coming! Thanks man...