Keyboard Magazine "In The Studio" Keyboard and Compressor/Limiters
In this months column I'd like to discuss the use of compressor/limiters with keyboards.
Compression was introduced in recording to help the recording engineer cope with the problems recording live music. Because most symphony orchestras could well exceed 100+ db and tape could only handle around 75 db of dynamic range, an engineer had to be awfully quick at the faders ( his keyboard ) or be lucky enough to have a compressor and know how to use it.
It should be easy to understand what a compressor is being that it is a VCA. For the VCA in a compressor to react, a certain level has to be reached. This is called the threshold. When the input level of a compressor exceeds the threshold set, the compressor's VCA starts to reduce the input level that has exceeded that threshold. The amount the signal is decreased is determined by the ratio. If an input exceeds the threshold set by + 10 db, and the ratio is 2 : 1, the compressor's VCA will restrict the input level to + 5 db above the threshold set.
Most compressor limiters have at least three basic controls. They are the threshold adjust, the compression ratio amount, and the output gain. To learn how to get the best out of a comp/limiter we'll start with the threshold adjust. If for instance you would only like to trim off the peaks from your synthesizer that feed into your stage amplifier, you would play your loudest passages and then set the threshold to react to those levels. Next check the softer passages. Be sure the threshold indicators not being triggered. If it is flickering, you're threshold is set too low and you will be compressing the whole program. The next thing to decide upon is the amount of compression ratio. Notice that is the ratio is set to 1 : 1 nothing audible is happening to the program. If you turn the ratio up to 2 : 1 and the input is above the threshold you should hear the compression working slightly. The purpose of the ratio is the difference between compression and limiting. Lower ratio 's from 2 : 1 to 6 : 1 are considered compression and higher ratios above 10 : 1 are considered limiting. A good definition of a compressor vs. a limiter is as follows :
A compressor is a VCA amplifier whose gain decreases as its input level increases.
A limiter is a compressor whose output level stays the same, regardless of its input level.
The art of knowing when to use compression vs. limiting is something of a matter of taste. Acoustic piano and very percussive synth sounds sound excellent with their sometimes harsh peaks removed. But beware, if you set the threshold to low, you can remove all the percussive qualities from the initial attack. The better comp/limiters have adjustable attack and release times which are invaluable for situations mentioned above. If you are not fortunate enough to have adjustable attack and release on you unit, you're just going to have experiment with different threshold settings and ratios.
One of the biggest mistakes made concerning compression is having the threshold set too low! Although this can produce a very unique sound it can also add much unwanted noise. Lets take a look how to avoid this. First the reason we are using compression is because we want to smooth an instrument out. This will will make it more enjoyable to the listeners and allow for is volume to be louder. This can be great for a solo synth part. It has to be loud to be dramatic, but it shouldn't be offensive. Instead of just limiting the peaks we want to control the solo synth's overall dynamic range into a narrow amount of loudness. To do this we use a ratio around 2 : 1 to 4 : 1 and reduce the threshold so that almost all the notes played are above the threshold. But here's where the trouble can start. If that instrument's output is noisy, we can going to amplify that noise into the same dynamic range. So you have to be careful not to set the threshold so low that it picks up the noise. If a noise gate is placed in front of the comp/limiter lower thresholds can be obtained. I intend to cover noise gates in a future column.
Now the last control that we haven't covered yet is the output gain adjust. This is a most important function. If you have been reducing your input signal's output, it has to be brought up in level again. After you've smoothed out the peaks, you're going to have to raise the output amplifier to bring that compressed signal back into the mix. Be careful because once again this stage can add noise to your signal if you abuse it! If you don't have enough range to boost the signal, go back and check your input threshold. Try boosting the volume of the instrument you're compressing and then re-setting the threshold and output levels again. Too low of a level will cause noise. Whenever you have to amplify a signal excessively you are going to add noise in the process.
One of the side effects of using a compressor/limiter is called pumping or breathing. These effects can become audibly obtrusive especially when using higher compression ratios. Remember the higher the ratio the more the gain change. When a signal passes the threshold level the compressor turns on. Its VCA goes to work and starts to reduce the signal by an amount equal to the ratio's setting. This action is called gain reduction. When the input signal falls below the threshold, the compressor's VCA starts reducing the amount of gain reduction. With high input levels and high compression ratios the amount of gain reduction could easily be around + 10 db. If the gain reduction changes rapidly, a pumping or breathing effect would take place. On the other hand if the compressor you're working with has a slow release time, ( the time it takes for the compressor to reset itself ) the notes that follow and are under the threshold level will also be reduced in gain. The sooner you get to know your unit's capabilities, you can learn how to set the threshold differently to overcome pumping and breathing sounds. A most unusual effect can be created with a compressor with a very fast attack time and a slow release. This back-wards sounding effect is most noticeable on a drum set, especially on the cymbls.
One of the best uses of compression is on synthesizer bass parts. This is probably the best instrument to work with to get to learn the effects of compression. In the studio it is common practice to record the bass guitar or bass synthesizer with compression on it. Because of the low frequencies and the percussiveness of the higher frequencies, the bass is a perfect example. Experiment with high and low ratio amounts. Listen to the difference it makes on the low vs. the higher frequencies. Try also different threshold levels. Notice when the bass line really jumps out at you where you've set the threshold and ratio amounts and you will begin to understand why. Listen also for when the bass part sounds least percussive and take note of the threshold and ratio settings. It will begin to become very clear after a short while of experimentation, of how the threshold and ratio interact with each other. If you have a compressor with adjustable attack and release settings, try a slow attack. This can even bring those lines out more.
Where is the best place to insert a compressor/limiter into the signal path? Well if you only have one unit it might be best to use it for all your keyboards in a live situation. By inserting it between your mixer and amplifier is the most common. If you only need compression or peak limiting on only one or two instruments, try using one of the effects sends. If your console has more than one bus, assign all the keyboards that you wish to be compressed to that bus. If you have a stereo unit, you can have a stage setting and give your sound man a separate compression setting.
All in all, not only will this make your soundman's job easier but your overall sound will be smoother.