Loudspeakers maybe in danger of getting damaged by excessive excursion. In these cases, we are concerned about controlling the peak levels of signals, such that the needed processing tends to be some form of limiting rather than compression. Reduce the dynamic range on a signal. The dynamic range when expressed in decibels, as is commonly done is the difference between the loudest and the softest signal.
If we attenuate the peaks out of signal, we are reducing its dynamic range. Since many devices are peak limited power amplifiers, recorders , this allows us to increase the RMS level of the signal. Some compressors provide the ability to select between compressing based on the detection of average RMS, the most common option or instantaneous peak levels.
The way to detect RMS levels may also vary: higher quality compressors detect real RMS, while cheaper ones only approximate it. Which brings us to defining what a limiter is. A limiter is really just a form of compressor. We could say that compressing is smooth attenuation, whereas limiting is doing it in an abrupt manner.
Often we will come across compressors that feature dedicated limiters, thus offering simultaneous compression and limiting from a single unit. Typically, the term limiter is also associated to faster times, particularly for attack, so as to avoid exceeding a specific signal maximum at all times. Controls Compression is a difficult task that may require very different characteristics depending of the type of signal.
Numerous controls are therefore needed. For the first four bars, Knee is set to 0. With this setting, the knee affects the onset of compression as the signal approaches the threshold. For the last five bars, Knee is set to 8. Because this is an eight-bar loop and the display has enough room to show nine bars, the display catches the same bar before and after turning up the knee. The red and green arrows show how this particular hit goes from uncompressed to gentle compression.
As you can see, Ozone Dynamics has a lot going on. This display shows controls for a separate Compressor and Limiter, to allow for two stages of dynamics control. You can set these parameters for up to four frequency ranges, so for example you can compress bass frequencies hard while leaving highs alone. Remember, we said that the threshold activates the compressor, not compression in general.
Otherwise, if the signal crosses the threshold often, the compressor will create unpleasant artifacts as it rapidly turns on and off.
The attack time is the amount of time the compressor will go from zero compression to full compression based on the ratio and threshold settings. The signal will eventually drop below the threshold, meaning compression has to stop. The release time determines how long it takes for the compressor to stop compression.
See this in the guitar tracks shown below, where the upper waveform shows the stereo signal before compression and the lower one shows it after compression. However, when we do this, the entire signal is amplified, so the quieter parts get amplified right along with the louder parts. The signal will have a narrower dynamic range, but a higher RMS level an average— see the Pro Audio Essentials video here — and if there is noticeable noise in the background the noise floor , it will be more audible.
Additionally, compressors can be used to add color to a sound. Each compressor is unique, with different analog circuits and digital algorithms being used. Compressors are also important for controlling the dynamics of live-recorded instruments and vocals. These tend to vary quite widely in level over the course of a performance, so some compression can help make the level more consistent.
You can also use compressors to shape transients in sounds like drums. A transient is the first part of any musical sound, where the instrument is hit, bowed, blown into, plucked, etc. Transients carry some of the essential information the human ear needs to determine what the sound is, so playing with them can create a wide variety of musical effects.
For example, lower faster attack times can be used to attenuate the initial transient like the crack of the snare or the beater snap of the kick drum , making the tail of the drum hit more prominent. More commonly, drum compression uses increased slower attack times. A slow attack lets the initial transient slip through untouched while the compressor is still getting started compressing. This will make the transient pop out even more, making drums punchier.
This screenshot from iZotope Nectar Pro illustrates this technique nicely. The orange line shows how much compression is being applied at any given moment. You can see that the long attack is allowing the sharp transients of drum hits to slip through and be heard at full dynamics.
Similarly, stretching out or tightening up the release time of a compressor affects how notes trail away. In the screenshot above, the release time of roughly ms allows the compression to ease back to zero in a very musical way before the next hit. There are two other techniques worth mentioning where compression is concerned. The first is parallel compression , which has become much more commonly known and popular in recent years. Parallel Compression is a technique that involves mixing a lightly compressed signal with a heavily compressed and sometimes high-pass filtered version of the same signal.
This allows for a smoother result, with crisp and level sound in the high end without any loud peaks or squashed transients. For example, inserting a compressor on a bass track that reacts to the kick drum will compress the bass every time the kick drum is hit. Another great example of sidechaining is an effect called a de-esser. In a de-esser, a vocal is run through a compressor, and the sidechain input controlling it is the exact same vocal — after being run through an EQ to isolate the hissy, essy parts of vocal sounds that we call sibilance.
This way, sibilance causes the vocal to compress a bit and makes the sibilance itself less audible. The sibilance teaches the compressor how to remove itself. Nectar Pro has a dedicated De-esser module in its signal chain, that looks like this:. The limiter serves as a ceiling which signal cannot pass.
If the signal hits this ceiling, it will be harshly compressed so that it does not pass above. You may be wondering if a limiter attenuates the loudest parts of a signal, how is it any different from a compressor? Essentially, a limiter is just a compressor with a very high ratio. Eventually, that compression amounts to an impermeable ceiling. We send three signals through it, at levels of 2 dB, 4 dB, and 8 dB over the threshold.
With this ratio, the compressor would output signals at levels of 1 dB, 2 dB, and 4 dB over the threshold. Closer to each other in level, but still not so consistent. However, if we turned the ratio up to quite high , the compressor would output signals at levels of 0.
These signals are now much closer to each other and much closer to the threshold level itself. Every limiter will have at least one parameter: gain. This is used to boost signal until it hits the ceiling and is compressed. Some limiters will have an adjustable threshold level, which is also often referred to as the ceiling.
If your limiter does not have this capability, you can always compensate for the added gain with a dedicated gain plug-in or at the channel fader. Most limiters will have a release time parameter as well.
Not all limiters will have an adjustable attack time, however. Now that we know a limiter is essentially a compressor with a high ratio, take a look at our compression output level equation again:.
If you plan to send your mix to a mastering engineer, however, you would want to remove the limiter. Some people will also run a limiter on individual tracks that need some heavy-duty dynamic control, like a bass guitar. Limiting is not unheard of on buses, either, such as at the end of a vocal chain for a bit of extra control on subtler settings. A limiter vs compressor is really just a different flavor of the same thing.
Limiters turn down the signal way more heavily and are mostly employed to keep a mix or instrument at one solid level with no overages above a set output level. Compressors are generally less abrasive and more usable for a variety applications.
Compressors work great for less heavy-handed, more creative dynamics processing. A limiter will also generally have faster attack and release times than a compressor so it can respond to very sudden transient peaks with ease. What does an audio compressor do? These ratios represent how much the compressor will attenuate an audio signal over a certain threshold.
For example, if you have the threshold set to dB and a ratio of , the compressor will turn down a signal 4dB over the threshold -6dB to 1dB over the threshold -9dB. In most cases, this keeps us safely below digital zero while still imparting their smooth levelling properties onto our signal obviously with a bit of level matching on the way out to make up for the resultant dip in RMS volume that tends to happen in the attenuation process. This high ratio is to — excuse the pun — limit the audio from going above a set volume, as opposed to just taming it.
Another subtle difference between the two is that limiters typically have faster attack and release times than compressors do.
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