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Compressor reduces a signal's dynamic range but the same can also be done by riding/automating the volume fader.

Compressors add coloration but other than that is it any different from fader riding?

  • There is a separate category of "riding" plugins such as Quiet Art WaveRider and Waves Vocal Rider. The rider plugins mimic manual fader riding, trying to even out volume changes, reducing or even eliminating the need for volume automation. If you try to avoid fader riding and volume automation by using a compressor, you will squash the life out of your tracks. Here's some info on the difference between "riders" vs compressors gearslutz.com/board/so-much-gear-so-little-time/… soundonsound.com/reviews/quiet-arts-wave-rider-2 – piiperi Reinstate Monica Oct 14 '20 at 20:30
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    I always imagine a compressor as a little guy inside the desk who is automatically riding the faders, but at potentially superhuman speed, with incredible accuracy. – Neil Oct 15 '20 at 8:45
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Let's compare gain/fader/wave riding (plugins or mixing engineers) and compressors

What they have in common:

  • Both change the gain of a signal
  • Both are used to even out volume changes (but not only that)

What compressors do, but riding doesn't or shouldn't

  • Compressors can and are used for shaping a sound's characteristics or timbre. The time window that a compressor takes into account is so short and its reactions are so rapid that it can change the attack part of a note significantly, making it sound like a different or differently played instrument. This is done particularly with drum sounds, for example to make them sound more punchy or snappy. This sort of character-change is distortion. (actually, ANY kind of time-variant gain change is distortion, but proper fader riding should never sound like distortion)
  • With extremely fast operation, a compressor can even be said to change the sound's waveform, not only its "envelope". Turn a sine wave into a square wave.
  • A compressor can be configured to mechanically react only to a certain limited frequency range in the input signal by filtering or EQing its key/sidechain input, but when riding you usually listen to the signal as a whole - possibly taking more abstract things into account such as excessively snappy or harsh "esses" i.e. sibilants. A compressor can be used as a de-esser by attenuating the signal when the sibilant frequencies raise above a threshold, but then the compressor is dedicated to only that purpose, not doing everything else at the same time as well.
  • A compressor can be configured to duck/attenuate with some other channel/instrument as its key/sidechain input, for example the kick drum can be used to duck everything else, creating a pumping sound. Or just to let the attack part of the drum to be heard by briefly making other things quieter when the drum hits. I don't think you'd do this kind of mechanical thing by riding a fader. Someone might do it with volume automation though.

What riding does but compressors don't

  • Compressors don't silence out quiet parts. (A gate or expander could do that.) But that can be an important goal in fader riding and automation - to remove unnecessary noise in places with no meaningful signal.
  • Compressors don't have a target level - a constant level that you try to maintain by either adding or removing gain, when riding. Instead, compressors have a "do not exceed" threshold, an upper limit above which the signal is attenuated, and a make-up gain.
  • When riding, you might reduce annoying weird or nasal sounds that you notice by understanding the signal such as vocals.
  • Compressors don't take other instruments into account for raising a level (the make-up gain doesn't follow anything), but a rider might raise a solo instrument or lead vocal according to the general loudness level of the music, to keep the solo element in front all the time.
  • A rider can listen to notes or phrases and keep track of which part of the signal constitutes a note or other similar musical entity, and not change the gain inside such an entity, only between them. But a compressor doesn't care about what's a note or a phrase or musically meaningful element, it does its thing mechanically even if it makes the performance sound unnatural.
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You are right that the basic effect that a compressor has on the signal is simply to change its gain, just as you are doing when you ride or automate the volume fader.

The 'selling point' of a device called a compressor is that for the purposes of gain reduction it does that riding for you, so that you don't have to. Additionally, a compressor can make its gain reduction much more reliably and with faster rates of change than anyone is likely to be able to do riding a fader. That's one reason that compressors may be able to add coloration - because they can respond so fast, they can change the attack characteristics of sounds they're applied to.

Essentially, the automation introduces possibilities that couldn't be realistically achieved by riding a fader, or (in a sensible amount of time) by drawing a curve.

Edit - my answer here is quite simple in what I saw as the spirit of the question - for more details please see piiperi Reinstate Monica's excellent answer.

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    True! By the time you've heard something too low or high in volume, you can't react quickly enough to compensate. And even if you could, levels would vary. +1. – Tim Oct 14 '20 at 12:32
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    It might be worth mentioning that the speed and automatic nature of a compressor comes along with some distortion, which might be pleasing, but also highlights an advantage of fader riding over compression, depending on the situation. In practice, most engineers with compress and ride the fader (automate) for key tracks like lead vocals and solos. – Todd Wilcox Oct 14 '20 at 19:33
  • @ToddWilcox perhaps the point is that a human fader-riding engineer has the ability to 'look ahead' (assuming that they've heard the track before) and is therefore not reliant on a fast 'attack' to keep the levels of instrumental phrases down, as they can fade-down in anticipation. IOW it's the look-ahead ability that allows slower gain changes and thus the avoidance of distortion? – topo Reinstate Monica Oct 15 '20 at 8:59
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The difference in using a compressor and riding the faders is evident in the results of each each procedure. Riding the faders results in the equal lowering of the level of an instrument or a track, that is, when the fader is pulled back, both the loud and quiet parts of the signal are attenuated equally. When using a compressor, it can be adjusted to attenuate the loudest (peak) parts of the signal to avoid overdriving the amplifier or recording device, while at the same time allowing us to boost the weaker more delicate parts of the signal into prominence. The result can be increased sustain on an instrument, with an appearance of increased overall volume, without overdriving the amplifier or recording medium. Riding the faders results in maintaining the original dynamics of the signal, whereas the use of compression reduces the original dynamics of the signal. Both techniques have their applications in music production.

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The difference is that it does it automatically. Also, with already-recorded material it might be able to 'look ahead' so it anticipates rather than reacts. (Which is the exact equivalent of a mixing engineer knowing the track or reading the score of course.)

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