Q What does a compressor's auto-release control do?

Sound Advice : Recording




Hugh Robjohns



I know it changes settings according to the source material, but how does the 'automatic recovery' function on a compressor actually work?



Bob Rogers via email



SOS Technical Editor Hugh Robjohns replies: The first thing to consider is why an auto-recovery function is useful at all. A slow recovery time provides very gentle changes in loudness, which are perceived as controlled but 'un-dynamic', which is often a good reason for choosing a slow setting. There's a risk, though, that some sudden, large, transient peaks will cause substantial gain-reduction that takes ages to recover, effectively 'punching a hole' in the audio. This can be an interesting effect, but it's often highly undesirable!



A very fast recovery time, on the other hand, produces an aggressively 'bouncy' or 'pumpy' signal, and those rapid gain changes are perceived as being very loud. A heavily compressed signal with a fast recovery time will always sound louder than the same signal with a slow recovery time — even though the peak levels are exactly the same. The downside of a fast recovery time is that the resulting rapid changes in any ambient noise level can become very obvious and may be perceived as unpleasant — the effect we know as 'pumping'.



So, if we want to make the signal sound loud (the most common reason for employing a compressor!) we need a fast recovery time, but if we also want to avoid ambience pumping we need a slow recovery time. The usual compromise is to find a single setting that achieves a reasonable balance between these two requirements.



The dual-stage or auto-recovery system is an attempt to provide both fast and slow recovery times simultaneously, handling the loudest signal transients with a fast recovery to maintain the perceived loudness, while cushioning the quieter signals with a slow recovery time to avoid pumping. Hopefully, this will give the best of both worlds with none of the drawbacks, and avoid the inherent compromises of a fixed recovery time.



You've probably heard it in action, but have you ever wondered what the 'auto' setting on a compressor actually does to your signal?To understand how this auto-recovery mode works, you must examine how the standard fixed attack and recovery (or release) times are typically established. The side-chain circuitry starts by rectifying the audio signal to convert it into a varying DC voltage, which represents either the signal peak or average (RMS) amplitude, depending on design. When that DC voltage exceeds the compressor's threshold level it's allowed to charge a 'time-constant' capacitor through a resistor, so that the voltage across the capacitor builds relatively slowly (taking a few milliseconds, typically). This slowly building voltage is used to control the compressor's gain-reducing element, causing it to increase the audio signal attenuation at a controlled rate called the 'Attack Time'. Altering the value of the resistor changes the charging time, allowing the attack time to be adjusted manually.



When the audio signal level falls below the threshold value, the capacitor is allowed to discharge through a second resistor, controlling the recovery or release time. As the capacitor discharges, the voltage across it falls, and the gain-reducing element restores the program signal level back to normal. Usefully, the capacitor charges and discharges with an exponential curve, and that means the voltage change can be easily engineered to have a logarithmic response, and that's ideally suited to the way we perceive loudness. In other words, it ensures that the program loudness through the attack and recovery phases changes smoothly and progressively, just like riding a mixer's fader up and down.



The auto-recovery mode works in exactly the same way, but with two (or sometimes three) time-constant circuits connected together, each with a different discharge resistor value (see diagram). The idea is that the loudest two-thirds (or thereabouts) of the signal voltage is used to charge the top capacitor, which is arranged to have a very fast discharge time. In this way the level changes in the loudest parts of the signal are handled with a very fast recovery time (typically 30ms), which maintains that perception of loudness.



Quieter elements of the signal charge the lower capacitor, which is arranged to have a much slower discharge time. So once the signal falls below the compressor's threshold level, the recovery rate is initially very fast, but then it slows so that the last 4dB or so of gain reduction is released very slowly (maybe with a 700ms recovery time). This ensures that the level of ambient noise is restored subtly, without audible pumping. Additional time-constant networks are sometimes used to introduce a more gradual change in the recovery rates, although the simple two-stage arrangement is extremely effective and widely employed.



So, really, an automatic release is often a good option if you're experiencing unwanted pumping of the signal from a long release, but it's less likely to be suitable with high ratios when you're approaching limiting — when you'll pretty much always want a relatively fast release.  

Published in SOS September 2014

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Q. What's the best order for recording a band?

Sound Advice : Recording



Paul White



I'm going to be recording my band's new album and it's my first big project, so I'd like a bit of advice. It's a rhythm & blues band consisting of drums, rhythm guitar, lead guitar, acoustic guitar, bass guitar and lead and backing vocals. I have a limited number of mics, so I'll be recording sources separately. The question is: in what order do I do this? The band are pretty tight, but I don't think the drummer would like playing to a click track with no reference to the song itself. I seem to get mixed views on this: I know the normal way is drums first, but what would be best for this band?



Via SOS web site



SOS Editor In Chief Paul White replies: It always gives the best feel if the band play together, even if it's only drums plus a rough mix as a guide of the other instruments and a vocal, recorded down one mic, as these can be replaced later. Of course, you also need to minimise spill, so using POD‑style devices and headphones for guitars and bass can help in getting the guide parts down and avoids the need for a mic, though you may need a small mixer. Otherwise, put the drums in a different room from the guitar amps. Some people manage by recording just the drums and bass together, but trying to do the drums on their own is asking for problems, as the feel will never be quite right.



If the music is of a type where the drummer is happy to play to a click track, you could always record the separate parts to a click or guide drum loop, then have the drummer put the drums on last. But, again, you could lose all your feel that way.



SOS Reviews Editor Matt Houghton adds: With a genre like rhythm & blues, the feel of the playing is so critical to getting a good result. There are plenty of great recordings where most parts were played with everyone in the room to get a good vibe. As Paul suggests, you need the rhythm section, in particular, to be really hitting the groove, and with that in mind, I would certainly want to track at least the drums and bass together as a starting point. You could have both playing in the same room, using a solid gobo or two to provide separation between the kit and the bass amp, with the amp separated from the kit, but with the bassist standing in a position where he and the drummer have good visual communication. Alternatively, put the bassist in the room, but DI his bass, perhaps running it through an amp simulator for monitoring purposes. If you get an amazing take, it would be simple enough to use a good amp simulator such as IK's Ampeg SVX or Softube's Vintage Bass Room, or re‑amping, to make it fit in the mix, but you also have the opportunity of laying a new bass part over the drums.



While you could take the 'playing together' principle further, and put guitars, keys, vocalists and whatever else you wish in the same room, you'll often find that achieving acceptable separation becomes problematic, and maybe even a problem that outweighs the benefits of having everyone perform together. In my experience, you're better off having a vocalist and guitarist either in a different room than the drums, or in the control room, with the guitar DI'd and/or run through an amp simulator, so that the whole group is doing a live take into the monitor mix. You can always track those parts, in case you get a moment of magic — with the guitars, as with the bass, amp simulations or re‑amping are valid approaches here — but you'll still have the option to overdub those parts later, and many musicians will be glad of the chance to try a few different takes.



What else you record in what order will depend on the other musicians. Does the vocalist want to hear the other parts? Do other musicians take their cue off the vocals? If you make sure you record the guide parts as you go, those questions become less of an issue, and while I often advocate starting with vocals (or other primary elements) when mixing, I don't usually find that it makes such a difference when recording.Recording the various instruments in a band in the right order is important, as it will affect the feel of the track. Photo: Richard EcclestoneRecording the various instruments in a band in the right order is important, as it will affect the feel of the track. Photo: Richard Ecclestone



When it comes to tempo and click tracks, your approach will depend very much on the band in question. When you say 'tight', that might mean that the drummer can keep a rock‑solid tempo, or it might just mean that all the musicians can keep in good time with each other. In my experience, some bands that sound tight can actually accelerate and slow down considerably during a track, which may or may not be a good thing. The one thing that I would say, though, is that the thresholdsfor what seems acceptable when playing live is different than when listening to a record that gets played again and again. So having a click track or guide track for the drummer might be useful. Really, the best advice I can give is to discuss this with the drummer, go with what they feel comfortable with and simply be alert to any problems.



If you do choose to use a click track, my advice would be to feed it to the drummer alone, and then have everyone else lock in with him or her, which is what a good live band will usually do, after all. And do make sure that the headphones don't leak that sound into the overhead mics, which is something I hear a lot on material sent in to SOS!



Of course, it's perfectly possible to overdub drum parts, but I invariably find that when doing this you lose almost all of the magical glue that holds a track together. In this style, more than most, that will probably prove unacceptable. In fact, I can only recall one occasion where I've done it and obtained a satisfactory result.    

Published in SOS December 2010

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