All about producing and mastering audio for disc, the web and beyond
Tuesday, May 8, 2012
Multi-Band Compression Today I’d like to spend just a little bit of time talking about multi-band compression. Multi-band compression is something that’s been at play in audio for decades. It first came into play in radio station broadcasts. More recently, beginning in the early 1990’s as DSP became cheaper and more affordable, we were able to deploy multi-band compression affordably so that recording, mixing and mastering engineers could use it. What’s good about multi-band compression? Probably the main thing that multi-band compression allows for and that gives you an advantage as opposed to using one single band across the spectrum of compression is that you can set independent attack and release times for different parts of the spectrum. The reason that’s important is that a bass waveform has a much longer period to it. The fundamental frequency of a kick drum in a hip-hop tune is going to take 50–70ms for the low-frequency transient to get through. If you don’t want your compressor to be chomping at the very beginning of that low-frequency transient, you need to make sure to give the attack time enough time before the compressor kicks in so it doesn’t restrain the low end. Of course, 50-70ms is going to be a very long period of time for a compressor to wait if you are thinking about trying to also compress a jangly, bright acoustic guitar or something like that. You might have a tune that has a deep kick drum and a jangly guitar and it’s hard to find a single compression setting that’s going to work equally well in both those parts of the spectrum. A multi-band compressor allows you to divide the spectrum up into typically three or four different sections, apply different time constants, different attacks, different release to each part of the spectrum and to optimize the performance of the different segments of the compressor to the various instruments in a mix. What does this mean for your audio? In theory, it means you can get a compressed signal that’s tailored more to the program, and also because you can restrict the dynamics a little bit more effectively without hearing the compression per se, it theoretically gives the possibility of getting a louder sounding result in your mastering work. Potential problems with multi-band compression Multi-band compression is problematic for a couple of reasons that most people don’t really think about. First of all in order to divide the audio, you have to run through a series of filters – hi-pass, low-pass etc. Every time you run audio through a filter, you lose something in terms of fidelity. That goes a little bit against the credo of the mastering engineer. The idea of mastering is to always make something come out sounding better than how it went in, at least from one perspective. Running through the crossovers that are adding possible a little bit of ringing, a little bit of distortion and a little bit of noise flies in the face of trying to make stuff sound better. The minute you turn on a multi-band compressor, even if you have linear phase filters that are well designed you’re still going to get some change to the audio and it’s usually not a flattering change. That’s problem number one to my way of thinking. Problem number two is that I believe there’s something important about the proportion of harmonics in the sound of any given instrument. For a moment, I’d like to segregate this conversation so I’m making it clear – if you’re talking about dance music or music concrete – electronic music that doesn’t refer to an instrument that you might hear played acoustically then all bets are off in terms of maintaining the integrity of the original instrument. But let’s say you’ve got a recording that’s got a bass, and a guitar, and a vocal, and assume that I’m correct in saying that there’s something important about the proportion of harmonics to the fundamental frequency of any instrument. The minute you put a crossover in a compressor and start doing something different with one range of an instrument and another range of that same instrument, you’re going to start to skew the relationship the fundamental and the early harmonics of that instrument which give it the warm, full, clear part of its sound and the higher harmonics which give it some edge or brilliance. At first it can be a very seductive phenomenon that you get when you get this increased sense of brilliance and control and so on, but to my ear usually what comes out of a multi-band compressor – the instruments themselves – don’t sound as good as how they sounded going in to the multi-band compressor. So keeping that in mind, I think most mastering engineers who work at the top of the craft use multi-band compression sparingly if at all. It’s just something to keep in mind when you turn on a multi-band compressor, just for a moment focus your attention on the individual instruments in the mix. Listen to the bass, listen to the guitar before and after, listen to the vocal and make sure you aren’t harming that instrument in a certain way that ultimately means the thing isn’t going to sound as good as when it went in.