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Automated Mixing (part 4)

Fader automation comes in three flavours of which the two most popular are VCAs and moving faders...


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Fader Automation

This is where we get serious. Fader automation comes in three flavours of which the two most popular are VCAs and moving faders. Let me explain: a VCA is a voltage controlled amplifier or attenuator and it can control the level of a signal in response to a separately input DC voltage. A variant of the VCA is sometimes known as a DCA where the signal level is controlled by a digital input signal. It amounts to much the same thing in operational terms. In a VCA automation system the source of the controlling voltage is the fader that you move with your finger. No audio signal passes through this fader, instead its level is controlled indirectly by the gain control element of the VCA. One point that is worth making straight away is that you can’t expect the performance of a voltage controlled amplifier to be every bit as good as a normal amplifier or attenuator. This is because the circuitry is much more complex, and although modern high quality VCAs sound pretty good to me and to most people, top engineers will sometimes still have that lurking shadow of a doubt that they are not getting absolutely the best sound possible.

To the untrained eye, the distinguishing characteristic of a VCA automation system is that when the computer is playing back the fader moves, the faders don’t physically move. They don’t have to since all the level control is going on out of sight in the VCAs. The alternative is a moving fader automation system where the faders literally do move under computer control. Here each fader has a motor and the computer can zip it to the correct position almost as quickly, and probably more accurately, than you can do it by hand. In a moving fader system the audio does pass through the fader and therefore the quality can be every bit as good as a non-automated system. If your short term memory is good, you will recall that I mentioned three types of automation system. The third type uses VCAs to control the level of the audio and moving faders to provide instantaneous visual verification of the level settings. This third type is more popular in post production than in music recording so I’ll do no more than mention it.

Either system will probably employ a computer, usually a standard Atari, Mac or PC, to control the levels and there are many points that VCAs and moving faders have in common. The simplest way of automating a mix is to use scenes or snapshots. In snapshot automation you will set the faders in positions that suit the various sections of the song, and record - or ‘write’ - each as a snapshot, which you will then get the computer to replay in sync with the timecode on the tape. The system will probably have an adjustable glide time so that changes between snapshots are smooth and not sudden. Snapshot automation is a great convenience but it doesn’t address every problem. The alternative is to set rough fader positions which will be good for most of the song and write these into the computer. You probably won’t have to play through the whole song to do this since there is bound to be a function to let you extend one set of fader positions right through to the end of the mix. Now you can set about writing moves for the individual faders, based upon those starting positions or completely different if you wish. The most important track in any song is the vocal so you will probably give this most of your attention. There are two problems here: the first is that the vocal probably varies in level anyway and you will need to even this out. The second is that you will need to ride the fader to suit the artistic demands of the mix as the song runs through.

By David Mellor Thursday January 1, 2004