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Automated Mixing (part 4)

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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.

David Mellor

Acoustics & Studio Design

Acoustics & Studio Design

The NLE AudioPedia series, our video-based audio encyclopedia, is an invaluable resource for sound engineers, musicians, students, educators and all audio enthusiasts. This second installment is about Acoustics & Studio Design.

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David Mellor