The amount of times I we explain gain structure to customers is incredible.
This article from the Scratch Live Forums is a great little explanation for all of you who still cant do it!
Gain Theory
What is gain?
Gain is the process of
increasing the amplitude of your signal, or 'making it louder'. The
reverse process, making your signal quieter, is known as attenuation.
Headroom
Headroom,
as the name implies, is how much room there is between the average
level of your signal and the 'ceiling' - the maximum allowable level in
a given part of your system.
Headroom equates to how much gain you can apply before clipping
Clipping
Clipping
describes what happens to a signal when it's amplitude is greater than
the system will allow. Clipping can result in unpleasant loud
distortion.
Clipping is irreversible
Noise
Noise
is technically any unwanted portion of your signal. Generally in DJing,
it will be hiss, hum, or perhaps crosstalk interference from other
signals.
An analog audio signal will always accumulate
some noise
during transmission, and minimising how much is best achieved by
keeping the signal as loud (hot) as possible while also avoiding
clipping. Signal to noise ratio (SNR) is the measure of how hot or high
the desired audio signal is above the noise floor of the system or
piece of equipment. Higher ratios meaning less background noise.
Poor gain structuring is almost always the cause of excessive noise in a system.
The noise portion of your signal will be affected by gain in the same
way as the rest, therefore it is less desirable for a quiet signal to
be transmitted than a loud one, if possible, while avoiding clipping.
For example a turntable phono output is
very low level, and if transmitted for more than a couple of meters via standard cables, will accumulate a large amount of noise
proportional to the material content.
When that signal is later turned up to line level by a phono pre-amp,
the result will be very noisy. Note the relevance of this to the SL 1
Thru outputs, if you use normal vinyl in your Scratch LIVE setup. Try
to use the shortest cables you can from the SL 1 to the mixer phono
inputs.
Putting Theory Into Practice
The ideal is to limit the points
in a system at which gain or attenuation is applied to two; One gain at
the input, and one attenuation before the power amplification. This
minimizes the possibility of poor signal to noise ratio in your system.
Metering
Meters tell you the level of your audio, usually the color red indicates clipping, or that you're approaching clipping.
In Scratch LIVE there are two meters, one for each deck. There are also
three gain controls, one for each deck individually as well as one
master gain affecting both sets of outputs equally. They indicate the
level of each output just before the signal is sent to your hardware.
If the meters are sitting just below the red, your signal is as loud as
it can be without clipping. This is generally what you will want.
Track Gain Tags
Each
audio file in your library can have a gain value associated with it. If
you have files which are too quiet, or too loud, you can correct this
using the track gain knob in Scratch LIVE, and that setting will be
saved into the gain tag.
In Scratch LIVE 1.7 we introduced an Auto Gain feature, which does this step automatically. Check it out in the manual.
SSL Master Gain
If your tracks peak just under maximum level, the master gain in Scratch LIVE should remain at 12'oclock.
Do
not be tempted to crank the master gain "into the red"
to get some more level into your mixer; all you will be doing is
clipping your audio, distorting the music before it even exits your
computer.
It should be noted that the track and master gains inside Scratch LIVE make up a single
overallSSL gain (or attenuation) value for each deck's output. So in practice
you should set your SSL master gain so most of the tracks in your
library are just below clipping on the SSL meters, and then adjust
individual tracks as needed to bring them to the right level.
Mixer Input Gains
In the signal flow diagram, you can see some
other sources coming into the mixer, normal vinyl from a turntable, and
CDs. Your input gains on the mixer are a good place to even up
different sources.
Use the input gains in conjunction with the meters on your mixer to
make sure the loudest track you will play doesn't clip the mixer at all.
As usual, get it into the yellow or orange, not the red.
Mixer Channel Faders
With correctly set input gains and master
output on your mixer, the channel faders should purely be a tool to add
dynamics to your performance, and aid in smoothly transitioning between
tracks. You should be able to have your track faders all the way up
without distorting or going into the red on your mixer's output meters.
Mixer Master Output Gain
Even though it's being mentioned last,
you should try and set the mixer master output gain first thing when
you're setting up, or during sound check.
Play the loudest track in your playlist, set the input gain into the
mixer so it's not clipping internally, then set the master output gain
to the loudest level you're going to hit during you're set - or to the
specifications of the sound technician (if you have that luxury)
Final thoughts
Well, that's it for this introduction to gain
structure. Once you have the basic ideas in mind, you should find the
practical implications of gain structuring becoming more apparent as
you continue DJing.
There are many good
books on audio theory available, and a ton of information available online, including the fabulous
Rane Professional Audio Reference, and of course, other DJs right here on the
Scratch LIVE forum :-)
There are various ways to bridge the gap between vinyl records and computers, as we saw last week. You can 