21st August 2007
Lives for gear
Joined: Mar 2006
Originally Posted by Lagerfeldt
This is an often misunderstood subject with much voodoo talk.
I'll try to write as clear as possible.
First the explanation: your master bus is overloading (clipping) because it is the sum of all your channels. Let' say, if your master bus can only contain a digit as large as 10, and all of your 16 faders each containing a value of 1, your master bus will be overloading with a value of 6. Get it?
As for how to avoid this situation, or if avoiding it is necessary at all:
The quick version: to be on the safe side, do not clip your master bus (or your channels if you require fidelity in your sound). This is done by lowering all your channel faders until no clipping occurs in your master bus.
The longer version:
In a floating point sequencer (this is how it calculates data) there is no - and I repeat - no difference in sound quality between A. raising your faders (clipping the master bus) and then lowering the master fader until it stops clipping, and B. lowering your faders and raising your master fader (or leaving it at unity). This is easily proven with a null test (a phase flip test where identical files will null out when played simultaneously).
But the story doesn't end there. Because that's rarely how we mix in a computer. We like output to busses and sum compress (e.g. drums) or in general add plug-ins to the bus and/or master bus. And then you have a problem if you're coming in too hot, even though the fader doesn't show any clipping going on.
That's because now you're overloading the input side of the plug-in, and that's another story. You can test this by inserting a limiter plug-in such as the Waves L1/L2 on a bus or master. Make sure the master is actually overloading but you've lowered the fader so it doesn't show. Notice how the limiter is already active without you having adjusted the threshold.
Finally there's no reason to aim for such hot mixes in 24 bits. A peak of around -6 to -3 dBFS is more than adequate. Most mastering engineers prefer a mix that doesn't peak any higher than that.
Very thorough and complete explanation Lagerfeldt.