Know how to use the Pro Tools master fader properly and you need never suffer clipping in your mixes again.
In my travels through the world of Pro Tools over the years, I have found that the humble master fader is a part of the mixer that is very often misunderstood and misused. It definitely shouldn't be used as a monitor volume control, and it also works differently in the HD and LE versions of Pro Tools. The master fader and mix bus behind it are no different, in many ways, from their counterparts on an analogue mixer, and moving to the digital world hasn't magically taken away the restriction of headroom or changed the way maths works when you add things together!
Let me clarify what I mean here by headroom. It is the maximum signal level any signal chain can handle before distortion takes place. In the analogue world, headroom was usually limited by the power-supply voltage rails, but in the digital world it is set by the word length used to encode the signal. Each sample is represented as a series of binary bits, each of which can either be set to one or zero. When you reach the point where all the bits are set to one, you have reached the limit of the system's headroom. It simply has no way of representing a signal that goes any higher, so when it comes to the digital‑to‑analogue conversion you will end up with the familiar clipped waveform that sounds very distorted. This point is often referred to as 0dBFS, where the 'FS' stands for Full Scale. But this headroom problem really starts to show up when we start adding signals together — otherwise known as mixing.
For ease of explanation, let's imagine we use a four‑bit recording system, so our 0dBFS point is reached when a sample has a full house of ones: 1111. That, on its own, is OK, but if we add another full‑scale signal from another input, we have nowhere to go, because 1111 plus 1111 is 11110: a five‑bit number, which cannot be represented in a four‑bit recording system.
The solution is to make our mixing engine employ a greater word length than is used to represent the recorded audio. Most DAWs can record 24‑bit audio, but to ensure sufficient headroom, their mixers typically work with 32‑bit, 48‑bit or even 64‑bit numbers.
LE Versus HD
Because Pro Tools LE systems use the computer host CPUs to undertake all the audio processing, the LE mixer employs 32‑bit floating‑point arithmetic. Without getting into all the maths behind this, it means that there is loads of internal headroom in the LE mixer. The HD mixer is a different ball game, as it has to use the TDM chips on the Pro Tools cards, and these chips don't support floating‑point arithmetic. So, to be safe, the Pro Tools designers use a 48‑bit mixer to provide the appropriate headroom. Even so, you can still run into problems, as we shall see. You are much less likely to run into headroom problems on an LE system, but even so, the tips I am about to outline are good for both LE and HD users to adopt, to help with headroom management throughout the Pro Tools mixer.
The Humble Master Fader
Take a look at the master fader channel in the first screen, below.
The first and second screenshots demonstrate that the clip light on the master fader (lit in the first screen above) indicates overload of the output stage, not the mix bus itself. The combined level of all the signals coming into the mix bus is the same in both cases, but bringing the master fader down (second screen, below) reduces clipping and eliminates audible distortion on the output. 
The first thing to point out is the clip light. This lights to indicate that the output signal is hot enough to clip the main output converters — but this is not the same as clipping the mix bus itself as I will shortly demonstrate. The master fader enables Pro Tools to take a 24‑bit window of the 32 or 48 bits on the mix bus and send it to the appropriate output, so there is no need to drop the signal levels feeding the mix bus from all the tracks. As we have discovered, the mix bus has loads of headroom: just drop the master fader and all will be well.
If you want to see for yourself, here's a experiment you can try at home on either an LE or HD rig.
1. Turn your monitors off and unplug any headphones, as what we about to do will be very loud!
2. Create a mono Aux track and insert the Signal Generator plug‑in, which you will find in the 'Other' group of plug‑ins. Set the Signal Generator to Sine Wave and the level at ‑20dB, which should be the default settings.
3. Duplicate this track 50 times using the Duplicate command in the Track menu.
4. Enable the ALL Mix Group. Push up one of the Aux track faders to the top (+12dB) and all the faders should follow.
5. Now, very carefully, turn up your monitors just a little bit. You will hear a very distorted sine‑wave tone.
6. Create a new stereo master fader using the New option from the Track menu. You will see that the level meters on the master fader are all the way up into the red.
7. Now bring down the Master Fader until it stops clipping and you will hear that the tone stops being distorted too. Don't forget to clear the clip lights by clicking them.
When we bring down the master fader in the last step, we are not changing the levels going into the mix bus, but we eventually reach a point where audible distortion and visible clipping no longer take place. This demonstrates that both were the result of the output stage being overloaded, not the mix bus. Across the whole Pro Tools mixer, the clip lights show you when you are overloading the input stage (tracks), inserts (plug‑ins) and output stage (master fader).
The Aux Bottleneck
This super mix-bus system is great, but what happens when we use an Aux track as a submix group? Unfortunately, it doesn't work in the same way. Things are much better in the LE systems because of the 32‑bit floating‑point arithmetic, but even so, it isn't infallible, so the tips I am about to share with you are well worth using to help you manage your mixer headroom efficiently.
Take a look at the screenshot.
Aux channel faders don't work in the same way as master faders. In this example, the combined level of all the signals going into Bus 1‑2 is hot enough to clip Aux 2's input (above) regardless of the fader setting on that channel. To avoid clipping, I need to add another master fader (below) that directly controls the level of Bus 1‑2 before it reaches the input to Aux 2.
Aux channel faders don't work in the same way as master faders. To avoid clipping, I need to add another master fader that directly controls the level of Bus 1‑2 before it reaches the input to Aux 2. I have routed 25 tracks through Bus 1‑2, and you can see that the subgroup track 'Aux 2' is clipping, even though the master fader track isn't. Every time you create a bus, it is a 48‑bit mixer bus, just like any bus in the Pro Tools mixer. However, to bring it back into the main mix, it has to be reduced to 24 bits, as all tracks are 24‑bit input devices. The result is that if you mix enough hot signals together via a subgroup, the Aux track acting as a subgroup master will clip. Don't panic, though. All is not lost. Did you know that you can actually put master faders into the mixer in other places apart from the main outputs?
Create another master fader, but this time change its routing from the main output to Bus 1‑2. Now bring down the level of this new master fader, and you'll see the clipping disappear, as in the screen above right. Notice that I haven't touched the subgroup fader, just the new Bus 1‑2 master fader. This works just as the master fader does on the main output: it provides an adjustable 24‑bit window on the Bus 1‑2 subgroup before the subgroup gets reintroduced into the mixer via the 24‑bit subgroup track 'Aux 2', so I am no longer overloading the input to 'Aux 2'.
Master Fader Inserts
There is a way in which the master fader is different from all the other track types in Pro Tools, and that is that the inserts are post‑fade. Normally, on audio and Aux tracks, you want the insert points to be pre‑fade, so that adjusting the fader doesn't change the level of the signal going into a plug‑in. However, with the master fader you need the inserts to be post‑fade, so that any brick‑wall limiters or dithering you might add are not affected by trimming the master fader.
Talking of dither, here's another tip for you. If you are going to bounce your mix onto another track (see my article on this in the July 2008 issue of Sound On Sound: /sos/jul08/articles/ptworkshop_0708.htm) and you plan to export the bounced file as a 16‑bit file ready for CD mastering, don't apply dither in your final plug‑in chain, as Pro Tools will automatically dither the exported file without asking. If you add a dither plug‑in or use the dither feature of your favourite mastering limiter, all you are doing is adding another unnecessary layer of low‑level noise (which is all that dither is). It's a shame that you have no control over what sort of dithering and noise‑shaping Digidesign are adding for you at the export stage. Digidesign (or should I say Avid now?), can you please make this dither an option in the 'Export Selected As Files' window?
I hope this has been helpful, and I do encourage you to have a go at the experiments for yourself, so you can see how you can make the Pro Tools mixer work the best for you. The take‑home messages is that yes, it is OK to just bring down the master fader a bit if you get a clip on its level meter, and you can fix clipping subgroups by adding an additional master fader track to the relevant subgroup bus.
