Pro Tools allows several different types of track/channel, most of which are fairly obvious in their purpose and operation: audio track, MIDI track, aux input, instrument trac; these all work more or less as you would expect if you have any experience of recording, DAWs and sequencing in general.
There is one type which is a bit different, however: the Master Fader. Many people assume that they automatically know exactly how this works, and what it is for, by direct analogy with the master fader found on a hardware mixer. This is not the whole story, however, and often results in some important misconceptions. Before we look at this in detail, let’s make sure we understand how the other track types work, at least in the audio domain.
Audio tracks contain audio (which may have got there by recording on to those tracks, or by importing existing an track). As such they have an input and an output corresponding to the number of audio channels on the track (mono/stereo/surround); both input and output may be either direct to hardware inputs/outputs or to an internal bus. The tracks signal path may include inserts (both software plug-ins and hardware inserts, both of which are always pre-fader) and sends (which split the signal off and send it somewhere other than the track’s main output; again this can be either a physical output or an internal bus, and can be either pre- or post-fader).
Aux inputs work in exactly the same way as audio tracks, except that they don’t contain any recorded audio, but rather process audio from an input source, which may be a physical input or an internal bus. Inserts and sends work exactly the same way as for an audio track, and the output can be to a physical output or to an internal bus.
In the simplest type of stereo ‘in the box’ session, all audio ultimately makes its way to the same pair of outputs. Some tracks may go straight there; others may be routed via aux inputs in order to be processed together; some tracks may include sends which go to aux inputs on which processing (for example reverb) is placed, which then proceed to the main outputs. Looking at this in reverse, both physical outputs and buses may (and usually do) combine input from several sources. Any number of these may be set to the same output, and any number may be set to the same input. There are no restrictions on this except for (most!) configurations resulting in a feedback loop, which Pro Tools automatically blocks.
It is perfectly possible to mix an audio-based session audio using just these two types of track. Master faders, however, are incredibly useful, and for some types of mixing they are essential. To use them properly we must understand some important differences with the other types of track.
Let’s consider the simplest use of a master fader by adding one to our hypothetical session consisting of both audio tracks and aux inputs (above). The master fader is added to the session and assigned to the main outputs. Note ‘assigned to’: this is an important distinction. Try assigning another master fader to the same outputs… Pro Tools won’t let you. This is a crucial difference. Rather than simply minding its own business and processing whatever audio it is fed (like an aux input), a master fader controls the output of an existing audio path, be it a physical output or an internal bus. It stands to reason that you can’t have two master faders fighting for control of the same path.
This master fader now controls the master output level. It also has insert points, to which hardware inserts and plug-ins can be added. However it is important to note that these are post fader. This means that any adjustments to the fader will affect the input level to these plug-ins, so for example if you put a bus compressor here over your mix, using the master fader to perform a fade will also immediately affect the sound of the mix, as it will soon stop crossing the compressor’s threshold. For this reason, it is generally advisable to place only metering plug-ins (rather than processing) here. To apply plug-ins to the mix, it is generally a better idea to route everything that would be going to your master outs to an aux input instead. Place your processing here, and then route this to your master outs.
Master faders really come into their own when used to control paths other than the main outputs. For example, you may be using “aux sends” to create headphone mixes. Each “aux send” can have its own master fader, so you can quickly adjust the over-all volume going to an individual headphone output, without having to adjust “aux sends” on many different tracks and without risking changing the balance.
You can also use a master fader to control the level of an internal bus. So if many tracks are being sent to the same reverb, you can tweak the level reaching the aux input hosting the reverb, again without affecting the relative balance of individual sends.
One of the most useful applications of the master fader: if your entire mix is sent to an aux input for over-all compression or EQ, there is a risk of clipping occurring in these plug-ins. In native systems, there is a further risk that you would never see this clipping on a meter, as the aux input’s metering, whether pre- or post-fader, is always post-plugins. If you assign a master fader to the bus, any clipping at this stage will be visible on its meter. But that’s not the best part: on a native or HDX system (which use floating point processing), simply bringing the master fader down will prevent the clipping from occurring in the first place, again without affecting the mix in any other way. (This will not work on older HD/TDM systems which use fixed-point processing, but on the other hand it is less important as insert points have clip indicators on HD systems anyway.)