This is an excerpt from my column, “The Pro Tools Corner” at audioMIDI.com, it is pretty old (2008) so I have since updated it to reflect newer version of Pro Tools hardware and software.

Updated 2012: Pro Tools 10 introduced HDX which uses a 64bit floating point mix bus and 32bit insert chains on it’s DSP based AAX plug-ins, effectively making it sonically identical to the Pro Tools Native system mixbus, RTAS and AAX native plug-ins. Older Pro Tools HD TDM (accell and process) cards will continue to use the 48 bit fixed mixer and 24-bit insert points until they are not supported in Pro Tools 11.

Update 2: As of Pro Tools 9 it was announced that Pro Tools Native uses a 64 bit floating point mix bus for internal summing and 32 bit float insert points, this was only revealed when the Pro Tools HD native card was released. Prior to this, it was stated that Pro Tools LE had a 32 bit floating point summing mixer, but it has come to my attention that it has been 64 bits for quite some time now (even before Pro Tools 8), although this was never revealed most likely due to marketing concerns over a perceived difference between the 48-bit TDM mixer found in the more expensive legacy HD systems.

Master Faders Demystified: Part 1

Often confused with a generic “volume control,” the master fader is one of Pro Tools most confusing, yet supremely critical features. Is it a master volume? Does it add an additional gain stage like an auxiliary track? Why don’t they have “inputs”? By bridging the extended headroom capabilities of the mixer with the 24 bit reality of your D/A converters, the master fader serves as a final output trim, allowing the mixer to optimize the signal coming off the high resolution internal mix-bus as it exits the system into the interface’s converters and ultimately the analog domain. In this two part series, we will take a look at the mysterious master fader and I will attempt to demystify its usage in Pro Tools LE/M-powered as well as Pro Tools HD.

Understanding mixer headroom

Before we can get into the nuts and bolts of master faders, it is important to review the concept of “headroom” and how it relates to the Pro Tools mixer. Generally, the term headroom is defined as “the dynamic range between the normal operating level and the maximum output level or clip point.” In an analog system, this concept is a bit grey in practice, as a good analog mixing console has a boat-load of headroom and can even sound subjectively better when pushed passed the “clip” point. In a digital system however, things are a bit more black and white. Measured in dBFS, a digital system’s clip point is hard at 0 dBFS. Meaning any signal that exceeds a value of 0dBFS is gone. Not subjectively saturated, or smoothly compressed like the heralded non-linearities analog tape or tube gear, but hard clipped in that nasty, “digital distortion” kind of way.

So how exactly does this relate to the digital recoding process and the pro tools mixer? Well, when we record audio into Pro Tools, we generally do so at a resolution of 24 bits. Meaning that each sample of audio we capture uses a 24 bit word, or 24 1s and 0s to describe the signal’s amplitude at that point in time. Likewise, in a 24 bit digital system, we can’t measure amplitude louder than what is represented by a full code sample (all 1s). We hear the ramifications of this all the time when we record too hot into the system; the input signal’s amplitude exceeds the maximum amplitude that can be described by the 24bit system and the loudest part of the waveform is clipped off, or thrown away at the converter, yielding that nasty digital distortion. But clipping doesn’t only occur at the recording stage, it can also occur when lots of very loud signals are summed together inside a digital mix bus.

Imagine a 4 bit recording system, if you were to take two words at full code and combine them together: “1111” + “1111”, you would need a 5th headroom bit to describe the resulting output of “11110.” Or think of it this way, if you had 2 one-gallon buckets and they were each nearly full of water, it would make sense that you would need a larger bucket to combine the two smaller buckets into one, without spilling any water. The same is true for a mix bus. If we want to add many “hot,” or near full-code 24 bit signals together, we would need a mixer with a higher precision (say 32 bits, 48 or 64 bits), or more “headroom,” for them to sum nicely without having to turn each down individually as it enters the mix bus. This is exactly what Pro Tools provides for us. In Pro Tools LE/M-powered and Pro Tools 9/10 Native, the mixer sums signals together using 64-bit floating point math, using 56 fixed bits (called the mantissa) with 8 additional exponent bits (think scientific notation) to scale and maintain precision as signals are added across it. Without getting into the complexities of floating point arithmetic, it is safe to say that the Pro Tools Native mixer has a ton of head room, so much that you are not likely to clip the internal mix bus, even if you had 256 24 bit tracks of full code audio, with each fader at +12dB running through it. The Pro Tools HD TDM mixer is a different story, we’ll get to that later.

Enter the master fader

At this point you must being thinking, “that’s BS, I have totally clipped the pro tools mixer. I see clip lights all the time and I hear distortion at my output.” Technically speaking, what you have clipped is your converter, which does have a maximum output of 24 bits (clipping at 0 dBFS), and is therefore fairly easy to clip with even a few hot signals in the mix. You can also clip signals during recording, as your ADC can only “see” or measure a maximum of 24 bits of signal on input, and occasionally you can clip plug-ins (as they have their own internal calculations going on). This is exactly where master faders come in. Master faders allow you to meter the summation of all the tracks in a mix as well as trim the output before the signal is truncated and exits the system to the converters at 24 bits again. Think of it as a “bit selector” of sorts. If you had a system with an internal resolution of 48 bits and the output could only “see” 24 of those, the master fader effectively puts a handle on which output bits you choose to leave the system with.  If you didn’t have that handle, you would have to individually reduce the level of each track as it enters the mixer to avoid clipping the converter. On top of this additional headroom, the mixer also allows for “footroom” bits to preserve the resolution of lower level signals that were attenuated significantly by the mixers level control. In other words, you need not be concerned about “eating into” a signals bit depth by reducing the volume of an individual track in the pro tools mixer.

How to use the master fader as an output control

In Pro Tools Native, LE or M-Powered, implementing the master fader is a fairly painless procedure. Simply create a new stereo master fader (Tracks > New) and use it to monitor the overall output of your mix. It should automatically set itself to control your main outputs (generally A1-2) and If the clip indicator lights up, it’s time to pull down the master fader until you are leaving the system with a nice, un-clipped output. Likewise, if the summation of many tracks seems to weak, you can kick up the master fader to optimize the output. Contrary to many rumors, the master fader doesn’t add any sonic color to your mix, nor does leaving it at unity gain or not using one in your session benefit you in any way.

At no time should you use the master fader as a “volume control” for your monitors, that is what your Mbox 2’s, 003’s, (or whatever PT interface you use) monitor or headphone level control is for. Your speakers/headphones are analog devices and their input level should be controlled in the analog domain, not the digital one.

Exercise: Sine Wave Test

To demonstrate the master fader’s purpose, try the following:

  1. Turn your monitors all the way down and please take off the headphones!
  2. Create a new mono aux track and insert the “signal generator” plug-in (found under “other”)
  3. Duplicate the new signal generator track 50 times (Track > Duplicate)
  4. Enable the “ALL” group and turn one of the aux track’s level controls up to 12dB (all should rise to 12dB)
  5. Carefully turn up your monitors just a bit, listen to the extremely distorted sine signal.
  6. Create a new stereo master fader set to your main outputs (Track > New)Trim down the master fader until it stops clipping, you may have to clear clip indicators Opt + C (Mac) Alt + C (PC).

What is happening? The combined input of all those sine waves into the mix bus is clipping the output at the converter but not the internal mix bus, the master fader allows us to recover and exit the system with a clean tone. Even if we were to submix some of the tracks into a bus and back into a new aux track, the signal master fader at output has us covered in the native world of (Pro Tools HD TDM is a totally different set up, stayed tuned).

So why do individual track clip lights matter? When recording into the system these help us avoid clipping the converter’s input and are critical in determining the input trim of your preamps. Once a signal has already been captured, during mixing, they really don’t mean anything, as long as you manage your final output (again this is different in the 48-bit HD TDM mixer). However, they do matter If you are mixing to a summing mixer, especially if you are setting each track’s output in the mixer to a discrete interface output. Remember, you can clip the converter at input and output, but it is highly unlikely that you will clip the internal mix bus in Pro Tools.

Coming up next…

In Pro Tools HD TDM (Not Pro Tools HD Native, that uses the 64 bit floating point mixer) the story is a bit different, because the mixer runs at a fixed 48-bit precision and is truncated back to 24 bits at each insert or input point in the mixer, headroom can be a little more challenging to manage. Next time we will look at good practices for master faders in Pro Tools HD TDM and cover some tips for using Master Fader inserts with “mastering” style effects and dither in your mixes.

Read Part 2