PRO TOOLS INTERNAL MIXING MYTHS

(Much of this document was simply copied and pasted from Digidesign's White Paper on this topic found HERE. It was so relevant, that we decided to include it here.)

The level of the dither added to summed signals inside the Pro Tools 48-bit mixer is around -144 dB, which is below the noise floor of the converters. Consider that our ears can deal with a dynamic range of around 120 dB (from the threshold of audibility to the threshold of pain). This gives you an idea of how low -144 dB is in terms of audibility; you’d have to have your monitor system cranked up to extremely high levels to even hear any sound that might exist in a noise floor this low.

The benefit is that not only is a tremendous amount of headroom for summing signals provided, but even more accommodation is made for preserving the low level details in the audio for when channel faders are set far below unity. The internal Pro Tools mixer accomplishes summing of an enormous number of tracks with absolutely no distortion. Why is this so important? Because there is a lot of debate about the summing accuracy of the Pro Tools mixer. This topic is discussed at great length in several audio forums and there are almost as many opinions as there are posters. Unfortunately, there are also many misconceptions surrounding the issue, we wanted to show exactly how the Pro Tools mixer works to help.

At the summing stage, Master Faders perform in a similar way to channel faders in terms of resolution. The final gain stage is in effect a master fader, which is always present; and, when the Pro Tools UI displays a Master Fader, the user is given a “handle” to adjust the range of the final gain stage output. As a built-in part of the mixer, Master Faders don’tconsume any additional DSP resources, so they are the best way to adjust the final output of an internal bus or external pair without losing precision.

DOES LOWERING THE MASTER FADER COMPROMISE RESOLUTION?

This brings up another important point: while you can sum hundreds of “real world” tracks in the Pro Tools mixer without clipping the “input side” of the mix bus, you can easily clip the “output” side of the bus (when you go to a 24-bit DAC or 24-bit digital output, or submix signals to an Aux Input). That’s why it’s so important and useful to utilize Master Faders—they utilize no DSP, provide metering that lets you know if you’re clipping on the way to your destination, and allow you to trim the output level of your sub- or final-mix without losing any resolution (because resolution is preserved by the 48-bit mixer). Because Master Faders preserve resolution when trimming, you don’t have to trim down all of the contributing faders to the mix to obtain the optimal output level. This is just like a well-designed analog console—you trim down the master fader to avoid clipping the output circuitry in the console while the mix bus headroom allows the input faders to stay in their sweet spot.

CAN DYNAMIC RANGE BE ENHANCED BY MIXING TO STEMS?

There’s also the notion that you can increase dynamic range by mixing to stems and using an external summing mixer. Consider the analog audio is leaving the Pro Tools mixing environment via a 192 I/O’s digital to analog converters, which deliver about 116 dB of dynamic range. From there, you are subject to the performance of the downstream analog devices, which may affect the sound in a subjectively pleasing way by imparting their own non-linear characteristics, but there isn’t a way to increase dynamic range—it just isn’t physically possible.

DELAY COMPENSATION AND OUTPUT MONITORING

Phase problems have also been an issue in the past, so it should also be mentioned that since Pro Tools version 6.4, the entire mixer uses Automatic Delay Compensation which ensures that signals remain phase coherent, no matter what routing techniques and plug-ins are used. In versions of Pro Tools before 6.4, double bussing and using different plug-ins and analog inserts could create phasing issues that, if not manually compensated for, would wreak havoc in the mix. By enabling Automatic Delay Compensation, the entire mixer dynamically adjusts itself to accommodate channel to channel differences caused by processing and inserts.

Even though lowering faders to very low levels is possible without losing bits, the system works best with analog volume control before connecting to your monitoring system. Avoid connecting the outputs of the 192 I/O interfaces directly to the monitors and pulling down channel faders or using a Master Fader like a volume control to achieve normal listening levels as this causes the converters to operate at very low levels where their non-linearity is most pronounced, producing mixes with extremely low levels (see the info about optimizing DAC levels above).

The main point to remember is that not only can you mix with confidence completely within the Pro Tools environment, or ‘in the box,’ but you can produce absolutely stunning mixes that are not possible in any other way. We’re not trying to persuade you to work one way or the other—your workflow may include a ‘hybrid’ mixing environment, and that’s great. Just be sure you are comparing apples to apples, attribute what you hear to the correct components, and know that the Pro Tools mixer is in no way degrading the sound you work so hard to get.

One forum you should check out is www.3daudioinc.com. Lynn Fuston created a set of CDs called the “Awesome DawSum”. Even though it was done in 2003, it’s still very appropriate and ear opening. Lynn and a team of volunteers carefully made comparable mixes of a tune on 29 different platforms, including analog mixers, digital mixers, and DAWs. Listen for yourself and compare the tracks, then look at the key to reveal what was what; the results are fascinating and not at all what you’d expect!