Thursday, November 02, 2006

Dilemma - Mixing In The Box part 3

Welcome to November! It's raining like... well, it's raining like everyone thinks it does in Seattle. That's OK, I work in a nice warm room with tube gear and no windows.

To briefly recap - mixing within Pro Tools wasn't working for me, so I bussed out into an analog console and that sounded better. Why did it sound better? This is the subject of much debate and more than a few well written treatises by knowledgeable folk. Here, then, are my findings. (Which, when combined with $5, will get you a cuppa coffee at Starbuck's.)

One could assume that the Pro Tools hardware just gets overloaded and sounds "bad". We all know that that's not true, the HD192 I/O is a great sounding interface. One might think that the internal summing bus in Pro Tools can't handle the amount of information that needs to be processed when mixing >64 channels down into 2. But that's not true, since the Pro Tools stereo summing bus is a 48 bit bus and has the capacity to sum many more channels than 64 without screwing up the output. As Roger Nichols points out in Sound On Sound, at that point in the process digital summing is just math. From that standpoint, Pro Tools hardware does the math pretty efficiently via proprietary DSP cards. Certainly faster than many host-based systems. But even those do the math correctly, it just takes a bit longer.

So what's the deal, you might ask? Here's the deal, and it's not flattering, because it points to basic engineering skills (or a lack of attention to same).

Part 1. In recording to Pro Tools or any DAW, we feel compelled to seek the highest possible input levels. Maybe it's a throwback to 16 bit recording days, when a -20dB input level yielded a 12 bit recording (Eeewwww!). In a 24 bit world, a -20 input still results in >20 bits. Whatever the reason, we record into our DAWs really hot. Hotter than we ever did to analog tape. This results (in my case) in >64 channels each tracked at the analog equivalent of +14dB VU. That's a lot of signal, even for an analog mixer! That in itself is not a bad thing, as long as you plan to manage that kind of level. The problem is...

Part 2. Metering on a DAW. Let's look at the metering on the Pro Tools stereo master fader. You immediately notice that it's post-insert. Meaning that you're only metering after that secret combo of Fairchild/1176/and L1 plug-ins that we all use. What's happening with gain? Who the heck knows! Why can't we meter a) at the input, b) at each gain stage, and c) post-insert? (DAW makers, please read this...) With >64 faders, all up near the top of their throw, the input to the stereo summing bus is pretty formidable. How do most of us fix it? Pull down the master fader to -30dB. What's wrong with that? (Remember your basic unity gain theory and how to structure gain in a console?) I hear distortion in Pro Tools because I'm slamming the bejeezus out of the summing bus. Duh. Same goes for the aux busses. 20 channels of high-transient drums recorded at 99% modulation means a ton of signal at the input of the aux master. Ergo: distortion, lack of headroom, lack of dynamic range, bad sounding mix.

How do we fix it? I wish it were easy, but it requires that you actually look at the meters on the DAW to check levels pre-insert. Don't know about you, but I'm kinda lazy and loath those extra keystrokes required to engage and disengage plug-ins. Also, there typically aren't enough insert slots in PT to insert 3 meters per channel, and you can't view them all even if you could. The obvious solution is to pull down the individual track faders when setting up your mix. Same as we used to do on an analog console. Help the summing bus do its work by leaving some headroom. Trust your ears to tell you if there's something wrong, but use your eyes to double check the meters. We can't calibrate if we don't know what the levels are! Simple stuff.

In the meantime, maybe we can lobby the DAW manufacturers to include more metering options, or low-latency metering plugs, or more insert slots on each channel, or the ability to look at more than one plug at a time, so we can really do our jobs properly. And make better music in the process.


QED. Thanks for reading!

MBM

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