Image via realhd-audio.com
A couple of weeks ago, we revisited the great debate on analog tape as a medium; this week we set another rampant misconception of the audio world in our sights: the analog vs. digital headroom debate. Mythology, or perhaps misunderstanding, are two other words I'd use for this topic. Unfortunately, the pro-audio world is steeped in these nuggets of folklore, which is maybe what causes such great debate among professionals and amateurs alike.
The audio realm stands at an interesting crossroads, with one foot rooted in art and the other in science. One side subjective, the other objective. This duality can muddy the waters quickly, as quantifiable, measurable data often must yield to the simple fact that if it sounds good, it is good.
On our virtual chopping block today? The concept of headroom and what it means to you.
Obligatory disclaimer/sidebar for the zealots: As with the tape debate, analog consoles certainly do "a thing." I'm not saying they don't. The non-linearities of the circuitry can add a certain "mojo" to a sound in some situations and are not necessarily obsolete. There's certainly a nostalgia about walking in a studio and seeing that massive desk in the center of the control room, meters and lights aglow. There's a speed to the workflow on a console that makes for quick results (why do you think most DAWs are setup like an analog console?). I love reaching out and grabbing a knob and a fader when I can. The fact remains however, they’re no longer a necessity. For the monastic-like reverence of the cloistered few, I say, "Smoke 'em if you got 'em," but before you assume you absolutely need to have an analog console to get a certain sound, or that an analog console is hands-down superior to mixing in the box, I invite you to consider the following.
What is headroom?
This is a term that gets thrown out all the time, and I honestly find that a shocking number of my students, clients, and even fellow engineers/mixers/musicians don't quite understand the concept.
At its simplest, headroom is the amount of audio range between the point of nominal signal-to-noise ratio in an audio path, and the point at which an audio system can no longer handle the signal (clipping/distortion). In analog gear, the application is such that this unseen buffer allows momentary loud transients in source material through an audio system without distorting or effecting the signal. As we all know, much analog gear can be "run in the red," and when those transistors, transformers, and tubes overload, sometimes wonderful things happen. On the other hand, when digital gets run into the danger zone and you run out of word length to store data, the resulting biting sound is one that has only a small handful of applications.
In the digital realm, since your bit depth is capped, does this mean that your headroom must be capped and limited as well? In a sense, this is true – digital has no "headroom," per se. But as with most aspects of the audio world, you have to look at the headroom in analog and digital differently. It's not an apples-to-apples comparison, and too many people look at it as such.
A different kind of headroom
In the digital realm, when a signal clips or maxes out the number of bits that can be stored, digital distortion is the result. That being said, there are a number of ways to avoid this.
Let's take a look at the built-in Pro Tools digital mixer as our example here. (For the record, this is quite similar in every other DAW as well; Logic, Cubase, Studio One, even Reaper and all the others out there can get the job done, too, so no hate to the other DAWs out there. Pro Tools is just my personal DAW of choice and still the go-to DAW in the pro audio world.) When analog signal enters the digital audio world of Pro Tools, it usually passes through a converter capable of transcribing audio in to a 16- or preferably 24-bit format. (We'll leave out 32-bit floating for now, as it's not that common in practice yet, and most hardware is unable to harness the advantage of this bit depth). For those who may also be questioning, this has nothing to do with 32-bit vs. 64-bit applications, so don't let that confuse you.
Once you have entered the Pro Tools mixer, a very interesting thing happens. Once audio is being bussed around digitally, the plugins and bus architecture actually functions on a 32-bit floating point format (or 48-bit in the case of most HD systems). Now, this doesn’t mean your audio is being upgraded in resolution, or that it's losing quality from upsampling; it applies to the way these digital clips are being combined and processed. To perhaps oversimplify this, there's actually a sort of digital headroom built in for processing and bussing in the box.
If you're feeling lost at this point, let's take a step back and look at things another way. Bit depth can also be mathematically converted to dynamic range in dB. For example:
- 24 bit = 144 dB (this is roughly equivalent to the operating dynamic range of studio equipment in the analog realm)
- 32 bit = 192 dB
- 48 bit = 288 dB!
So what does that have to do with anything, and what does it all mean?
Bowling for strikes
In terms of mixing headroom, a console has definitive power rails, meaning there's a finite threshold to how hot you can run the signal. While hitting the ceiling in a console can be more sonically pleasing than hitting the ceiling in the box, digital offers a unique and often misunderstood feature. In Pro Tools, master faders can be applied in numerous ways to your internal signal flow (that's right, they're not just for your master bus). These master faders have no loss in resolution sonically when applied. With the proper application of master faders, your headroom in a digital mix environment is actually almost endless. The enemies, if they even really warrant being called that, are the 24-bit internal bottlenecks that people set up for themselves and have no idea how to negotiate that inevitably cause people to give up on mixing in the box. With the proper knowledge and planning, the ceiling should never be an issue in Pro Tools, because you as the user actually define what the ceiling is.
When it comes to distortion, working with digital in-the-box mixers is like bowling, and analog console signal flow is bumper-bowling. In the digital realm, if you cross that threshold, it's going to be a straight gutter ball; there's no wiggle room. In the analog realm, if you take things to the red, you've got a cushion to play with. In fact, sometimes if you're looking for a strike, it might be desirable to drive things as hard as you can into the bumper. In the digital realm, you can define your headroom window, and you can take yourself back from the brink of distortion easily and non-destructively. The stakes are higher because the distortion of digital is unwanted, and it's less forgiving, but you're given the tools you need to avoid it and have it work for you.
For those interested in further reading into the specifics of how a floating-point digital mixer works, here's a great white paper on the subject. It's a little older, from the DigiDesign days of Pro Tools, but the architecture and principles still apply.
So the next time you hear someone say they don't like mixing in the box because digital doesn't have the same headroom as analog, they're not wrong, but what they're really saying is they don't understand their tools. We always tend to fear what we don't understand, and of course, there's also that whole thing about a good carpenter not blaming his or her tools. If you like the "sound" of a console, that's one thing; there are also other ways to go about that, but that's a completely different discussion for another time. If you're pointing the finger at headroom though, unfortunately, your headroom woes are not digital's fault.
For more tips on getting the best sound every time, check out more from our resident "Angry Sound Guy."
Aaron Staniulis is not only a freelance live sound and recording engineer, but also an accomplished musician, singer, and songwriter. He has spent equal time on both sides of the microphone working for and playing alongside everyone from local bar cover bands to major label recording artists, in venues stretching from tens to tens of thousands of people. Having seen both sides at all levels gives him the perfect perspective for shedding light on the "Angry Sound Guy." You can find out more about what he’s up to at aaronstaniulis.com.
