I don't need to spell out the fact that countless recordists who work on digital systems are on the quest to get a warmer sound. Nor do I need to delve into the 10,000 tools, tricks, and techniques employing transformers, tapes, and tubes to help warm things up. The problem is abundantly clear, yet never quite as succinctly stated as when audio guru and Crane Song founder, Dave Hill, said that, "[Pro Tools] sometimes has the problem of being too pure." In an act of humble problem-solving, Avid openly embraced this obvious truth about digital recording and employed the guru himself to design their solution: the new Harmonically Enhanced Algorithm Technology, or HEAT.

The predecessor to HEAT is Dave's revered Crane Song Phoenix plug-in suite, which emulates the electronics and EQ curves of tape machines as well as the saturation characteristics of tape itself. Of course, Phoenix doesn't sound exactly like tape, but it behaves a lot like tape by smoothing over transients and adding a very musical layer of harmonic complexity. Phoenix is quite subtle, but it adds just enough harmonic content to really warm up digital mixes.

Only available in TDM format, Phoenix quickly became the secret weapon of many professional mixers working on Pro Tools HD systems. At one point, I even knew a few who would have admitted that Phoenix was really the main reason they were still working in Pro Tools. The more you talked to these Phoenix devotees, the more you realized they were putting instances of Phoenix on every individual track in order to get their mixes happening. But why was that? Why not just put one instance on the stereo bus? Well, consider how analog tape machines and consoles manage audio; each track has its own lane on the tape, its own output card on the tape machine, and its own channel on the console. With each track hitting these different stages, a vastly complex array of tiny harmonic distortions is introduced into the mix. By using many instances of Phoenix across many tracks, a digital mix will begin to behave more like an analog mix, because each track is able to pick up its own harmonic enhancement by interacting with Phoenix's algorithms. The result is mix that takes on a subtle but very complex harmonic richness that a single instance of the plug-in on the L/R bus just can't achieve.

Avid's R&D team took a good hard look at how these pros were using Phoenix across their mixes and decided to get Dave Hill himself to integrate the Phoenix-esque HEAT algorithm directly into Pro Tools HD as the first "global plug-in," meaning that HEAT can be applied across all tracks simultaneously from a single interface. Take note that this is not just one instance of the plug-in. When you turn HEAT on, you are inserting an individual instance of the plug-in on every audio track in your mix. (HEAT doesn't work on MIDI, Instrument, or Aux tracks.) This global integration seriously simplifies things.

When you open the HEAT panel to the right of the mix window, you'll find the simplest controls: drive and tone knobs; a bypass switch; and an orange light that glows dynamically with the music. At the top of each individual audio track, you'll find a bypass toggle and a pre toggle that moves the HEAT plug-in between its default position as last in the insert chain to first. There are also individual glowing lights for each track, so you can get a visual sense of how hard each track is hitting HEAT.

The drive and tone knobs each have five positions to either the left or the right. When you click the drive knob to the left, HEAT emulates analog tape's nonlinear distortion, adding odd harmonics (third, fifth, and beyond as you push it). Turning the drive control to the right has a more aggressive effect, adding another layer of even harmonics found in triode-based tube circuits. The tone knob does basically what you'd think it would do, emphasizing or de-emphasizing the high-end richness and detail. The tone and drive knobs are also highly interactive, so they don't quite act the same way on every mix - and that's where the fun really begins.

Because HEAT reacts non-linearly in relation to the frequency content and dynamic range of a sound, it will have a unique, and somewhat unpredictable, reaction to each track in a mix, much like an analog system. A bass might get fatter while a drum overhead mic might pick up some sizzle; a vocal might seem to come forward while a keyboard takes on a warmer sonic aura; a brash acoustic guitar might be rounded off while a snare drum starts to pop more. You just have to play around and see what happens. With that said, it's important to keep in mind that, like Phoenix, HEAT is a pretty subtle thing. It's not like you put it on, and suddenly your mix becomes an unpredictable mess. In fact, HEAT is very capable of unifying a mix ("sonic glue", it's often called), but there's just no way of knowing exactly how that's going to work out until you try it.

You also have to watch how hard you hit HEAT because its effect is level-dependent (much like an analog system's). HEAT will "crunch out" pretty quickly if you hit it too hot and drive it too hard. I got my best results leaving a good bit of headroom, as it seemed to my ears that HEAT was more interesting when signals were dynamically changing their tone as they danced in and out of HEAT, as opposed to just being more statically enhanced. Of course, my way of using HEAT shouldn't be taken as a norm of any kind; your approach might differ based on genre, taste, philosophy, and technique. Still, I do recommend that one figures out where HEAT starts to become most dynamic because you can then get inside the nuances of the sound where small changes make the biggest differences.

As advertised, when mixing in-the-box with HEAT, the "too pure" problem begins to fade away as a warmer and more harmonically-complex image emerges. With HEAT, Pro Tools actually has tone and vibe - or I should say tones and vibes, as the various drive and tone combinations really do pack in a number of different sounds, despite their simplicity. For example, if you're familiar with Neve and API consoles, you may have noticed that Neves are, on average, a bit softer in the middle compared to an API, which has a very punchy center image. With HEAT, it's possible to push the sound in either of those directions. Backing off on the tone a click or two and pushing the drive to the right, I found a Neve-ish sound in many cases. Bumping the tone up a click and turning the drive to the left, the center of the mix seemed to punch in an API-ish way. Again, it's not an exact science here, and each mix reacted differently. But within the realm of subtle imaging, you can learn to steer HEAT in different directions.

One aspect of the sonic image that improves with HEAT is the front-to-back imaging of the mix. For example, I was mixing a beautiful track with female voice floating in a drum-less world of upright bass, Rhodes, pedal steel, and acoustic guitars. I wanted the voice to hang right in that magical place in the foreground while still inviting the listener inward toward the music. I got close to my end goal summing through my API "Mini Legacy" console (which I hope to cover in a future Tape Op), and when it finally went to tape (1/2'' 2track Studer A80 RC MkII running ATR at 250 nWb/m at 30 ips - for those who care) the image picked up that great 3D quality that makes the time and effort of maintaining my Studer worth it. I then ran the same exact mix without the Studer in the chain but with HEAT applied. As I pushed the drive knob to the left for tape emulation, a very different kind of depth emerged in the mix. The HEAT mix was also quite deep, but with less space between the various elements. And this makes sense because HEAT was adding harmonic content to all the source tracks, rather than relying on the print path for a final layer of varnish.

There was also a pretty big level increase as I ran HEAT in this case, and this is a somewhat bothersome issue that has bugged other users as well. The volume jump is due to the fact that there is more harmonic content, which makes sense. But, it's hard not to like the louder mix when A/B'ing, so I found myself riding my monitor volume to compensate when making judgment calls. Also frustrating is that when you start to bypass HEAT on individual tracks, you will hear them drop in relative level, throwing your balances off. Lastly, whatever is downstream - digital bus, analog console, or otherwise - will be seeing more level when HEAT is applied. Eventually, I'd like to see an auto-trim on the output similar to that found on SoundToys Decapitator.

On another mix, I tried to work solely in-the-box by globally applying HEAT to drums, bass, piano, and sleigh bells. In this case, I had tracked with some pretty lush sounding ribbon mics through API preamps and various compressors, so I had fairly soft transients and a good deal of harmonic interest already. With HEAT applied, everything just got a bit too thick - a rare case of too much glue in a digital recording. Instead of using HEAT in this case, I ended up stemming out to my console and hitting it just gently enough to add the depth and punch without things getting too crunchy or gooey. This experience begs some questions about digital summing with HEAT versus analog summing, and I really can't make any strong conclusions at this point, other than to say that my impression is that HEAT can only add more sound to a mix while analog summing can sometimes add more space. Your mileage may vary.

Interestingly, though, this case of HEAT not working for me brings up one of its greatest strengths: its ability to fatten up the unfortunately anemic sound that plagues so many digital tracks these days. HEAT is the ideal solution for doing this because you only have to turn it on and find a setting that works. For example, I used HEAT to fatten up some wafer-thin 16bit files, and I have to say that this was a nearly miraculous moment for these poor starving tracks. I could hear them each in their Oliver Twist voice saying, "Could I have some more, please?" Turning the drive knob again to the left for tape emulation and the tone down a bit to round things off, the tracks sounded so much better so quickly that I was able to dive right in and really enjoy mixing the tune - and that can only make for a better mix.

If you're serious about mixing in the box, and you've spent the big bucks to get Pro Tools HD, then you should definitely plunk down the additional $500 for HEAT. It will add a whole new dimension of analog-esque mojo to your digital mixes, and the global plug-in format is the most intuitive, elegant, and efficient execution of analog simulation I've yet to see. Hats off to Avid and Dave Hill for taking a really huge step forward on behalf of digital mixing. ($495 street; www.avid.com)

Tape Op is a bi-monthly magazine devoted to the art of record making.

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