Pro Tools HD Accel Card

HD Accel is the latest processor card for Pro Tools HD TDM systems. We investigate, and try to clear up some of the confusion surrounding the release.

As you'll probably have seen in SOS's News pages, Digidesign have released their latest DSP card, with the traditional promise of doubled mixing/plug-in capacity per card. With the DSP power doubling 18 months since the appearance of the previous card, I'm supposed to mention Moore's Law at this point — but I won't. Rather than a being completely new system, the HD Accel PCI cards are designed to slot into Digidesign's top-of-the-range Pro Tools HD system. The 'master' HD Core card is unchanged, so the basic single-card HD system is the same as it was before. However, on new systems, additional cards will now be Accels instead of the original HD Process cards.

In addition to the extra plug-in power, HD rigs with these new cards will benefit from 50 percent higher voice counts, meaning a new maximum of 192 audio tracks at 44.1 or 48 kHz. If you've already got an HD system but have no spare slots (or you just want the maximum possible DSP) there's a reduced-cost exchange program for swapping out original Process boards. As usual there's an extra concession if you bought into HD recently (since July 20th).

So that's the basics, but existing HD owners seem a little confused and uncertain about what to make of Accel. Let's look a little deeper at the new technology, and the practical matters such as plug-ins, compatibility, and whether to exchange Process cards.

Pick & Mix

First the technical bit (bear with me, because this should make what follows clearer). Accel cards sport seven audio processors that run at a little more than twice the speed of the DSPs on HD Core and Process cards. Four of the new chips have access to a generous amount of on-card RAM. Alongside the seven faster chips, the cards have a further two processors identical to those found on the original HD cards. These two older chips are intended to share the work of audio recording and playback, and also provide compatibility with plug-ins that have not been updated to run on the new chips. Here's where a whole bunch of confusion has arisen. Due to an unexpected technical 'banana skin' it turned out that existing HD plug-ins didn't work on these two 'compatibility' chips! A simple fix is required for plug-ins to work (the code just has to be recompiled based on the new Digi Software Developers' Kit). Ideally, existing plug-ins will be updated so that they work on the new chips (and are optimised for extra instances) as well as the old ones. In fact, all Digi's own plug-ins — and those co-developed by them — are already at this stage.

Digidesign's Impact mix-buss compressor is the first plug-in to take advantage of Accel's faster DSP chips.What's confused people is that most independent developers will probably do a two-stage update: a quick one that supports the older chips, and then a complete optimised reworking to support all chips. So the thing to be aware of is that a plug-in that has been updated for Accel may actually just be compatible with the two older chips. Those thinking about replacing Process cards (rather than simply installing additional Accel cards) might actually see a drop in DSP resources until their plug-ins are fully updated. This should be taken into account if you have a non-updated plug-in that you tend to use many times in each mix. Digidesign's on-line third-party software catalogue holds up-to-date info on individual plug-in support, and you should refer to this to help make the decision whether to defer a hardware upgrade.

Pro Tools veterans may start to hear another alarm bell following some of what they've just read, and several concerns have indeed been voiced, most publicly on Digi's User Conference message board. People who've experienced Mix TDM systems will remember possible problems associated with the combination of different DSP processors, and amounts of processing memory. In certain circumstances, when you had a packed mixer with some plug-ins that could run on any type of chip alongside other plug-ins that needed a specific chip, it was possible that Pro Tools would allocate its DSPs poorly when reopening the session. The result could be that your session was unable to reload all the plug-ins you were originally using. HD Core and original Process cards have just one DSP chip type, configured the same way across the board, so this can't happen. With Accel, the worry is that this type of 'DSP shuffling' scenario might return. Digidesign have responded to concern about this issue by saying that Pro Tools's built-in DSP manager is now much smarter and can foresee problems and prevent them from occurring.

The DSP Arms Race

For years the challenge for manufacturers of DSP cards has been to keep their systems significantly ahead of contemporary host-based systems. We're at the point now where a huge amount can be achieved with an unaided off-the-shelf computer, and Digi are clearly aware of this given their broad expansion into the host market. Accel-boosted systems are certainly going to enable larger mixes and more plug-ins, meaning that high-end Pro Tools-based music and film mixing rooms are going to be very tempted. The real story here, though, seems to be a shift in emphasis. A fully stocked HD — or even Mix — system can already handle a large majority of recording and mixing jobs, so with Accel it looks like Digi will likely be shooting for quality as much quantity.

Expect to see new plug-ins emerging that can do things previously only viable in expensive outboard gear, and doing them at very high sample rates and in surround. With the right third-party development, the Accel era could see the kings of the pro mixing environment, such as TC and Lexicon units, start coming under the same pressure as hardware samplers and synths.