Having watched the Avid Webinar on the new Avid Hybrid Engine coming to HDX Pro Tools users before the end of the month, James Richmond considers how the Hybrid Engine will change the way he works with his Pro Tools Ultimate HDX system.

The History

In the early days of Pro Tools, DSP acceleration was essential in computer audio systems. For example, Mike Thornton reminded me that his first Pro Tools HD system was using Nubus cards running on a PowerPC 7100 Mac computer with a 66MHz clock. That computer could run 16 tracks of audio with DSP powered plugins. You can learn more in our History of Pro Tools - 1994 to 2000 article.

Compare that now to my 16-core Mac Pro with processors running at 3.2GHz, the difference is huge. However as we have all found, as CPUs have become faster and more powerful and the amount of RAM available has increased, more and more of the audio processing can be handled on the computer’s own processors, rather than shipping it off to dedicated processors.

Tracks And Voices

• A voice is used when you stream audio from disk to the Pro Tools mixer

• A voice is required for every audio path - a mono path requires one voice, a stereo path requires 2 voices and so on up to a 7.1.2 path requiring 10 voices.

• An audio path could be playing back from disk, recording onto disk or routing audio out of the Pro Tools DSP mixer to go to a native plugin.

• Aux Inputs don’t require voices because paths within the Pro Tools mixer do not use up the voice count.

• The voice count is tied to the sample rate. At 44.1/48K you have a voice count per HDX card of 256. At 96kHz, that drops to 128 voices and at 192kHz that drops again to 64 voices.

The Problem

As an Avid HDX1 owner, it has sometimes been a frustrating experience juggling the 256 available voices. In my project studio/mix room I mostly record myself or one other person so I don’t really need the capability of hundreds of simultaneous inputs I would get from an HDX2 or HDX3 configuration and my 2019 Mac Pro has more than enough native power to do the job so I have largely limped through my productions using a minimal amount of DSP processing and doing as much as I can natively. The reality is with computers like my Mac Pro the raw power of the modern CPU far outstrips that of the dedicated audio DSPs.

In case you were wondering, a fully native version of Pro Tools would be unsuitable for me as I still need more than 32 simultaneous inputs and I do very much prefer the ultra-low latency of HDX to any of the other available alternatives.

The problem with HDX1 has always been down to voice count.

At a 48 kHz sample rate, my HDX1 card gives me 256 voices, which is halved at 96 kHz to 128 and halved again at 192 kHz to 64 voices.

This really isn’t enough when you get to a large mix, and especially if you’ve been careless and used a mixture of DSP and native plugins on channels, in the wrong way as this also ‘costs’ voices.

Remember the mixer for HDX is DSP driven so it is fairly easy to tap out your available DSP resources, which has seen me playing the oh-so-exciting game of plugin conversion hopscotch at the later stages of a mix and generally getting quite grumpy about it.

The Solution?

I’ve been reading about Avid’s Hybrid Engine, currently available for Pro Tools Carbon, with some degree of envy and have just watched today’s Avid webinar where they detailed the upcoming Hybrid Engine for HDX users.

A summary of the Hybrid Engine is essentially the Pro Tools mixer will now be done natively, running on the computer’s processors, rather than eat up DSP chips. Then, as you can on Pro Tools Carbon, you are able to select specific tracks to use DSP as you need to, for tasks such as tracking or overdubbing audio.

One click converts a track from native to DSP, with any channels downstream also switching to DSP mode automatically, with any native plugin without a DSP equivalent being bypassed.

That way you get ultra-low latency on the channels you need, shown by the tracks in the lower half of the diagram above running on the HDX DSP cards, and all the native performance your computer can provide for everything else, shown by the tracks on the top half of the diagram running natively on the host computer’s processors.

When Julian Rodgers took his first look at Pro Tools Carbon, and the Hybrid Engine that powers Carbon, he presented an excellent video, which explains exactly how the Hybrid Engine works…

With the HDX Hybrid Engine, which Avid say will be released by the end of June you will be able to have the following…

• 512 Aux Inputs

• 1024 MIDI Tracks

• 512 Instruments Tracks

• 512 Master Faders

• 512 Routing Folder Tracks

• 2000 Basic Folder Tracks

• 128 VCA Master Faders and 64 Video Tracks

• 2048 Mono Audio Tracks

And those 2048 voices will be available regardless of the number of HDX cards you have and regardless of the sample rate you choose to use.

What The Hybrid Engine Will Mean For Me

The Hybrid Engine for HDX will transform my workflow, both how it was and how I have wanted it to develop for some time now but couldn’t due to voice count limitations.

First of all, the voice count for an HDX1 user like myself will go from 256 voices at 48 kHz up to a frankly stonking 2048 voices. 

Terrific!

Not only that the maximum number of tracks is also going up to 2048.

Brilliant!

Whilst I am still working in stereo the main issue I’ve had is how voices get chewed up when using a mixture of DSP and native plugins on one track. Some of my workflow processes has meant this was a necessity. Soon it will be a thing of the past and I can’t wait. 

The Mac Pro tower has a 16 core processor with loads of RAM, so since upgrading, I’ve never really needed DSP on the mix bus, I used HDX because the low latency for tracking was so good and accept the voice count limitations of an HDX1 system. The only reason I’d need to go beyond a single HDX card will soon be if I wish to exceed more than 64 channels of IO.

In the webinar, Avid showed the difference in resource allocation between an HDX 3 system, firstly in what they are calling ‘classic mode’, which is what we have now and the same session using the Hybrid Engine…

Note that because the Pro Tools Mixer is now running on the host computer’s processors that the plugins in the session only take up the DSP on one of the 3 cards, in Hybrid Engine mode, whereas in classic mode the Pro Tools mixer is consuming the DSP on 2 of the cards.

Also in classic mode, the voice count is 405 mainly due to needing to route audio in and out of the DSP mixer to go to native plugins, whereas when running the Hybrid Engine with the mixer running natively, those ‘hops from DSP to Native no longer eat up so many voices, so the example session uses just 262 voices instead of 45 voices when in classic mode.

What you may not be aware of is that on an HDX system in classic mode, putting a native plugin before a DSP plugin does not use up additional voices, putting a native plugin after a DSP plugin does use additional voices. To try and offset this ‘cost’, Pro Tools HDX has an additional hidden 64 voices so that these hops don’t eat into the 256 voice limit as quickly. The other problem is those hops out of the DSP mixer cost latency too, set by the hardware buffer size.

All of this goes away by running the Pro Tools mixer on the host processors in hybrid mode, leaving the precious DSP resources to handle the paths that need to be at near-zero-latency.

Another massive change for me will be that it allows me to consider, for the first time, to be able to move to a HDX based surround sound format for mixing (look for a forthcoming article on this soon). Because of the way the voices in Pro Tools work it is extremely difficult to do large mixes in surround with HDX1, especially in Dolby Atmos. Voices get used up very quickly indeed and the only way around it was to buy more HDX cards and even then the voice count can be very quickly eaten up.

2048 Voices At No Additional Cost

As a Pro Tools Ultimate user with a current update plan, this update won’t cost me a penny and it is going to transform my workflow.

Right near the end of the webinar Avid did explain that there may be times when it might be beneficial to turn off the HDX Hybrid Engine.

• Running a complex tracking session where you want all the audio paths to be near zero latency as long as you can still work within the limitations of the DSP powered Pro Tools mixer.

• Running HDX on an older computer with less native DSP power, you might prefer to opt for the predictability of using the classic HDX engine within its limitations.

• Running a separate Pro Tools recorder with simple routing and no plugins

I hope to provide more information about it as soon as I have a chance to use it for myself.

Pro Tools Tech Talk - Introducing The Hybrid Engine For Pro Tools HDX

If you missed the Hybrid Engine Webinar that I watched live recently then here it is…

What About You?

Are you an HDX user? Are you looking forward to being able to use the Hybrid Engine? What are you going to do with all those tracks? Do tell in in the comments below…