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Pro Tools | Carbon - First Look

What Is A 'Hybrid' Audio Interface? By Sam Inglis
Published December 2020

Pro Tools | Carbon might be the biggest news from the world of music technology this year. But how does Avid's "hybrid audio interface" work?

Sound On Sound received the first review unit to leave Avid HQ, and Editor In Chief Sam Inglis has been putting it through its paces. Read Sam's in-depth, hands-on test report here:

In this video, he talks us through Avid's radical solution to the problem of latency.

For as long as I can remember, people have been telling me that hardware DSP is on the way out. “Computers are so fast now, they don’t need help to run plug-ins. DSP is just a giant dongle. Cast off your DSP shackles and join me in the glorious future where everything runs natively!” they said. In 1998. And 1999. And every year since!

But like Rasputin and black mould in the bathroom, DSP refuses to die. Even though most of us don’t need it to run plug-ins at mixdown any more you can find DSP, or something that does the same job, in almost every multi-channel audio interface currently on the market. The reason it’s there? Latency.

What is latency? It’s a delay between sound going into a computer from a microphone or instrument, and coming out of our monitors or headphones. It’s a fact of life with digital recording, and it’s a pain in the arse. If the delay is longer than a few milliseconds musicians start to hear it and it affects their performance. So what do we do about it?

Well, the anti-DSP brigade will say this is a solved problem. Buy a fast computer and a Thunderbolt interface, run it at a very low buffer size, and you can monitor through your recording software with piflingly low latency. And that’s true… up to a point. When you’re tracking into an empty project, no problem. But you’re not always doing that.

What happens when you’re putting the finishing touches to your hundred-track mix, and the singer suddenly decides she wants to recut her vocal? Maybe then you can’t reduce the buffer size without running out of CPU resources. Or maybe all those fancy native plug-ins you’re using all need long lookahead times to do their stuff, and you’re fighting 100 milliseconds of delay compensation.

That’s when it’s DSP to the rescue. Because the DSP in our audio interface creates a direct monitoring path that bypasses the recording software and cuts out latency. Come back, digital signal processing, all is forgiven.

But there’s a price to pay for bypassing latency. Suddenly we’re not just dealing with one piece of software. We’ve also got to use another mixer to set up our monitor balance and we’re constantly tabbing between two windows and muting and unmuting things all the time. Computers were supposed to make our lives easier, not make everything really confusing.

But what if our recording software could control the DSP directly, so that when we record-arm a track it automatically gets switched to a DSP monitor path without us having to do anything? Now there’s an idea. It’s an idea that Avid are calling a hybrid system and it’s the basis of their new Pro Tools vertical line Carbon.

It’s not a new idea. Avid did it before in their 003 and HD Native, and there are quite a few other implementations around too. But usually they’re a compromise. Yes you get low latency monitoring without having to use two mixer windows, but you can’t monitor through plug-ins, and you have to work within quite a limited mixer configuration. It’s still not like working on an inline console where you can set up a really fancy rough mix during tracking, and hear the same thing on playback.

So what’s different about Pro Tools Carbon? Well, in a nutshell, it gives us DSP monitoring within a native DAW, without the compromises. The DAW is Pro Tools, obviously, and the key to this new-found power is this button here (mouse circles button - see video). When I engage input monitoring or record arm a track, you can see that it turns green. That means this track is now running in DSP mode, and I’m hearing the input routed directly through the interface at very low latency.

That’s something you can already do with other systems like ASIO Direct Monitoring from Steinberg, but look what happens when I record-enable my mandolin track. Not only does the track I’m recording to get put into DSP mode, but also the two tracks next to it. Why’s that? Because this track isn’t routed directly to the output. It’s routed to an auxiliary input track via a bus. It’s also sent to another aux which is giving me exactly the right blend of FX to sculpt my unique mandolin tone. And when I record enable the mandolin track, Pro Tools knows to put those tracks into DSP mode too.

You’ll also notice that I don’t lose my signature mandolin tone when this happens. That’s because all these plug-ins exist in two formats. There are AAX Native versions that run within the native Pro Tools mixer. And there are AAX DSP versions that can be hosted on the DSP chips in Carbon. When I switch tracks into DSP mode, Pro Tools automatically swaps the native ones out for DSP ones, and my mandolin tonequest is unaffected. When I’ve finished recording, I can just put those tracks back into native mode and free up the DSP resources for the next part, if I need to.

So the cool thing, and the unique thing about Carbon is that I don’t need to change the way I work in order to take advantage of low latency DSP monitoring. I don’t need to keep tabbing back and forth between windows. I don’t need to imagine what my mandolin will eventually sound like when I can put native plug-ins on it. I don’t need to worry about where my tracks are routed or whether my session has delay compensation going on. Everything that I put into record enable goes seamlessly into low-latency mode and I can just get on with creating my seminal mandolin concept album. Even if I’ve got a ton of fancy routing going on and I’m creating four different cue mixes with elaborate effects for fussy musicians, they can still hear everything at sub 1ms latency.

How is it different from Pro Tools HDX? Well in HDX the entire mixer runs in DSP. You can use native plug-ins within the mixer but each time you do that you’re using up an input and output path to send the audio to the computer, and you’re adding some latency. In Carbon the mixer runs natively except for tracks that are in DSP mode, and on those tracks you can only use DSP plug-ins. So in HDX the DSP is a mixing platform, but in Carbon really it’s there for monitoring live inputs. You wouldn’t usually leave tracks in DSP mode when you’re not recording, and if you did you’d run out of DSP quite fast.

So that’s what Avid mean by a hybrid audio interface. How successful is it? Well to find out you’ll have to read the exclusive full review in the December 2020 issue of Sound On Sound. And now if you’ll excuse me I have a place in mandolin history to secure. Thanks for watching!