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04/29/2021
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Pro Tools Carbon Review

In little less than a decade the audio interface has mutated from basic I/O between a computer and audio system to the nerve-centre of most production setups. Back in the day your audio interface was primarily for moving audio in and out of an analogue console, with access to its microphone preamps and inserts for outboard processing, and monitoring from either the console or the two-track recorder – be that tape, DAT, VHS, or even cassette. In those early days, additional equipment took care of monitor switching, talkback, headphone sends, and perhaps additional metering. Nowadays, all of these functions and more have been rolled into a single stand-alone unit. Mixing ‘in-the-box’ is without doubt the driving force here. As CPU speeds multiplied and computer-based processing became a viable alternative to outboard processing, analogue mixing consoles made their reluctant way onto Ebay for a pittance of their original worth – the more avant-garde were carved up and racked for channel strips, while the rest were plonked into rehearsal rooms and skip-bins.

CARBON DATING
One device leading the charge on this evolution for budget conscious producers was Digidesign’s humble Digi 001. That was 22 years ago, but back then Digidesign (now Avid) cleverly unleashed a device that offered monitoring level control, microphone preamps, Midi I/O, and the propensity to record, edit, and mix entirely in the digital domain using Pro Tools and CPU-based plugins – all for a little less than two grand. Digi 001 users ditched their 16-channel analogue consoles and DAT recorders, upgraded to a Power Mac G4 and were suddenly mixing with all the convenience that in-the-box production entails: instant recall, a single unified digital gain structure without electrical interference and noise, way less maintenance and far greater portability. The more well-heeled and professional outfits at the time had been moving down this road with DSP-assisted Digidesign systems costing 10 times the cost of a Digi 001 – but now in-the-box mixing and Pro Tools was available to the many.

The ensuing decade saw multiple variations on the Digi 001 theme, with similar devices from the likes of Apogee, MOTU, Presonus, RME, and indeed Digidesign’s successors to the 001 – the progressively predictable 002 and 003 rack units. All followed the same theme: eight analogue I/O with ADAT expansion and a handful of mic preamps, along with monitoring and headphone outputs. However, all these devices were constrained by one single caveat; native DSP processing, for which there are two downsides. Firstly, the number of native processing plugins you can instigate is limited by the processing speed of your CPU, with virtual instruments and high-end reverbs chewing up scads more CPU juice than standard EQs and dynamics processors. Secondly – and this is the big one – native CPU DSP introduces monitoring latency from the live recording inputs: analogue audio comes in, the CPU juggles it for an arbitrary amount of time, then the signal is spat out the other end some milliseconds after it was initially heard by the analogue input. This is problematic. If your vocalist is hearing their voice a few milliseconds late it can, and will, affect their performance. However, there was a workaround (don’t we love those?). All of the aforementioned interface manufacturers devised methods of monitoring the ‘live input’ signal directly from the interface, rather than waiting for the live signal to meander via the CPU and DAW application. With great misfortune, this negates the ability to include processing on a recording track during a live take – find me a vocalist that doesn’t want at least a smidgeon of reverb in the headphones! The workaround was a separate interface-specific monitoring application. For the engineer this scenario involved flitting between the DAW and the interface-specific app to re-enable direct monitoring. There’s been hundreds of articles penned as to how many milliseconds are too many for acceptable record monitoring, but, at the end of the day, it’s a right royal pain. For the talent it becomes a roadblock to immediacy – waiting on the engineer to switch to a ‘direct monitoring’ mode. And for the engineer it’s just added confusion. Native CPU latency and its workarounds can utterly stifle a session.
In stark comparison, remember those $20k DSP-assisted Pro Tools systems I mentioned earlier? There’s no latency roadblock. What enters the mic is heard via the system less than an imperceivable millisecond later – and less at higher sample-rates. All the time, every time, regardless of the speed of the CPU, and you can load that input track with plugins should you wish. That’s why those interfaces go for the big bucks. Dedicated DSP ain’t cheap.

As I outlined initially, the entire concept of the audio interface has evolved from basic I/O into the ‘nerve-centre’ paradigm. Now a single unit typically takes up those multiple outboard duties. Alongside a plethora of I/O you’ll find multiple headphone outputs, switching between different monitor pairs, dedicated talkback microphones (or the option to add talkback via a mic input), metering, high impedance instrument inputs and more. Which brings us to the pinnacle of the concept: Avid’s latest all-in-one ‘production centre’ interface: Pro Tools | Carbon.
CARBON AVB
Unlike many interface choices, Carbon doesn’t connect to your host computer via USB or Thunderbolt. Avid has taken the Audio Video Bridge (AVB) route, as MOTU has with its professional line of interfaces and as Presonus with its live audio products. Connection is via a Cat-6 ethernet cable, or if you’re using a Thunderbolt-equipped Mac you can use Apple’s Thunderbolt-to-Ethernet adaptor. Avid has limited Carbon’s compatibility to Thunderbolt equipped Macs only and macOS 10.15.6 (Catalina). I assume, as is the Avid way, the company is treading the compatibility line with an abundance of caution. While AVB is an open standard that can function in Windows environments, and via USB 2 and 3, not all ethernet chipsets support AVB, and USB2 connection would possibly not play nicely with Carbon. As it happens, the ethernet chipset used by Apple supports AVB so this is where the compatibility cards fall. Equally, Carbon runs Pro Tools only. Why’s that? Oh do read on…

Here’s the big deal. Carbon has in-built DSP. Carbon is touted by Avid as a hybrid device in the sense that it can use native CPU-based plugins, just like any other native-based interface, but it can also utilise DSP-assisted plugins courtesy of its eight built in HDX DSP chips. These are the very same DSP chips you’ll find on an HDX core card, albeit eight rather than 18. What this means is that Carbon can provide sub-millisecond latency when recording, even when using plugins instantiated on record tracks, into Pro Tools – precisely like those expensive systems I mentioned earlier.

Now I hear the shouting. Yes, I know there are other audio interfaces on the market offering onboard DSP. Universal Audio’s Apollo range immediately springs to mind but there are others – they’re all at it – but those systems still require a separate mixer/monitoring application to monitor audio directly from the interface to alleviate latency. Carbon, on the other hand, combined with Pro Tools, avoids this conundrum completely by virtue of Avid’s AAX plugin format. Because AAX plugins run in both the native CPU arena and on Avid’s DSP processors, it’s transparently simple to swap from one processing base to the other (CPU or DSP) and maintain the exact same sonic effect. Everything is done within Pro Tools. Conversely, with the Universal Audio example, UA’s plugins run on UA’s DSP – and nowhere else.

How’s all this integrate? Within Pro Tools the switch between DSP and native AAX is virtually seamless. Each channel strip within the Pro Tools mixer has a small lightning bolt icon. Clicking the lightning bolt icon to ‘on’ switches the channel and and any instantiated AAX plugins over to Carbon’s DSP mode for latency-free monitoring, then switching it off flips the channel and plugins back into native CPU processing. It couldn’t be any simpler. Absolutely no need to flip over to that third-party mixer app for latency-free monitoring. Pro Tools also takes care of any plugins that may be instantiated downstream that are associated with the DSP-assisted record channel, such as effects returns and subgroups. If there happens to be plugins on those returns and subgroups that can’t be converted to DSP-assisted processors, you can switch those channels to DSP safe mode – in which case they’ll remain in the native DSP domain. In some cases this may be perfectly acceptable, for example, a reverb send isn’t going to suffer from a few milliseconds of pre-delay on an auxiliary track. It really is the best of both worlds. Super low latency when you need it, alongside a plethora of native processing and seamless switching between each.
CARBON LOADING
As for the mechanicals of Carbon, there’s a bunch going on. The single rack unit Carbon houses eight of Avid’s 126dB (A-weighted) THD+N 105dB (0.00056%) 20Hz–20kHz mic preamps using combo Amphenol (!) connectors. Spec-wise these are decidedly respectable and are in a similar ballpark to the Pro Tools | HD Omni mic preamps. What’s unique to the Carbon mic pre lineup is that preamps five to eight offer variable impedance switching, as do the two front panel instrument inputs. The mic preamps offer three input impedances (1kΩ, 5kΩ and 50kΩ) while the instrument inputs provide five options (32kΩ, 72kΩ, 90kΩ, 230kΩ and 1MΩ). Line level I/O is via DB25 connectors, and their 125dB (A-weighted) dynamic range surpasses the HD Omni’s line input specification of 118dB greatly. Next up are dedicated 6.5mm TRS jack monitoring outputs with 120dB (A-weighted) dynamic range and THD+N of -110dB (0.0003%) – again, virtually identical to the HD Omni specification give or take some inconsequential decimal places. An additional two monitor pairs can be fed from the output D-subs, and assigned within the Hardware Setup window within Pro Tools. These can be switched via dedicated buttons on the front panel. I’d have liked to see a digital coax output for some dedicated upmarket D-to-A monitoring, but I guess you could do this via the ADAT ports if you’re that keen. Bear in mind this is a production station, not a mastering rig. ADAT expansion follows the standard S/MUX layout, so your sample-rate I/O mileage will vary accordingly. BNC wordclock ports are available, then there’s two ‘NET’ ethernet ports. One for connection to the host computer, with the other I assume for expansion options in the future. AVB is, of course, an infinitely expandable platform – so expansion might include things like external fader controllers, additional mic preamps, bridging to Dante networks, or more. Have a look at what Presonus and MOTU are creating using the AVB protocol and you’ll see the potential.

Out front there are two instrument inputs, mic/instrument gain pots, buttons for impedance switching, phantom power, polarity invert, eight I/O LED bar-graph meters, left/right monitoring meters, monitor switching and level control, mute and dimming buttons, and (wait for it) four headphone outputs. Four! Enough for your smaller ensembles. These are rated at 100mW into 300Ω and 200mW into 32Ω – so plenty loud enough.
SOFTWARE
Software supplied includes a one-year subscription to Pro Tools with a perpetual license and 116 AAX plugins (including about 70 AAX DSP plugins). These include UVI Falcon, Celemony Melodyne 5 Essential, the Avid Complete Plugin Bundle, and HEAT (ya gotta love HEAT!), three free months of Antares Auto-Tune Unlimited, including Auto-Tune Hybrid, and a 5.4GB sound library. Plus you get Arturia Rev PLATE-140, Purple Audio MC77, Embody Immerse Virtual Studio, McDSP 6050 Ultimate Channel Strip HD, Native Instruments Vintage Organs, UVI Model D, along with Brainworx bx_console N, bx–rockrack and bx_masterdesk.

CONCLUSION
Overall, it’s a killer package to my mind. Until now there’s been nothing like Carbon, and certainly not at this price. I must admit I’ve been a little spoilt over the years as I’ve predominantly owned and worked with Digidesign HD and TDM DSP assisted systems, so on the few occasions that I’ve been forced to record in native systems I’ve been left disappointed with the direct monitoring fiasco. For me it’s a big fat horse-fly in the ointment – I don’t know how you guys with native systems put up with it. So if your DAW of choice is Pro Tools and you’re still in the Apple world, six grand is a no-brainer compared with a base level HDX system at 14 grand. Everything’s built into the one box: monitoring, mic pre’s, instrument inputs, headphone outs, the fabulously creative option to play with impedance loadings, and no latency issues ever. Plus you can run this from a qualified MacBook; no aging cheese-grater or ten grand Mac Pro tower required. It’s the best thing since sliced bread – go get one.