Audio Interfaces: The HDX Factor

Avid’s Pro Tools HDX, like its HD predecessor, has always been a fairly ‘closed’ system: it uses a proprietary plug–in format (AAX), requires an Avid PCIe or Thunderbolt card (either HDX or Native) to run, and is intended — by Avid, at least — to be used with the company’s own A–D/D–A converter interfaces. Most other DAWs will usually work with a variety of plug–in types (VST or AU being the commonest), while almost all — including the non–HD version of Pro Tools — are quite happy to pipe audio through interfaces from almost any manufacturer, as long as ASIO or Core Audio drivers exist.

This aspect of Pro Tools HDX has prompted some complaints, but it has the advantage of guaranteed performance and stability: a necessity for professional facilities, where a flaky system could spell disaster in terms of angry clients and lost revenue. Because Avid only officially support their own interfaces (namely the HD Omni, HD I/O, HD MADI and Pre), they are able to focus all their testing and troubleshooting on just a few devices, unlike other DAWs, which have to be able to cope with interface drivers that have been coded by different companies and with varying degrees of competence.

Pass The Support

Although the DigiLink and DigiLink Mini connectors are a proprietary Avid protocol, many third–party manufacturers make interfaces that use them.The operative word there, though, is ‘officially’: there are now many companies making converters and interfaces that, while not explicitly sanctioned by Avid, either sport, or can be made to sport (via expansion cards), an Avid DigiLink or Mini DigiLink socket, for connection to an HDX or HD Native card. And although such devices are officially ‘unsupported’ by Avid, third–party interface manufacturers do perform rigorous testing themselves, and almost always provide detailed information on which systems and versions of Pro Tools they are known to work well with.

The question remains, though: why would you opt for one of these ‘unofficial’ interfaces when going down Avid’s preferred route guarantees at least some recourse should tragedy befall your HDX system?

Third Party Animal

One reason is mentioned not once, but twice, in the name of this very magazine. People will almost always express a preference for the sound of one converter over another, and while pretty much all HDX–compatible interfaces are of a high standard — it is, after all, an expensive, ‘professional’ system — great import is often placed on the subtle differences in sonic character imposed by different interfaces.

Given that they all perform well, though, perhaps a bigger reason lies in the differing feature sets. Some interfaces offer more headphone outputs than others, say, while some offer a greater number of mic preamps (some, like those on the Apogee Symphony I/O, are even remotely controllable).

Star Of Avid

One final issue to be aware of concerns latency. HDX systems are capable of running at extremely low latencies thanks to the DSP cards they use, but an often–overlooked fact is that A–D and D–A converters impose their own latency, independently of that caused by the combination of a computer and interface. Since Pro Tools HDX always assumes it is working with an Avid converter, its automatic delay compensation is tuned to negate the specific amount of latency that is inherent to HD I/O, Omni I/O and Pre interfaces. This could potentially cause problems when using a third–party interface, as the assumed delay incurred by audio leaving through an analogue output and returning through an analogue input could well be different to the reality (Lynx’s Aurora is the exception here, as it has been specifically designed to incur the same amount of latency as Avid’s own interfaces). Don’t let that put you off too much, however: the very slight differences in latency between converters are only ever likely to present a problem if you are wont to use complex routing of outboard hardware.