I have a fairly cheap and cheerful audio interface that has ADAT ports to allow me to add more inputs and outputs. If I were to use these to hook up a more expensive interface with better mic preamps, will I see an increase in quality, or can the cheap interface’s converters still compromise the sound quality anyway?
SOS Forum post
SOS Technical Editor Hugh Robjohns replies: Assuming you connect a higher-quality device you should see an improvement because you’re not at the mercy of any A-D/D-A conversion! Inside your cheap (or any other) interface, there are a bunch of A-D and D-A converters that convert between digital and analogue signals for the unit’s onboard analogue inputs and outputs. These have nothing to do with the interface’s digital (typically AES, S/PDIF and/or ADAT) inputs and outputs.
The digital signals to and from these converters and all of the digital ports are routed through some form of DSP, which handles the task of data reformatting needed to translate between the computer’s USB / Firewire / Thunderbolt ‘language’ and the ‘language’ required by the interface’s converters and digital ports. Think of it not as ‘conversion’, but as ‘reformatting’ or ‘repackaging’. The digital audio data from the ADAT socket is completely unmolested; it’s just repackaged differently as it is passed on to the computer. It is most definitely not converted to analogue and then re-encoded back to digital!
So, in the scenario you describe, the cheaper audio interface is providing the interface between the incoming ADAT data from the higher-quality device and the cheaper device’s USB or Firewire (or whatever other protocol) connection to your computer. The digital audio from the source remains as digital audio throughout that ‘translation’ process, and the only ‘conversion’ that could potentially be incurred is a sample-rate conversion, if the interface provides that facility — and even that remains entirely within the digital domain. Few interfaces offer that facility anyway, which is why it is important to set the clocking options correctly on the interface and external ADAT source!
Ideally, assuming you’re using the preamps on both your cheap interface and the ADAT-equipped mic preamp box, the cheap audio interface would be set as the clock master, the expensive (external ADAT source) one as the slave, and a synchronising word clock signal taken from the cheap interface’s clock-out port to the expensive interface clock-in. That assumes that the A-Ds and D-As in the cheap interface perform best when running on their internal crystal clock, while the external ADAT source has a decent clock-recovery system. If you’re not using any of the analogue inputs on your cheaper interface and your interface allows, you should set the higher-quality unit as the clock master.