SOS Forum Post
Staff Writer Chris Mayes-Wright replies: This question is becoming more common because of the popularity of ADAT inputs and outputs on affordable audio interfaces like yours.
The ADAT protocol allows eight channels of audio at up to 24-bit/48kHz to be streamed down a fibre-optic cable. If your soundcard or audio interface has ADAT optical inputs and outputs, you can use them to get more channels of I/O to use with your DAW. Of course, you need something to take the analogue mic, instrument or line input and turn it into the digital ADAT output, and vice versa if you want more analogue outputs.
There are quite a few products on the market that supply A-D and/or D-A conversion, with a wide range of input and output options to boot. At the cheapest end, the 1U Behringer ADA8000 supplies eight phantom powered mic and balanced line-level inputs with balanced line outputs and ADAT optical in and out. BNC wordclock comes as standard and it's also very easy on the wallet, available at under £200 in UK shops. M-Audio's Octane is a 2U, eight-channel preamp with a built-in analogue to digital converter. Compared to the ADA8000, the Octane is twice the price, at £419 in the UK, although you can probably find cheaper street prices. It only has ADAT output — providing A-D conversion but no D-A — but it has some useful additional features, such as an M&S matrix and phase invert switches. Also, the quality of the mic preamps is arguably better than the Behringer's. But, as my heating engineer told me recently, you get what you pay for!
If you've got more money to spend, Focusrite offer two preamps with ADAT options — the Octopre and Octopre LE, which both feature eight decent preamps and occupy 1U of rack space. The Octopre LE has all the features of the Behringer ADA8000 plus switchable low-cut filters on all channels, two instrument inputs and a rather fetching blue VU meter that can be selected to show the output from each preamp separately. However, the ADAT input and output is an optional extra and in the UK the whole package, including the ADAT option board, will cost £478. The standard Focusrite Octopre is similar, although it has individually switchable phantom power and dynamics processing on each channel. Like the Octopre LE, the standard version also needs a digital option board to provide ADAT in and out and there is a further option with additional AES-EBU and S/PDIF connectivity. In the UK, the Octopre with ADAT-only board will cost you £798.
Further up the price ladder is the Mackie Onyx 800R, which offers eight Onyx preamps with plenty of extras, including a feature-packed digital output section that supports sample rates up to 192kHz (with lower ADAT channel counts). It costs around £1000 on the street. Even further up is the Audient ASP008, which supplies eight top-flight preamps and ADAT output for £1228. This is beyond the reach of most people's budget, but it's worth mentioning as you can win one in this month's competition, along with Audient's great Sumo summing mixer (check out page 66 for more details).
Another thing to consider when buying any device with digital inputs or outputs, after the quality of the analogue circuitry, is how you will sync it up to your audio interface, as digital audio transfer always requires sychronisation — whether it's from word clock, or down the same cable as the audio signal.
If you want to use your ADAT-equipped preamp as the master clock, you need to be able to trust that the clock will not transmit excessive jitter — arteficts in the clock pulse signal that can cause audible glitches — to other devices in the sync loop. Conversely, if you want to use your preamp as a slave device, you can — depending on the features of the unit — use an alternative input to receive your sync signal. These include ADAT (of course), AES-EBU or S/PDIF inputs, or a dedicated word clock, which is usually on a BNC connector. If you have a device that you specifically want to use as a master clock, you should ensure that your preamp will be able to sync to it. Referring back to my wise heating man, you get what you pay for here as well, as some low-end digital devices are known to kick out the odd glitch here and there.