I read with interest Paul White's FAQ in last month's SOS about digital audio. One of the questions was whether you can mix digital signals by wiring the cables, which of course you can't. My question is, do you know if anybody manufactures a very simple digital mixer which would allow me to mix together six or eight signals, either S/PDIF or AES/EBU, into one single digital signal? I don't need any EQ or mixing facilities, I just need to merge the signals. I am trying to save inputs on my 02R desk, which uses up channels for effects returns. So I want to take four stereo digital outs from four different effects units, merge them into one digital signal, and then feed this digitally to the 02R, thereby freeing up eight analogue inputs. I thought of using a small analogue mixer, and then using a A‑D to convert the output, but it seems a bit long‑winded...
Leszek Gasiorek
Technical Editor Hugh Robjohns replies: Sadly, I can't think of anyone who makes that kind of 'budget digital mixing' product. The problem is that you need a DSP to combine digital signals, as it requires big sums, followed by re‑dithering to the required output resolution. The first might be trivial but the second is certainly not!
The O2R has digital I/O expansion card options, but I assume your digital inputs are already in use and/or your expansion slots are already full. The analogue mixer trick is therefore the only practical solution I can recommend, especially as you already use the analogue route at the moment. A little Spirit Folio or Mackie will give you perfectly adequate quality for a very small outlay and there are plenty of no‑frills A‑D converters around at reasonable prices these days.
The only other alternative would be to look for something like an old Yamaha Promix 01. You might be able to pick one up for little more than than the cost of a Folio or Mackie and an A‑D, and you would gain a whole bunch of channels and onboard FX into the bargain. If memory serves, though, the digital output is only at 44.1kHz, which may not be flexible enough for you.