first of all, congrats for pulling this off - I still have no clue how you are piping this many channels through a single I²S bus!
I ordered two RCA Octos from Amazon UK and I'm now measuring them. For "consumer" sound, they are pretty ok, but for professional use, there appear to be number of limitations.
First, my measuring setup:
- Octo card running on an RPi 3 Jessie with a jack2 sound server and an eight-channel zita network-to-jack bridge. It resamples and is thus not bit-transparent, but measures perfectly.
- RCA outs via an 8way RCA->TS jack loom into an RME Micstasy (input impedance 5.6kOhm) at 10dB gain
- measuring PC with RME HDSPm running jack2
- jaaa as signal generator and analyser
- Output level: I'm sending -3dbFS white noise in and measure between -15.6 and -16 dBFS at the Mictasy's MADI output, Micstasy has FS at +21dBu, the analog gain is at 10dB. That means the level is reasonably close to the nominal -10dBu for consumer line interfaces, although not terribly well matched with a maximum deviation of 0.3dB between channels. This result is consistent between both Octos I've seen.
- Output 7 is about 11dB down compared to the other channels, and this is again consistent across both Octos, suggesting a design issue.
- The left and right channels are consistently swapped, i.e. the white RCA is right or odd, and the red is left or even.
- The frequency response is blameless pretty much from DC to Fs/2.
- The noise floor is quite high, -85dB at 47Hz.
- The noise spectrum has clear harmonics at multples of 375 Hz, suggesting a power issue.
- I'm seeing noise peaks whenever the RPi goes to work, such as by issuing an "ls" or "ps aux" command, again suggesting power issues, strong interference from the RPi circuits, or maybe timing issues on the I²S bus introducing spurious LSBs?
- Distortion and crosstalk seem reasonable, I'm currently preparing more detailed measurements, and will look at the performance of the line inputs as time permits.
The HifiBerry XLR outperforms the Octo RCA by orders of magnitude, alas at a lower channel count. I couldn't get my hands on the balanced version of the Octo for a fairer comparison, but I doubt its noise performance will be much better, given the load-dependent spikes I'm seeing.
Looking at the PCB, some components seem to be shifted a bit - if this is hand-soldered, OMG respect how is it even possible? But if it's factory made, whatever fab is doing it should really ratchet up their quality control (and seeing the design company's domain name misspelled on the PCB doesn't exactly increase my confidence...)
It's certainly an impressive project, and I would be prepared to back an updated version (at a higher cost if necessary) if the bugs mentioned above get shaken out and the noise performance is improved by at least 10dB.
I would also welcome any attempt to trade the pretty much useless 96kHz option for more dynamic range. I didn't look at the digital data on the I²S bus (wouldn't know how), but the noise suggests that the Octo uses a word length of 16-bit or less, and that's not optimal.
Also, I wonder if having the outputs go to DC is such a wise thing to do... then again, I'm not an electronics person, but a sound engineer.