[LAU] What audio interface to use for a Linux-powered surround preamp?

[Jörn Nettingsmeier]

On 12/15/2011 02:59 AM, Johan Herland wrote:
>   2. Decode and/or upsample the input (if necessary) into a suitable
> format, typically 96kHz or 192kHz 24-bit PCM in 8 channels (for a 7.1
> system).

out of curiousity, what is the benefit of upsampling? is there something 
peculiar about power dacs that would make it useful or mandatory?

>   - A HDMI switch with audio split (SPDIF) and RS-232 control.

hdmi to spdif means that you either get only two channels, or lossy 
encoding. plus you will need to think about the clocking structure. 
usually this will mean that your audio card will have to slave itself to 
the incoming hdmi/spdif/whatever.

>   - A suitable audio interface with at least 8 digital outputs.

tough one. there are a number of cheap options with ADAT, but you will 
need two at 96k due to s/muxing, and four at 192k. for the latter, the 
only option i know of is the rme raydat.
iirc, rme also has a card with native aes/ebu outs, and it should be 
working ootb with the normal driver, but you may well find yourself to 
be the only user ever of this card under linux. so be prepared to get in 
touch with adi and get your kernel compilation toolchain ready :-D

the vendor is moderately linux-friendly, at least they are providing 
specs and not actively making our lives harder than necessary. plus the 
gear tends to be reliable and flexible.

>   - Since this is still in a proof-of-concept phase, I'd like to spend
> not more than about $1000 on the interface.
>
>
> So far I've been browsing interfaces like:
>
>   - MOTU 828mk3 (capable, but seems to be poorly supported by Linux)

don't go for firewire. it's great for mobile use, but a) it's on the way 
out, b) it requires a very carefully tuned low-latency system, and c) it 
totally does not work with frequency scaling and you will burn a quarter 
of the available cpu cycles just to get the audio data in and out. 
pretty deadly for an embedded and possibly passively cooled system.

>   - Focusrite Saffire PRO 40 (seems similar to RME Multiface II, but
> cheaper. Unsure how well it is supported by Linux)

it is well supported, but see above.

> What other audio interfaces should I check out? Are there other things
> I should keep in mind when picking an audio interface?

unfortunately, the only answer for truly professional audio hardware 
under linux is rme. not that their gear is bad, but it sure is 
expensive, and i wonder what we're going to do if they ever turn evil... 
time for those open hardware projects...