[LAU] Open Source Audio Interface (was Successor/replacement for RME HDSP+Multiface?)

[Len Ovens]

On Sun, 31 Aug 2014, Moshe Werner wrote:

> all the recent talk about less audio interfaces being supported for Linux got me
> thinking.
> First I want to mention that I'm neither a programmer nor an electrical engineer, but
> what if we could develop our own AI?

This is not the first time for this idea. There are one or two people 
working on it. The idea that seems to be the best is an ethernet connected 
AI because this seems to be the digital interface that stays around and is 
best supported. The idea is to use an arm based board with a netjack 
master and built in audio IF. The only project I know of is to at first 
provide stereo i/o as a proof of concept.

> I imagine a modular approach with options to customize and add different features like
> building blocks.
> One user needs a lot of preamps, the other works mostly over ADAT or Madi...

A few comments about ADAT, while the format of the messages is well known, 
it is not open. Licencing may be a problem. True MADI uses obsolete HW. 
There are a number of audio over ethernet protocols with various openness 
or not.

Just a quick note about DIY HW. It is not generally cheaper than buying 
the same capability made by someone else. There are premade solutions out 
there, but they are not in the range of a lot of linux musicians.

> We could even implement a DSP chip with LV2 effects to lessen the CPU load.

Almost any arm based board would be able to add some DSP capabilities. It 
may also be possible to use a DSP board instead of an arm board, but the 
learning curve for programing it may be higher... like having a running up 
to date linux kernel and jackd running on it for example.

> Maybe this is just wishful thinking, but it would solve one of the biggest problems for
> Linux audio musicians.

The biggest limit to audio interfaces in basement studio is getting over 
the fact that a good audio interface is going to cost more than the 
computer it works with. This is first of all a psycological problem not 
technical.

Most of us are looking for those ~$500 solutions. That is, we expect to 
get 4 to 8 i/os with mic pres for $500 or so. That is why ADAT looks good 
to us. The Audio Science 8 i/o PCIe card is just over $1k but just has 
line in (my D66 is line level too BTW) so then you need mic pres. Most of 
us have supplied these with a mixer of some sort.

Really, an audio interface is two parts, The adc/dac, and the preamps. It 
is easy enough to do the line level input, even balanced.

Going one step up, there are a lot of mic preamps around that provide a 
lot more control than a mixer or 8 input audio IF. Most of them have 
s/pdif or aes3 out. So maybe an audio interface that provides that makes 
more sense. However, the reason the generic mic pres work for most people 
is that they are using a set of dynamic mics with a few condeser mics 
(probably low end like mine) and the pre doesn't matter as much as it 
might with a ribbon mic for example.

So the problem is less technical than it might seem. (on the technical 
end, using netjack as an audio interface does use more CPU than the same 
alsa card used locally) To make such a project worth while it needs to 
have a wide appeal, this means cheap... I can't think of any other way to 
put it. The goal ends up being 8 i/os with 8 mic pre for around $500. 
Having something more stable and lower latency than USB2 might give you 
another $200 (maybe more) to play with, but writing OSx/win drivers would 
give a lot more room (netjack does this OOTB).

Another idea might be to start looking at asking for linux drivers for 
stage end digital snakes. 24/12 i/o for $2200 is already within the range 
cost wise (per channel) we are looking at. and there may be cheaper ones 
like this one:
http://www.sweetwater.com/store/detail/S16
for $900.
I think it may make more sense to write drivers for these kinds of things 
even if it means adding a second ethernet card to the computer (or letting 
the driver take over the one on the laptop)

Just some quick thoughts, perhaps not as complete as they could be.

--
Len Ovens
www.ovenwerks.net