Re: [FFADO-devel] USB controller and (MOTU) hardware mix buses

[Jonathan W. <jw...@ju...>]

Hi Patrick

> First, when I use the ffado-mixer, I get XRUNs.  This wasn't the case with
> an earlier version of ffado and Linux.  Has anyone else experienced this?
> I will try and do some more diagnostics of this later and send more
> details in a separate email.

I haven't personally, no.  Unfortunately it could be almost anything, and
without knowing the details of the earlier setup and your current one it's
very hard to guess what the likely candidates might be.  Since there's been
little in the way of low-level changes to ffado-mixer or the streaming
system in recent years I'd be tending to focus on the other aspects of the
system.  Below you'll find some idle speculation. :-)

My first suspicion would be any proprietory binary video drivers you may
have in use.  The thing which ffado-mixer does which jackd/ffado doesn't is
create video traffic, so this is a reasonable first guess.

I assume that the computer hardware in use is the same as you've used
successfully on previous occasions.  If not, I'd be checking interrupt
assignments for the firewire card and how they relate to other things in the
system.

Does your current distribution give you a CONFIG_PREEMPT kernel (aka "low
latency desktop" setting in the kernel configuration)?  On a significant
amount of hardware these days the realtime-patched kernel isn't required for
reliable ffado operation, but CONFIG_PREEMPT tends to be.

Have you confirmed that you never get xruns at any time when ffado-mixer is
not running?  In other words, is ffado-mixer the only thing which triggers
xruns for you?

> Second, controlling the hardware mix buses via the ffado-mixer UI is a
> little awkward for a live show.  I'd like to use a controller to control
> these mixes, if possible.
> 
> Can/does that work with any equipment now?

I don't believe anyone's tried that before.

> Would there be any conflict when using Ardour for the recording?

I guess that would depend on what the controller did and how it interacted
with the streaming system.  If all you're doing is controlling the onboard
mixer I can't see that it would cause ardour any issues.

> Does the MIDI data map to the same protocol as the ffado-mixer, in which
> case I'd be likely to have the XRUNs?

There is only one bitstream flowing from the computer to the MOTU.  This is
primarily for audio data (as one would expect), but the MIDI data is sent as
a subchannel of this.

As to whether you'd have xruns, that would depend entirely on what is
causing them.  As I said above, there's insufficient information to
determine this at this point in time.

Having said that, the mere presence of an active midi channel won't change
much at the protocol level - the midi subchannel is present in the bitstream
regardless of whether it's in use or not.  Of course the computer itself
will be doing extra work (driving the MIDI software and so on), and if that
is part of your problem then it may cause xruns.

All the above applies to software controllers.  If on the other hand you're
thinking of getting a MIDI controller and plugging it straight into the MOTU
then that's entirely different (but I can't remember of the 896HD has MIDI
control of its mixer, so I can't be sure that this is possible to do).  In
this case the computer plays no part in the control system so at least in
theory the operation of the computer is oblivious to the presence of the
controller.  In this scenario it's hard to see how a hardware MIDI
controller could interfer in any way with what the computer is doing.

> Are there some controllers that work and others that don't, or some that
> folks on this list would recommend?  I'd like to (this time) buy from
> someone that is Linux friendly.  Budget is up to US $400.

What kind of control surface are you looking for - faders, trigger pads, a
2-octave keyboard?  It really depends on how you envisage controlling the
device.

> If the software isn't in place now, I'd be interested in helping develop
> it.

I'm not sure what you mean by the software being in place and how you see
this interacting with whatever control surface you end up getting.  Given
what you've said in totality I'm guessing that you intend to get a midi
controller and send its data into the PC (either via the MOTU's MIDI port,
or some other MIDI input on your computer).  You then want some software on
the PC which reads this control data and manipulates the 896HD's mixer as
required.  Is this vaguely accurate?

If it is then the only software which you require which is not yet in
existance is the bridging software from MIDI to the interface.  This would
not be hard to write and would have only a weak binding to ffado itself. 
You may or may not be aware that ffado-mixer is really just a PyQt skin
around dbus controls provided by ffado-dbus-server.  So this new software
would just need to do is take incoming MIDI control messages, translate them
into the (straight-forward) form expected by ffado-dbus-server's dbus nodes,
and send them out to the applicable dbus nodes.

Regards
  jonathan