Re: Multichannel audio playback

[David Wright <deblis@lionunicorn.co.uk>]

On Fri 30 Apr 2021 at 09:24:53 (-0400), Dan Ritter wrote:
> David Wright wrote: 
> > On Wed 28 Apr 2021 at 18:21:27 (+0000), Rodolfo Medina wrote:
> > > Now it works: 
> > > 
> > > $ ecasound -a:1,2,3 -i:sndfile,3canali.wav -o alsaplugin,2,0
> > > **************************************************************************
> > > *        ecasound v2.9.1 (C) 1997-2014 Kai Vehmanen and others    
> > > **************************************************************************
> > > (eca-chainsetup) Chainsetup "untitled-chainsetup"
> > > (eca-chainsetup) NOTE: Real-time configuration, but insufficient privileges
> > > ... to utilize real-time scheduling (SCHED_FIFO). With small buffersizes,
> > > ... this may cause audible glitches during processing.
> > > (eca-chainsetup) "rt" buffering mode selected.
> > > (audioio_sndfile) Using libsndfile to open file "3canali.wav" for reading.
> > > (eca-chainsetup) NOTE: using existing audio parameters -f:s16_le,3,48000
> > > ... for object 'sndfile' (tried to open with -f:s16_le,2,44100).
> > > (eca-chainsetup) Opened input "sndfile", mode "read". Format: s16_le,
> > > ... channels 3, srate 48000, interleaved (locked params).
> > > (eca-chainsetup) Opened output "alsaplugin", mode "write". Format: s16_le,
> > > ... channels 2, srate 48000, interleaved.
> > > - [ Connected chainsetup: "untitled-chainsetup" ] ------------------------
> > > - [ Controller/Starting batch processing ] -------------------------------
> > > - [ Engine - Driver start ] ----------------------------------------------
> > > 
> > > - [ Engine - Processing finished ] ---------------------------------------
> > > - [ Controller/Batch processing finished (0) ] ---------------------------
> > > - [ Controller/Processing stopped (cond) ] -------------------------------
> > > - [ Engine exiting ] -----------------------------------------------------
> > > (eca-control-objects) Disconnecting chainsetup:  "untitled-chainsetup".
> > 
> > > Let's try to go to problem 2).
> > 
> > It's difficult to know what you've actually done from this description.
> > I know nothing about the UMC404HD apart from the two downloaded PDFs,
> > 8849567252510.pdf and download_259085.pdf. I know nothing about Ecasound.
> > 
> > Let me first reveal my ignorance by asking: does Ecasound run on your
> > computer, or on the Behringer?
> 
> The Behringer is a USB-attached sound card. It happens to have
> four inputs and four outputs. This is not particularly unusual.

Ah, I see. When you put it like that, it throws a different light on
what the box really is. In terms of this computer, all those input
and output sockets are the equivalent of my little 3.5mm jacks
(but with phantom power), and the gain knobs are alsamixer sliders.

> > I'm going to assume the former, that you're playing a file on the
> > computer in Ecasound, which then enters the Behringer via the USB
> > connection.
> 
> Correct.

And so, by the sounds of the OP's solution post, the only
functionality of the Behringer being used is the four better quality
DACs (rather than my two), with four balanced outputs (rather than my
two unbalanced ones through a 3.5mm jack).

> > How are these loudspeakers connected to the Behringer: which type of
> > connectors are used, in which sockets, and what's the pinout at each
> > end of the cables? What type of input do these loudspeakers expect
> > to be driven by: presumably not an unbalanced 50W into 8Ω!
> 
> Completely irrelevant. The Behringer produces four single
> channel outputs at line level suitable for an amp or preamp. We
> can assume that the original questioner has things hooked up
> properly.

I didn't assume that because the OP wrote on Tuesday:

   "on the back of Behringer there are two groups of `playback
   outputs': group A and group B.  Each of the two groups has 4 holes:
   2 black 6.35mm, one red RCA and one white RCA.  Please help also to
   understand how to use them for my special purpose.  Besides, on
   Behringer's back there is one more group of outputs called `Main
   Out' and including two 6.35mm and two XLR."

> > > Actually, the Behringer seems to behave in
> > > stereo mode.

That, and didier's post, made me think the OP might have been using
the two Main outputs rather than three of the four playback outputs
(where playback here means Play).

> > Do you mean that all three channels have been mixed into two channels,
> > and this is what you hear on your loudspeakers 1 and 2?
> > 
> > Isn't that what these lines might indicate:
> > 
> > > (eca-chainsetup) Opened input "sndfile", mode "read". Format: s16_le,
> > > ... channels 3, srate 48000, interleaved (locked params).
> >                ↑
> > > (eca-chainsetup) Opened output "alsaplugin", mode "write". Format: s16_le,
> > > ... channels 2, srate 48000, interleaved.
> 
> Yes, this means that ecasound is writing 2 channels, which is
> what the question is all about.

As it turns out, yes. I presume the previous command (which "worked":
ecasound -a:1,2,3 -i:sndfile,3canali.wav -o alsaplugin,2,0) was using
the default number of output channels, likely two, and now corrected
with -f:16,3,44100.
           ↑

> The questioner wants ecasound to write 4 channels, which could
> be:
> 
> ecasound using the wrong ALSA output
> 
> or
> 
> ALSA not presenting an acceptable 4 channel output, so ecasound
> falls back.

So now it appears to be working for the OP, I have odd questions
about the unit:

. The least mentioned feature of the Behringer is the Inserts on the
  back. Do you know how they work?

. There's ½GB of compressed software that goes with this device.
  Are these programs various people's version of what ecasound is?

. Would it help to see what kind of difference there is between a
  sophisticated card like the Behringer and a run-of-the-mill
  soundcard, by listing the ALSA Controls with   amixer -c 2 scontents
  as I asked about earlier?

. On one of my PCs, I can record (Capture) the output of the Stereo Mix,
  which is the sound being played by the computer from, say. a browser:

    Simple mixer control 'Input Source',0
    Capabilities: cenum
    Items: 'Mic' 'Line' 'Stereo Mix'
    Item0: 'Stereo Mix'

  Would this unit be capable of providing that facility (which the
  rest of my surviving computers cannot do)?

Cheers,
David.