Non-continous channel map?

Non-continous channel map?

[Pierre Ossman]

Hi!

ALSA uses a rather strict default channel map, just a simple list. This 
is insufficient for the polypaudio plug-in I wrote, as the polypaudio 
server might have an output of e.g. just the center channel and a 
subwoofer. I'm wondering how to properly represent this in ALSA.

For the PCM, there is no problem as we as clients decide which channels 
are to be set up. The server will then need to map these to actual hardware.

For mixer, however, we're modifying the server hardware so we will need 
to adapt to whatever channels it has. So provided I have a device with 
channels "left", "right", "center" and "subwoofer", should I:

  a) Just expose "left" and "right" since "rear left" isn't present (the 
next in ALSA's list).

  b) Expose all channels until I can cover my four and simply filter out 
the ones that aren't present on the server.

  c) Ignore ALSA's list completely and leave it to the user to figure 
out what the channel map is.

  d) Something else?

Input appreciated.

-- 
Pierre Ossman                Telephone: +46-13-21 46 00
Cendio AB                    Web: http://www.cendio.com


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Re: Non-continous channel map?

[Clemens Ladisch]

Pierre Ossman wrote:
> ALSA uses a rather strict default channel map, just a simple list.

Does it?  AFAIK ALSA devices just have a specific number of channels and
leave the interpretation to the application.

The default channel order is just a convention.


HTH
Clemens


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Re: Non-continous channel map?

[Pierre Ossman]

Clemens Ladisch wrote:
> Pierre Ossman wrote:
>> ALSA uses a rather strict default channel map, just a simple list.
> 
> Does it?  AFAIK ALSA devices just have a specific number of channels and
> leave the interpretation to the application.
> 
> The default channel order is just a convention.
> 

A convention that isn't followed is just as bad, if not worse, than no 
convention at all.

So what you're saying is that any application that wants to output more 
than one channel must have the user inform it of how channels are mapped?

-- 
Pierre Ossman                Telephone: +46-13-21 46 00
Cendio AB                    Web: http://www.cendio.com


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Re: Non-continous channel map?

[Takashi Iwai]

At Wed, 03 May 2006 14:43:51 +0200,
Pierre Ossman wrote:
> 
> Clemens Ladisch wrote:
> > Pierre Ossman wrote:
> >> ALSA uses a rather strict default channel map, just a simple list.
> > 
> > Does it?  AFAIK ALSA devices just have a specific number of channels and
> > leave the interpretation to the application.
> > 
> > The default channel order is just a convention.
> > 
> 
> A convention that isn't followed is just as bad, if not worse, than no 
> convention at all.

Most apps do follow the convention :)


Takashi


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Re: Non-continous channel map?

[Takashi Iwai]

At Thu, 27 Apr 2006 07:52:35 +0200,
Pierre Ossman wrote:
> 
> Hi!
> 
> ALSA uses a rather strict default channel map, just a simple list. This 
> is insufficient for the polypaudio plug-in I wrote, as the polypaudio 
> server might have an output of e.g. just the center channel and a 
> subwoofer. I'm wondering how to properly represent this in ALSA.
> 
> For the PCM, there is no problem as we as clients decide which channels 
> are to be set up. The server will then need to map these to actual hardware.
> 
> For mixer, however, we're modifying the server hardware so we will need 
> to adapt to whatever channels it has. So provided I have a device with 
> channels "left", "right", "center" and "subwoofer", should I:
> 
>   a) Just expose "left" and "right" since "rear left" isn't present (the 
> next in ALSA's list).
> 
>   b) Expose all channels until I can cover my four and simply filter out 
> the ones that aren't present on the server.
> 
>   c) Ignore ALSA's list completely and leave it to the user to figure 
> out what the channel map is.
> 
>   d) Something else?
> 
> Input appreciated.

I vote for (b).  The method is simple enough.

Does the server-side configuration change on the fly, or the setup is
fixed once after the server-client connection is established?
In theory, ALSA controls can dynamically added/removed.  But this may
rather confuse some applications that don't support the dynamic change
of control elements.


Takashi


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Re: Non-continous channel map?

[Pierre Ossman]

Takashi Iwai wrote:
> 
> I vote for (b).  The method is simple enough.

Ok. I'll start implementing that then. :)

> 
> Does the server-side configuration change on the fly, or the setup is
> fixed once after the server-client connection is established?

Yes and no. A back end specifies its channel map during creation, so 
it's static during its lifetime. But we identify the back end using a 
name, so it is possible that "our" back end could get removed and 
another loaded with the same name.

It is possible to get notifications when this happens though, so we can 
handle this gracefully.

> In theory, ALSA controls can dynamically added/removed.  But this may
> rather confuse some applications that don't support the dynamic change
> of control elements.

Something that Polypaudio itself is probably guilty of. ;)

Rgds
-- 
Pierre Ossman                Telephone: +46-13-21 46 00
Cendio AB                    Web: http://www.cendio.com


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