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On Tue, 13.11.07 12:28, Takashi Iwai (tiwai@xxxxxxx) wrote:
Hi!
> > aplay -L suggest to use "surround71:" (and *only* that) as plugin for my
> > usb webcam that only has a single mono channel. What's going wrong?
> > Any idea where I should be looking?
>
> I guess it's C-Media one? Its controller accepts only 8-channel
> interleaved output even if the hardware supports only less channels.
> Perhaps to save a few cent for the controller chip.
It's a cheap Philips webcam which really has only a single channel. If
I try to open the device with "arecord", i get:
<snip>
$ arecord -D surround71:CARD=U0x4710x311,DEV=0 -r 44100 -c 8 -f s16_le
Recording WAVE 'stdin' : Signed 16 bit Little Endian, Rate 44100 Hz, Channels 8
arecord: set_params:879: Broken configuration for this PCM: no configurations available
</snip>
i.e. the device string that arecord -L lists for the device is not
working at all. Actually, speaking of it, on my machine with 5 sound
devices "aplay/arecord -L" generate almost exclusively total
rubbish -- only for one device the list makes sense. Here's the
full list:
<snip>
$ aplay -L | grep -v "^ "
front:CARD=AudioPCI,DEV=0
surround40:CARD=AudioPCI,DEV=0
iec958:CARD=AudioPCI,DEV=0
null
front:CARD=V8237,DEV=0
surround40:CARD=V8237,DEV=0
surround41:CARD=V8237,DEV=0
surround50:CARD=V8237,DEV=0
surround51:CARD=V8237,DEV=0
iec958:CARD=V8237,DEV=0
surround41:CARD=U0x4710x311,DEV=0
surround50:CARD=U0x4710x311,DEV=0
surround51:CARD=U0x4710x311,DEV=0
surround71:CARD=U0x4710x311,DEV=0
surround41:CARD=Audio,DEV=0
surround50:CARD=Audio,DEV=0
surround51:CARD=Audio,DEV=0
surround71:CARD=Audio,DEV=0
</snip>
And this is the output of "arecord -L":
<snip>
$ arecord -L | grep -v "^ "
front:CARD=AudioPCI,DEV=0
rear:CARD=AudioPCI,DEV=0
surround40:CARD=AudioPCI,DEV=0
iec958:CARD=AudioPCI,DEV=0
null
front:CARD=V8237,DEV=0
surround40:CARD=V8237,DEV=0
surround41:CARD=V8237,DEV=0
surround50:CARD=V8237,DEV=0
surround51:CARD=V8237,DEV=0
iec958:CARD=V8237,DEV=0
surround41:CARD=Audio,DEV=0
surround50:CARD=Audio,DEV=0
surround51:CARD=Audio,DEV=0
surround71:CARD=Audio,DEV=0
</snip>
For comparison, here's what /proc/asound/pcm contains:
<snip>
00-01: ES1371/2 : ES1371 DAC1 : playback 1
00-00: ES1371/1 : ES1371 DAC2/ADC : playback 1 : capture 1
01-01: VIA 8237 : VIA 8237 : playback 1 : capture 1
01-00: VIA 8237 : VIA 8237 : playback 4 : capture 1
02-01: Bt87x Analog : Bt87x Analog : capture 1
02-00: Bt87x Digital : Bt87x Digital : capture 1
03-00: USB Audio : USB Audio : capture 1
04-00: USB Audio : USB Audio : playback 1 : capture 1
</snip>
First of all, the -L lists miss one device: my Bt878 tv card. It's
hwoever shown properly in /proc/asound/pcm, and works fine if
accessed.
Then, as mentioned above, it lists the mono-only webcam (U0x4710x311)
with the four surround suggestions -- which doesn't make any sense at
all. And only on "aplay -L" -- although it is a record only
channel. If I try to open the audio device with any of the four
suggestions I just get the "configuration error" back that is shown
above. The way to open it is "hw:CARD=U0x4710x311,DEV=0" -- and
of course only with arecord.
Then, it doesn't list either "front:" nor "iec958" for my USB surround
sound card (The one listed as "Audio") -- both of which happen to work
fine. However, it does list "surround71:", which is quite surprising,
since the sound card doesn't have that many output ports -- but I
guess that is one of the cases where the hardware just doesn't have
soldered all outputs the device theoretically supports. Opening the
device as surround71: works fine, BTW.
Then, for the "AudioPCI" card, it lists a "surround40:" device, which
when opened just yields:
<snip>
$ aplay -D surround40:CARD=AudioPCI,DEV=0
ALSA lib setup.c:555:(add_elem) Cannot obtain info for CTL elem (MIXER,'AC97 2ch->4ch Copy Switch',0,0,0): No such file or directory
aplay: main:546: audio open error: No such file or directory
</snip>
And it also lists a "iec958" device for the same "AudioPCI", which
just yields this:
<snip>
$ aplay -D iec958:CARD=AudioPCI,DEV=0
ALSA lib setup.c:555:(add_elem) Cannot obtain info for CTL elem (PCM,'IEC958 Playback PCM Stream',0,0,0): No such file or directory
aplay: main:546: audio open error: No such file or directory
</snip>
The hardware has no SPDIF and is stereo only.
And then, surprisingly it lists a "rear:" device -- but only for
recording (!). Not surprisingly, this one doesn't work either.
The devices listed for the V8237 are the only ones which fully work
and are complete.
So, in summary. For the five audio cards I have connected to my
machine, "aplay -L" and "arecord -L" show 11 device hints that don't
work at all. And it misses 6 device hints that are important.
I am not sure what to make of this, besides that the name hinting
system is just badly broken.
This is alsa-lib 1.0.15 on kernel 2.6.22, btw.
Five more things I am wondering about, though:
1) Wouldn't it make sense to have a "mono:" device similar to the
other surround/front/rear devices? The single use case would be the
mono-only usb webcam I mentioned above.
2) If I understood correctly, then on some devices it is possible to
open a analog output and a SPDIF output simultaneously and
independently. On others however, they are exclusive to each
other. The name hinting system doesn't really distuinguish
those cases. Any idea how I could detect this? The reason is, I'd like to
open both SPDIF and analog out in PulseAudio at the same time -- if
it is possible. If it is not I want to give the user a quick way to
switch the outputs.
3) Some newer soundcards lack a hw volume control if I understood
correctly. Unfortunately I don't have access to them myself. But
apparently if PulseAudio is run on those devices, volume control in
PulseAudio doesn't work at all. Apparently ALSA uses softvol to add
a software volume control (attenuation) to the mixer on those
soundcards. PA finds that control and makes use of it -- however,
since it accesses the PCM via the raw devices (hw:, surround41: and
so on) softvol doesn't work: i.e. the volume that people can change
through the PA mixer is not reflected in any way in the audio
output.
Now, is there a way I can detect for a mixer element if it is
"real" or if it is implemented only for the "plughw" PCM? As it
seems the "hw:0" mixer device does contain "virtual" tracks, isn't that a
bit strange? Shouldn't the virtual mixer tracks be available only
in a mixer device "plughw:0"? That at least would be symmetric to
the PCM case, wouldn't it?
4) If I open an audio device with "plughw:" i can disable the software
resampling that takes place via
"snd_pcm_hw_params_get_rate_resample()". However, there is no
equivalent for disabling the channel number adjustment or the
sample format conversion. This would be very useful in PA however,
since this would allow me to use only the parts of plughw I am
interested in (softvol), and disable all the rest (resampling,
conversion, channel remixing).
5) I really dislike the fact that ALSA insists to do the channel
reordering for me if I access the audio device via
"surround51:". In the interest of having the shortest pipeline
possible with no redundant steps in PA, I'd prefer if I could open
the device as hw:0, and query the suggested channel ordering of the
HW, instead of always having ALSA around doing the rearranging.
Lennart
--
Lennart Poettering Red Hat, Inc.
lennart [at] poettering [dot] net ICQ# 11060553
http://0pointer.net/lennart/ GnuPG 0x1A015CC4
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