On 23. 06. 22 10:24, Takashi Sakamoto wrote:
Hi,
On Thu, Jun 23, 2022, at 16:18, Jaroslav Kysela wrote:
On 23. 06. 22 7:58, Takashi Iwai wrote:
Even with this patch, there is a remaining problem, which is not present
in the MS2109. The sound sample values range from 0x0000 to 0x7fff, with
silence around 0x4000, i.e. 15-bit-ish audio. This actually sounds OK to
the ear (although half as loud as it should be), but looks odd when
looking at the waveform, and makes volume meters always think the sound
is very loud.
To convert to s16le, I can bitshift one bit left, and subtract 32768.
I'm told that this isn't something that can or should be done in the
kernel, but should be in userspace. Any more advice on how to fix this
remaining quirk would be very welcome.
Ouch, this is painful. We haven't had any devices that require a
15 bit unsigned format, and maybe we don't want to add it to the
common standard format just for one funky device, either. Such data
processing could be done in alsa-lib, but for the proper interaction
with the user-space, the kernel should provide some information so
that user-space can process the data accordingly. However, we have no
proper way defined for it generically, so far.
Maybe an easy way would be to create an alsa-lib external plugin, and
apply it per device. Jaroslav, could it be done via UCM?
I agree that we may start with a special plugin for this format. The UCM can
use any alsa-lib configuration now. So PA/PW should work with this very
specific hardware when properly configured.
Note that we have SNDRV_PCM_FORMAT_SPECIAL for such cases. It will imply that
the applications will fail when the special conversion plugin is not used. The
minor issue may be with the silence routines (which is already with the
improper format).
I think the combination of format/subformat is available in the case.
Actually the odd frame format is within 16 bit slot, so SNDRV_PCM_FORMAT_U16
or so is available. Then we can define new subformat to notify userspace; e.g.
SNDRV_PCM_SUBFORMAT_MODEL_SPECIFIC.
The cons is that we need to add new subformat, thus update asound.h. On the
other hand, the pros is that existent userspace stuffs still handle the odd format
and user can hear playback sound at least even if the loudness is not expected.
It's a question if the cutted half-wave samples should be sent to D/A in this
case. Also, if the high-bit (U16) is set, the resulting sample value for D/A
is completely incorrect. It's not only about loudness. My opinion is to not
use the U16 sample format for this and handle this hw as special one until the
format is more common (which I do not expect) to extend the format list.
Jaroslav
--
Jaroslav Kysela <perex@xxxxxxxx>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.
