Documentation/sound/cards/cmipci.rst

Source file repositories/reference/linux-study-clean/Documentation/sound/cards/cmipci.rst

File Facts

System
Linux kernel
Corpus path
Documentation/sound/cards/cmipci.rst
Extension
.rst
Size
9337 bytes
Lines
273
Domain
Support Tooling And Documentation
Bucket
Documentation
Inferred role
Support Tooling And Documentation: documentation
Status
atlas-only

Why This File Exists

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

Dependency Surface

Detected Declarations

Annotated Snippet

=================================================
Brief Notes on C-Media 8338/8738/8768/8770 Driver
=================================================

Takashi Iwai <tiwai@suse.de>


Front/Rear Multi-channel Playback
---------------------------------

CM8x38 chip can use ADC as the second DAC so that two different stereo
channels can be used for front/rear playbacks.  Since there are two
DACs, both streams are handled independently unlike the 4/6ch multi-
channel playbacks in the section below.

As default, ALSA driver assigns the first PCM device (i.e. hw:0,0 for
card#0) for front and 4/6ch playbacks, while the second PCM device
(hw:0,1) is assigned to the second DAC for rear playback.

There are slight differences between the two DACs:

- The first DAC supports U8 and S16LE formats, while the second DAC
  supports only S16LE.
- The second DAC supports only two channel stereo.

Please note that the CM8x38 DAC doesn't support continuous playback
rate but only fixed rates: 5512, 8000, 11025, 16000, 22050, 32000,
44100 and 48000 Hz.

The rear output can be heard only when "Four Channel Mode" switch is
disabled.  Otherwise no signal will be routed to the rear speakers.
As default it's turned on.

.. WARNING::
  When "Four Channel Mode" switch is off, the output from rear speakers
  will be FULL VOLUME regardless of Master and PCM volumes [#]_.
  This might damage your audio equipment.  Please disconnect speakers
  before your turn off this switch.


.. [#]
  Well.. I once got the output with correct volume (i.e. same with the
  front one) and was so excited.  It was even with "Four Channel" bit
  on and "double DAC" mode.  Actually I could hear separate 4 channels
  from front and rear speakers!  But.. after reboot, all was gone.
  It's a very pity that I didn't save the register dump at that
  time..  Maybe there is an unknown register to achieve this...

If your card has an extra output jack for the rear output, the rear
playback should be routed there as default.  If not, there is a
control switch in the driver "Line-In As Rear", which you can change
via alsamixer or somewhat else.  When this switch is on, line-in jack
is used as rear output.

There are two more controls regarding to the rear output.
The "Exchange DAC" switch is used to exchange front and rear playback
routes, i.e. the 2nd DAC is output from front output.


4/6 Multi-Channel Playback
--------------------------

The recent CM8738 chips support for the 4/6 multi-channel playback
function.  This is useful especially for AC3 decoding.

When the multi-channel is supported, the driver name has a suffix
"-MC" such like "CMI8738-MC6".  You can check this name from
/proc/asound/cards.

When the 4/6-ch output is enabled, the second DAC accepts up to 6 (or

Annotation

Implementation Notes