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This article describes how to use C-Media CMI series PCI cards with ALSA

TakashiIwai is the maintainer of the code for this card.



There has at March 07 been a thread running on the Ubuntu forums for 2 + years reporting that microphone sound levels are so low with the cmi8738 chip and the cmipci driver as to be unusable. I've just experienced this with both a motherboard sound chip and a PCI card with a cmi8738 chip. Is there any chance of this issue ever being resolved? - Chris Dunn

This card performs well as a playback device but has poor recording ability. The driver supports spdif I/O for cards that have it available.

  • Patrick Shirkey

Actually I have no problems recording, even at small buffer sizes (spdif on dio2448, no soft-monitoring). - Tim Orford

I agree with Patrick - The card produces a loud hissing sound when capturing from Line-In. Haven't tried capturing SPDIF. - Michael Stevens

I got loud hissing when I tried to record from the microphone, but got good quality sound when I recorded from the line-in and put the microphone through a preamp. - John Kawakami

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 arbitrary playback rates but only certain fixed rates: 5512, 8000, 11025, 16000, 22050, 32000, 44100 and 48000Hz.

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.


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 front DAC accepts up to 6 (or 4) channels. This is different from the dual DACs described in the previous section. While the dual DAC supports two different rates or formats, the 4/6-ch playback supports only the same condition for all channels.

For using 4/6 channel playback, you need to specify the PCM channels as you like and set the format S16LE. For example, for playback with 4 channels,

snd_pcm_hw_params_set_access(pcm, hw, SND_PCM_ACCESS_RW_INTERLEAVED);
// or mmap if you like
snd_pcm_hw_params_set_format(pcm, hw, SND_PCM_FORMAT_S16_LE);
snd_pcm_hw_params_set_channels(pcm, hw, 4);

and use the interleaved 4 channel data.

There is a control switch, "Line-In As Bass". As you can imagine from its name, the line-in jack is used for the bass (5th and 6th channels) output.

Volume Control

I found one solution to adjust the volume for all channels when using routing to map stereo to 5.1 as the dmix setup does not work with latest 1.0.11rc3 alsa release. On my CMI8738-MC6 the Master Volume only affects the front channel. Since the other channels are on "hw:0,1" and I found no way to adjust the levels on this, one can use "softvol" to do a virtual volume adjustment.

Drawback: Master Volume still does not affect all channels, but the custom "SoftMaster" item can be used to set the volume for all channels.

Here is my .asoundrc:

pcm.softvol {
    type softvol
    slave {
        pcm "hw:0,1"
    control {
        name "SoftMaster"

pcm.dsp0 {
    type plug
    slave.pcm "softvol"
    slave.channels 6
    route_policy duplicate

pcm.!default {
    type plug
    slave.pcm "softvol"
    slave.channels 6
    route_policy duplicate

Here it is folks! A working CMI8738-mc6 config file that supports 5.1, upmixing, dmix, and now, softvol. You can control the volume of your center/lfe channels! This one enables default upmixing of 2.0->5.1 as we all real 5.1 streams to be dmixed at the same time, so you can watch a DVD in 5.1 (analog) and have other sounds still coming from all speakers.

# 6 channel dmix:
pcm.dmix6 {
    type dmix
    ipc_key 1024
    ipc_key_add_uid false
    ipc_perm 0660
    slave {
        pcm "hw:0,1"
        rate 48000
        channels 6
        period_time 0
        period_size 1024
        buffer_time 0
        buffer_size 5120

# upmixing: 
pcm.ch51dup {
    type route
    slave.pcm dmix6
    slave.channels 6
    ttable.0.0 1
    ttable.1.1 1
    ttable.0.2 1
    ttable.1.3 1
    ttable.0.4 0.5
    ttable.1.4 0.5
    ttable.0.5 0.5
    ttable.1.5 0.5

pcm.duplex {
    type asym
    playback.pcm "ch51dup" # upmix first
    capture.pcm "hw:0"

# change default device:
pcm.!default {
    type softvol 
    slave.pcm "duplex"
    control {
        name "Software Master"
        card 0

# for aoss
pcm.dsp "duplex"

pcm.dsp1 "duplex"

Digital I/O

The CM8x38 provides the excellent SPDIF capability with very cheap price (yes, that's the reason I bought the card :)

The SPDIF playback and capture are done via the third PCM device (hw:0,2). Usually this is assigned to the PCM device "spdif". The available rates are 44100 and 48000Hz. For playback with aplay, you can do:

% aplay -Dhw:0,2 foo.wav
% aplay -Dspdif foo.wav

So far, only S16LE format is supported. Still no 24bit. Sorry, not enough info for this.

The playback and capture over SPDIF use normal DAC and ADC, respectively, so you cannot playback both analog and digital streams simultaneously.

To enable SPDIF output, you need to turn on "IEC958 Output Switch" control via mixer or alsactl. Then you'll see the red light on from the card so you know that's working obviously :) The SPDIF input is always enabled, so you can hear SPDIF input data from line-out with "IEC958 In Monitor" switch at any time (see below).

You can play via SPDIF even with the first device (hw:0,0), but SPDIF is enabled only when the proper format (S16LE), sample rate (441100 or 48000) and channels (2) are used. Otherwise it's turned off. (Also don't forget to turn on "IEC958 Output Switch", too.)

Additionally there are relevant control switches:

"IEC958 Mix Analog" - Mix analog PCM playback and FM-OPL/3 streams and
    output through SPDIF.  This switch appears only on old chip
    models (CM8738 033 and 037).
    Note: without this control you can output PCM to SPDIF.
    This is "mixing" of streams, so e.g. it's not for AC3 output
    (see the next section).

"IEC958 In Select" - Select SPDIF input, the internal CD-in (false)
    and the external input (true).  This switch appears only on
    the chip models 039 or later.

"IEC958 Loop" - SPDIF input data is loop back into SPDIF
    output (aka bypass)

"IEC958 Copyright" - Set the copyright bit.

"IEC958 5V" - Select 0.5V (coax) or 5V (optical) interface.
    On some cards this doesn't work and you need to change the
    configuration with hardware dip-switch.

"IEC958 In Monitor" - SPDIF input is routed to DAC.

"IEC958 In Phase Inverse" - Set SPDIF input format as inverse.
    [FIXME: this doesn't work on all chips..]

"IEC958 In Valid"   - Set input validity flag detection.

Note: When "PCM Playback Switch" is on, you'll hear the digital output stream through analog line-out.

The AC3 (RAW DIGITAL) Output

The driver supports raw digital (typically AC3) input and output over SPDIF. This can be toggled via IEC958 playback control, but usually you need to access it via alsa-lib. See alsa-lib documents for more details.

In the raw digital mode, the "PCM Playback Switch" is automatically turned off so that non-audio data is heard from the analog line-out. Similarly the following switches are off: "IEC958 Mix Analog" and "IEC958 Loop". The switches are resumed after closing the SPDIF PCM device automatically to the previous state.

Analog Mixer Interface

The mixer interface on CM8x38 is similar to SB16. There are Master, PCM, Synth, CD, Line, Mic and PC Speaker playback volumes. Synth, CD, Line and Mic have playback and capture switches, too, as well as SB16.

In addition to the standard SB mixer, CM8x38 provides more functions.

  • PCM playback switch
  • PCM capture switch (to capture the data sent to DAC)
  • Mic Boost switch
  • Mic capture volume
  • Aux playback volume/switch and capture switch
  • 3D control switch

MIDI Controller

The MPU401-UART interface is disabled by default. You need to set module option "mpu_port" properly (eg "0x330") to enable it.

There is _no_ hardware wavetable function on this chip (except for OPL3 synth below). What's said as MIDI synth on Windows is a software synthesizer emulation. On Linux, you can use softsynths such as TiMidity for playing MIDI music. See the pages PlayingMidiFiles and recording MIDI files for more details.

FM OPL/3 Synth

The FM OPL/3 is disabled by default. To enable it, set "fm_port" module option to "0x388". You must also load the snd-opl3-synth kernel module, with

modprobe snd-opl3-synth

Don't forget to load some instruments, otherwise you won't hear any sound at all from your OPL/3 synth. Load instruments with

sbiload -p65:0 --opl3 std.o3 drums.o3

You need to specify the full path to the files std.o3 and drums.o3. To verify the port of your OPL/3, type aconnect -o. Those files are provided with the kde midi player kmid.

A more complete version, which always uses the correct port:

sbiload -p `pmidi -l |grep OPL3 |awk '{print $1}'` --opl3 -P /path/to/files std.o3 drums.o3

sbiload is found in alsa-tools. Debian doesn't have an alsa-tools package in sarge, but you can get it from the AGNULLA project, from

Else, you can get sbiload directly from CVS by typing

cvs -d '' login
[hit Enter for empty password]
cvs -z3 -d '' co alsa-tools/seq/sbiload

The quality of the OPL cell seems to be very strange at least on CMI8738. This is verified under DOS also. Perhaps there is some PCI chipset dependency, or CMI8738 has a hardware bug. Has anyone gotten good OPL3 sound from a CMI8x38 chip?

On my card (Leadtek Winfast) the OPL sound seems fine, comparable to that of the old Soundblaster cards.

Joystick and Modem

The joystick and modem should be available by enabling the control switch "Joystick" and "Modem" in /etc/asound.state respectively. But I myself have never tested them yet.

I (another I, than the previous one :) have tested it and I had to load the "ns558" module as well in order to get the gameport enabled. Not sure if the "gameport" and "analog" modules are required as well, but they are loaded here as well.

Debugging Information

The registers are shown in /proc/asound/cardX/cmipci. If you have any problem (especially unexpected behavior of mixer), please attach the output of this /proc file together with the bug report.

In some hardware configurations, card echoes the beginning of the signal instead of smooth playing. A workaround for some motherboards (e.g., Intel 865PE-based GA-8IPE1000) is to install the card into the last PCI slot, not in some middle.

Quick Install

NB. If you are using cvs then you need to type

make build

instead of

./configure  "or"  ./cvscompile

(if you are using rh9 with a "newer" kernel look at this link if you have problems installing the modules: )

In a shell type these commands:

Make a directory to store the alsa source code in.

cd /usr/src
mkdir alsa
cd alsa
cp /downloads/alsa-* .

Now unzip and install the alsa-driver package

bunzip2 alsa-driver-xxx
tar -xf alsa-driver-xxx
cd alsa-driver-xxx
./configure --with-snd-card=cmipci --with-sequencer=yes;make;make install

Now unzip and install the alsa-lib package

bunzip2 alsa-lib-xxx
tar -xf alsa-lib-xxx
cd ..
cd alsa-lib-xxx
./configure;make;make install

Now unzip and install the alsa-utils package

cd ..
bunzip2 alsa-utils-xxx
tar -xf alsa-utils-xxx
cd alsa-utils-xxx
./configure;make;make install

Now insert the modules into the kernel space.

modprobe snd-cmipci;modprobe snd-pcm-oss;modprobe snd-mixer-oss;modprobe snd-seq-oss

Now adjust your soundcards volume levels. All mixer channels are muted by default. You must use a native mixer program to unmute appropriate channels,for example alsamixer from the alsa-utils package

  • NB. Some soundcards don't utilise the alsamixer program so you will need to learn how to use the amixer program

You can also look at the utils/alsasound file. This script is designed for the RedHat distribution, but it can be used with other distributions which use System V style rc init scripts. This will allow you to load your modules at boot time. Of course if you want to do this you could just compile them into the kernel instead and save yourself the hassle of coming to terms with the rc init scripts.

Setting up modprobe and kmod support

NB. Before you send a mail complaining that "I don't have /etc/modules.conf, where do I find it ....." ,The /etc/conf.modules has been deprecated with a few distro's so in your case, it may still be /etc/conf.modules. Basically they are both same, but recent version of modutils uses /etc/modules.conf instead. Nothing to worry about as such, optionally please update to latest version of modutils. This should solve your problem.

Here's the example for this card. Copy and paste this to the bottom of your /etc/modules.conf file.

# ALSA portion
alias char-major-116 snd
alias snd-card-0 snd-cmipci
options snd-cmipci snd_enable_midi=1 

# OSS/Free portion
alias char-major-14 soundcore
alias sound-slot-0 snd-card-0

# card #1
alias sound-service-0-0 snd-mixer-oss
alias sound-service-0-1 snd-seq-oss
alias sound-service-0-3 snd-pcm-oss
alias sound-service-0-12 snd-pcm-oss

To copy and paste the above to your /etc/modules.conf file follow these instructions.

Options for the snd-cmipci module

The ALSA module snd-cmipci is for C-Media CMI8338 and 8738 PCI soundcards. This module supports autoprobe and multiple chips up to a maximum of 8. The available options for this module are:

mpu_port - 0x300,0x310,0x320,0x330,-1=disable (default)
fm_port - 0x388,-1=disable (default)
snd_dac_frame_size - max playback frame size in kB 4-128kB
snd_adc_frame_size - max capture frame size in kB 4-128kB
snd_enable_midi - 1 = enable, 0 = disable default
snd_enable_fm - 1 = enable, 0 = disable default

See also

Retrieved from ""

Category: ALSA modules