From low-art Gloominati

I finally had a reason to get into ALSA in more depth, so this page will summarize what I learned as well as describe my dual card setup.... but not right now, b/c I am tired :). For now I am just dropping random bits of info here.

[edit] Current setup in /etc/asound.conf

# Here is a diagram of what this config does:
#
#                                                                           'nforce'              nForce2,0
#                                                  4-channel             _______________         ____________
#             SOFTVOL               ROUTE            MULTI          0L  |     PLUG      |       |            |
#          ______________         __________       _________   /------->|    convert    |------>|            |
#   0L    |              |  0L   |          |  0L |         | /         | rate & format |       | Sound card |
# ------->|   Software   |------>|\---------|---->|---------|-      1R  |    for HW     |       |            |
#         |    volume    |       | \        |     |         |  /------->|               |------>|            |
#   1R    |  attenuation |  1R   |  \       |  1R |         | /         |_______________|       |____________|
# ------->|              |------>|---\------|---->|---------|-           _______________
#         |______________|       |\   \     |  2L |         |       0L  |     PLUG      |        ____________
#            'softvol'           | \   \----|---->|---------|---------->|    convert    |------>|            |
#                                |  \       |  3R |         |       1R  | rate & format |       | Sound card |
#                                |   \------|---->|---------|---------->|     for HW    |------>|            |
#                                |__________|     |_________|           |_______________|       |____________|
#                                   'both'          'multi'               'audiophile'             M2496,0
#                               
#

################################################## BEGIN ##################################################

# Give our cards friendly aliases. Note that we are wrapping the cards with 'plug' plugin
# in order to perform automatic conversion of the stream from the original format into 
# the format suitable for each individual card.
#
pcm.nforce "plughw:nForce2,0"
pcm.audiophile "plughw:M2496,0"

pcm.!default main_device

# 'asym' plugin allows us to create assymetrical devices which have different 
# interfaces handling playback and recording streams.
# This configuration ensures that playback is sent through the complicated routing,
# but recording streams are fetched directly from Audiophile
#
pcm.main_device {
	type asym
	playback.pcm "softvol"
	capture.pcm "audiophile"
}

# This device acts as a software volume control
pcm.softvol {
    type            softvol
    slave {
        pcm         "both"
    }
    control {
        name        "SoftMaster" 	# This is the name of the mixer control
        card        M2496
    }
}

# Here we are using the 'route' plugin to send the signal unchanged to a 
# 4-channel device. Since the incoming stream is only 2-channel, we use
# the transfer table to copy channel 0 slave's input channel 0, 
# channel 1 to slave's 1, and again channel 0 to slave's 2 and channel 1
# to slave's 3. The last column (1.0) is the volume, which we are leaving unchanged
#
pcm.both {
	type route
	slave {
		pcm multi
		channels 4
	}
	ttable.0.0 1.0
	ttable.1.1 1.0
	ttable.0.2 1.0
	ttable.1.3 1.0
}

# 'multi' plugin allows us to create many slave devices and then distribute the stream's
# channels between the slaves.
# Here we define Slave a for the nforce card (headphones) and Slave b for the Audiophile.
# Then we use channel binding to send channels 0 and 1 to the headphones and channels
# 2 and 3 to the speakers
#
pcm.multi {
	type multi
	slaves.a {
		pcm "nforce"
		channels 2
	}
	slaves.b {
		pcm "audiophile"
		channels 2
	}
	bindings.0.slave a
	bindings.0.channel 0
	bindings.1.slave a
	bindings.1.channel 1
	bindings.2.slave b
	bindings.2.channel 0
	bindings.3.slave b
	bindings.3.channel 1
}

pcm.dsp0 {
    type plug
    slave.pcm "softvol"
}

ctl.main_device {
    type hw
    card M2496
}

[edit] How can I find which processes are using Alsa devices?

fuser -v /dev/snd/*

[edit] Misc

http://www.volkerschatz.com/noise/alsa.html

http://www.alsa-project.org/alsa-doc/alsa-lib/pcm_plugins.html

http://www.mythtv.org/pipermail/mythtv-users/2005-June/091327.html

http://quicktoots.linuxaudio.org/toots/el-cheapo/

/usr/share/alsa/alsa.conf

• Use alsaplayer for detailed error messages when the setup doesn't work at all.
• Use aplay -v to see all of the routing details when we can write to whatever device we are testing.