Suse Linux Professional 9.2 Upgrade [Strong Encryption 128 Bit] 00662644457260 User Manual
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00662644457260
15.1
The ALSA PCM Types
As of version 0.9 of the Advanced Linux Sound Architecture (ALSA), the concept for
PCM devices was fundamentally modified and expanded. PCM is an acronym
for “Pulse Code Modulation” and designates the digital output interfaces when
relating to sound cards. The user can influence the way ALSA addresses the
sound card by selecting a specific PCM type. The main PCM types are
PCM devices was fundamentally modified and expanded. PCM is an acronym
for “Pulse Code Modulation” and designates the digital output interfaces when
relating to sound cards. The user can influence the way ALSA addresses the
sound card by selecting a specific PCM type. The main PCM types are
hw
and
plughw
.
To understand the difference between the two types, consider how a PCM de-
vice is opened. It must be opened with specific settings for at least the following
parameters: sample format, sample frequency, number of channels, number of
periods (previously referred to as fragments), and size of a period. For example,
an application may attempt to play a WAV file with a sample frequency of 44.1
kHz although the sound card does not support this frequency. In this case, ALSA
can automatically convert the data in the plug-in layer to a format supported by
the sound card. The conversion affects the following parameters: sample format,
sample frequency, and number of channels.
vice is opened. It must be opened with specific settings for at least the following
parameters: sample format, sample frequency, number of channels, number of
periods (previously referred to as fragments), and size of a period. For example,
an application may attempt to play a WAV file with a sample frequency of 44.1
kHz although the sound card does not support this frequency. In this case, ALSA
can automatically convert the data in the plug-in layer to a format supported by
the sound card. The conversion affects the following parameters: sample format,
sample frequency, and number of channels.
Activate the plug-in layer by selecting the PCM type
plughw
. If the PCM type
hw
is selected, ALSA tries to open the PCM devices directly with the parameters
required by the application. The complete designator for a PCM device consists
of the PCM type followed by a colon, the card number, and the device number,
for example,
of the PCM type followed by a colon, the card number, and the device number,
for example,
plughw:0,0
.
The
dmix
output plug-in is a recent addition to the PCM types. It allows appli-
cations to share access to a PCM device on sound cards that do not support this
natively. The complete identifier in this case is
natively. The complete identifier in this case is
plug:dmix
. More information
about
dmix
can be found at
http://alsa.opensrc.org/index.php?page=
DmixPlugin
.
15.2
Mixers
Mixers provide a convenient means of controlling the volume and balance of the
sound output and input of computers. The main difference between the vari-
ous mixers consists in the outer appearance of the user interface. However, there
are a number of mixers that are designed for specific hardware. One example is
envy24control, a mixer for the Envy 24 sound chip. Another one is hdspmixer,
which is for RME Hammerfall cards. From the mixers available, select the one
that best suits your needs.
sound output and input of computers. The main difference between the vari-
ous mixers consists in the outer appearance of the user interface. However, there
are a number of mixers that are designed for specific hardware. One example is
envy24control, a mixer for the Envy 24 sound chip. Another one is hdspmixer,
which is for RME Hammerfall cards. From the mixers available, select the one
that best suits your needs.
176
15.1. The ALSA PCM Types