audio(7D) Devices audio(7D)NAMEaudio - common audio framework
DESCRIPTION
The audio driver provides common support routines for audio devices in
Solaris.
The audio framework supports multiple personalities, allowing for
devices to be accessed with different programming interfaces.
The audio framework also provides a number of facilities, such as mix‐
ing of audio streams, and data format and sample rate conversion.
Overview
The audio framework provides a software mixing engine (audio mixer) for
all audio devices, allowing more than one process to play or record
audio at the same time.
Multi-Stream Codecs
The audio mixer supports multi-stream Codecs. These devices have DSP
engines that provide sample rate conversion, hardware mixing, and
other features. The use of such hardware features is opaque to applica‐
tions.
Backward Compatibility
It is not possible to disable the mixing function. Applications must
not assume that they have exclusive access to the audio device.
Audio Formats
Digital audio data represents a quantized approximation of an analog
audio signal waveform. In the simplest case, these quantized numbers
represent the amplitude of the input waveform at particular sampling
intervals. To achieve the best approximation of an input signal, the
highest possible sampling frequency and precision should be used. How‐
ever, increased accuracy comes at a cost of increased data storage
requirements. For instance, one minute of monaural audio recorded in u-
Law format (pronounced mew-law) at 8 KHz requires nearly 0.5 megabytes
of storage, while the standard Compact Disc audio format (stereo 16-bit
linear PCM data sampled at 44.1 KHz) requires approximately 10
megabytes per minute.
An audio data format is characterized in the audio driver by four
parameters: sample Rate, encoding, precision, and channels. Refer to
the device-specific manual pages for a list of the audio formats that
each device supports. In addition to the formats that the audio device
supports directly, other formats provide higher data compression.
Applications can convert audio data to and from these formats when
playing or recording.
Sample Rate
Sample rate is a number that represents the sampling frequency (in sam‐
ples per second) of the audio data.
The audio mixer always configures the hardware for the highest possible
sample rate for both play and record. This ensures that none of the
audio streams require compute-intensive low pass filtering. The result
is that high sample rate audio streams are not degraded by filtering.
Sample rate conversion can be a compute-intensive operation, dependin‐
gon the number of channels and a device's sample rate. For example, an
8KHz signal can be easily converted to 48KHz, requiring a low cost up
sampling by 6. However, converting from 44.1KHz to 48KHz is computer
intensive because it must be up sampled by 160 and then down sampled by
147. This is only done using integer multipliers.
Applications can greatly reduce the impact of sample rate conversion by
carefully picking the sample rate. Applications should always use the
highest sample rate the device supports. An application can also do its
own sample rate conversion (to take advantage of floating point and
accelerated instructions) or use small integers for up and down sam‐
pling.
All modern audio devices run at 48 kHz or a multiple thereof, hence
just using 48 kHz can be a reasonable compromise if the application is
not prepared to select higher sample rates.
Encodings
An encoding parameter specifies the audiodata representation. u-Law
encoding corresponds to CCITT G.711, and is the standard for voice data
used by telephone companies in the United States, Canada, and Japan. A-
Law encoding is also part of CCITT G.711 and is the standard encoding
for telephony elsewhere in the world. A-Law and u-Law audio data are
sampled at a rate of 8000 samples per second with 12-bit precision,
with the data compressed to 8-bit samples. The resulting audio data
quality is equivalent to that of stan dard analog telephone service.
Linear Pulse Code Modulation (PCM) is an uncompressed, signed audio
format in which sample values are directly proportional to audio signal
voltages. Each sample is a 2's complement number that represents a pos‐
itive or negative amplitude.
Precision
Precision indicates the number of bits used to store each audio sample.
For instance, u-Law and A-Law data are stored with 8-bit precision. PCM
data can be stored at various precisions, though 16-bit is the most
common.
Channels
Multiple channels of audio can be interleaved at sample boundaries. A
sample frame consists of a single sample from each active channel. For
example, a sample frame of stereo 16-bit PCM data consists of 2 16-bit
samples, corresponding to the left and right channel data. The audio
mixer sets the hardware to the maximum number of channels supported. If
a mono signal is played or recorded, it is mixed on the first two (usu‐
ally the left and right) channel only. Silence is mixed on the remain‐
ing channels.
Supported Formats
The audio mixer supports the following audio formats:
Encoding Precision Channels
Signed Linear PCM 32-bit Mono or Stereo
Signed Linear PCM 16-bit Mono or Stereo
Signed Linear PCM 8-bit Mono or Stereo
u-Law 8-bit Mono or Stereo
A-Law 8-bit Mono or Stereo
The audio mixer converts all audio streams to 24-bit Linear PCM before
mixing. After mixing, conversion is made to the best possible Codec
format. The conversion process is not compute intensive and audio
applications can choose the encoding format that best meets their
needs.
The mixer discards the low order 8 bits of 32-bit Signed Linear PCM in
order to perform mixing. (This is done to allow for possible overflows
to fit into 32-bits when mixing multiple streams together.) Hence, the
maximum effective precision is 24-bits.
FILES
/kernel/drv/audio 32-bit kernel driver module
/kernel/drv/amd64/audio 64-bit x86 kernel driver module
/kernel/drv/sparcv9/audio 64-bit SPARC kernel driver module
/kernel/drv/audio.conf audio configuration file
ATTRIBUTES
See attributes(5) for a description of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ATTRIBUTE TYPE │ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Architecture │SPARC, x86 │
├─────────────────────────────┼─────────────────────────────┤
│Availability │driver/audio │
├─────────────────────────────┼─────────────────────────────┤
│Interface Stability │Uncommitted │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOioctl(2), attributes(5), audio(7I), dsp(7I)SunOS 5.11 3 Aug 2009 audio(7D)
