1audio(7D) Devices audio(7D)
2
6 audio - common audio framework
7
9 The audio driver provides common support routines for audio devices in
10 Solaris.
11 The audio framework supports multiple personalities, allowing for
14 devices to be accessed with different programming interfaces.
15 The audio framework also provides a number of facilities, such as mix‐
18 ing of audio streams, and data format and sample rate conversion.
19 Overview
21 The audio framework provides a software mixing engine (audio mixer) for
22 all audio devices, allowing more than one process to play or record
23 audio at the same time.
24 Multi-Stream Codecs
26 The audio mixer supports multi-stream Codecs. These devices have DSP
27 engines that provide sample rate conversion, hardware mixing, and
28 other features. The use of such hardware features is opaque to applica‐
29 tions.
30 Backward Compatibility
32 It is not possible to disable the mixing function. Applications must
33 not assume that they have exclusive access to the audio device.
34 Audio Formats
36 Digital audio data represents a quantized approximation of an analog
37 audio signal waveform. In the simplest case, these quantized numbers
38 represent the amplitude of the input waveform at particular sampling
39 intervals. To achieve the best approximation of an input signal, the
40 highest possible sampling frequency and precision should be used. How‐
41 ever, increased accuracy comes at a cost of increased data storage
42 requirements. For instance, one minute of monaural audio recorded in u-
43 Law format (pronounced mew-law) at 8 KHz requires nearly 0.5 megabytes
44 of storage, while the standard Compact Disc audio format (stereo 16-bit
45 linear PCM data sampled at 44.1 KHz) requires approximately 10
46 megabytes per minute.
47 An audio data format is characterized in the audio driver by four
50 parameters: sample Rate, encoding, precision, and channels. Refer to
51 the device-specific manual pages for a list of the audio formats that
52 each device supports. In addition to the formats that the audio device
53 supports directly, other formats provide higher data compression.
54 Applications can convert audio data to and from these formats when
55 playing or recording.
56 Sample Rate
58 Sample rate is a number that represents the sampling frequency (in sam‐
59 ples per second) of the audio data.
60 The audio mixer always configures the hardware for the highest possible
63 sample rate for both play and record. This ensures that none of the
64 audio streams require compute-intensive low pass filtering. The result
65 is that high sample rate audio streams are not degraded by filtering.
66 Sample rate conversion can be a compute-intensive operation, dependin‐
69 gon the number of channels and a device's sample rate. For example, an
70 8KHz signal can be easily converted to 48KHz, requiring a low cost up
71 sampling by 6. However, converting from 44.1KHz to 48KHz is computer
72 intensive because it must be up sampled by 160 and then down sampled by
73 147. This is only done using integer multipliers.
74 Applications can greatly reduce the impact of sample rate conversion by
77 carefully picking the sample rate. Applications should always use the
78 highest sample rate the device supports. An application can also do its
79 own sample rate conversion (to take advantage of floating point and
80 accelerated instructions) or use small integers for up and down sam‐
81 pling.
82 All modern audio devices run at 48 kHz or a multiple thereof, hence
85 just using 48 kHz can be a reasonable compromise if the application is
86 not prepared to select higher sample rates.
87 Encodings
89 An encoding parameter specifies the audiodata representation. u-Law
90 encoding corresponds to CCITT G.711, and is the standard for voice data
91 used by telephone companies in the United States, Canada, and Japan. A-
92 Law encoding is also part of CCITT G.711 and is the standard encoding
93 for telephony elsewhere in the world. A-Law and u-Law audio data are
94 sampled at a rate of 8000 samples per second with 12-bit precision,
95 with the data compressed to 8-bit samples. The resulting audio data
96 quality is equivalent to that of stan dard analog telephone service.
97 Linear Pulse Code Modulation (PCM) is an uncompressed, signed audio
100 format in which sample values are directly proportional to audio signal
101 voltages. Each sample is a 2's complement number that represents a pos‐
102 itive or negative amplitude.
103 Precision
105 Precision indicates the number of bits used to store each audio sample.
106 For instance, u-Law and A-Law data are stored with 8-bit precision. PCM
107 data can be stored at various precisions, though 16-bit is the most
108 common.
109 Channels
111 Multiple channels of audio can be interleaved at sample boundaries. A
112 sample frame consists of a single sample from each active channel. For
113 example, a sample frame of stereo 16-bit PCM data consists of 2 16-bit
114 samples, corresponding to the left and right channel data. The audio
115 mixer sets the hardware to the maximum number of channels supported. If
116 a mono signal is played or recorded, it is mixed on the first two (usu‐
117 ally the left and right) channel only. Silence is mixed on the remain‐
118 ing channels.
119 Supported Formats
121 The audio mixer supports the following audio formats:
122 Encoding Precision Channels
124 Signed Linear PCM 32-bit Mono or Stereo
125 Signed Linear PCM 16-bit Mono or Stereo
126 Signed Linear PCM 8-bit Mono or Stereo
127 u-Law 8-bit Mono or Stereo
128 A-Law 8-bit Mono or Stereo
129 The audio mixer converts all audio streams to 24-bit Linear PCM before
134 mixing. After mixing, conversion is made to the best possible Codec
135 format. The conversion process is not compute intensive and audio
136 applications can choose the encoding format that best meets their
137 needs.
138 The mixer discards the low order 8 bits of 32-bit Signed Linear PCM in
141 order to perform mixing. (This is done to allow for possible overflows
142 to fit into 32-bits when mixing multiple streams together.) Hence, the
143 maximum effective precision is 24-bits.
144
146 /kernel/drv/audio 32-bit kernel driver module
147 /kernel/drv/amd64/audio 64-bit x86 kernel driver module
150 /kernel/drv/sparcv9/audio 64-bit SPARC kernel driver module
153 /kernel/drv/audio.conf audio configuration file
156
159 See attributes(5) for a description of the following attributes:
160 ┌─────────────────────────────┬─────────────────────────────┐
165 │ATTRIBUTE TYPE │ATTRIBUTE VALUE │
166 ├─────────────────────────────┼─────────────────────────────┤
167 │Architecture │SPARC, x86 │
168 ├─────────────────────────────┼─────────────────────────────┤
169 │Availability │SUNWaudd │
170 ├─────────────────────────────┼─────────────────────────────┤
171 │Interface Stability │Uncommitted │
172 └─────────────────────────────┴─────────────────────────────┘
173
175 ioctl(2), attributes(5), audio(7I), dsp(7I)
176SunOS 5.11 3 Aug 2009 audio(7D)