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If applicable, add the following below this CDDL HEADER, with the 5 .\" fields enclosed by brackets "[]" replaced with your own identifying information: Portions Copyright [yyyy] [name of copyright owner] 6 .TH AUDIO 7D "Aug 3, 2009" 7 .SH NAME 8 audio \- common audio framework 9 .SH DESCRIPTION 10 .sp 11 .LP 12 The \fBaudio\fR driver provides common support routines for audio devices in 13 Solaris. 14 .sp 15 .LP 16 The audio framework supports multiple \fBpersonalities\fR, allowing for devices 17 to be accessed with different programming interfaces. 18 .sp 19 .LP 20 The audio framework also provides a number of facilities, such as mixing of 21 audio streams, and data format and sample rate conversion. 22 .SS "Overview" 23 .sp 24 .LP 25 The audio framework provides a software mixing engine (audio mixer) for all 26 audio devices, allowing more than one process to play or record audio at the 27 same time. 28 .SS "Multi-Stream Codecs" 29 .sp 30 .LP 31 The audio mixer supports multi-stream Codecs. These devices have DSP engines 32 that provide sample rate conversion, hardware mixing, and other features. The 33 use of such hardware features is opaque to applications. 34 .SS "Backward Compatibility" 35 .sp 36 .LP 37 It is not possible to disable the mixing function. Applications must not assume 38 that they have exclusive access to the audio device. 39 .SS "Audio Formats" 40 .sp 41 .LP 42 Digital audio data represents a quantized approximation of an analog audio 43 signal waveform. In the simplest case, these quantized numbers represent the 44 amplitude of the input waveform at particular sampling intervals. To achieve 45 the best approximation of an input signal, the highest possible sampling 46 frequency and precision should be used. However, increased accuracy comes at a 47 cost of increased data storage requirements. For instance, one minute of 48 monaural audio recorded in u-Law format (pronounced \fBmew-law\fR) at 8 KHz 49 requires nearly 0.5 megabytes of storage, while the standard Compact Disc audio 50 format (stereo 16-bit linear PCM data sampled at 44.1 KHz) requires 51 approximately 10 megabytes per minute. 52 .sp 53 .LP 54 An audio data format is characterized in the audio driver by four parameters: 55 sample Rate, encoding, precision, and channels. Refer to the device-specific 56 manual pages for a list of the audio formats that each device supports. In 57 addition to the formats that the audio device supports directly, other formats 58 provide higher data compression. Applications can convert audio data to and 59 from these formats when playing or recording. 60 .SS "Sample Rate" 61 .sp 62 .LP 63 Sample rate is a number that represents the sampling frequency (in samples per 64 second) of the audio data. 65 .sp 66 .LP 67 The audio mixer always configures the hardware for the highest possible sample 68 rate for both play and record. This ensures that none of the audio streams 69 require compute-intensive low pass filtering. The result is that high sample 70 rate audio streams are not degraded by filtering. 71 .sp 72 .LP 73 Sample rate conversion can be a compute-intensive operation, dependingon the 74 number of channels and a device's sample rate. For example, an 8KHz signal can 75 be easily converted to 48KHz, requiring a low cost up sampling by 6. However, 76 converting from 44.1KHz to 48KHz is computer intensive because it must be up 77 sampled by 160 and then down sampled by 147. This is only done using integer 78 multipliers. 79 .sp 80 .LP 81 Applications can greatly reduce the impact of sample rate conversion by 82 carefully picking the sample rate. Applications should always use the highest 83 sample rate the device supports. An application can also do its own sample rate 84 conversion (to take advantage of floating point and accelerated instructions) 85 or use small integers for up and down sampling. 86 .sp 87 .LP 88 All modern audio devices run at 48 kHz or a multiple thereof, hence just using 89 48 kHz can be a reasonable compromise if the application is not prepared to 90 select higher sample rates. 91 .SS "Encodings" 92 .sp 93 .LP 94 An encoding parameter specifies the audiodata representation. u-Law encoding 95 corresponds to CCITT G.711, and is the standard for voice data used by 96 telephone companies in the United States, Canada, and Japan. A-Law encoding is 97 also part of CCITT G.711 and is the standard encoding for telephony elsewhere 98 in the world. A-Law and u-Law audio data are sampled at a rate of 8000 samples 99 per second with 12-bit precision, with the data compressed to 8-bit samples. 100 The resulting audio data quality is equivalent to that of stan dard analog 101 telephone service. 102 .sp 103 .LP 104 Linear Pulse Code Modulation (PCM) is an uncompressed, signed audio format in 105 which sample values are directly proportional to audio signal voltages. Each 106 sample is a 2's complement number that represents a positive or negative 107 amplitude. 108 .SS "Precision" 109 .sp 110 .LP 111 Precision indicates the number of bits used to store each audio sample. For 112 instance, u-Law and A-Law data are stored with 8-bit precision. PCM data can be 113 stored at various precisions, though 16-bit is the most common. 114 .SS "Channels" 115 .sp 116 .LP 117 Multiple channels of audio can be interleaved at sample boundaries. A sample 118 frame consists of a single sample from each active channel. For example, a 119 sample frame of stereo 16-bit PCM data consists of 2 16-bit samples, 120 corresponding to the left and right channel data. The audio mixer sets the 121 hardware to the maximum number of channels supported. If a mono signal is 122 played or recorded, it is mixed on the first two (usually the left and right) 123 channel only. Silence is mixed on the remaining channels. 124 .SS "Supported Formats" 125 .sp 126 .LP 127 The audio mixer supports the following audio formats: 128 .sp 129 .in +2 130 .nf 131 Encoding Precision Channels 132 Signed Linear PCM 32-bit Mono or Stereo 133 Signed Linear PCM 16-bit Mono or Stereo 134 Signed Linear PCM 8-bit Mono or Stereo 135 u-Law 8-bit Mono or Stereo 136 A-Law 8-bit Mono or Stereo 137 .fi 138 .in -2 139 .sp 140 141 .sp 142 .LP 143 The audio mixer converts all audio streams to 24-bit Linear PCM before mixing. 144 After mixing, conversion is made to the best possible Codec format. The 145 conversion process is not compute intensive and audio applications can choose 146 the encoding format that best meets their needs. 147 .sp 148 .LP 149 The mixer discards the low order 8 bits of 32-bit Signed Linear PCM in order to 150 perform mixing. (This is done to allow for possible overflows to fit into 151 32-bits when mixing multiple streams together.) Hence, the maximum effective 152 precision is 24-bits. 153 .SH FILES 154 .sp 155 .ne 2 156 .na 157 \fB\fB/kernel/drv/audio\fR\fR 158 .ad 159 .RS 29n 160 32-bit kernel driver module 161 .RE 162 163 .sp 164 .ne 2 165 .na 166 \fB\fB/kernel/drv/amd64/audio\fR\fR 167 .ad 168 .RS 29n 169 64-bit x86 kernel driver module 170 .RE 171 172 .sp 173 .ne 2 174 .na 175 \fB\fB/kernel/drv/sparcv9/audio\fR\fR 176 .ad 177 .RS 29n 178 64-bit SPARC kernel driver module 179 .RE 180 181 .sp 182 .ne 2 183 .na 184 \fB\fB/kernel/drv/audio.conf\fR\fR 185 .ad 186 .RS 29n 187 \fBaudio\fR configuration file 188 .RE 189 190 .SH ATTRIBUTES 191 .sp 192 .LP 193 See \fBattributes\fR(5) for a description of the following attributes: 194 .sp 195 196 .sp 197 .TS 198 box; 199 l | l 200 l | l . 201 ATTRIBUTE TYPE ATTRIBUTE VALUE 202 _ 203 Architecture SPARC, x86 204 _ 205 Interface Stability Uncommitted 206 .TE 207 208 .SH SEE ALSO 209 .sp 210 .LP 211 \fBioctl\fR(2), \fBattributes\fR(5), \fBaudio\fR(7I), \fBdsp\fR(7I)