File Formats au(4) NAME au - AU audio file format SYNOPSIS #include DESCRIPTION An AU audio file is composed of three parts: a header, an optional description field, and a contiguous segment of audio data. The header is 24 bytes, and the description field is at least 4 bytes. Therefore, the offset for most AU files is 28 bytes. However, some people store additional data in the AU header. The AU audio structure members and audio data are stored big endian. That is, it starts with the most significant byte, regardless of the native byte order of the machine architec- ture on which an application may be running. Therefore, multi-byte audio data may require byte reversal for proper playback on different processor architectures. See the macro section for properly reading and writing the AU audio struc- ture members. The AU header is defined by the following structure: struct aufilehdr { uint32t aumagic; /* magic number (.snd) */ uint32t auoffset; /* byte offset to start of audio data */ uint32t audatasize; /* data length in bytes */ uint32t auencoding; /* data encoding */ uint32t ausamplerate; /* samples per second */ uint32t auchannels; /* number of interleaved channels */ }; typedef struct aufilehdr aufilehdrt; The aumagic field always contains the following constant for an AU audio file: AUDIOAUFILEMAGIC ( 0x2e736e64 ) /* ".snd" */ The auoffset field contains the length of the audio file header plus the variable length info field. Consequently, it can be interpreted as the offset from the start of the file to the start of the audio data. SunOS 5.11 Last change: 15 Jan 2001 1 File Formats au(4) The audatasize field contains the length, in bytes, of the audio data segment. If this length is not known when the header is written, it should be set to AUDIOAUNKNOWNSIZE, defined as follows: AUDIOAUNKNOWNSIZE ( ~0 ) /* (unsigned) -1 */ When the audatasize field contains AUDIOAUNKNOWNSIZE, the length of the audio data can be determined by subtract- ing auoffset from the total length of the file. The encoding field contains one of the following enumerated keys: AUDIOAUENCODINGULAW /* 8-bit u-law */ AUDIOAUENCODINGLINEAR8 /* 8-bit linear PCM */ AUDIOAUENCODINGLINEAR16 /* 16-bit linear PCM */ AUDIOAUENCODINGLINEAR24 /* 24-bit linear PCM */ AUDIOAUENCODINGLINEAR32 /* 32-bit linear PCM */ AUDIOAUENCODINGFLOAT /* Floating point */ AUDIOAUENCODINGDOUBLE /* Double precision float */ AUDIOAUENCODINGFRAGMENTED /* Fragmented sample data */ AUDIOAUENCODINGDSP /* DSP program */ AUDIOAUENCODINGFIXED8 /* 8-bit fixed point */ AUDIOAUENCODINGFIXED16 /* 16-bit fixed point */ AUDIOAUENCODINGFIXED24 /* 24-bit fixed point */ AUDIOAUENCODINGFIXED32 /* 32-bit fixed point */ AUDIOAUENCODINGEMPHASIS /* 16-bit linear with emphasis */ AUDIOAUENCODINGCOMPRESED /* 16-bit linear compressed */ AUDIOAUENCODINGEMPCOMP /* 16-bit linear with emphasis and compression */ AUDIOAUENCODINGMUSICKIT /* Music kit DSP commands */ AUDIOAUENCODINGADPCMG721 /* CIT G.721 ADPCM */ AUDIOAUENCODINGADPCMG722 /* CIT G.722 ADPCM */ AUDIOAUENCODINGADPCMG7233 /* CIT G.723.3 ADPCM */ AUDIOAUENCODINGADPCMG7235 /* CIT G.723.5 ADPCM */ AUDIOAUENCODINGALAW /* 8-bit A-law G.711 */ All of the linear encoding formats are signed integers cen- tered at zero. The ausamplerate field contains the audio file's sampling rate in samples per second. Some common sample rates include 8000, 11025, 22050, 44100, and 48000 samples per second. SunOS 5.11 Last change: 15 Jan 2001 2 File Formats au(4) The auchannels field contains the number of interleaved data channels. For monaural data, this value is set to one. For stereo data, this value is set to two. More than two data channels can be interleaved, but such formats are currently unsupported by the Solaris audio driver architec- ture. For a stereo sound file, the first sample is the left track and the second sample is the right track. The optional info field is a variable length annotation field that can be either text or data. If it is a text description of the sound, then it should be NUL terminated. However, some older files might not be terminated properly. The size of the info field is set when the structure is created and cannot be enlarged later. Macros Accessing all of the AU audio structure members should be done through the supplied AUDIOAUFILE2HOST and AUDIOAUHOST2FILE macros. By always using these macros, code will be byte-order independent. See the example below. EXAMPLES Example 1 Displaying Header Information for a Sound File The following program reads and displays the header informa- tion for an AU sound file. The AUDIOAUFILE2HOST macro ensures that this information will always be in the proper byte order. void main(void) { aufilehdrt hdr; aufilehdrt local; int fd; char *name = "bark.au"; if ((fd = open(name, ORDONLY)) < 0) { printf("can't open file %s\n", name); exit(1); } (void) read(fd, &hdr, sizeof (hdr)); AUDIOAUFILE2HOST(&hdr.aumagic, &local.aumagic); AUDIOAUFILE2HOST(&hdr.auoffset, &local.auoffset); AUDIOAUFILE2HOST(&hdr.audatasize, &local.audatasize); AUDIOAUFILE2HOST(&hdr.auencoding, &local.auencoding); AUDIOAUFILE2HOST(&hdr.ausamplerate, &local.ausamplerate); AUDIOAUFILE2HOST(&hdr.auchannels, &local.auchannels); SunOS 5.11 Last change: 15 Jan 2001 3 File Formats au(4) printf("Magic = %x\n", local.aumagic); printf("Offset = %d\n", local.auoffset); printf("Number of data bytes = %d\n", local.audatasize); printf("Sound format = %d\n", local.auencoding); printf("Sample rate = %d\n", local.ausamplerate); printf("Number of channels = %d\n", local.auchannels); (void) close(fd); } ATRIBUTES See attributes(5) for descriptions of the following attri- butes: ATRIBUTE TYPE ATRIBUTE VALUE Availability SUNWaudh Stability Level Evolving SEE ALSO attributes(5) NOTES Some older AU audio files are incorrectly coded with info strings that are not properly NUL-terminated. Thus, appli- cations should always use the auoffset value to find the end of the info data and the beginning of the audio data. SunOS 5.11 Last change: 15 Jan 2001 4
