Inside Macintosh: QuickTime

The Sound Description Structure

A sound description structure contains information that defines the characteristics of one or more sound samples. Data in the sound description structure indicates the type of compression that was used, the sample size, the rate at which samples were obtained, and so on. Sound media handlers use the information in the sound description structure when they process the sound samples.

See the chapter "Image Compression Manager" for a description of the image description structure, which contains information that defines the characteristics of an image.

The SoundDescription data type defines the layout of a sound description structure. See "Media Functions," which begins on Media Functions , for more information about sound media handlers.



 
struct SoundDescription 

{

    long    descSize;           /* number of bytes in this structure */

    long    dataFormat;         /* format of the sound data */

    long    resvd1;             /* reserved--set to 0 */

    short   resvd2;             /* reserved--set to 0 */

    short   dataRefIndex;       /* reserved--set to 1 */

    short   version;            /* reserved--set to 0 */

    short   revlevel;           /* reserved--set to 0 */

    long    vendor;             /* reserved--set to 0 */

    short   numChannels;        /* number of channels used by sample */

    short   sampleSize;         /* number of bits in each sample */

    short   compressionID;      /* reserved--set to 0 */

    short   packetSize;         /* reserved--set to 0 */

    Fixed   sampleRate;         /* rate at which samples were obtained */

};

descSize: Defines the total size, in bytes, of this sound description structure.
dataFormat: Describes the format of the sound data. Possible values include:; Some older movie files sometimes have a zero value in this field. You should assume that this is the same as the 'raw ' value.
resvd1: Reserved for Apple. Set this field to 0 in any sound description structures you create.
resvd2: Reserved for Apple. Set this field to 0 in any sound description structures you create.
dataRefIndex: Reserved for Apple. Set this field to 0 in any sound description structures you create.
version: Reserved for Apple. Set this field to 0 in any sound description structures you create.
revLevel: Reserved for Apple. Set this field to 0 in any sound description structures you create.
vendor: Reserved for Apple. Set this field to 0 in any sound description structures you create.
numChannels: Indicates the number of sound channels used by the sound sample. Set this field to 1 for monaural sounds; set it to 2 for stereo sounds.
sampleSize: Specifies the number of bits in each sound sample. Set this field to 8 for 8-bit sound; set it to 16 for 16-bit sound.
compressionID: Reserved for Apple. Set this field to 0 in any sound description structures you create.
packetSize: Reserved for Apple. Set this field to 0 in any sound description structures you create.
sampleRate: Indicates the rate at which the sound samples were obtained. Sound media handlers use this value to influence the natural playback speed of the sound described by this sound description structure. This field contains an unsigned, fixed-point number that specifies the number of samples collected per second. Some common values include:

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	'raw '	Sound samples are stored uncompressed, in offset-binary format (that is, sample data values range from 0 to 255).
	'twos'	Sound samples are stored uncompressed, in twos-complement format (that is, sample data values range from -128 to 127). The Sound Manager uses this format when it creates sound files in Audio Interchange File Format (AIFF).
	'MAC3'	Sound samples have been compressed by the Sound Manager at a ratio of 3:1.
	'MAC6'	Sound samples have been compressed by the Sound Manager at a ratio of 6:1.

	0x15BBA2E8	Specifies a sample rate of 5563.6363 samples per second.
	0x1CFA2E8B	Specifies a sample rate of 7418.1818 samples per second.
	0x2B7745D1	Specifies a sample rate of 11127.2727 samples per second.
	0x56EE8BA3	Specifies a sample rate of 22254.5454 samples per second.
	0xAC440000	Specifies a sample rate of 44100.0000 samples per second.