Inside Macintosh: Sound /

Chapter 2 - Sound Manager / Sound Manager Reference

Sound Manager Routines / Compressing and Expanding Audio Data

Exp1to3

You can use the Exp1to3 procedure to expand a buffer of sound samples you previously have compressed with the Comp3to1 procedure.

PROCEDURE Exp1to3 (inBuffer: Ptr; outBuffer: Ptr; cnt: LongInt;
                   inState: Ptr; outState: Ptr; 
                   numChannels: LongInt; whichChannel: LongInt);

inBuffer

A pointer to a buffer of packets to be expanded.

outBuffer

A pointer to a buffer where the expanded samples will be written.

cnt

The number of packets to expand.

inState

A pointer to a 128-byte buffer from which the input state of the algorithm is read, or NIL. To initialize the algorithm, this buffer should be filled with zeros.

outState

A pointer to a 128-byte buffer to which the output state of the algorithm is written, or NIL. This buffer might be the same as that specified by the inState parameter.

numChannels

The number of channels in the buffer pointed to by the inBuffer parameter.

whichChannel

The channel to expand, when numChannels is greater than 1. This parameter must be in the range of 1 to numChannels.

DESCRIPTION

The Exp1to3 procedure expands cnt packets of sound stored in the buffer specified by inBuffer and places the result in the buffer specified by outBuffer, whose size must be at least cnt packets * 2 bytes per packet * 3, or cnt * 6 bytes. If numChannels is greater than 1, then the compressed sound must be stored in interleaved format on a packet basis.

If you expand compressed sound data that includes multiple sound channels, you retain only one channel of sound, which you specify in the whichChannel parameter. Thus, if you use the Exp1to3 procedure to expand three-channel sound, the output buffer will be the same size as the input buffer since only one channel is retained. To retain multiple channels of sound after expansion, you must call the Exp1to3 procedure for each channel to be expanded and then interleave the expanded sound data on a sample basis.

The Exp1to3 procedure expands every packet of sampled-sound data to exactly 6 bytes.

You can use the inState and outState parameters to allow the MACE compression routines to preserve information about algorithms across calls. Alternatively, you may pass NIL state buffers and let the Sound Manager allocate the buffers internally.

SPECIAL CONSIDERATIONS

Because the Exp1to3 procedure might allocate memory, you should not call it at interrupt time.

ASSEMBLY-LANGUAGE INFORMATION

The trap macro and routine selector for the Exp1to3 procedure are

Trap macro	Selector
_SoundDispatch	$00080010