Compressing and Expanding Sounds

Some of the capabilities provided by MACE are transparently available to

your application. For example, if you pass the SndPlay function a handle to an

'snd ' resource that contains a compressed sampled sound, the sampled sound

synthesizer automatically expands the sound data for playback in real time.

Your application does not need to know whether the 'snd ' resource contains

compressed or noncompressed samples when it calls SndPlay. This is because

sufficient information is in the resource itself to allow the synthesizer to

determine whether it should expand the data samples.

However, aside from expansion playback, all of the MACE capabilities need to

be specifically requested by your application. For example, you can use the

procedures Comp3to1 or Comp6to1 if you want to compress a sampled

sound (for example, to create an 'snd ' resource containing compressed audio

data). And you can use the procedures Exp1to3 and Exp1to6 to expand

compressed audio data.

All of these procedures require you to specify both an input and an output

buffer, from and to which the sampled sound data to be converted is read and

written. Your application must allocate the appropriate amount of storage for

each buffer. For example, if you want to expand a buffer of compressed

sampled sound data by using Exp1to6, the output buffer must be at least six

times the size of the input buffer.

When calling these routines, you must also specify addresses of two small

buffers (128 bytes each) that the Sound Manager uses to maintain state

information about the compression or expansion process. When you first call a

MACE routine, the state buffers should be filled with zeros to initialize the

state information. You can pass NULL for both buffers if you do not want to save

state information across calls to the MACE routines. The code example below

illustrates the use of the Comp3to1 procedure.

Because the numChannels and the whichChannel parameters are both set to 1,

Comp3to1 compresses monophonic audio data.

// Compressing audio data

// Assuming inclusion of MacHeaders

#include <Sound.h>

// Prototype routine like this prior to calling it

void CompressBy3(Ptr,Ptr,long);

void CompressBy3 (Ptr inBuf, Ptr outBuf, long numSamp)

{

Ptr myInState; // input state buffer

Ptr myOutState; // output state buffer

// Allocate both state buffers

myInState = NewPtrClear(128);

myOutState = NewPtrClear(128);

// Check that both buffers are non-nil i.e. non-zero

if ( myInState && myOutState )

// Compress audio data

Comp3to1(inBuf, outBuf, numSamp, myInState, myOutState, 1, 1);

}