Playing Sampled Sounds
You can play a sampled sound by calling the SndPlay function and passing it a handle to an 'snd ' resource that contains a sampled sound header. To gain
greater control over the sound output, you can use a number of sound
commands, including bufferCmd, soundCmd, rateCmd, and getRateCmd. Both
bufferCmd and soundCmd specify a pointer to a sampled sound header that is
locked into memory. The soundCmd command is used to install a sampled sound
into a channel as a voice and is discussed in the section entitled
To play a sampled sound in one-shot mode (without any looping), use the
bufferCmd command. The pointer in the param2 field of the sound command is
the location of a sampled sound header. A bufferCmd command is queued in the
channel until the preceding commands have been processed. If the bufferCmd
command is contained within an 'snd ' resource, the high bit of the command
must be set. If the sound was loaded in from an 'snd ' resource, your application
is expected to unlock this resource and allow it to be purged after using it.
You can use the bufferCmd command to handle compressed sound samples in
addition to sounds that are not compressed. To expand and play back a buffer of
compressed samples, you pass the sampled sound synthesizer a bufferCmd
command where param2 points to a compressed sound header.
Note: Using the bufferCmd command to play several consecutive compressed samples on the Macintosh Plus and the Macintosh SE is
not guaranteed to work without an audible pause or click.
To play sampled sounds that are not compressed, pass bufferCmd a standard or
extended sound header. The extended sound header is used for stereo sampled
sounds. The standard sampled sound header is used for all other noncompressed
sampled sounds.
You can divide large sampled sounds into multiple buffers and then issue
successive bufferCmd commands to play that sound. In this case, each buffer
must contain a sampled sound header. Except as noted just above, the sound will
play smoothly, without audible gaps. It is usually much easier, however, to
play large sampled sounds from disk by using the play-from-disk routines or
Note: If a sound is playing and you send a bufferCmd command by
command will not play. You should send all bufferCmd commands by
using SndDoCommand, or else you should first stop the sound that repeatedly until the scChannelBusy flag of the sound-channel status
record turns FALSE.
Table below lists approximate values for the most common sample rates. When
you specify a value in the sampleRate field of a sound header, you should use
the values in the third column of the Table.
Table Sample Rates
sampleRate
Rate (kHz) Rate (Hz) value (Fixed)
5 kHz 5563.6363 0x15BBA2E8
7 kHz 7418.1818 0x1CFA2E8B
11 kHz 11127.2727 0x2B7745D1
22 kHz 22254.5454 0x56EE8BA3
44 kHz 44100.0000 0xAC440000
To store and operate on compressed audio data, you use a structure called the
logical extension of the sound header.
The following code example illustrates the structure of an 'snd ' resource that
contains compressed sound data.
Listing: An 'snd ' resource containing compressed sound data
data 'snd ' (9004, "Sample 1", purgeable) {
0x"0001" //format type
0x"0001" //number of synthesizers
0x"0005" //resource ID of first synthesizer
0x"00000380" //initialization option: initMACE3 + initMono
0x"0001" //number of sound commands that follow (1)
0x"8051" //cmd: bufferCmd
0x"0000" // param1: unused
0x"00000014" // param2: offset to sound header (20 bytes)
//compressed sound header follows:
0x"00000000" //pointer to data (it follows immediately)
0x"00000001" //number of channels in sample
0x"56EE8BA3" //sampling rate of this sound (22 kHz)
0x"00000000" //starting of the sample's loop point; not used
0x"00000000" //ending of the sample's loop point; not used
0x"FE" //compressed sample encoding
0x"00" //baseFrequency; not used
0x"00006590" //number of frames in sample (26,000)
0x"400DADDD1745D145826B" //AIFFSampleRate (22 kHz in l
//extended type)
0x"00000000" //markerChunk; NULL for 'snd ' resource
0x"00000000" //futureUse1; NULL for 'snd ' resource
0x"00000000" //futureUse2; NULL for 'snd ' resource
0x"00000000" // stateVars; NULL for 'snd ' resource
0x"00000000" //leftOverBlockPtr; not used here
0x"0003" // compressionID, 3 means 3:1
0x"0010" //packetSize, packetSize for 3:1 is 16 bits
0x"000B" //snthID is 11
0x"0008" // sampleSize, sound was 8-bit before processing
0x"2F 85 81 32 64 87 33 86" //the compressed sound data
0x"6F 48 6D 65 72 6B 82 88
0x"91 FE 8D 8E 86 4E 7C E9
0x"6F 6D 71 70 7E 79 4F 83
0x"59 8F 8F 65" //rest of data omitted in this example
};
This resource has the same general structure as the 'snd ' resource
illustrated in The Format 1 'snd ' Resource in the section entitled
header is replaced by the compressed sound header. This example resource
specifies a monophonic sound compressed by using the 3:1 compression
algorithm.