Playing Frequencies
You can play frequencies one at a time by using the SndDoCommand or sound plays for a specified duration at a specified frequency. You can use any of
the three available playback synthesizers to play the sound. If you use the
wave-table synthesizer or sampled sound synthesizer, then a voice must
previously have been installed in the channel. (See the section called
tables and sampled sounds as voices.)
You can also play frequencies by issuing the freqCmd command, which is
identical to the freqDurationCmd command, except that no duration is specified
when you issue freqCmd.
Note: A freqDurationCmd command continues playing until an other command is available in the sound channel. Therefore, to play a single
frequency for a specified duration, you should issue freqDurationCmd
followed immediately by quietCmd.
on quietCmd.
When you use the freqDurationCmd command and are using a sampled sound as
the voice, freqDurationCmd starts at the beginning of a sampled sound. The
freqDurationCmd command plays the sound between the loop points specified
in the sampled sound header to extend the sound to the specified duration. There
must be an ending-point for the loop specified in the header in order for
freqDurationCmd to work properly.
The structure of a freqDurationCmd command is slightly different from that
of most other sound commands. The param1 field contains the duration of the
sound, specified in half- milliseconds. (A duration of 2000 represents a
duration of 1 second. The maximum duration is a duration of 32,767, or about
16 seconds.) The param2 field specifies the frequency of the sound. The
frequency is specified as a MIDI value. The following Listing demonstrates the
use of the freqDurationCmd command.
//Listing: Using the freqDurationCmd command
// Assuming inclusion of MacHeaders
#include <Sound.h>
// Prototype your routine like this prior to calling it
void MyPlayFreqDuration (void);
void MyPlayFreqDuration()
{
SndChannelPtr mySndChan; [TOKEN:12079] pointer to a sound channel
SndCommand mySndCmd; [TOKEN:12079] a sound command // Prototype for error handling routine
mySndCmd.cmd = freqDurationCmd;
mySndCmd. param1 = 2000; // duration in half- milliseconds
mySndCmd. param2 = 60; [TOKEN:12079] frequency
// play the sound
// assume that mySndChan points to a valid sound channel
if ( myErr )
DoError( myErr);
}
The Table below shows the decimal values that can be sent with a
freqDurationCmd or freqCmd command. Middle C is represented by a value of
60. These values correspond to MIDI values.
To calculate a duration, use the following formula:
duration = (2000/(repetitions per minute/60)) * repetitions per sound
You can rest a channel for a specified duration by issuing a restCmd command.
The duration, specified in half- milliseconds, is passed in the param1 field of
the sound command.
Table MIDI values
A A# B C C# D D# E F F# G G#
Interval 1 1 2 3 4 5 6 7 8
Interval 2 9 10 11 12 13 14 15 16 17 18 19 20
Interval 3 21 22 23 24 25 26 27 28 29 30 31 32
Interval 4 33 34 35 36 37 38 39 40 41 42 43 44
Interval 5 45 46 47 48 49 50 51 52 53 54 55 56
Interval 6 57 58 59 60 61 62 63 64 65 66 67 68
Interval 7 69 70 71 72 73 74 75 76 77 78 79 80
Interval 8 81 82 83 84 85 86 87 88 89 90 91 92
Interval 9 93 94 95 96 97 98 99 100 101 102 103 104
Interval 10 105 106 107 108 109 110 111 112 113 114 115 116
Interval 11 117 118 119 120 121 122 123 124 125 126 127