developer's mag
main page
article
part 1
part 2
part 3
part 4
part 5
part 6
part 7
part 8
part 9
5 - Tone Duration
Determining duration can be somewhat trickier. The SDK documentation lets you calculate the duration for the structure's values of 'man' and 'exp' (mantissa and exponent) via the following equation:Interval in seconds = ( (man) * 4 ^ (15-(exp)) ) / 1,000,000
In practice, however, most of the values are uninteresting for general tones, being either extremely short or extremely long - the more practical values for 'exp' are:
| Useful Exponent Values and Durations for piezo_tone Structure | |||||||||||||||
| piezo_tone.exp = 3 | 16 seconds | piezo_tone.exp = 4 | 4 seconds | piezo_tone.exp = 5 | 1 second | ||||||||||
| piezo_tone.exp = 6 | 1/4 second | piezo_tone.exp = 7 | 1/16 second | piezo_tone.exp = 8 | 1/64 second | ||||||||||
The 'man' value is simply multiplied by the time value generated for each exponent. For example, a tone duration of about one second would be man=1, exp=5 (1 by 1 sec), or man=4, exp=6 (4 by 1/4 sec). According to the SDK, just the lower four bits of 'man' are valid, meaning that only 0 through 15 are usable. Taken together, these values should take care of most audio needs.
As an example to wrap this all up, if we wanted to play a Middle C note for 1/2 second, we would set the following:
struct piezo_tone info; info.tone =62; // approximately middle c info man =2; // 2 times 1/4 sec for exp, or 1/2 sec info.exp =6; // 1/4 sec info.unused=0; // always set to zero! piezo_play_1tone(info,0); // simple tone-no flags
The sample code accompanying this article includes a button 'keyboard', allowing you to play one octave of notes in this manner.
Previous Section
Next Section