For a typical output at volume 64, with maximum data values of -128 to 127, the voltage
output is between +.4 volts and -.4 volts. Some volume levels and the corresponding
decibel values are shown in Table 5-2.
Table 5-2: Volume Values
VOLUME DECIBEL VALUE
64 0 (maximum volume)
48 -2.5
32 -6.0
16 -12.0 (12db down from the
volume at maximum level)
For any volume setting from 64 to 0, you write the value into bits 5-0 of AUD0VOL. For
example:
SETAUDOVOLUME:
LEA CUSTOM,a0
MOVE.W #48,AUD0VOL(a0)
The decibels are shown as negative values from a maximum of 0 because this is the way a
recording device, such as a tape recorder, shows the recording level. Usually, the recorder
has a dial showing 0 as the optimum recording level. Anything less than the optimum
value is shown as a minus quantity.
SELECTING THE DATA OUTPUT RATE
The pitch of the sound produced by the waveform depends upon its frequency. To tell the
system what frequency to use, you need to specify the sampling period. The sampling
period specifies the number of system clock ticks, or timing intervals, that should elapse
between each sample (byte of audio data) fed to the digital-to-analog converter in the
audio channel. There is a period register for each audio channel. The value of the period
register is used for count-down purposes; each time the register counts down to 0,
another sample is retrieved from the waveform data set for output. In units, the period
value represents clock ticks per sample. The minimum period value you should use is 124
ticks per sample NTSC (123 PAL) and the maximum is 65535. These limits apply to both
PAL and NTSC machines. For high-quality sound, there are other constraints on the
sampling period (see the section called "Producing High-quality Sound").
NOTE
A low period value corresponds to a higher frequency sound and a high period value
corresponds to a lower frequency sound.
- Audio Hardware 137 -
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