LIMITATIONS ON SELECTION OF SAMPLING PERIOD
The sampling period is limited by the number of DMA cycles allocated to an audio channel.
Each audio channel is allocated one DMA slot per horizontal scan line of the screen
display. An audio channel can retrieve two data samples during each horizontal scan line.
The following calculation gives the maximum sampling rate in samples per second.
2 samples/line * 262.5 frames/frame * 59.94 frames/second
= 31,469 samples/second
The Figure of 31,469 is a theoretical maximum. In order to save buffers, the hardware is
designed to handle 28,867 samples/second. The system timing interval is 279.365
nanoseconds, or .279365 microseconds. The maximum sampling rate of 28,867 samples
per second is 34.642 microseconds per sample (1/28,867 = .000034642). The formula for
calculating the sampling period is:
sample interval clock constant
Period value = --------------- = --------------
clock interval samples per second
Thus, the minimum period value is derived by dividing 34.642 microseconds per sample
by the number of microseconds per interval:
34.642 microseconds/sample
Maximum period = -------------------------- = 124 timing intervals/sample
0.279365 microseconds/interval
or:
3,579,545 ticks/second
Minimum period = ---------------------- =124 ticks/sample
28,867 samples/second
Therefore, a value of at least 124 must be written into the period register to assure that
the audio system DMA will be able to retrieve the next data sample. If the period value is
below 124, by the time the cycle count has reached 0, the audio DMA will not have had
enough time to retrieve the next data sample and the previous sample will be reused.
28,867 samples/second is also the maximum sampling rate for PAL systems. Thus, for
PAL systems, a value of at least 123 ticks/sample must be written into the period register.
CLOCK VALUES
NTSC PAL UNITS
Clock Constant 3579545 3546895 ticks per second
Clock Interval 0.279365 0.281937 microseconds per interval
- 138 Audio Hardware -
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