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The source wave could be prepared with this in mind, using some specialised software.
However, ignoring this and outputting "normal" linear 4-bit data will generally sound
good, but significantly quieter than it would be on a linear scale.
The quality depends on the rate at which data is sent to the chip; on most systems, the
limit is more likely to be memory space than CPU speed (8kHz 4-bit audio will fit 4.1
seconds into 16KB).
Advanced PCM
Instead of setting the volumes on all three tone channels in unison, it is possible to
instead set the levels on all three independently. In theory this allows for 816 unique
output levels, although they are not distributed regularly or linearly, and are
concentrated in the lower half of the waveform; this is equivalent to about 9.7 bits of
resolution (but "costs" 12 bits per sample for storage).
If only two channels are used (so the source can be 8 bits per sample rather than 12, to
save ROM space), 136 unique levels are possible (7.1 bits of resolution), which seems to
be a better trade-off. By outputting 4 bits to two channels and the other 4 bits to the
third, the volume range for this 8-bit variation can be boosted at the expense of the
uniformity of coverage.
No known software uses this technique.
Pulse Width Modulation
This works by outputting pulses at constant volume whose pattern gives the effect of
different frequencies and volumes. This is also how samples are played on PC internal
speakers and some CD players. On the SN76489, this is done by:
Setting all 3 tone channels to frequency 0x000
At rapid, closely timed intervals, setting the output volume of all 3 to either 0xf (off) or
0x0 (full) depending on values stored in ROM.
All three channels are used to get maximum volume.
This is equivalent to dithering the sound to one bit per sample instead of however many
bits per sample are in the input data. Thus, an 8kHz 8-bit sample can be output as a
64kHz 1-bit sample and it will sound much the same. It is somewhat dependent on the
output frequency being above the range of hearing.
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Summary of Contents for Franky
Page 1: ...Instruction Manual ...
