Changes

* [[SP0256 Pin-Outs]]

The voice generator relies on the Amplitude, Pitch, F0..F5, and B0..B5 registers, which are processed like so:

Another important register is the Repeat counter, which indicates when the next opcode shall be executed (and which may then load new values into the above registers).

The SP0256 is (usually) driven by a 3.12MHz oscillator, and it uses 7bit PWM output. So the sample rate should be 3.12MHz/128, probably further divided by two:

That (guessed/undocumented) samplerate does more or less match the specifications that say that (unwanted PWM-) noise is above 10kHz, and that wanted pitch can be up to 5kHz. And the repeat timings do more or less match the timings shown in the allophone list.

The 8bit amplitude register defines the volume in floating point form,

Note that (aside from noise) the AL2 ROM uses only one pitch value: 5Bh aka 91 decimal (meaning that all vowels are using the same base frequency, and they differ only by using different filter settings).

Noise is activated when setting pitch=0. The timings are then same as when pitch=64, but instead of outputting HIGH and NULL levels, the hardware does now randomly output HIGH or LOW levels, for example, pitch=0 and repeat=5:

The exact random algorithm is unknown (probably some shift/xor stuff?), the random levels seem to be output on each sample (not only on the first sample of a repeat). Like normal pitch, the noise is passed to the 6 filters.

The pause command sets amplitude=0. The timings are then same as when pitch=64, but the output is always NULL, for example, pause and repeat=5:

Pause does reset the filters to 0, so the silence is not affected by filters.

As shown above, the amplitude/pitch/noise output is passed through six digital filter stages (using the F0..F5 and B0..B5 registers), each stage looks like so: