Reply 60 of 61, by stripwax
Demystifying a couple of other things: different versions of the datasheet have meaningfully different contents I think. At least one version states this (which I didn't realise until just now):
If the frequency channel controlled is off (NE = FE = 0) the envelope will appear at the output, which provides an alternative "non-square" tone capability. In this event the frequency will be the envelope rate, which provided the rate is from the frequency channel, will be a maximum of 1kHz. Higher frequencies of up to 2kHz can be obtained by the envelope resolution being halved from 16 levels to 8 levels. (Rates quotes are based on an 8MHz clock)
So I guess my note above about whether this is by design or undocumented can now be disregarded 😀
There's also a note in the datasheet that says:
The SAA1099 uses pulse width modulation to achieve amplitude and envelope levels ... The amplitude and envelope signals chop the output at a minimum rate of 62.5kHz.
That puzzled me until just now - I think that refers to the following combination:
Frequency enabled, Noise enabled, Amplitude set to 2 and Envelope set to 1.
Under this scenario, per my table above, the combination of the amplitude + envelope patterns ANDed together yields one pulse every 64 cycles; and by mixing both frequency and noise then the pulse will occur only on even periods (since odd periods contribute noise); assuming the noise is generating a zero level then the combination yields just one pulse every 128 cycles, or 62.5kHz at 8MHz clock.