I’ve been working on the controller part of the Speech256. The current status is that is it mostly working: it sends pitch, duration, voiced/unvoiced and filter coefficients control signals to the source-filter model.
To save command ROM space, the designers of the SP0256 command ROM chose to encode the filter coefficients in a 8-bit sign-magnitude format instead of the 10-bit format needed by the filter engine. To be clear, the Speech256 project does not use a copy of the SP0256 ROM; it uses a custom format. However, the bit-depth reduction technique is used to keep the control ROM small.
The SP0256 contains an expansion circuit that translates the 8-bit format into the 10-bit format. The SP0256 seems to use the same translation table as the SP0250, which is documented in its Applications Manual:
The table pertains to the (7-bit) magnitude part of the (8-bit) sign-magnitude only: positive and negative numbers are translated by the same table, keeping the sign untouched.
Although the manual claims it uses a lookup ROM, the content looks like a piece-wise linear curve which can therefore be implemented without a ROM. I found four lines (C1 .. C4) that represent the table quite accurately:
The following code snippet will translate a 7-bit magnitude into a 9-bit magnitude according to the table …
if x < 38
if x < 69
return 301 + (x-38)*4;
if x < 97
return 425 + (x-69)*2;
if x < 128
return 481 + (x-97);
… except for x=2, where the result should be 12 instead of 16. Note that the C1 line also has a slight offset error but I think this translation function is most likely good enough.
The convenient factors-of-two scaling found, makes me suspect the function was meant to be implemented directly in logic, not in a ROM.
Time is running out, so I’d better get on implementing the above!