[sdiy] DX7 chip die photos

[David Riley]

On Nov 13, 2021, at 10:40 PM, Brian Willoughby <brianw at audiobanshee.com> wrote:
> 
> Thanks, Mike, this is great info!
> 
> I like the way the chip designers saved cost by adding log values rather than multiplying linear numbers.
> 
> One question I'm left with is regarding the second use of logarithms: envelopes. Considering that the logarithm of zero is Not a Number, how is it that any of the Yamaha FM operators close their envelopes off to zero amplitude? It seems like amplitudes could get extremely small, depending upon the bit depth, but could never hit zero. This seems particularly troublesome with modulators, more so than mixing operator outputs, because you could never get a pure sine without reducing the modulator amplitude to zero.
> 
> Hopefully Ken's future installments will be written after he has uncovered more details, and perhaps my questions will be answered. (unless a math-head here already knows what I'm missing)

My guess would be that any sane implementation that wanted to actually be able to specify zero for envelope purposes would have a value (say, INT_MAX) mapped to an output of zero.  I don't recall the details of this particular chip, though.  Generally, one would also employ saturating adders in this case because if you're dividing down to zero, you want it to hit an endstop rather than rolling around back to the start.


- Dave