[sdiy] Reverse engineer an EPROM encoding?

[cheater00 .]

Dear Grant,


On Wed, Oct 16, 2013 at 7:09 PM, MTG <grant at musictechnologiesgroup.com> wrote:
> So I'm still banging away on this Kawai R100 drum machine when time permits.
> The idea is to be able to make fresh sound ROMs for it. It's a great 12-bit
> drum machine. The unit uses a couple of ASICs where Linn and Oberheim, et al
> used counters. Here is the jist:
>
> AGU -> ROM -> DGU -> DAC (12-bit linear?)  [DAC312/AM6012]
>
> I assume AGU is Address Generation Unit and DGU is Data...  The schematic is
> widely available, such as
> http://www.burnkit2600.com/manuals/KAWAI_R-100_SRV.pdf .
>
> Now if you assume the whole thing is linear and you code up a ramp waveform
> and stuff it in and EPROM and press play, this is what you get out. NOT
> ramp. Some EXPONENTIAL thing. The sign bit is understood and the output wave
> is single-ended on the +15v rail (centered about half way).
>
> http://musictechnologiesgroup.com/blog/wp-content/uploads/2013/10/R100_first_ramp.jpg
>
> So there was this theory floated around on the web that they used some
> 12-bit DAT encoding method (http://www.rfc-editor.org/rfc/rfc3190.txt) in
> the ROMs that was expanded by the DGU. And as unlikely as that seems, it's
> not too far off. But just far enough to sound bad if you try and account for
> it.  RFC 3190 describes something not unlike uLaw where you have big Chord
> bits and smaller Step bits.
>
> I coded up something that becomes a ramp if you run it through RFC 3190.
> Simulate if you will. And this is what you get. It should be a nice ramp.
> But it's still not.
>
> http://musictechnologiesgroup.com/blog/wp-content/uploads/2013/10/R100_non_ramp.jpg
>
> It has flattened out significantly though, so it's a step in the right
> direction. So I wonder if anyone is aware of any other 12-bit encoding
> schemes they might have used?? DAT didn't come out until after the R100 so I
> think that was a red herring.
>
> GB

I suggest that you attach a linear DAC to read the same bus that is
feeding the DGU, then use the output of that DAC and the typical DAC
to an oscilloscope in XY mode. This will tell you if you're even
dealing with a transformation or if the DGU has got logic inside. A
dual-trace mode should provide interesting information as well.

Afterwards, instead of reading off a ROM, set up a simple data feed
where you can flip the bits yourself, and see what happens when you
do.

Cheers,
D.