[sdiy] Reverse engineer an EPROM encoding?

[John Ames]

The waveform produced by the linear data almost makes me think it's
some variety of floating-point just based on the way it's almost
completely flat for about 3/4 of the up and down phases...that could
be where it starts to increment the exponent enough to give it
significant scaling. But I have a hard time imagining that they'd have
used floating-point DACs for this purpose; Yamaha did on the DXes, but
that was probably more because the precision was needed for
good-sounding FM.

On 10/16/13, 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
>
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