[sdiy] Reverse engineer an EPROM encoding?

[Ove Ridé]

Could it be a delta encoding?

On 16 October 2013 19:09, 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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-- 
/Ove

Blog: <http://blog.gg8.se/>

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