Saturday, October 10, 2015

DR 220 ROM

The DR 220's ROM is partially audio data and partially pattern/miscellaneous data.

The 220 comes in two flavors: 220A(coustic) and 220E(lectric). Most of the differences can be toggled and are contained in the gate array(IC1). The rest are in the sound ROM(IC2).

I haven't bothered breaking down the data portion since it doesn't appear to contain anything too interesting. Some simple tests suggest that it holds pattern/song data, instrument names for the lcd and instrument volumes. The volumes can be increased beyond what the normal interface allows, but it's buggy and can distort.

The audio is encoded as mono, signed, 8bit-packed, 12bit mu-law PCM @ 25KHz. A partial departure from the Roland standard.

None of the address lines nor data lines are jumbled and no samples are interleaved. It's pretty straightforward. Mu-law encoding is the big hurdle.

Mu-law is a logarithmic encoding that was used on some older drum machines: Linn, Oberheim, etc. The idea of it is to exploit our non-linear perception of sound. The encoding gives more precision to lower(quieter) amplitudes that we have better perception of. Conversely is has to remove some detail from higher(louder) amplitudes that we can't discern as well. It's a tradeoff for the more "valuable" part of the signal.

The nuts and bolts of the encoding work like scientific notation. Each number gets translated into a significand(mantissa) and an exponent. In the 220's case, the four least significant bits of each byte are the mantissa, the next most significant three are the exponent and the most significant is the sign bit. For whatever reason, the sign bit is nonstandard for mu-law. If the bit is set the byte must be inverted before being decoded.

1 comment:

  1. nice work!
    did you ever make a script to do it the other way around, otherwise i'm about to reinvent the wheel.