OPL Decapsulation - Report by Olli Niemitalo

[NoobDog]

Hi All!

Olli Niemitalo just posted this to the music-dsp list. As i am currently a very proud ownder / builder of a midibox-fm (right now the minimum version, next month i'll start with the "UI"), i just thought some readers of this forum might be interested in his findings. i asked for permission to reproduce his post here, and he agreed. here we go!

Post sent by Olli Niemitalo to music dsp mailinglist 18.4.08:

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Me and Matthew Gambrell have an interest in emulation of YM3812, which

is the OPL2 sound chip found in the Adlib and 8-bit Sound Blaster

cards. A later derivative OPL3 is found in early 16-bit Sound

Blasters. We sent one YM3812 and one YMF262 (OPL3) to MEFAS for

decapsulation; the cost was around 90 USD each. They indicated that

the chips would still be operational after decapsulation, but we had

no need to test this. Looking at the revealed YM3812 die surface with

a microscope turned out two ROM's. See:

http://docs.google.com/Doc?id=dd8kqn9f_13cqjkf4gp

The contents could be read bit-by-bit. The first ROM was a log-sin

waveform table, containing one quarter of a sine wave, 256 samples

long. The second ROM was an exponential table, 256 samples long. There

were no other ROM's larger than 16 samples. This is strong evidence

that YM3812 produces the sound without any multiplications, using for

frequency modulated (actually phase modulated) synthesis the formula:

out = exp(logsin(phase2 + exp(logsin(phase1) + gain1)) + gain2)

There was first some difficulty understanding the ROM's because they

were compressed. Every second sample was stored as a difference value

to the previous. Once this was understood, we were able to derive

formulas that met our expectations and re-created the contents of the

ROM tables exactly (yes, they used the same rounding and everything at

Yamaha, back then when they created these tables).

Exponential table:

x = 0..255, y = round((power(2, x/256)-1)*1024)

When such a table is used for calculation of the exponential, the table

is read at the position given by the 8 LSB's of the input. The value +

1024 (the hidden bit) is then the significand of the floating point output

and the yet unused MSB's of the input are the exponential of the

floating point output. Indeed, YM3812 sends the audio to the YM3014B

DAC in floating point, so it is quite possible that summing of voices

is done in floating point also.

Log-sin table:

x = 0..255, y = round(-log(sin((x+0.5)*pi/256/2))/log(2)*256)

This is the first (rising) quarter of sine wave. The rest can be

constructed by flipping all the bits of x and/or by changing the sign

of the samples.

I hope this will be useful for anyone writing a good emulator. Also,

there are no extra ROM's on the YMF262 (OPL3), so the additional

waveform is probably simply the exponential table.

-olli

[Jaicen]

That is truly awesome! Anybody fancy donating a (broken) SID for the same treatment?

[Ashiman]

WOW , FINALLY I GET DAC DIE'S ON MY WALL HELL YEAH  :D

[Wilba]

That is truly awesome! Anybody fancy donating a (broken) SID for the same treatment?

I have plenty. I don't know what will be gained by it though... other than pretty pictures.

[Enth]

For pretty pictures, you could go here :p

http://www.digital-circuits.org/sid/CSG%20Micrographs/

[sebiiksbcs]

What was this law about increase in transistors every 4 years like? Moore's law?

[ilmenator]

originally one year, later changed to two: Moore's Law