info on Yamaha YM2164 FM(?) chip?

[Richard Atkinson]

On Mon, 11 Dec 2000, edanderson wrote:

> I recently picked up a Korg 707 synth for cheap, and I was hoping to
> do some kind of simple mods to it.  A best I can tell, this digital FM
> beastie has this Yamaha YM2164 4-op FM chip as its heart, chained up
> to a YM3012 DAC.  Does anyone know anything about how this synth
> works.  All I've been able to dig up so far is a pinout of the DAC.

I'd be very interested in seeing the datasheet for this chip as well, as I
have recently encountered it in my experiments with MSX computers.

The Yamaha CX5 and later computers came with an FM sound expander (SFG-01)
in the base of the computer. This contains a MIDI interface, simple
non-velocity keyboard interface and an FM chip, the YM2151 OPM. This is
the same chip used in Sega's System 16 arcade machines. Its specs are 4
operators, 8 algorithms, 8 voices and two output channels (simple on/off
mixing, no panning).

Later when MSX disk systems appeared, Yamaha released an upgraded sound
expander which could be installed in the computers by removing the old one
- this was called SFG-05. Originally I thought it was simply a software
upgrade, as the enhancements advertised by Yamaha were disk compatibility
and a better MIDI implementation. However, when I opened the case up not
only did I find a different firmware (ROM) chip but the FM chip was
different too - the YM2164 OPP. So at the very least I'd say the OPP has
the capabilities of the OPM; maybe more.

Do you have the specs of the FM capabilities of the Korg?

> I was hoping to perhaps do some kind of decimator mod (chop the least
> significant bits off for extra grunge-effect) but since the DAC input
> is (AFAIK) a serial stream this seems like it might be difficult.
> 
> Any thoughts, or should I just paint the case and be done with it?

The DAC output is floating point; it has a 10 bit mantissa and 3 bit
exponent (from memory). It would be possible to feed this directly into a
PC and do all manipulations from there. A linear 16 bit convertor is
enough to play all the information in the bit stream.


Richard