[sdiy] FM Synth Designs

[Batz Goodfortune]

Y-ellow Ben 'n' all.

At 02:51 PM 8/13/02 +0800, ben wrote:
>which chip is that? is rom only? would it be possible to change the
>wavetable?
>
>just sniffin for a dx mod

In fact it was widely rumored at one time that Robyn Whittle use to pop the 
top off the OPs chip and somehow expose the wave rom section to UV and 
change it randomly. I have no idea how this might work but it's something I 
wouldn't even entertain.

There is no outboard ROM chip if that's what you were thinking. Had there 
have been I would probably have been the first to throw in a rotary switch 
and be able to select between a bank of different wave forms. The 
wavetable/ROM is in the OPs chip. This chip is responsible for the time 
slicing and assembling of the final output wave form and then spitting it out.

The other chip is known as the EG chip. Since a vast proportion of it is 
devoted to the functioning of the 96 envelope generators. Registers that 
keep track of the levels etc and present all the control data to the OPs chip.

You could think of them as the two halves of just about any synth. Analogue 
or digital control. You have on one side the control generators and on the 
other side you have the audio generators. Both do a fair bit of data 
crunching but unlike a micro, they only do the functions they were designed 
to. They are hard-wired logic. (The best Kind IMHO) If you look at how 
these chips are laid out, especially the EG chip, it's like looking at 
everything you know about FM reduced to hardware. There are hardly any 
corners cut. A bunch of registers, adders and subtractors etc. When the OPs 
chip asks for a piece of data next in line the EGs supply it. Even if that 
data is zero because there's no notes playing.

All the CPU has to do is load up the voice data into those registers as 
required, then wait till a note is struck. It doesn't even have to read the 
keyboard or anything because there's another micro to do all that. That 
micro (6801) presents data to the main micro as if it were incoming MIDI 
data, this is added to any actual MIDI data and then the micro tells the EG 
chip to start EG-ing.

There are 256 register addresses assigned to the EG chip. I can't remember 
if all of them are used but certainly a great deal are. There are 160 odd 
adjustable parameters in a DX patch and most of them have registers in the 
EG chip.

The OPs chip works out the position and amplitude of each operator of each 
voice, adds all 16 results together and produces the composite wave form. 
There is no analogue mixing in an original DX7.

I know I harp on about this stuff but honestly, I find this stuff to be a 
work of art in itself. No-one does this stuff anymore but for a brief time 
in the mid 80s, this was the future. Then DSPs came along and the line 
between software and hardware was increasingly blurred. Xilinx anyone?

Be absolutely Icebox.

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