Pure FM Synths.. How? What? Why?

[vinyl_junkie]

May be a stupid question but how did all the early FM synths like the DX'es produce the raw sound/waveforms?

They didn't use DSP's and neither did they use analogue oscillators. Digital oscillators sure but what and how exactly?
How is this digital oscillator used by them (Custom Yamaha VLSI chip) different from using a DSP? It's still a CPU performing mathematical equations no?

[CS_TBL]

Wasn't it actually phase distortion? A table with a sine being read out for a specific phase?

[karmag]

Wasn't it actually phase distortion? A table with a sine being read out for a specific phase?

Yes, it's essentially a wavetable. (And don't confuse "phase modulation" with "phase distortion" - PD is just a marketing term, from Casio even.) :-)


Here's something that was posted to the music-dsp list a few years ago. It has to do with the OPL2/OPL3 chips that were used in PC soundcards, but it's probably the same techniques that are involved for all of Yamaha's chips.

Date: Fri Apr 18 02:44:09 2008
Subject: [music-dsp] YM3812 reverse engineering

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.

[snip]

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)

[snip]

For more professional parts, the kinds used in the synths, you might expect to see variations on this theme, perhaps longer tables or wider data words; apparently the OPL chips have 11-bit resolution per operator, I think most synths after the first DX7s have higher resolution than that.

Some more comments on his blog. Also, chip p**n.

[commodorejohn]

May be a stupid question but how did all the early FM synths like the DX'es produce the raw sound/waveforms?

They didn't use DSP's and neither did they use analogue oscillators. Digital oscillators sure but what and how exactly?
How is this digital oscillator used by them (Custom Yamaha VLSI chip) different from using a DSP? It's still a CPU performing mathematical equations no?

It's not a CPU doing the sound generation - it's a custom logic IC handling it. The CPU only does control functions. You could do FM on a CPU, but it wouldn't have been cost-effective at the time - even emulating a lesser Yamaha chip in software requires a good deal more horsepower than the DX7's CPU has to offer.

[vinyl_junkie]

Great info, thanks!

[Hallu]

This is a good reason why owning vintage FM synths is just as practical as most vintage analog synths for long term use - all through hole design (even though SMT is easier to do DIY repair, some cranks demand through hole tho) and widely found IC chips (arguably easier to find than most analog chips, since a lot of FM synths shared the exact same chip with different circuit designs - in this regard it mirrors Yamaha's CS line).

All the early digital circuit design information is pretty interesting, especially when you look at some of the larger and more intimidating pieces of gear from this era. The DX line is beautiful in its simplicity, but the extent to which something like the Fairlight filter -> output chain delves into is pretty astounding.

[calaverasgrande]

This is a good reason why owning vintage FM synths is just as practical as most vintage analog synths for long term use - all through hole design (even though SMT is easier to do DIY repair, some cranks demand through hole tho) and widely found IC chips (arguably easier to find than most analog chips, since a lot of FM synths shared the exact same chip with different circuit designs - in this regard it mirrors Yamaha's CS line).

All the early digital circuit design information is pretty interesting, especially when you look at some of the larger and more intimidating pieces of gear from this era. The DX line is beautiful in its simplicity, but the extent to which something like the Fairlight filter -> output chain delves into is pretty astounding.

how is sMT easier than through hole?
I can buy the tools to fix through hole circuits at any radioshack for $10 total.
SMT, IIRC, requires specific desoldering equipment to rework parts.

[Hallu]

This is a good reason why owning vintage FM synths is just as practical as most vintage analog synths for long term use - all through hole design (even though SMT is easier to do DIY repair, some cranks demand through hole tho) and widely found IC chips (arguably easier to find than most analog chips, since a lot of FM synths shared the exact same chip with different circuit designs - in this regard it mirrors Yamaha's CS line).

All the early digital circuit design information is pretty interesting, especially when you look at some of the larger and more intimidating pieces of gear from this era. The DX line is beautiful in its simplicity, but the extent to which something like the Fairlight filter -> output chain delves into is pretty astounding.

how is sMT easier than through hole?
I can buy the tools to fix through hole circuits at any radioshack for $10 total.
SMT, IIRC, requires specific desoldering equipment to rework parts.

The tools you need for either are a bit specific, thats negligible.

With through-hole you have to align the component, hold it steady and solder at the same time.
With SMT you do every piece in the same steps - 1) blob a small thing of solder on one of the connection points, it is okay if it cools 2) pick up component with tweezers in one hand, keep iron in other 3) hover component over connection point, re-heat solder with tip of iron 4) place component down 5) leisurely solder in other connection points

With SMT you can also just glob on solder everywhere, and then remove all the excess with a braid and (given that you don't burn the PCB) everything will work 100%.

Its actually quite forgiving to do compared to through hole, it is just a different set of steps.

[calaverasgrande]

I dunno, I use a panavise with a circuit board holder. I dont have to hold the components in place when I solder them. I just tilt it at an angle so gravity does that for me.
Then I have one hand to feed solder and one to hold an iron.
The main differnce between the two for me is that I can barely read normal sized components. SMT is so small I can't even see it without a loupe.