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Author Topic: Yamaha FM chip question  (Read 3291 times)

12rockers

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Yamaha FM chip question
« on: June 21, 2014, 12:53:05 AM »

I have been puzzling over a Yamaha FM chip. The chip is YM3420BF, and the type is 'OPU'. You find this chip in the PSR-16 and PSS-480.

The PSS-480 sounds similar to other PSS-n80 keyboards, but it's quite different because of this OPU chip. The chip integrates a DAC, so it doesn't need a separate DAC chip like other PSS-n80 keyboards (or indeed like most Yamaha FM keyboards from toy to pro level).

The interesting thing about YM3420BF is that not only does it have an analog output, putting out the converted audio signal, but it has 4 separate 'outputs' as well. I call them 'outputs' because that's how they're referred to in the service manual of a different YM3420BF based machine I could get hold of.

I'm fascinated by these 'outputs'. What are they, and what do they do? Are they putting out 4 different numbers that can be converted to an audio signal by a DAC chip? If so, what are these numbers? 4 different mono oscillator outputs, or 2 stereo digital outputs? Or 2 stereo digital oscillator outputs and an LED driver? Or separate auto-accompaniment and/or drums from main voice digital outputs? Or any combinations of these? They could be anything.

As I don't have a PSS-480 could someone please have a look at theirs, and let me know how these pins are connected on the YM3420BF:

Pin 46 'channel A'
Pin 47 'channel B'
Pin 48 'channel C'
Pin 49 'channel D'
Pin 50 'feedback'
Pin 42 Analog Output

Right now I believe there are 2 possibilities. Either pins 46 to 50 inclusive are connected to the DAC chip and pin 42 is disconnected, OR pin 42 is connected to the op-amp and pins 46 to 50 are just left unused and disconnected.

Once we know which situation is the case, it will be possible to strike PSS-480 from the 'unbendable' list. I hope pins 46 to 50 inclusive are connected to a DAC chip in the PSS-480. If so, we can do a lot to enhance the FM synth capability of this keyboard.

Thanks in advance.
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iwillbeacircuitbender

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Re: Yamaha FM chip question
« Reply #1 on: August 02, 2015, 01:19:15 PM »

I think it might have to do with the DAC, might have to look at the service manual to find out
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juno6

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Re: Yamaha FM chip question
« Reply #2 on: July 08, 2016, 06:43:19 PM »



The interesting thing about YM3420BF is that not only does it have an analog output, putting out the converted audio signal, but it has 4 separate 'outputs' as well. I call them 'outputs' because that's how they're referred to in the service manual of a different YM3420BF based machine I could get hold of.

I'm fascinated by these 'outputs'. What are they, and what do they do? Are they putting out 4 different numbers that can be converted to an audio signal by a DAC chip? If so, what are these numbers? 4 different mono oscillator outputs, or 2 stereo digital outputs? Or 2 stereo digital oscillator outputs and an LED driver? Or separate auto-accompaniment and/or drums from main voice digital outputs? Or any combinations of these? They could be anything.

Hi... I just bought a PSR-36, which apparently uses the YM3420BF. I think I have an idea of what those outputs are... one thing that called my attention about the PSR-36 is that when you use the digital synth section, depending on slider position, sounds get super stereo... before reading your post here I began wondering what could be happening and thought about the synth using two chips or (crazy idea) that the chip had individual outputs for each carrier operator (summed from each voice). So depending on certain slider position, the operators spread over the stereo field. As sliders make different things depending on which patch you begin with, sometimes is brillance and sometimes detune what makes that effect. I had this keyboard for a day or so, maybe Ill continue discovering more things. So far Im fascinated... I have over 50 synths, but Im really having lots of fun from this one, and sounds really crude and full of personality.
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