Music Module DAC

Par Parn

Paladin (803)

Portrait de Parn

08-04-2022, 21:15

Earlier today I was looking at the MSX I/O ports overview at Grauw's MSX Assembly Page, and something caught my attention.

It says that port #0A is used by the Music Module DAC. I tried to find info about this, but I couldn't. I'm just curious, does this mean that the Music Module (and just the Philips Music Module, not any MSX-Audio cartridge) has some kind of DAC in addition to the one that converts the sound chip's data into audio? Or is it some way of accessing the already existing DAC chip? How does one use it?

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Par Grauw

Ascended (10623)

Portrait de Grauw

08-04-2022, 21:44

It’s a separate 8-bit raw PCM DAC, only present in the Music Module and not the other MSX-AUDIO-likes. Similar to the PCM DAC that’s present in the turboR, although more basic and without timing functions.

I’m a bit puzzled by the reasons for its presence since the Y8950 chip also has the ability to output raw PCM at a higher resolution (3.10 bits floating point). But, it is there, we have it so we can use it. The only downside is that the Music Module DAC is hooked up in quite a lazy way so it pollutes the low I/O area with a lot of mirroring (just like the MIDI is too).

After enabling its output via Y8950 GPIO 0, the CPU can just output (unsigned?) 8-bit PCM data to it.

See also (in Dutch) this short note on the MCCM MILC.

Par Wlcracks

Champion (475)

Portrait de Wlcracks

09-04-2022, 07:47

I think its for the realtime sound FX in the rom, maybe bit faster using single out and to use MIC/ADC of the Y8950 and additional 8 bit DAC at same time.
Still amazing hardware available for MSX at the time...
All the information about the hardware is available in the Philips service doc.
The DAC is easy to use, just write the 8 bit value to port. Works only for AC, not a static value.

Par Parn

Paladin (803)

Portrait de Parn

09-04-2022, 12:47

Thanks, Grauw. I now faintly remember having read something about it, a long time ago. Truly a puzzling feature, I wonder if anyone ever made use of it. Its sheer simplicity could be a boon, though.