A board · sheet 4/8 · manual page 6

The sound sheet

A second computer whose only job is to play music, sealed inside one package, taking orders one byte at a time.

Draft. This sheet was read from the original 1985 drawing and cross-checked against MAME, but it has not yet been walked pin by pin against a physical board. The logic described here is what the drawing shows.
▸This sheet is part ofHow the sound works

The subsystem followed end to end, across every sheet that builds it.

What this sheet is for

Two YM2203 sound chips at 13E and 14E, their DACs at 13D and 14D, an amplifier, and the sound CPU that drives all of it.

The 6809 does not play music. It writes a single byte to a latch and forgets about it. Everything after that happens on its own clock, in its own address space, with its own RAM.

What is inside 13K

Capcom's drawing shows a sealed custom module at 13K: a DIP package containing the sound Z80, its RAM and its address decoding. You cannot probe inside it and the drawing does not show you.

The photographed board has none of that. A socketed Zilog Z84C0020PEC, a Toshiba TMM2016BP 2K static RAM beside it, and discrete decoding do exactly the same job in the open. It is the same substitution pattern as Commando's custom decryptor being replaced by a PAL riser, and it makes this board far easier to understand than a factory one.

One detail from the module worth knowing: A15 never leaves it. The sound CPU's address space is entirely local, which is why the sound side can be reasoned about on its own.

13K — sealed custom moduleZ80the sound CPU2K RAMits own scratchpaddecoderA15 never leaves6809writes one bytelatch2x YM220313E, 14E

Capcom sealed the whole sound computer into one package at 13K. On this bootleg board it is opened up: a socketed Z80, a separate 2K RAM and discrete decoding do the same job in the open. The 6809 never talks to the YM2203s directly; it writes one byte to a latch and walks away.

Two YM2203s, not one

Each YM2203 gives three FM channels and three square-wave channels. Two of them is six and six, which is what the score needs. They share the 1.5MHz clock derived from the same crystal as the video, so the music cannot drift against the picture.

The YM3014 parts at 13D and 14D are the DACs. The YM2203 has no analogue output of its own; it hands out a serial stream and the 3014 turns it into a voltage.

Every chip on this sheet

Click any of them to open the schematic with that chip selected in both panels.

12DLM32412H74LS37413DYM301413EYM220313KCUSTOM (Capcom module, contains the Z80 sound CPU)14DYM301414EYM220314H27256(HA13001)HA13001VR1RGP102 10KVR2RGP102 2K
← open the schematic for this sheet