Snes APU on a PC
One of the most important component inside the Snes is the
Audio Processing Unit (APU). The APU is responsible for
playing music and sound effects. The APU is made with a processor (SPC700),
some memory (64Kb), a DSP and a Digital/Analog converter. The Snes's
main CPU communicates with the APU using 4 ports.
Here is a page explaining in greater details what's inside the APU:
How sound on the SNES works
The .spc format Snes music files are 'save states' of the APU
components, similar to the save states of the whole Snes we obtain
by pressing F2 in zsnes, but this time, only the APU memory, CPU
and DSP registers are saved. This is sufficient to playback game music
because most games simply load a playback program in the spc which once
started, plays music endlessly. When asked, the program also plays sound
We can playback .spc files with a spc player(standalone or plugin). A spc
player is a simplified Snes emulator (APU emulation only). I had been
hoping for a long time to be able to use a real APU to play spc's to
get a perfect reproduction of how it sounded on a real Snes (Of course
emulators can do a better job than the real Snes APU, by using higher
sample rates, better resampling, better interpolation, etc... but here,
it is a matter of authenticity and cool factor).
I found how to connect a Snes APU on a parallel port on
, as well as a simple program to upload .spc files to the module
and play them back.
Here is the schematic:
And here is a file containing the pinout of the Snes APU:
The schematic and pinout comes from the archive
If you build this project, please contact me. I'd
like to add pictures to the
For the 5 volts power supply, you can use a lm7805 voltage regulator. The module
consumes 62mA from digital VCC when idle or playing and 36mA when reset is low.
The module always consumes 1.3mA from Audio VCC.
A user reports that his unit would not work until he grounded the APU chassis.
I built this project in a simple plastic box. In the left part, we can see the
small PCB holding the 74ls32 and we can also see the DB25. Look at
the next picture for more details about the right part.
Here we can see the APU connector (taken from a Snes), the RCA outputs, the 1/4 Jack
output for headphones, the power led, and 2 leds to reflect the state of the R/W signals.
On the RCA connector, I installed 1uf capacitors to get rid of the DC offset
(around 1.2 volts IIRC) coming out from the APU. The headphones jack uses the 50 ohms
series resistors as on the original schematic. Apparently, my headphones do not like
the DC offset, and using the same capacitors does not work either. I need to add
a small amplifier...
Once closed, here's what this nice music box looks like.
I first tried using CaitSith2 software, but it did not work. By looking at the signals
with an oscilloscope, I deduced that the computer did not wait long enough for
the logic levels to settle(rising was slow, this is probably due to the way my
parallel port is designed). Fortunately, the source code was available. Adding
small delays at the right places made it work.
The user interface is written in VB, and the low level parts are in C/C++ in a separate dll.
I had to recompile the dll to add the deleays. If you get errors such as "Error Loading SPC"
my dll after confirming that everything is wired correctly.
Here is my dll:
and the file I modified:
I did not want to be forced to run Windows in order to use the APU, so
once everything was working well, I adapted the dll code for Linux (just a iopl call
and a macros for doing port io) and I rewrote the VB code in C.(ouch..)
Xmms plugin screenshot:
The source code for my Linux program is available here:
|March 30, 2005
- Added ppdev support (parallel port control from user space.
Not necessary to be root anymore if your kernel supports it.).
- Added xmms support (ppdev only)
- Added color to the command line interface
- Added a -d command line argument for debugging
- Status line now enabled by default. Use -s to disable it
- Code is now clearly licensed as GPL
|March 6, 2005
This new version can be compiled with DJGPP to build an executable
usable on DOS/win9x (wont work with 2000/NT/XP since it needs to
access io ports directly. You may be able to use it with the help
PortTalk - A Windows NT I/O Port Device Driver
Here is a precompiled DOS version:
|March 5, 2005
No features have changed in this version. I just cleaned and
clarified the code that manipulates the .spc file (the part converted
from VB) by using #defines instead of hardcoded hex values. I have
also added some comments about what is done. The code is now
much easier to understand!
This is a simple command line tool to load a given .spc file
into the snes apu using the interface described on this page. It
partially supports id666 tags, stops the song after the specified
timeout (from the tag or argument), displays the title,
remaining time, etc...
I'm not done yet with this project. Here are a few things I'd like to do:
xmms plugin for playing spc files on the APU. DONE
- Make a PC independant version. Right now, if it is powered with a battery we can unplug
it from the PC once the tune is loaded and bring the APU elsewhere. I would like something
similar to this
SID player, but more
- Study the possibility to hack snes9x or zsnes to use the APU instead of emulating it.
Here is a recording of a tune played with my APU:
The best places to get game soundtracks in spc format are:
Zophar's spc archive.
I cannot be held responsible for any damages that could occur to you
or your equipment while following the procedures present on this page.
Also, I GIVE ABSOLUTELY NO WARRANTY on the correctness and usability
of the informations on this page. Please note, however, that the procedures
above have worked in my case without any damages or problems.
Now you cannot say that I did not warn you :)