Thursday, October 1, 2009

Last minute addition: Odyssey 2 (P824x) VDC chip!

Building a software product and analyzing hardware are two very important facets of a product like chipsounds. Do mostly the analysis and you have a very techy app that no one understands, do only the former and you miss that that hands on feeling and the smell of printed circuit boards.

So this weekend I took a little break from coding (which is very close to done anyway) And looked at the few consoles in my lot that are not fully analyzed yet (I will add more chips in updates). And decided that chipsounds 1.0 required the Odyssey2.

The interesting thing is that there is no sound-coder-friendly documentation on the P8244/P8245 online yet, so I figured might as well publish everything I've learned so far.

My research consisted in confirming Sören Gust's info from g7kbios.pdf and to go further in trying various combination of tones and timbres that would be possible on the console itself without using IRQ based resynchs and also to check on MESS's source code theory of the "OR"ING of tone and noise, and to check for their claim of a 16 bit LFSR used for the noise channel.

On top of his great commented Bios document, Gust kindly provided the community with example assembler "Hello Worlds" for the console and even an improved the BIOS sound player which I then used to run tests on the "real thing", using a ripped apart Baseball Cart and a 2732 eprom (2k code is mirrored twice)... don't have 2716's at hand yet

The available docs mention a 24 bit shift register that is clocked by only two dividers: 983Hz 3933Hz, without any concrete info on how those two frequencies are achieved by the hardware from the master clocks in the first place.

So what is the main clock divisors for this chip?

Before trying to hack in assembly/eprom, I played with the few cartridges I had, and found out that pressing (and holding) the reset button stalls the CPU, but not the VDC, which keeps playing the current tone (or noise) - very handy indeed.

Long tone segments were recorded by tapping the audio signal at the junction of the main board and the RF box. (a tad bit of LP filtering at this point but nothing heavy)

Various NTSC Recorded pitches (Bidule FFT/PVOC Loudest freq analysis)

Assuming 1966.572266Hz came from this pattern: 101010101010101010101010 this means a high freq of would be generated from a clock of 3933.144532 (2x) Which tends to confirm the 3933 value. (note: 983 is just 3933/4)

The preliminary docs for the Intel VDC included in Arnim Läuger's
mcs-48.pdf Mentions the 3933 value comes from 15734/4 but nothing else. Looking around for the source of this frequency:

NTSC television scans 525 lines about 29.97* times a second = 15734.25)
SO -> Precise (theoretic) NTSC values would then be:
15734.25/4 = 3933.5625 (high from now on)
15734.25/16 = 983.390625 (low from now on)

(PAL television scans 625 lines * 25 times a second = 15625)
Precise (theoretic) PAL values would then be:
15625/4 = 3906.25
15625/16 = 976.5625
(would love a Videopak machine to confirm)


Using Gust's improved BIOS player:

We can verify that:

1)DAC is linear, from recordings of a programmed ramp of the 16 volume values

2)Noise is a LFSR-based 16383 bit long repeating pattern.
I haven't yet taken the time to find the exact LFSR formula,
but i've added it as a table.

3)We can indeed generate any looping 24 bit patterns we want:
Example: 0xFFFF00 (2/3 duty)
0x080000 (1/24 duty)
0x001403 (000000000001010000111011 or padded TIA pattern)
Byte Order check with output:
0x010203 (000000010000001000000011)
Recorded output: from left to right starts with:

Listen to those here

4) Possible pitches (using 50% duty as guide):

24 bit
000000000000111111111111:(low) 40.9746Hz
000000000000111111111111:(high) 163.8984Hz

000000111111000000111111:(low) 81.9492Hz
000000111111000000111111:(high) 327.7969Hz

000011110000111100001111:(low) 122.9238Hz
000011110000111100001111:(high) 491.6953Hz

000111000111000111000111:(low) 163.8984Hz
000111000111000111000111:(high) 655.5938Hz

001100110011001100110011:(low) 245.8477Hz
001100110011001100110011:(high) 983.3906Hz

010101010101010101010101:(low) 491.6953Hz
010101010101010101010101:(high) 1966.7813Hz

That's all well and good, but what about 24/8=3 bits?
yes that would make another pitch frequency available,
but could only play 011 and 001 (1/3 and 2/3 duty)

011011011011011011011011:(low) 327.7969Hz
011011011011011011011011:(high) 1311.1875Hz New freq!!!!

in pitch order:
000000000000111111111111:(low) 40.9746Hz MIDI:28( E2): 41.20344543
000000111111000000111111:(low) 81.9492Hz MIDI:40( E3): 82.40689087
000011110000111100001111:(low) 122.9238Hz MIDI:47( B3): 123.4708252
000000000000111111111111:(high) 163.8984Hz MIDI:52( E4): 164.8137817
000111000111000111000111:(low) 163.8984Hz MIDI:52( E4): 164.8137817
001100110011001100110011:(low) 245.8477Hz MIDI:59( B4): 246.9416504
011011011011011011011011:(low) 327.7969Hz MIDI:64( E5): 329.6275635
000000111111000000111111:(high) 327.7969Hz MIDI:64( E5): 329.6275635
010101010101010101010101:(low) 491.6953Hz MIDI:71( B5): 493.8833008
000011110000111100001111:(high) 491.6953Hz MIDI:71( B5): 493.8833008
000111000111000111000111:(high) 655.5938Hz MIDI:76( E6): 659.2551270
001100110011001100110011:(high) 983.3906Hz MIDI:83( B6): 987.7666016
011011011011011011011011:(high) 1311.1875Hz MIDI:88( E7): 1318.510254
010101010101010101010101:(high) 1966.7813Hz MIDI:95( B7): 1975.533203

So there are 10 UNIQUE 50/50 pitches, plus one 2/3 pitch, but LOTS more different _timbres_ using other patterns. The BIOS engineers only included the 10 "different" (to the ear) 50/50 ones...

At least they found a right divider/pattern length to generate a mostly tuned E5 Scale
( -9.6417 to -7.6867 cents off Equal Temperament in NTSC)
(-21.7044 to -19.7494 cents off Equal Temperament in PAL)

Just enough for a PO-LING :)
Back to possible Timbres Lets fill some timbre void shall we?
A pitch is perceived by the repetition of a pattern:

2^24= 16777216 possible timbres for 40.9746Hz and 163.898560Hz
2^12= 4096 possible timbres for 81.951134Hz and 327.796722Hz
2^8 = 256 possible timbres for 122.919807Hz and 491.694153Hz
2^6 = 64 possible timbres for 163.898621Hz and 655.593628Hz
2^4 = 16 possible timbres for 245.850220Hz and 983.390808Hz
2^3 = 8 possible timbres for 327.7969Hz and 1311.1875Hz
2^2 = 4 possible timbres for 491.696228Hz and 1966.781128Hz

case 2^2 can only really do 50/50 since 01 and 10 sounds the same
... in a monophonic console. 00 is silence; 11 is a DC offset click

case 2^4 following the previous logic can only do 25% 50% and 75% duty...

We did not map them all in chipsounds, but this could be added in the future in a more specific oscillator ...


  1. How easy is it to drop in new chips as you figure them out? Is the code of your synth modular enough that it's pretty simple for you to add one?

  2. Most of the time yes.

    In the case of the 02, it didn't require any mods to the software code, just a new table for the noise and a "behavior" text definition in the form of a SFZ 2.x file.

  3. When you talk about an "E5 scale," do you mean a major scale that starts at the fifth octave E note?

  4. nope i mean a "power chord scale" which is just the E's and B's from

    40.9746Hz MIDI:28( E2):
    1966.7813Hz MIDI:95( B7)

  5. Oh! Then I _do_ understand your chart after all!

  6. are the editscreens different for every chip?

    (ilove your products, enjoy bidule on a daily base. thanks ! )

  7. Only one sub edit screen is different per chip. The rest is standardized.

  8. I don't know if the odyssey has any video sync based interrupt, but if it has, maybe it's possible to dynamically change the waveform every wave cycle with some tight timing to produce other pitches (since the frequency is based on video timing anyway). I don't know, just throwing out uninformed ideas, really!

  9. i would need to check on the annotated BIOS again, but i think that the 24 bit register needs to be fully flushed before its changed.

    But such timed IRQ syncs tricks are used on may other chips, i wouldn't be surprised if something could be done about it.

  10. Just found this page... Lots of interesting information! Off the top of my head, the Killer Bees cartridge sounds different to the standard cartridge sounds, may with some envelope effects. Dunno if you're still doing anything with this, but might be worth taking a sample of the sounds and looking at the bit pattern. Cheers

  11. Hi Simon. Been a while since I looked into it. Apparently Killer Bees use the Voice addon, which is something we emulate in chipspeech. Otherwise, indeed the arcadeish sounds feels like rapid envs on the noises.