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Idea to quickly emulate another sound board
Posted: Sat Dec 19, 2015 6:24 pm
by stefano
Around the 1982 a book called "66 programmi per zx81" proposed a cool way to connect a 555 based oscillator to the ZX81 and get a cheap and simple sound board. The book is available online as well as a re-arrangement of the circuit (the board is now called "Octavia").
I thought at a quick hack to get it emulated: to use the already existing AY emulation and convert the bytes being poked to the sound card to AY registers.
The conversion expression is not very good (can you find a better one?) but the idea looks nice.. feel free to use it into your favorite emulator: here's my code for the SZ81:
if (Address>8191 && Address<16384) {
extern void sound_ay_write(int reg,int val);
/* keep only audio channel 0 */
sound_ay_write(7,254);
/* set volume to zero if "makic key" found to disable audio */
if ((Data & 15) == 15) {sound_ay_write(8,0);}
else {
/* enable the 555 oscillator */
int val=170*Data*Data/7040-39227*Data/3520+1450;
sound_ay_write(8,7); /* raise volume */
sound_ay_write(0,(val)&255);
sound_ay_write(1,(val>>8)&15);
sound_ay_write(6,0);
}
}
..I can provide a (partial) tone conversion table, in case a maths would want to challenge himself
Re: Idea to quickly emulate another sound board
Posted: Sun Dec 20, 2015 9:43 pm
by stefano
Reference to the sound card:
Original book (in good resolution) Sound card is described starting at page 95:
http://www.microatena.it/scheda_libro.php?id=198&ord=N
..there is an error in the sound table in page 100, note #47 should be played by POKEing value 237 rather than 372.
Remake:
http://zx81.ordi5.free.fr/brico/son/index.htm
I understood that every bit in the value has a different "weight" than expected, I'm still trying to decode it in a better way, but few demos sound good already, even if music wouldn't play right.
Re: Idea to quickly emulate another sound board
Posted: Sun Dec 20, 2015 10:02 pm
by 1024MAK
stefano wrote:R..there is an error in the sound table in page 100, note #47 should be played by POKEing value 237 rather than 372.
Yes, 372 does not fit in an 8 bit byte
Mark
Re: Idea to quickly emulate another sound board
Posted: Mon Dec 21, 2015 2:22 pm
by stefano
This should explain why the current conversion function is not accurate: I provided a simple parabolic conversion while the sigle bits weight differently. Here's the diagram of the right values based on a conversion table. you can see the 4 octaves split in 4 note blocks.
[img=0]octavia-zonx.jpg[/img]
Re: Idea to quickly emulate another sound board
Posted: Tue Dec 22, 2015 3:27 pm
by stefano
Ok, I partially decoded the board logic and got a better simulation of the oscillator. Bit 6 and 7 are octave multipliers, bit 4 and 5 shift the frequency in one of the possible three positions for the 4 notes in the 12 semi-tones scale.
Bit 1..4, inverted, determine the reference frequency for the note and normally are not combined (but for the higer 2 notes).
this code snippets fits into SZ81, in file "zx81.c", just before the line:
Code: Select all
if (Address>8191 && Address<16384 && zx81.shadowROM && zx81.protectROM) return;
Here it is:
Code: Select all
if (Address>8191 && Address<16384) {
extern void sound_ay_write(int reg,int val);
/* keep only audio channel 0 */
sound_ay_write(7,254);
/* set volume to zero if "makic key" found to disable audio */
if ((Data & 15) == 15) {sound_ay_write(8,0);}
else {
/* enable the 555 oscillator */
float freq=(60.0 + 5.4*(float)(~Data>>3&1) + 9.4*(float)(~Data>>2&1) + 13.2*(float)(~Data>>1&1) + 22.1*(float)(~Data&1) + 17.5*(float)(Data>>4&1) + 42.1*(float)(Data>>5&1)) * (float)(1<<(Data>>6));
unsigned int val=(1625000/(16*freq));
/* raise volume */
sound_ay_write(8,7);
sound_ay_write(0,(val)&255);
sound_ay_write(1,(val>>8)&15);
sound_ay_write(6,0);
}
}
23/12.. edited the expression for freq to make it more suitable for music. Tuning is not perfect but it is close to the results I'd likely get building the circuit myself
I'm not sure on how to name this board, I like the "Octavia" name but the its original description is probably historically correct: "50 notes music interface" by Gaetano Marano.
Re: Idea to quickly emulate another sound board
Posted: Fri Jan 15, 2016 3:38 pm
by stefano
More sound demos, they integrate the ones already published by XavSnap.
Re: Idea to quickly emulate another sound board
Posted: Sun Jun 20, 2021 10:21 am
by stefano
I just found this article:
http://www.muzines.co.uk/articles/zx81-sequencer/3220
Similar idea, different implementation. In the article it looks that the valid values to POKE had to be <127. 5 octaves, though!