Covox

[sirmorris]

First iteration of a Covox interface for the ZX81 is working now.

Woohoo!

Lessons learned:
- I should NEVER solder anything when having a fever. I had to redo everything later.
- There is no IN/OUT in ZX81 basic, but PEEK/POKE is. Doing a memory-mapped device is easier to test.
- When doing a memory-mapped device, don't try to write to ROM-area and expect the data to be what you want.
- A 74x574 does NOT have an active low clock signal. I've used these quite a lot (a long time ago), but somehow I did not remember this...
- ULA:s can be slightly broken. Mine could not read 2 columns of the keyboard, and the keyboard was not faulty. The leads are even shorter now.

All excellent lessons I can't wait to hear the results

[1024MAK]

I have heard a more charitable explanation - that writing to somewhere 'non-changable' was another clever optimisation which saved a few bytes in the ROM. I can't remember where I heard this, though.

Well, the 16k/48k/+ ZX Spectrum also does the same thing (writes to the first five ROM locations, 0000h to 0005h), but then a lot of the maths routines are very similar, the ROM code having evolved from the ZX81 ROM.

Mark

[stefano]

>olofsen wrote:
>By the way, why does the BASIC write to ROM?
>
>This is a bug in the firmware or "BIOS".
>I think there are only a few candidates of BASIC instructions doing it. I found for example PRINT does try to write to the ROM. >I found this during testing with an EEPROM instead of a ROM.

Perhaps it is a bit out of topic but the Spectrum has a similar bug, it writes over the higher ROM locations.
Luckily it happens only while listing/editing a BASIC program, in a particular condition making a different scroll-up happen.

[yerzmyey]

Hmmmm.
Well, Covox music is still being played by CPU, after all.
Will the ZX81 handle this amount of work?
It's hard even on ZX Spectrum, where Covox is fairly popular.
4 channels of MOD go on ZX with "not any stunning quality".
Also, digital stuff takes faaaaaaar more RAM than 32Kb.

[nollkolltroll]

I'm doing direct digital synthesis, and no it is not great quality. But it works! I enjoy writing it, you'll get to listen and try later when I'm done.