Differences new ZX Evo without VDAC and emulator

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Postby chernandezba » Sat, 07.04.2018 13:25:55

Hi
I have recently purchased a ZX Evolution rev C, without the VDAC module.I'm not sure about the differences on having vdac or not, and what are the differences between 2-bit,3-bit,4-bit or 5-bit vdac (register VDVER)
But what I really see is differences between emulation and my real evo. I attach some photos running on my real Evo, and one screenshot of ZEsarUX emulator. Seems the Evo is doing some kind of dithering...

Why these differences?
Thanks
Cesar
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IMG_7645_comp.jpg
IMG_7655_comp.jpg
IMG_7657_comp.jpg
keepcalm_real_zxevo.jpg
keepcalm_emulator.png
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ZEsarUX
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https://github.com/chernandezba/zesarux
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Postby TS-Labs » Sat, 07.04.2018 19:37:44

Cheers to ZX-Evo designers. If there's no VDAC there's only RGB2 output on ZX-Evo (6 bit). In ancient times we "invented" so called "chunked PWM" which gives sandy output as on your photos. It emulates RGB5 in a lame way. It can only give 0-24 grayscale where 24-31 values give same output, which is obvious from the fact that 0b11 DAC value cannot be modulated by any chunks.
That's why VDAC1 and 2 devices are produced. VDACx gives true RGB5 output (i.e. 15 bit color) with linear grayscale 0-31. They insert into IDE connector instead of HDDs. Yep, this way.
We didn't bother with the PWM in the emulator, so it assumes linear 0-31 scale everytime.
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Postby WBC » Sat, 07.04.2018 19:46:27

TS-Labs wrote:We didn't bother with the PWM in the emulator, so it assumes linear 0-31 scale everytime.

no we does, UnrealSpeccy emulates non-linear gamma ramp for PWM modes (but doesn't emulate PWM artifacts)
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Postby chernandezba » Mon, 09.04.2018 15:09:07

So, as I undertand, the ZX Evolution rev c, has 64 colours, and the whole 15-bit colours are done with dithering

So how you get 3-bit per component (9-bit : 512 colours) with vdac? And what about 12 bit?
I suppose if you buy the vdac module you will get the full 15-bit palette (32768 colours), right?

But I'm confused with the previous information you gave me
viewtopic.php?f=29&t=758&start=75

Having the 15-bit palette, in which cases you have full 5-bit (0...31) values, or you have (0...24) values?

Thanks
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Postby TS-Labs » Tue, 10.04.2018 03:07:32

chernandezba wrote:So how you get 3-bit per component (9-bit : 512 colours) with vdac? And what about 12 bit?

We don't.
chernandezba wrote:I suppose if you buy the vdac module you will get the full 15-bit palette (32768 colours), right?

Right.
chernandezba wrote:But I'm confused with the previous information you gave me

256 color cells, 16 bit each.
bit 15 (0x8000) is used to distinct the old 0-24 PWM and linear RGB5.
When bit15=0 then values 0-24 are linear grayscale and values 24-31 are reproduced as 255 on output.
When bit15=1 then values 0-31 are linear grayscale.
RGB5 to RGB8 is a tricky convertion, but formulas used are as simple as possible.

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