ZX81 and TV RGB output

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squid4
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ZX81 and TV RGB output

Postby squid4 » Sat Feb 18, 2017 6:40 pm

I've added TV RGB (15khz) output to the Sinclair ZX81 core.

core.zip

This was a learning exercise for me with FPGAs, verilog and the development/debug environment, as I've never touch FPGAs before but have C/C++ experience. I'd be grateful if some Sinclair/CRT fan could test it to see if it works on their TV. It works fine on my Sony CRT. The ZX81 was my first ever computer. Thanks to the guys who ported this originally to the MiST.

There does appear to be a green cast to the output but I'm not sure if it's a problem with my timings or just how the ZX81 (ZXGATE) does its output.
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NML32
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Re: ZX81 and TV RGB output

Postby NML32 » Sat Feb 18, 2017 10:01 pm

I could be doing something wrong but I'm not having any luck loading a game.
I get video output on my LCD and my Sony PVM until I try and load a game.
I mount the game through F12 then do load "" and hit enter then my screen goes blank on my LCD (out of sync) and on my Sony PVM it goes weird. In either case the game never starts and I don't see any LEDs flashing on the MiST.

squid4
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Re: ZX81 and TV RGB output

Postby squid4 » Sun Feb 19, 2017 7:38 am

Did you press the third button to play the tape? The screen does go blank/weird when it's loading.

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Re: ZX81 and TV RGB output

Postby NML32 » Sun Feb 19, 2017 12:37 pm

squid4 wrote:Did you press the third button to play the tape? The screen does go blank/weird when it's loading.


Thanks, I wasn't pressing the play button.

rittwage
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Re: ZX81 and TV RGB output

Postby rittwage » Tue Feb 21, 2017 3:48 am

It seems like PAL and NTSC are backwards? PAL seems to be 60Hz and NTSC seems 50Hz...

Looks very close, though. The top of my screen is skewed out of sync in "PAL" mode. Maybe frequency is not perfect?

squid4
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Re: ZX81 and TV RGB output

Postby squid4 » Tue Feb 21, 2017 9:34 am

rittwage, does your CRT TV tell you the Hz? Mine doesn't. Is it definitely 60Hz for PAL, 50 for NTSC?

The top of the screen is skewed, that's a flaw in the original ZX81 video output.

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Re: ZX81 and TV RGB output

Postby Higgy » Tue Feb 21, 2017 6:43 pm

@squid4 - I play a lot of 60Hz NTSC on my CRT and I would agree, PAL & NTSC are swapped and should be changed.

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Re: ZX81 and TV RGB output

Postby rittwage » Wed Feb 22, 2017 1:32 am

My TV does not show the Hz, but judging by the flicker, I am reasonably sure they are reversed.

I will take a picture of the top of screen sync issue. My original ZX81 doesn't do that...

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Re: ZX81 and TV RGB output

Postby jerome777 » Wed Feb 22, 2017 12:45 pm

rittwage wrote:My TV does not show the Hz, but judging by the flicker, I am reasonably sure they are reversed.

I will take a picture of the top of screen sync issue. My original ZX81 doesn't do that...


Yes, they really are reversed, I remember having seen this on my PC VGA Monitor displaying briefly the frequency (a long time ago, I forgot to post it) :
when PAL is selected, the frequency is 60 Hz and when NTSC is selected, the frequency is 50 Hz :o

Could someone correct this in the file zx01_mist.v ?

https://github.com/mist-devel/mist-boar ... x01_mist.v

line 115 :

Code: Select all

"O1,Video standard,PAL,NTSC;",


line 148 :

Code: Select all

wire ntsc = status[1];

squid4
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Re: ZX81 and TV RGB output

Postby squid4 » Wed Feb 22, 2017 1:24 pm

I'm working on it.

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Re: ZX81 and TV RGB output

Postby squid4 » Wed Feb 22, 2017 7:22 pm

Could someone with a real ZX81 on the same CRT as the MiST measure the top and bottom of the display (where text is displayed) and compare it with measurements for the MiST? Or a screenshot of the two.

A simple basic program:
10 print "HELLO"
20 goto 10

I think there's a VGA timing problem.

Here's a test version with the NTSC/PAL switched around and better bottom borders.
zx01.zip
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rittwage
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Re: ZX81 and TV RGB output

Postby rittwage » Sat Feb 25, 2017 4:30 am

Hello,

50/60Hz is correct. Pictures looks great except the one very small defect.

Here is a photo of a real ZX81 and ZX01. ZX01 looks great, but top line is skewed if you look closely. It slants right...
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Re: ZX81 and TV RGB output

Postby DrOG » Sat Feb 25, 2017 8:30 am

I can confirm the above bug report: the SCART signal is not 100% standard!

Tried both versions avaiable on this page, the result was the same: distortion in the 'Video standard' line of the core's menu, and another at the limit of letters 'Z' and 'X' on the vertical 'ZX01' text.

The other modded cores (BBC Micro and Atari 2600) are fine, no such problem.

Some pictures taken from my TV's frame buffer:

Image

Image

squid4
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Re: ZX81 and TV RGB output

Postby squid4 » Sat Feb 25, 2017 5:42 pm

Thanks rittwage, that's very useful.


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