ZX81 Composite Video and RGBtoHDMI Adjustments

Trying to improve the picture from my ZX81 with Mutant Caterpillar ZXvid Composite Video board, when fed through an RGBtoHDMI. We check the voltages into the ZX81 and have a look at the video signal on an oscilloscope, then see how they change when the pots on the ZXvid are adjusted.

[@MikeLynx]

Great video and instructional. I learned a lot about both analogue composite signals and oscilloscopes! 👏

[@MrLurchsThings]

Big congrats on the 1K sub 👍

[@minombredepila1580]

Amazing video. Learnt a lot !!!. Thanks.

[@carledwards9477]

Congrats on the sub count going over 1000.
It's always good to see a smaller channel growing.
I found this to be a fascinating coverage of how signals come out of a computer and manifest on a screen.
It's way too easy to just look at something and ignore what is going on in the background.
Thanks for making it clearer!

[@kevinjoyce5076]

I've only just discovered this channel - fantastic. It's up there with Ben Eater for me. Very well made and informative. I notice you have a raspberry pi lurking around on your shelf. Have you not been tempted to do anything interesting with that?

[@mikeh_nz]

You might want to try out a zx8-ccb (which is the same wiring into zx81)
I think the voltage separation between the sync and back porch is greater.
I would find a video comparing the two signals to be interesting.
Of course you could also get a modern ula replacement that will give you a nicer formed signal with backporch
Congrats on the 1K

[@RudysRetroIntel]

Excellent video and work. How did you connect the RGB2HDMI adapter? I'm assuming you rigged it up to the composite jack. Thanks for sharing

[@lsbyte]

Yup - I just made up a short cable with a 6-pin IDC connector on one end and an RCA plug on the other end, as described here: github.com/IanSB/RGBtoHDMI/wiki/Cables
The ground (outside sleeve of the RCA) connects to pin 1 of the IDC and video (tip) connects to pin 4.
I used a Mutant Caterpillar mod on the ZX81 to get a composite image.
The problems I had in this video have been subsequently resolved by using a Clamp setting on the RGBtoHDMI, as that will automatically compensate for the varying voltage. I probably should do a short video about that but haven't had time!

[@RudysRetroIntel]

@@lsbyte Excellent! Thanks for sharing this information. Great work!

[@Spongman]

that's weird. i'm not sure why the rgb2hdmi requires a difference between the sync and blanking voltages. if it can detect the start of the sync, it can just wait the requisite time (line-sync + front-porch/2 = 4.7us + 1.65us/2 = 5.525us) and sample the black level then.

[@katalina1953]

...briinging back memories... then the QL😃😃😃

[@BM-jy6cb]

The amazing thing about this is that you're using a £300 scope to do what the original engineers could only have dreamed of back in the day. The ZX81, for all its acclaim is still a pile of poo though. ;⁠-⁠)

[@MrLurchsThings]

Pretty sure, in 1981 if all you could afford is a “pile of poo”, you’d be happy with it.

[@lsbyte]

It's all I could afford at that age. It was, however, enough to persuade my parents I could use the savings they'd put aside for me, from when I was born, to buy a Spectrum a year or so later.
All that said, I really don't remember it being as slow as that, and my fingers were certainly smaller!