Monday 19 September 2022

Minstrel 2 Pixel Synchronisation (Part 2)

In the previous post (http://blog.tynemouthsoftware.co.uk/2022/09/minstrel-2-pixel-synchronisation-part-1.html), I asked a series of questions, which led up to 

"Can I make the video output of the Minstrel 2 better?"

I came up with a solution that involved removing the 74LS08 chip that had been added to provide the missing back porch part of the video signal, and replacing it with a 74LS74 which did both the back porch and also pixel synchronisation. I made the modification a bit more permanent on my test board and tested it further.

It was looking promising, but the next question was a bit more ethical.

"Should the glitches be fixed?"

The fixes make the output much cleaner, but also better than the standard ZX80 circuit, so should they be fixed?

In the previous post I put forward the suggestion that maybe these glitches would not have been visible to a ZX80 user back in the 1980s. My theory being that the RF modulator and circuitry inside the TV would form a low pass filter and effectively get rid of those glitches.

"Can I recreate a 1980s setup to test that?"

In order to resolve this, I need to create such a setup.

Back in the early 1980s, I learned to program on a ZX81 displayed on a Binatone Starvision TV.

(not that actual ZX81, and not that actual TV, but the same models).

To test this properly I should have used my ZX80, but I can't get to that at the moment, so I went for a Minstrel instead.

This is a Minstrel 1. My first go at a ZX80 clone. This has the original video mixer and does not have the back porch circuitry, so I thought it was the fairest test.

That does not have RF output, so I borrowed the modulator on a ZX81 board. With the ULA disconnected it is not being driven, so I linked the input to the mixed composite video on the Minstrel 1. That should mean my setup now matches the ZX80 schematic.

I connected it up to the TV, tuned to channel 36 on the TV and I got a K!

I bodged up an old Minstrel keyboard to the Minstrel 1 and tried a few tests, including the inverted and non inverted goth battenburg I had used previously, and as I predicted, the glitch was not visible.

Even close up, I couldn't see any evidence of the glitch lines on the 1980s CRT.

A modern LCD TV is much higher resolution and the signal path has higher bandwidth, so the glitches are passed and displayed as thin grey lines.

Next, I tried Kong.

To keep it 1980s, I loaded it from tape.

And there was Kong himself, waving his arms about as usual, and with no glitches as far as I could see.

Again close up, no sign of the thin glitch lines seen on the LCD.

I also tried Breakout, and here for the first time, I could just make out the thin lines on the bottom half of the screen.

But nowhere near as visible as on a modern LCD TV.

So given the user would have barely seen the glitches on 1980s hardware, I think it is reasonable to remove them so they will not show up on modern hardware. (well I say modern, that LCD TV is probably 10 years old....).

I don't think that is going to far to improve the picture output.


These were previously Patreon exclusive posts, and as I rework them for this blog, it occurs to me that I tackled this problem in the digital domain. I wonder if an alternative solution would have been add a low pass filter to the video signal to replicate the effect of the UHF modulator and TV circuitry. This would have blurred the edges of the pixels a bit (horizontally only), and would probably have flattened out the glitches.

I think the sharper image with nice square pixels is a better solution, those who prefer the misery of the blurry version are welcome to find an original ZX80 and Binatone Starvision on ebay.


Next time (yes, there is a part 3, and maybe even a part 4), I look at two more questions, "Could I route the Minstrel 2 board any better than I did in 2016?" and "why are the chips the wrong way around?".

http://blog.tynemouthsoftware.co.uk/2022/09/minstrel-2-pixel-synchronisation-part-3.html


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A small spoiler for the next post, but the updated Minstrel 2 PCBs are available from my SellMyRetro store:

https://www.sellmyretro.com/offer/details/minstrel-2-zx80-compatible-computer-pcb-(green)-62001

Patreon

You can support me via Patreon, and get access to advance previews of posts like this and behind the scenes updates. This now also includes access to my new Patreon only Discord server for even more regular updates.

Now you know the setup I used to take the picture below with the patreon URL on.

https://www.patreon.com/tynemouthsoftware