exxos's Atari, Amiga & retro forum

Only monochrome CRTs usually cope without a backporch. Colour TVs use it for brightness gain control and take the low voltage as the base for the black level, so if you have no backporch it takes the normal signal level as the deepest of deep black. LCDs controllers seem to use the backporch as part of the horizontal sync and can't latch onto the start of the line without it, in my experience.

stephen_usher wrote: 03 Feb 2024 16:05 Only monochrome CRTs usually cope without a backporch. Colour TVs use it for brightness gain control and take the low voltage as the base for the black level, so if you have no backporch it takes the normal signal level as the deepest of deep black. LCDs controllers seem to use the backporch as part of the horizontal sync and can't latch onto the start of the line without it, in my experience.

Interesting, and certainly indicative of the problem I'm having.

I've posed the question of does it have a backporch or not on OSIWeb and asked the bloke I bought the PCB from what he used for his.

It could be the answer.

Surely you have a CRT which takes composite video laying around somewhere just to test?

LCDs are too clever for their own good.

Just a thought, might be a ridiculous one, would fitting a composite mod with back porch like they do on the zx81 help?

stephen_usher wrote: 03 Feb 2024 17:04 Surely you have a CRT which takes composite video laying around somewhere just to test?

LCDs are too clever for their own good.

No, I don't have any CRT displays with composite, at all.

PhilC wrote: 03 Feb 2024 17:41 Just a thought, might be a ridiculous one, would fitting a composite mod with back porch like they do on the zx81 help?

To get backporch on my ZX81 I had to get a replacement ULA that included it as an option. Not sure how I'd do it to this.

Yeah, it's difficult. Something would have to be triggered by the sync to generate another negative pulse before the system sends the line data, but from the look of things there's no time for a backporch in the timings, unless you use the falling edge of the sync pulse to trigger something to inject a black level voltage for a 5 microseconds before the data?

There should also be a 1.5uS front porch directly after the screen line too.

I s'pose you could use a couple of 555 timers to control the sync pulse. The current sync goes low, a timer starts and runs for 1.5uS before bringing the signal low. This triggers another timer running for 4.7uS (sync pulse) which turns the pull-down off. That should fit the timings for a 60Hz refresh rate (NTSC) scan line.

rubber_jonnie wrote: 03 Feb 2024 19:47

PhilC wrote: 03 Feb 2024 17:41 Just a thought, might be a ridiculous one, would fitting a composite mod with back porch like they do on the zx81 help?

To get backporch on my ZX81 I had to get a replacement ULA that included it as an option. Not sure how I'd do it to this.

One that I got was a small pcb, not sure how it worked but before it would not sync on composite video but after it would.

Then as you say, another is the ULA option which one of my ZX81s has.

@stephen_usher &@PhilC

I managed to pick up a small B&W TV on eBay for £40 that has composite input, so let's see how that does :)

Will it work? Don't know but it's a small price to pay and could be useful.

I also posted on OSIWeb about the signal, and Klyball, who designed the updated PCB (Mostly just a change in the key switches) has come back and said the signal looks good, so fingers crossed the B&W TV will work.

I guess I could try it on my TVs in the meantime or via my RetroTink, but I'm not confident in either.