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ZX81 HDMI interface using Raspberry Pi Zero

Posted: Sun Feb 03, 2019 4:51 pm
by IanSB
I've been contributing to an open source HDMI converter project primarily developed by Hoglet on the stardot forums for use with the BBC micro / Master 128 / Electron (I wrote the mode 7 deinterlace and genlock code) but we've been adapting it for other micros and I recently got it working with the ZX81 so I thought I'd post some info here:

Original thread:
https://stardot.org.uk/forums/viewtopic.php?f=3&t=14430

Most recent post about ZX81:
https://stardot.org.uk/forums/viewtopic ... 00#p226513

Unlike general purpose HDMI converters, this one digitally resamples and reconstructs the source video so you end up with perfect screen output on the LCD with no noise, smearing, ghosting etc. You can also configure it for integer scaling so one original ZX81 pixel occupies an exact block of LCD pixels with no interpolation. It also works with the old style ULA with no back porch blanking.
At the moment, the design is not suitable for multi level analogue video / RGB so is only suitable for systems like the BBC / electron / IBM PC CGA / Oric which have TTL RGB and systems with monochrome video like the ZX80/81, UK101, Nascom, Apple II, TRS80 etc. that can be converted to TTL with a simple comparator circuit.

Here are a few pictures:
(Modulator has been modified to pass through composite video from the ULA)
zx81-1.jpg
zx81-2.jpg
zx81-3.jpg
zx81-4.jpg
zx81-5.jpg

Re: ZX81 HDMI interface using Raspberry Pi Zero

Posted: Sat Feb 16, 2019 1:16 pm
by Fwapp
Very, very interesting!

I pretty crap these days at conjuring circuits up, could you post a schematic of the little circuit required? I'd love to give this a try. I have the CPLD pcb on the way.

Re: ZX81 HDMI interface using Raspberry Pi Zero

Posted: Sat Feb 16, 2019 2:25 pm
by Andy Rea
very nice indeed, any plans to sell this as a ready to use module ?

regards Andy

Re: ZX81 HDMI interface using Raspberry Pi Zero

Posted: Sun Feb 17, 2019 3:04 am
by IanSB
Fwapp wrote:
Sat Feb 16, 2019 1:16 pm
I pretty crap these days at conjuring circuits up, could you post a schematic of the little circuit required? I'd love to give this a try. I have the CPLD pcb on the way.
This is the circuit on the veroboard in addition to the pcb that's plugged into the Pi zero:

composite2rgb.jpg

Re: ZX81 HDMI interface using Raspberry Pi Zero

Posted: Sun Feb 17, 2019 10:42 am
by Fwapp
IanSB wrote:
Sun Feb 17, 2019 3:04 am
Fwapp wrote:
Sat Feb 16, 2019 1:16 pm
I pretty crap these days at conjuring circuits up, could you post a schematic of the little circuit required? I'd love to give this a try. I have the CPLD pcb on the way.
This is the circuit on the veroboard in addition to the pcb that's plugged into the Pi zero:


composite2rgb.jpg
Perfect! Thanks for that, I can easily knock that up!

Re: ZX81 HDMI interface using Raspberry Pi Zero

Posted: Sun Feb 17, 2019 11:45 am
by Fwapp
Just a thought, but if you already have a ZX CCB or ZX Vid comp video pcb installed, would you be able to bypass the comparator circuit altogether, and feed the signal directly into the RGB to HDMI?

Re: ZX81 HDMI interface using Raspberry Pi Zero

Posted: Sun Feb 17, 2019 12:21 pm
by 1024MAK
Fwapp wrote:
Sun Feb 17, 2019 11:45 am
Just a thought, but if you already have a ZX CCB or ZX Vid comp video pcb installed, would you be able to bypass the comparator circuit altogether, and feed the signal directly into the RGB to HDMI?
No, if you are saying what I think you are saying. As I understand it, the PI needs separate video and composite sync inputs, both at TTL levels.

However, I suspect a ZX CCB could be “abused”. You would need to solder wires on to the appropriate points to pick up the separated sync signal and the separated video signal. Then no additional comparator board would be needed.

This may also be possible with the ZX Vid (I don’t have one so am not sure).

Does the PI care about the content of the video signal during the sync period if the video signal is at TTL levels?

Mark

Re: ZX81 HDMI interface using Raspberry Pi Zero

Posted: Sun Feb 17, 2019 11:26 pm
by Fwapp
1024MAK wrote:
Sun Feb 17, 2019 12:21 pm
Fwapp wrote:
Sun Feb 17, 2019 11:45 am
Just a thought, but if you already have a ZX CCB or ZX Vid comp video pcb installed, would you be able to bypass the comparator circuit altogether, and feed the signal directly into the RGB to HDMI?
No, if you are saying what I think you are saying. As I understand it, the PI needs separate video and composite sync inputs, both at TTL levels.

However, I suspect a ZX CCB could be “abused”. You would need to solder wires on to the appropriate points to pick up the separated sync signal and the separated video signal. Then no additional comparator board would be needed.

This may also be possible with the ZX Vid (I don’t have one so am not sure).

Does the PI care about the content of the video signal during the sync period if the video signal is at TTL levels?

Mark
Thanks for the clarification. It was an idle wonder if the ccb/vid did a similar job to the comparator. There's plenty out there installed and it's be nice if the RGB2HDMI was an easy fit for this user base. I'm happy enough knocking that circuit up.