ZX81 Video Conditioning 

Bit late to the party here, but I've just built the 555 circuit and it works perfectly. Come back JoulesperCoulomb we miss your excellent videos.

Thank you for this circuit and the explanation. I built the one based on the 555 and got excellent results. My ZX81 is alive again!

Wow, Wow, Wow, Wow, Wow! Absolutely brilliant. I learn't about tv waveforms at college back in the late 80s and sadly have forgotten much of the detail since - amazing how a little prompting brought it back. Thankyou for such a superb explanation. Now to build the circuit and try it out on my newly-acquired ZX81 !!

You're videos are excellent, to the point, informative and without silly background music or advertisements. Thank you!

You make such superb videos. Everything you show is so very well thought out and explained. It must take you ages to get all the footage and edit it together, but thank goodness that you do. A big thank you for another great video. Cheers!!

I built this circuit for my Timex Sinclair 1000 and had a few difficulties. It works great now that I've figured it out. Just so any other poor souls try this on a Timex Sinclair 1000, the main thing you must know is you MUST attach the video input to pin 16 of the ULA. The USA builds do some magic between the ULA and the RF box and you can't take the signal from the wire that enters the RF box. Other notes: There are 2(3?) different ULA chips depending on your model. ULA 2C184E needs this circuit or some other video circuit to add back porch. ULA 2C210E does not need this circuit, but might need a simple transistor amplifier. www.classic-computers.org.nz/blog/2016-01-03-composite-video-for-zx81.htm I hope this helps!

Finally got around to building the circuit (only changed the 22uF cap to 10uF since I was lazy and didn't have any 22uF ones). Works a treat, picture on my flatscreen is very good, on my sony PVM broadcast monitor quality is on par with emulation. I'd post a before-after-picture but for some reason youtube won't let me paste the URL. Thanks a lot for this.

absolutely fascinating. I don't have a ZX81, yet, but this succeeded in explaining video signal generation to me

Your Videos read like a thriller and I cannot stop watching until the very end. Very educational stuff !!

It is nice to look at a video made by somebody who knows what he is talking about. Improving the video-signal with so little components is almost magic. Somewhere in the far distance I see two colours coming to the ZX81, in stead of black and white. If you are able to find the back-porch you can be skilful enough to transform the black and white signal into primary colours. Only two colours full screen of course, I know the limits of the ZX81. I am not asking for black, white and a form of grey, that would be impossible.

555 circuit was tested on a kit built ZX80 which suffered from a poor and fuzzy picture even with the brightness at 100% due to no back porch output. Image quality on the 80 is now very good, so much so that the brightness and contrast levels on the flatscreen test television are down in the 50% levels.

Excellent - I dont think this could have been explained any more perfectly. I'm currently building a ZX81 clone using an old ULA (without the back porch) and havent been able to understand whats going on with other approaches so thankyou :)

Built the 555 circuit for my ZX81. Works great :)

My ZX81 no longer produces a signal at the mic socket when saving, and I'm wondering if this is because of the composite mod, since isn't the video pin on the ULA also feeding the mic socket? Anyone else experienced this?

Everything's allright Sir? You haven't posted a video for a long time.

Sorry, for commenting on such an old video, but quick question: should this circuit also work on a ZX80?

I had to use a 100 Ohms resistor load in the video out. If not done the new TV sets do not detect the video signal. My ZX81 uses the new ULA with back porch. Thanks.

Exelent video, thank you.

1:35 _"Strictly, this is a luminance video signal, not a composite one."_ I am pretty sure that technically that's a luma signal, not luminance. And is the term "composite" so so precisely defined? The signal you show, after after all, is not just the luma information but is the composition of the luma and two sync signals. Combining the horizontal and vertical sync signals together (which is not correctly done with a simple XOR, though that usually works ok) is usually called "composite sync." I deal a lot with split and composite sync signals independent of the video signal. To try to reduce confusion with baseband single-channel signals that include composite sync, I've taken to using the terms CVBS (widely used, with many different ways of expanding that acronym) and MVBS to distinguish those with and without a colour signal mixed in. (This is independent of scan format; e.g. I use MVBS for both 60 Hz (old B/W NTSC) and 59.94 (newer colour NTSC) scan rates, so long as a color signal isn't present). Such quibbles aside, this video is well done and very informative.

I expected better picture without rf modulator, clearly video signal band is to low but i don't know the reason.

I just got a US TS1500. Would this apply? ULA is different? Not much of an electronics boffin, but would really like to connect this to a LCD monitor.

Great video :-)

That's weird as the Memotech MTX512 actually had a power pack that had a -5v supply as well as -12v, I think that the -5v supply is to make the pc produce the standard video signal that all televisions expect.

Well done!

Pictures as promised. Top: original modulator output, bottom: composite with 555 based back porch generator. Both taken on my Sony LCD, on the PVM broadcast monitor the picture look even better. imgur.com/aBfK6GI.jpg

Hmm, this circuit (the one using the 555) doesn't seem to work for me. My ZX81 doesn't have a back porch, and the line synchronization pulse is about 2.3 V, the rest is about 4V. To trigger the 555, we'd need to go below 1/3*5V (~1.667V). If anyone has a nice solution, I'm all ears. Otherwise I'll reply to myself when I have figured out something that works.

Brilliant, another masterful improvement to a classic system! I might need to do this mod to my ZX81 - need to get it out and check which revision it is. Do you know how to identify which ZX81s this mod is not required for - ie. what revision board / ULA?

From www.kolumbus.fi/pami1/video/pal_ntsc.html you can find all timing information of both PAL and NTSC video signals

I was recently gifted my first ZX81, but for some reason the black to white transistions "bleed" quite bad (white to black is fine) on both of my TVs (Sony PVM tube and KLV LCD). Would this be related to the back porch issue or is it a seperate issue? The board has been modified to output composite (but has no additional circuitry as found in various instructions online).

Great video, hope you don't mind, I've used a still from this video showing the video waveform in my blog post on improving the video signal on my ZX80 clone. blog.tynemouthsoftware.co.uk/2017/01/minstrel-zx80-clone-video-output.html