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Re: Composite mod issue: left side of screen missing - solved!

Posted: Tue Apr 13, 2021 8:03 pm
by patters
I went back and re-watched the Joulespercoulomb video and according to him the black pixels should be at 0V which they're some way off on your oscilloscope traces. Is there an easy fix for that? Is that what the additional 1K resistor was aiming to fix? I notice that the circuit with your fixes is still a bit different to his design.

Re: Composite mod issue: left side of screen missing - solved!

Posted: Wed Apr 14, 2021 1:46 am
by 1024MAK
No, it depends on which level you use as the reference point. And on which standard you use. The U.K. broadcasting standards are a bit different to the U.S.A. broadcasting standards. But most actual practical TV sets can cope with the levels from either, as long as they can cope with the other signal timing differences.

In a system like the ZX81, I consider the reference point to be the bottom (negative) of the sync pulse, which should be considered to be at 0V (as the ULA and any DC coupled circuitry can’t generate a negative signal voltage level). This is -40 IRE. Or about -0.3V if the blanking level/black level is considered to be the reference level (0V).

In some documents, the blanking level is the same as the back level. In other documents it’s as follows:
The blanking level should be 0 IRE.
The black level should be 7.5 IRE.

Both should be about 0.3V greater than the bottom (negative) of the sync pulse. So in a composite video signal from a ZX81, it should be about 0.3V.

The peak white level should be 100 IRE. As far as U.K. video signals are concerned, this is about 0.7V greater than the blanking level/black level. So in a composite video signal where we are using bottom (negative) of the sync pulse as 0V, the peak white level should be about 1V.

See here for a diagram showing the IRE levels.

The document defines IRE as:
An arbitrary unit of measurement equal to 1/100 of the excursion from blanking to reference white level. In NTSC systems, 100 IRE equals 714mV and 1-volt p-p equals 140 IRE.

And in on this web page, the diagram shows voltages.

Keep in mind, that in the TV set or monitor, there will be some circuitry to ‘restore the DC levels”. This circuitry then sorts out the sync signals and the rest of the waveform. As most TVs and monitors will use a capacitor to couple the video signal from the external source to the TV/monitors internal circuitry, the actual DC level at the input often does not matter. It’s the relative signal levels of the different parts of the signal waveform that matter. And the peak white level is even less critical, as the user controls (brightness and contrast) can compensate for this.

I hope that helps. And yes, it is a bit confusing, but that’s what happens when there are lots of variations and they change over time...

And yes, I do intend to experiment a bit more, and yes that’s why I suggested getting the extra parts. But if you are happy with the results that you are getting on screen, don’t worry about the actual signal voltage levels. Analogue TV systems are fairly forgiving.


Re: Composite mod issue: left side of screen missing - solved!

Posted: Wed Apr 14, 2021 11:13 am
by patters
Thanks for taking the time to write that out, very detailed.

One other observation I've noticed is that the remaining 10K trimmer seems to drift with temperature, or perhaps it's the ULA drifting as it warms up. The trimmer we removed seemed to set the duration of the back porch in the h-sync pulse. The remaining trimmer seems to alter the black level and if you go too low then the image loses sharpness and some very high frequency wobbling, and further still you get colour burst artifacting. So you want to set it as black as you can get without that happening (which is still not as black as the 210E ULA).

However, if you set this just right, and reassemble the Zeddy you find that it does drift when the computer has been on say 20mins. This caused me to have to open it up several more times to find a compromise tuning value, which has the effect of lighter blacks when first powered on. Do you think this is likely to be caused by more poor quality parts in the mod, or by a 40 year old ULA?

Re: Composite mod issue: left side of screen missing - solved!

Posted: Wed Apr 14, 2021 8:00 pm
by Lardo Boffin
Only anecdotal not technical but I have sold a lot of zeddies with ZX8-CCBs fitted and once tuned (to nearly always the same settings) I don’t need to further adjust them. I always soak test the ones I sell for two lots of 8 hours so would notice any warm up effects.

The only thing I have seen once warm is ‘sparkly’ pixels - where random pixels flash on and off - on one or two zeddies. Swapping the ROMs has always fixed this.

Re: Composite mod issue: left side of screen missing - solved!

Posted: Fri Apr 16, 2021 2:36 pm
by patters
The trimmer seems to have drifted now so all black pixels have ghosting, meaning I'll have to open the Zeddy up yet again.

I have decided out of curiosity to order a new composite mod from ZX Renew (£11) and give that a go, seeing as I was ordering an S-Video mod for Spectrum 48K.