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- Location: Looking forward to summer in Somerset, UK...
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.
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?
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.
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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.