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.
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.