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In the datasheet of the RF modulator UM1233 used in Oric-1 we can find the following typical waveform at the input:

enter image description here

I tried to analyse the following Oric-1 circuit applied  to this modulator:

enter image description here

To get the 2.6V as minimum voltage the pot value will be around 77 ohm.

However for this value the maximum value will be around 2.9V. This will not cover the waveform voltage range.

I know that the PROM TBP24S10 has a tristate output but this does not change the range.

What do I miss?

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The resistor network output voltages do not match the optimal modulator input voltages, and they do not have to match, as long as the modulator input voltages are within the range.

The antenna RF inputs of the TVs work under various signal strength conditions anyway, like due to range to various transmitting TV stations or good/bad weather conditions.

So there is an automatic gain control (AGC) block in the TV signal path that will amplify the signal by the necessary amount to get the demodulated composite video amplitude to the expected level.

For PAL signals, the expected composite signal amplitude is such that there is 300mV between sync tip level and blank/black level, and thus the white level is 700mV above black level.

As the RF signal uses negative amplitude modulation, the sync tip level voltage sets the maximum RF carrier amplitude level.

So everything in the receiver is relative to the maximum received RF carrier amplitude.

Even if the modulator input at sync tip level does not go to the rated minimum voltage of 2.6V but stays above it, it is still the strongest level of RF carrier it can produce at sync tip, so it simply looks like it was just sent with weaker RF power than it would be if it uses nominal 2.6V level.

All that matters is that the signal is DC biased with high enough DC voltage level to operate at the linear range of the modulator, between 2.6V and 3.4V.

And what the adjustable resistor does, is to allow setting the DC bias point, and the AC signal amplitude at the same time, simply to a level that works good enough to see a picture on TV. The lower the trimmer resistance, the lower the DC bias, but lower video signal amplitude too. Also the 24S10 has TTL output stage so it cannot push out much current, but it can pull more current, so I suspect that the trimmer resistor is used to actually set the voltage level of near white level, from where the 24S10 outputs can only pull the voltage down to black and sync levels.

So that is why the actual AC voltage amplitude at the RF modulator input does not matter much. It just has to have the correct relative amplitude levels, 30% for sync pulse amplitude and 70% for video to have a 100% composite video level.

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  • thank you for the answer. I still have some doubt. I know about the AGC at the receiver so if it is only to respect proportionnalities I still do not get the useness of the pot and why you are mentioning the 300mV and 700mV if only the proportionnality matters? Are you talking about the receiver side signal when you mention the black level? Mar 15 at 9:46
  • Yes, the restored signal after AGC will have the nominal voltages at the receiver. The pot is still useful for rough adjustment at the factory, maybe to account for tolerances of the components (resistors, supply voltage, or to adjust modulator output power to certain level that is know to work well with most TVs as the video signal level is pretty low at the modulator input, etc).
    – Justme
    Mar 15 at 11:46

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