RGBHV video signal voltage level question

[Electronizer]

Now that I have my Atari Jaguar working with the Sony KV-20XBR TV I got recently, I'd like to try programming an FPGA to generate some simple video signals. I'm using the book Designing Video Game Hardware in Verilog.

In the book, they present several examples that generate simple one-bit RGBHV output (i.e. RGB either on or off). I'm trying to figure out how to interface this to my Sony TV's RGB input. I'd like to limit the luminance to 75% of maximum, which if I understand correctly, is 0.75 x 0.7V = 0.525V.

The FPGA I'm using puts out 3.3V. So, if the input to the TV is terminated at 75 ohms, I need a series resistor that will drop 3.3 - 0.525V = 2.775V. This resistor will form a voltage divider with the 75 ohm termination at the TV, so that means I need something around 390 ohms.

Now, here's where I started getting confused. Because I don't want to fry anything on the Jaguar or in the TV, I took some measurements. I measured about 800 ohms on the red input to the TV. Definitely not 75 ohms!

Since the Jaguar is working nicely with the TV, I decided to look at its RGB output. I used an oscilloscope to measure the red signal while triggering on the composite sync. The red signal maxed out at about 1V, not 0.7V as I've seen listed online in various discussions about the standard. Also, there was a +1VDC offset for the red signal.

Jaguar RGB signals.jpg

So, now I'm not sure what to do. The sync signal output from the FPGA can probably be used with a 75ohm series resistor to match the impedance of the VGA cable I'm using since the Jaguar was putting out around 3V on the composite sync line. However, the RGB signals definitely need to be stepped down from 3.3V. I have two questions:

1. What R, G, B voltage should I aim for at the input to the TV in order to get 100% luma?

2. Once I know the voltage, should I recalculate my voltage divider resistor using 800 ohms termination at the TV?

[old_tv_nut]

Do you have a schematic?

1) The AC input impedance could be different from the DC if there is a coupling capacitor followed by another resistor.
2) Some early TV RGB inputs were logic level, not video level. Can you find out (or does someone here know) if that's the case with this set?

[old_tv_nut]

This manual specifes 1 v pp for video, but does not spec voltage for RGB
https://docs.sony.com/release/KV20XBR.PDF

[Electronic M]

It is good to have characteristic impedance on both ends of a transmission line so why not have a 75 ohm resistor across the FPGA source end of the cable...Also why not use a potentiometer for your output you could vary the output level to suit the TV and any other device you might feed it to in the future.

[Electronizer]

Ok, I think I may have a clue to this mystery. I still don’t have a schematic for my set, but I was able to find one for the KV-1311CR, a contemporary set that also has the 34-pin RGB connector. Interestingly, the 1311 also has a separate 8-pin digital RGB connector, which is selectable by a switch. Here is the schematic for the blue analog input:

624A602E-2AA5-416C-BCF3-50684B7FD54C.jpg

This input has 75 ohm termination, as expected. Now, here’s the schematic for the RGB digital inputs:

7DF5765A-A080-4198-96B2-F9C08BE470F6.jpg

These have a higher termination value, 470 ohms. In a post I found describing how to connect the KV-25XBR to an Amiga computer, they state that pin 21 of the RGB connector selects between digital RGB and analog RGB (no mechanical switch as on the 1311). I’m guessing it has a pull-up resistor on it to select analog RGB by default, so that it worked perfectly to display the signal coming from the Jaguar. However, when the TV was off and I measured the RGB resistance, the pull-up resistor had no effect and I was seeing the resistance of whatever electronically controlled switch they’re using to select between digital and analog RGB.

Now I’m wondering if I can just hook the 3.3V output of the FPGA directly to the monitor with no dropping resistor and pull pin 21 of the connector low in order to get a CGA display... The only issue is that it’s expecting 5V inputs, so the question is whether there will be enough voltage to drive the logic inputs (the AND gates).

[old_tv_nut]

I couldn't recall the input levels for TTL, but this ref says 0.0 to 0.8 low and 2.0 to 5.0 high:
https://www.allaboutcircuits.com/tex...oltage-levels/