resistordivider.md 3.7 KB

% New software project: resistor divider generator % Leah Rowe % 2020-09-20

I wrote a new program: resistor divider generator. It takes input voltage, desired output voltage, intended tolerance and, based on your input, creates a list of resistor pairs to use in a "resistor divider" circuit, for voltage division. You can find the generator, which is a web application, here:

https://retrofreedom.com/resistordivider/

I do a lot of work on retro gaming consoles. This includes RGB bypass mods. These old consoles typically use an old video encoder like a Sony CXA1145, which converts analog RGB into composite video and svideo, and also outputs 75ohm RGB (~1.42Vpp RGB lines).

While these encoders produce a decent quality picture output, on modern flatscreens the analog RGB outputs can be quite noisy and/or soft, as in not sharp, and generally look a bit off.

There are modern ICs that handle video buffering and amplification, namely:

The way to install these is as follows:

  • Check on the console mainboard or in schematics for the exact source of the analog RGB signal, if the console has one. For instance, Sega's VDP on Master Systems and Mega Drives.
  • Ensure that no components are connected to the source, so that there is no load on the line. This is important, because you need to get an accurate voltage reading
  • On an oscilloscope, get the point to point voltage of each RGB line on an all white screen. You will need to have a way to get a white screen, even though there is no output (you'll be doing the test blind)

Now you have your source voltage. Typically, the load limit on these RGB lines is very low, so it's important to use a buffer circuit such as THS7314. This reduces the load on the source. The output of that buffer can then be amplified; again, THS7314/7374 handles this.

An analog RGB signal coming out of a games console has these rules, if for 15kHz TV signal:

  • If RGB SCART cable has 75ohm series resistors in SCART cable, simply wire the output of the THS7314/7374 to the console's external video connector
  • If RGB SCART cable has no resistors, use 75ohm series resistors off of the RGB outputs

The TV has 75ohm pull-down resistors in it for each incoming RGB line. The voltage of the signal should be about 710mVpp, which is why SCART cables have these series resistors in them, for shaping the voltage.

NOTE: The above is based on studying Sega consoles

NOTE: PAL SNES uses 75ohm pull-downs on the RGB lines, which means that the voltages are going to be higher that 1.42Vpp. I've yet to study PAL SNES RGB bypass (I've done NTSC SNES RGB bypass before, which is more sega-like in the voltages on RGB outputs)

Obviously, on your console you might need to cut some traces and redirect some signals.

This ensures that the analog RGB voltages going into the TV will be correct. The correct voltage is between 640mV-780mVpp on an all white screen.

710mV is the ideal input to the THS chip. With knowledge of source Vpp reading, you can use a resistor divider to shape it into 710mV. The ideal impedance on the output of the divider should be about 0.9mA and the output impedance around 750-1000 ohms.

I will update this article later, to include more info and show examples. I have a few videos showing me doing this "RGB bypass" mod. For instance, see:

https://www.youtube.com/watch?v=DZFLhf3PUbI