A resistor divider (see diagram) divides a larger voltage into a smaller
mVout = (mVin * R2) / (R1 + R2)
This program generates a table of resistor pairs for your circuit. Click here to return to retrofreedom.com homepage
A video is available, showing how to use this program: https://www.youtube.com/watch?v=U3F1gT5qPYc
You can learn more about voltage dividers by reading the following Web page on WikiPedia.
This program is Free Software, released under the terms of the GNU Affero General Public License, either version 3 of the license as published by the Free Software Foundation, or (at your option) any later version as published by the Free Software Foundation. A copy of this license is included; see COPYING.txt. Source code is available via https://notabug.org/vimuser/resistordivider/
I, Leah Rowe, wrote this program from scratch to assist me in calculating the correct resistor values to use in attenuating analog RGB signals, for THS7374-based RGB bypass circuits on retro games consoles. I specialize in component level board repair and modifications on retro games consoles (e.g. Sony, Sega, Nintendo) in my lab, where I have an army of high-quality soldering and de-soldering equipment, plus equipment for case tooling (e.g. adding connectors for RGB mods), my home-made retrobrighting (de-yellowing) kit and a whole host of whacky zany things that assist me in my work. You can see my work on my YouTube channel and on my twitter at @n4of7. I do this work professionally, for money; you can send me your console for servicing, or buy a pre-modded console from me by visiting RetroFreedom.com.
Specifically, here is a list of some equipment that I use:
THS7374 mods replace the default video encoder (usually a Sony CXA1145) with the THS7374, which is more modern with more sophisticated noise filters built in, designed specifically with modern digital displays in mind such as LCD TVs, when using a digital to analog converter and upscaler solution. Analog noise including things like jailbars are much more visible on modern flatscreens; these old consoles were designed during the CRT era, and these noise patterns just simply were not possible to perceive on CRT TVs of the time.
RGB bypass boards exist already, using THS7314 or THS7374, for all well known retro gaming systems, so why did I write this app? Well, the PROBLEM is that the designers of those boards are getting an average reading of the voltages on these RGB signals, and using resistor values that are roughly correct, for attenuating the signals, but precision is generally never observed. My tool makes it easy to properly calculate precise resistor values, per main board, after getting a base reading of the signal source (using an oscilliscope). The REASON this matters is because even on the same board revisions for a given console, each board will generate analog RGB at slightly different voltages, so some people will be getting a picture that is too dark or too bright.
Additionally, most of these designers of RGB bypass circuits usually don't take into consideration the resistor tolerance rating, which is extremely important for analog circuitry. You ideally want 0.1% or lower tolerance. Generally, a console's RGB SCART cable will have 75ohm resistors in the SCART cable, or in the console. This, in concert with the 75ohm terminations to ground inside your TV (present on all analog inputs), halves the voltage. The correct voltage is 710mV at 75ohm level, and the THS7374/7314 has a fixed 6dB 2v/v gain; thus, we feed 710mV (at about 800ohm level) into the THS7374/7314, and out comes a 1420mV signal, which is then halved by the 75ohm+75ohm divider back down to 710mV going into your TV.
The THS7374 is a 4-line video driver, capable of driving a 75ohm load. It buffers the inputted video lines and amplifies them as described. This is important, because at the signal source (e.g. Sega VDP) there's usually not enough capability to drive a high load (e.g. 75ohm), so the signal is buffered through the 7374 and its amplitude doubled.
Info about these ICs can be found on these links:
For all intents and purposes, the THS7374 is the best chip to use. The 7314 is almost as good, but it has a hardwired 9MHz low-pass filter built in, which eliminates noise on the signal; however, this softens the image a bit. The filter ought to be disabled on CRT TVs and PVMs. Additionally, the OSSC (opensource scan converter) and Framemeister have superior low-pass filters in them already, as do most built-in upscalers that you typically see on a flatscreen TV. Thus, the THS7314/7374's built in low-pass filter is superfluous and should be disabled. The THS7374, unlike the THS7314, allows you to bypass the low-pass filter, so that it is unused, but you can easily turn it back on in those rare instances where you'd want it, and it has a stronger filter than the 7314.
In other words, you should ideally have just a single low-pass filter on your video chain, as close as possible to where the analog signal terminates (e.g. into/inside your TV or upscaler). However, in practise the 7314's LPF is quite weak and having it enabled is OK-ish; if you combine it with, say, an OSSC, the OSSC's low-pass filter can be disabled, or you can just enable the OSSC's low-pass filter, but the picture will be a bit softer than with THS7374 that has the LPF bypassed.
The diagram shown on this page is from WikiPedia under Creative Commons Zero license.
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