De-jitter For NESRGB
The timing NES video signal is slightly
non-standard. This was a novel way to reduce composite video
artefacts, but causes trouble with modern digital video scalers in TV
sets and monitors. In affected scalers, the symptom usually looks like
flickering at the top of the screen. Here's video
of the symptom on an LG television set.
Marqs on the Shmups forum, creator of the
OSSC video scaler, came up with a good solution. This involved
precisely controlling the clock to the PPU to make the video timing
standard compliant. I have implemented this feature into the NESRGB
version 2.0 hardware and firmware, released 10/2/2019.
If you have a previous version of the
NESRGB hardware and would like this de-jitter feature, I have created
a firmware update which adds this feature to the older boards (1.0,
1.2, 1.3, 1.4). Some modification to the NESRGB board itself is
required and the palette select feature was removed to make it fit. I
will not provide any support for this procedure. If you don't
understand this guide please do not attempt this mod (buy the new
De-jitter for NESRGB 1.X hardware installation guide
- Identify the hardware version. Find the text
on the board, where XX is the hardware version. For example, the
text NESRGB14 means hardware version 1.4.
- Update the firmware with an Altera USB Blaster
clone programmer (details here).
- The palette selection pads 1, 2, 3, now have a
different uses. They are 1. Sync output, 2. Sync input, 3. PPU clock
output. On hardware version 1.2, 1.3, 1.4 the palette pins each have
430 ohm resistors in series. These are there to protect the FPGA in
case the palette pins are connected incorrectly during installation.
Identify the resistors in series with pads 2 and 3 (marked red in
the diagram below).
- Replace each resistor with a jumper wire. You can
remove the surface mount resistors easily by adding extra solder to
each pad and holding the soldering iron tip in such a way as to heat
up both pads at once. Then slide it off the pads.
- Solder a wire between the palette pad 2 and the
- Isolate the PPU pin 18 (clock) from the NESRGB
board. I recommend cutting the small part of the pin off so it no
longer makes contact with the socket. I DO NOT recommend
bending the pin away from the socket at that causes the pin to
fatigue and possibly break off. Solder a wire between the isolated
PPU pin 18 and palette pad 3.
- Palette pad 1 is the new, corrected sync output.
You must use this signal instead of V, Y, CS#, PPUV to get the
benefit of the de-jitter feature. The 430 ohm series resistor
already on the board makes its suitable to connect directly to a 75
ohm sync input.
- If you only use RGB output you can finish here. If you also need to
use the sync outputs of the video encoder (composite video, s-video,
component video) you also need to connect the new sync signal to the
video encoder too. The video encoder is BH7236AF and the sync input is
pin 10. Most versions of the NESRGB hardware have a series resistor
between CS# and the encoder sync input. Remove this resistor and
connect the new sync signal by soldering a wire between palette pin 1
and video encoder BH7236AF pin 10. Note that if you leave the 430 ohm
resistor in place you should not use this signal to drive the video
encoder and an external monitor an the same time.
Last updated 26/8/2019.