8-bit acorn hardware • Re: RGB to HDMI using a Pi Zero and a small CPLD

I created a custom profile and it works well for 60Hz display. The text is sharp and noise free.

Sometimes the display gets noisy or the text starts dancing in a wavy manner! In this case I need to go Geometry menu and change the line length or frequency a little to get stable display again.

The reason for the instability is the way the hsync output circuit is designed. If you look at the highlighted schematic you posted and trace back the hsync signal from the header, it comes from a resistor/capacitor monostable pulse generator which is used to shape the hsync pulse.

This means that although the leading edge of the hsync pulse that triggers the monostable is derived from crystal accurate timing, the trailing edge is determined by the R/C value of the monostable itself and this drifts with temperature etc so is not stable. By default RGBtoHDMI uses the trailing edge to determine the sampling position so that results in the video calibration drifting.

The fix for this is to use the Leading edge by changing "Sync Edge" in the sampling menu to "Leading" and then adjust the h offset and calibration as required.

The Vsync output also has a monostable and that can cause vertical jitter. The fix for that is to set "Vsync Type" in the geometry menu to "Flywheel".

Also the DAC values are not set correctly and should be something like this although the RB values will need changing:
DAC-A: G Hi = Disabled
DAC-B: G Lo = Disabled
DAC-C: RB Hi = 232
DAC-D: RB Lo = 155
DAC-E: Sync = 100
DAC-F: G V Sync = 100
DAC-G: G Clamp = Disabled

The RB Hi value should be adjusted to the level half way between normal and bright video
The RB Lo value should be adjusted to the level half way between black and normal video
(You will need to display both video intensities to set that)

I also created a profile for 50Hz display but couldn't test it much. I'll post it later on when I fix a rolling issue in my computer's monitor.

A 50Hz profile can be created by starting from the 60Hz profile and then using analyse timing to set the correct line count etc in the custom profile menu followed by adjustment of the vertical offset and calibration if necessary.
Once the 50Hz profile is saved, put that and the 60Hz one in a sub-folder in the profiles folder and RGBtoHDMI should auto select the correct one.
(See other profiles for examples)

Here is a modified version of your profile above with the leading edge and flywheel set and the DAC values changed as above:Delete any existing saved profile as that will override this version if you update the profiles folder.

Statistics: Posted by IanB — Wed Jul 24, 2024 1:06 pm

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