Some time ago I designed an adaptor for the video NuLA which was intended to solder on top of the NuLA board picking up the 12 bits to be fed to a 12 bit RGBtoHDM via a ribbon cable but it had issues decoding mode 7 due to the way the NuLA mangles the mode 7 data. I was hoping that RobC would be able to make some firmware changes to work around the problem but that hasn't happened so far.
Recently I had a flash of inspiration and came up with a solution that allows the NuLA to work with RGBtoHDMI without requiring any firmware update:
There were several issues:
1. The 12 bit video signal was buffered by 74HC parts which should have been plenty fast enough for a 12 Mhz video signal but I was trying to sample at 144 Mhz to pick out a clean sample and that wasn't reliable so I changed to the faster 74VHC parts.
2. The sync signals from the 6845 drifted slightly with temperature, enough to take the signal out of calibration as the beeb warmed up so I have now added a latch circuit to the redesigned adapter that compensates for that.
3. I had to use the 6Mhz signal from the NuLA rather than the one generated by the beeb.
Here is a photo of the prototype showing the earlier adapter design soldered to the NuLA together with the breadboarded sync regenerator:This is the revised adapter design:It is designed to be soldered on top of the NuLA with 12 wires picking up the 12 digital bits from resistors on the NuLA board using short wires soldered to the J1- J12 pads.
Here are screencaps with standard mode7 coloursand NuLA mode 7 colours:I'm going to get a few boards assembled so if anyone is interested in one of those, please let me know as I'd like to get some confirmation of reliability across multiple machines.
You should make sure that your NuLA can output the 6Mhz signal which is present on JP1 as I don't know if that was included in all revisions of the firmware (It's the hole that has a wire soldered into it in my photo)
You could either check with an oscilloscope or wire it up to the 6Mhz input of the SAA5050 instead of the one generated by the beeb.If you already have an RGBtoHDMI it you will also need a 12 bit adapter which I can supply.
As the installation of this is not for the novice, there might eventually be an fitting option available via Jinxter if testing goes OK.
Recently I had a flash of inspiration and came up with a solution that allows the NuLA to work with RGBtoHDMI without requiring any firmware update:
There were several issues:
1. The 12 bit video signal was buffered by 74HC parts which should have been plenty fast enough for a 12 Mhz video signal but I was trying to sample at 144 Mhz to pick out a clean sample and that wasn't reliable so I changed to the faster 74VHC parts.
2. The sync signals from the 6845 drifted slightly with temperature, enough to take the signal out of calibration as the beeb warmed up so I have now added a latch circuit to the redesigned adapter that compensates for that.
3. I had to use the 6Mhz signal from the NuLA rather than the one generated by the beeb.
Here is a photo of the prototype showing the earlier adapter design soldered to the NuLA together with the breadboarded sync regenerator:This is the revised adapter design:It is designed to be soldered on top of the NuLA with 12 wires picking up the 12 digital bits from resistors on the NuLA board using short wires soldered to the J1- J12 pads.
Here are screencaps with standard mode7 coloursand NuLA mode 7 colours:I'm going to get a few boards assembled so if anyone is interested in one of those, please let me know as I'd like to get some confirmation of reliability across multiple machines.
You should make sure that your NuLA can output the 6Mhz signal which is present on JP1 as I don't know if that was included in all revisions of the firmware (It's the hole that has a wire soldered into it in my photo)
You could either check with an oscilloscope or wire it up to the 6Mhz input of the SAA5050 instead of the one generated by the beeb.If you already have an RGBtoHDMI it you will also need a 12 bit adapter which I can supply.
As the installation of this is not for the novice, there might eventually be an fitting option available via Jinxter if testing goes OK.
Statistics: Posted by IanB — Sat May 25, 2024 10:47 pm