FPGA based modification board for the Commodore 64 computer to produce YPbPr video output.
The C64 is the most iconic device of the 8-bit computer era and there is a huge library of software and hardware modifications available. Nevertheless the video output quality is inherently poor by modern standards, even with the use of the s-video option. This is specially true with LCD flat screens that amplify the visual artifacts even more.
The biggest problem here is the fact, that the VIC-II video chip directly creates a chrominance signal in the weird and strange way that is necessary for use with analog television systems. It is then basically impossible to truly convert it back to any form of RGB or other component signal (and I tried really, really hard). And even the luminance signal of the VIC-II is far from perfect and carries a lot of noise from various sources.
So the way to go was then to bypass the chrominance/luminance signal generation of the VIC-II and make a solution that computes a YPbPr signal directly from the digital information available inside the computer. As it turns out, it is enough to passively listen to just about 22 pins of the VIC-II to figure out what video signal it actually intends to generate. Using the information from these pins and some logic implemented in an FPGA, a pixel-perfect replication of the video image can be generated.
This mod evolved from a previous version that uses my generic A-Video board. It fixes the main shortcomings of this version, as it was a very ugly install and it needed big holes in the case for the three RCA plugs. It is compatible with the VIC-II adapter board and the firmware. This board now is designed to replace the RF modulator and use the existing holes in the case. No need to modify the outside of the C64 in any way.
The mod set consists of two main parts:
- VIC-II adapter This is installed between the existing VIC-II and its socket and sniffs all relevant signals and translates them to 3.3V logic levels for use by the FPGA.
- FPGA board This is connected via a ribbon cable to the adapter to receive the signal stream and provide a YPbPr output signal on a 4-pin TSSR jack. Additionally this board carries the necessary electronics to take over the functions of the removed RF modulator to amplify the composite and s-video signals (so the original A/V-jack is still functional).
The mod is intended to eventually support all revisions of the C64: PAL and NTSC, long boards and short boards and 5V or 12V supply voltage. The hardware is probably already compatible (tested with a long 12V-board and a short 5V-board), but for some variants of the VIC-II (specifically the 6567R56A that was used in the very first devices) I still need to adapt the firmware.
The mod was tested to work with the following variants of the VIC-II:
- 6569R5 (PAL)
- 8565R2 (PAL)
- 8562R4 (NTSC)
The mod board generates a YPbPr signal which can be switched to one of three modes using the onboard slider switch:
- 240p/288p progressive 60Hz/50Hz
- 480p/576p progressive 60Hz/50Hz using scanline doubling
- 480p/576p progressive 60Hz/50Hz with visual scanline effect
The signal is provided on a 4-pin TRRS-jack. The order of signals (tip to sleve) is as follows: Y, Pb, Pr, GND. This assignment seems to be a kind of standard, but you can use any breakout cable that converts TSSR to 3 RCA jacks, as long as the common GND is located at the sleeve.
Use the three-state switch at the back to select the video mode. The 240p/288p mode is not supported by all TVs and will probably create some de-interlacing artefacts. On the other hand, this mode is perfect to feed into a dedicated upscaler that can handle it (the famous 'Framemeister' or the 'OSSC' come to mind). The 480p/576p modes are best used with TVs and give quite a good picture already without any additional upscaler.
- Remove the VIC-II and put the adapter board into the socket. The correct orientation for the adapter is such that the ribbon cable either extends to the right or to the top (depending on your main board). On some main boards, this could require to either relocate some passives or to insert an additional IC-socket to rise the adapter above these passives.
- Put the VIC-II into the adapter. Make sure to put it in the original orientation.
- I have found that some boards have such a low-quality IC socket that it will not hold the adapter board tightly enough. If this is the case, you should replace it with a precision IC socket.
Before de-soldering the RF modulator, you can test the function of the main mod board. For this, temporarily connect one of GND3,GND4,GND5 somewhere to the C64 GND and the rightmost hole of RFCON2 to +5V. Connect the ribbon cable from the adapter to the FPGA board. The TSSR plug should then already output a proper YPbPr signal.
- Remove the RF modulator and remove all the solder from the pin holes.
- Install both 4-pin headers in the mainboard These pins will later go into the FPGA board to provide power and to carry the analog singnals in and out (to replicate the functionality of the RF modulator).
- Check into which holes in the FPGA board these pins will go. For installation on a "long" main board, these are the 8 holes at the edge, for a "short" main board, use the other set of holes.
- Temporarily stick the FPGA board onto the pins and use it as a means to align the remaining pins correctly to the main board (either two pins on each side if installed on a 'long' board or only two pins at the back if installed on a 'short' board) and solder them to the main board. These pins are needed to connect the GND as well as to provide the correct vertical spacing.
- Align the FPGA board vertically to the holes in the computer case and solder all pins.
- If your VIC-II uses the higher voltage (all 6xxx - variants), you need to short the JPLUM1 solder bridge.
- Connect the 20-pin ribbon cable to the FPGA board.
The 4-pin TSSR connector provides the YPbPr signal, and this can be connected to a TV or upscaler using different cabling options:
- A breakout adapter with a TSSR jack and 3 female RCA jacks. With this a standard YPbPr cable with male plugs can be attached.
- A cable with a TSSR jack and 3 male RCA plugs. This can be plugged directly into the YPbPr input on a TV.
- Some TVs also use a TSSR jack to input YPbPr, probably to safe cost and space. There it is possible to directly use a cable with two TSSR plugs on both sides (given that the TV uses the same pin assignment: Tip=Y, Ring 1=Pb, Ring 2=Pr, Sleeve=GND).
Everything to create the RF replacement board as well as the VIC adapter board (see the A-Video board repository) is completely open source and free to use for any purpose. Most parts are pretty standard, only the TSSR connector and the three-way switch are probably not that easy to source with every electronics retailer. There you possibly need to adjust the PCB layout to make your own parts fit.
Hint for assembling: The holes denoted GND1 - GND5 should not be populated with pin headers during assembly. It is intended that
pin headers that are installed into the C64 main board will stick up through the holes and are soldered as last step
of the installation.
Hint for testing: After the board is assembled and before installing it into the computer, you can already upload
a test pattern generator to the FPGA (the quartus folder that is directly located in the A-Video Board repository).
Do this with a USB-Blaster connected to the JTAG headers. You also must provide 5V-9V on the power pin
(the rightmost hole of RFCON2 - as seen from the edge of the board) and 0V to any of GND3, GND4, GND5.
The test pattern should show some fancy colors and three gradients to test each of the three signal components.
If the gradients look uneven or have less than 32 different steps each, you probably have some soldering
problems in the resistor networks.
For technical questions and also to share your experience with this modification, please use the issue tracking system of github. If you want to receive a set of bare PCBs, please contact me directly at reinhard.grafl (at) aon.at maybe I will have some to spare.