PAL to NTSC-50 Chroma Converter Project - Page 1
Forum section: electronics / projects

author: mbarszcz
author_rank: Contributor
timestamp: April 12, 2019, 09:44:29 pm
content: What I'd like to do is convert the separate Y/C PAL (50Hz) signals from a PAL Commodore 64 to Y/C NTSC-50 signals for display on a US Commodore 1702 Monitor. (I know there are PVMs and LCDs that can display PAL, but I'd like to try to see if I can get this to work to use an original commodore monitor). Also, the converters that converting PAL 50Hz to NTSC 60Hz are a waste of time, as the scrolling will never wind up looking right because of the non-integer framerate ratios. Using original PAL monitors is out of the question since they all run on 230V/50Hz and will be destroyed in shipping. Maybe there is a way to use the NTSC models.....The North American 1702 monitor can display a stable 50Hz signal from a PAL C64, just without any color because of the different color encoding standards. I'm not certain, but I believe if it was fed an NTSC 3.58MHz encoded chroma signal to go with the 50Hz 625 luma signal, it would display correctly.Does anyone know of a relatively straightforward way to convert the 4.43MHz PAL Chroma to 3.58MHz NTSC Chroma? Progrmaming an FPGA is certainly an valid option to accomplish such a task, but I have no experience with that and have serious doubts about my ability to learn how to program that for a hobby project.My initial thought (and I think still the best option) is convert the Y/C into RGBS (essentially decoding the color standard but preserving the color data and sync in the analog world), and then re-encoding back to NTSC again with the appropriate 3.58Mhz Crystal and an AD724. However, but I'm struggling to find a solution to convert from analog Y/C to analog RGBs. Maybe there are some discontinued chips that were used in old TVs ICs that could do this (PAL to RGB Decoders)? Anyone know of any? Readily available ICs all seem to go the other way, RGBS/RGBHV to Y/C or composite output for displaying video games/computer graphics on a TV. A few I've found for on Ebay either decode to component or like the relatively cheap Philips TDA3566, don't have a sync output (and the datasheet doesn't have a good pinout). I suppose I could use a sync seperator on the incoming luminancen to feed the outputted RGB + separated composite sync back into the encoder.My second thought was to use something like an Analog Device ADV7403 to convert the Y/C to BT.656 digital and then the ADV 7393 to convert back to Y/C. There seems to be two problems with this approach though. First, the chips are auto-standard sensing, so when they see a PAL signal, they go into PAL mode. This is more of an issue for the re-encoder (which can handle PAL-60 but not NTSC-50 ahh!). Secondly, I don't think is that NTSC's 525 lines @ 50i is really a valid standard, so it couldn't be forced on the A-D side even if it was a standard.The (discontinued) Cypress Technology CP-100N is the closest thing I've found and does convert PAL to NTSC-50, but it only supports a composite input, and not Y/C.Any suggestions on Circuits, ICs, or how I might otherwise be able to do this would be greatly appreciated (or if something already exists that accomplishes this, even better!)Matt

author: soldar
author_rank: Super Contributor
timestamp: April 13, 2019, 09:10:04 am
content: Have you considered tinkering with the monitor so you can feed it RGB? That might be easier.

author: Kjelt
author_rank: Super Contributor
timestamp: April 13, 2019, 05:26:30 pm
content: Yes Philips made PAL to RGB decoders later also all three standards (Pal/NTSC/secam) decodersn a single dip package IIRC.Search for TDA35xx chips.I made a PAL to RGB and NTSC to RGB decoder late 80s, let me see if I can find something in my old archive (long live paper, if it was digitized on a 5,25" floppy it would be gone by now)As I remember we used a small pcb from the Philips K12 crt tv's they were everywhere for scrap back then but hard to comeby nowadays.

author: Benta
author_rank: Super Contributor
timestamp: April 13, 2019, 06:40:43 pm
content: Combining Y/C or S-Video to composite is simple, a 470 pF cap from C to Y is enough. See here:https://en.wikipedia.org/wiki/S-Video

author: CommodoreChris
author_rank: Newbie
timestamp: May 02, 2020, 06:24:37 am
content: Hello Matt!! I found this thread and im thankful that i did! Because i have the same dilemma as you do! I have a PAL C64 that i want to use with an original Commodore 1702 monitor, however my 1702 is NTSC!! I was curious as to if you have had any success in this great endeavor! I am a simple hobbyist so alot of what was said in this thread goes over my head a bit. However i had an idea and i wanted to run it past anyone that may have an input about it.You mentioned the Cypress Technology CP-100N which does in fact convert a PAL signal into a NTSC 50hz signal, the only issue is that it that it only supports the inferior composite signal. So my idea was would it be possible to modify the Cypress Technology CP-100N and add a Y/C input and output jack?

author: Bud
author_rank: Super Contributor
timestamp: May 04, 2020, 02:46:19 am
content: You guys attacking it from a wrong angle. The way to convert a C64 to a different video standard (either way) is to replace the video chip (VIC) to the other standard one, change the crystal resonator in the clock oscillator and switch a jumper in the oscillator. I am considering this mod but have not done it yet , but seen posts on C64 forums from people who did. You can even buy these days a add-on universal board with a PAL <--> NTSC switch for C64, but it comes with no video chips so you need to source your own for the other standard.This would be your cheap solution. Performing standard conversion by conventional re-encoding methods would be expensive and extensive.Edit: refer to the attached fragment of C64 schematic. Note the jumper in top left and read the note in right bottom.