Does NTSC console benefit from DFO mod?

Post by **Flappyraccoon** » January 9th, 2017, 10:31 am

Hiya, as the title states I was just curious if there is any benefit to mod an NTSC console with the dual frequency oscillator modification. I installed my switchboard and have a modchip coming in the mail. Figured might as well do all the mods at once if it needs them. Cheers!

Oh and in case this changes anything I am going to be using an RGB scart cable into an OSSC. So I dunno?

Post by **TriMesh** » March 5th, 2017, 1:55 pm

The only practical impact in your setup is that with the stock NTSC oscillator the frame and line rates are about 1% off when you run PAL games and the dual oscillator setup fixes this so that the video rates are correct in both modes.

If you ever plan on using Y/C or composite outputs in PAL mode then the results vary depending on what model of console you have:

PU-18 or earlier, stock NTSC oscillator - Composite and Y/C outputs are broken in PAL mode (wrong subcarrier)
PU-18 or earlier, DFO mode - Composite and Y/C outputs are correct in both modes
On PU-20 or later, the subcarrier is always the one for the native video mode, with or without the DFO mod.

You can do an additional mod to the PU-20 and later than disconnects the subcarrier input on the decoder from the clock synthesizer chip and connects it back to the subcarrier output on the GPU, in which case they start to act the same way as the PU-18 and earlier.

Without this mod, the composite output from an NTSC console in PAL mode is not very useful - it's basically PAL but with a 3.58MHz NTSC subcarrier, and that's not a combination that's commonly supported.