Discrepancies between hel_gates_002 and hel_gfx_engine_040

mpark

Can someone please explain the different values of LNTSC and SNTSC in hel_gates_002 vs hel_gfx_engine_040?
hel_gates_002:

LNTSC         = 3584           ' NTSC color cycles per line (224) * 16
SNTSC         = 624            ' NTSC color cycles per sync (39) * 16

hel_gfx_engine_040:

LNTSC         = (227*16)       ' NTSC color cycles per line (220-227) * 16
SNTSC         = (67*16)        ' NTSC color cycles per sync (39-44) * 16

OK, 224 <-> 227, that's close enough, but 39 <-> 67 seems way off. Yet they both work -- how?

AndreL

You can have really long sync if you like, just think of it as less video time, that's all. Thats the beauty of NTSC, its VERY flexible. You can really fudge it alot as long as overall its 60 FPS and each line is 64uS, you can have a really long sync and a little video or a lot of video and the min sync required. For the HEL engine, I need a lot of time to do calcs to get everything on a scanline with 1 cog, sprites and tiles, so I had to really stretch the sync time out.

Andre'

mpark

Thanks for the reply.

I see what you mean. As I increase SNTSC, I can get more and more sprites on the same scan line without the picture tearing, but the overall image on screen becomes narrower and narrower. And the picture becomes a little wavy. I think I'm beginning to understand.

AndreL

Yup, pretty cool stuff.

Andre'