Vdp Signal

mickagame · Post by **mickagame** » Wed Oct 08, 2014 8:29 pm

I was surfing on forum when i find this pictures illustrating how the vdp signal of the sega genesis is composed.

flamewing · Post by **flamewing** » Wed Oct 08, 2014 10:14 pm

I am no expert, but I have to confess I don't think they are entirely accurate for the pixel counts per line (numbers for the lines seem fine). Using the best numbers I can find, I come up with the following for H40 mode in NTSC:

hsynch ~ 31.55 pixels
back porch ~ 30.20 pixels
front porch ~ 10.07 pixels
active line ~ 354.75 pixels

Numbers and formulas:
Horizontal rate: Fh = 9000000/572 Hz (about 15734.27 Hz)
Vertical rate: Fv = 60/1.001 Hz = Fh * 2 / 525 (about 59.94 Hz)
Line duration: Ls = 1/Fh = 572/9000000 s (about 63.555 µs)
Front porch: Fp = 1.5 µs
Hsynch pulse: HSp = 4.7 µs
Back porch: Bp = 4.5 µs
Active line period: 1/Ls - Fp - HSp - Bp (about 52.8555 µs)
MCLK = 53693175 Hz
H40 Pixel Clock = MCLK/8

For X µs, you have:
#pixels = (X / 1000000) * (53693175 / 8 )

The NTSC numbers come from the "Video Demystified" book, and the MCLK value comes from TmEE. I rounded off the numbers to 2 decimal places, but even this is a stretch as Fp + HSp + Bp has a tolerance of the order of 0.1 µs (about 1%).

Edit: correcting "8)" to "8 )".

Mask of Destiny · Post by **Mask of Destiny** » Thu Oct 09, 2014 12:00 am

I wouldn't use the official NTSC numbers. If you're not planning on actually broadcast a signal, you only need to get close enough for a TV to accept it and so it's quite likely that the Genesis/MD does not produce a perfectly compliant signal. Nemesis did some work finding the correlation between certain analog signal events and the HV counter which can be used to get the proper values. I don't have them in front of me, but that's what I would use to check that image.

r57shell · Post by **r57shell** » Thu Oct 09, 2014 7:13 am

I don't know why it's there, but we discuss it here. I think it must be enough, because it was enough for me

tomaitheous · Post by **tomaitheous** » Fri Dec 19, 2014 5:58 pm

The numbers are slightly off, but it gets the point across.

But yeah, I think part of the problem is assuming the horizontal scanrate is always going to be NTSC standard of 15734hz. Which I found not to be true for these older consoles.