Megadrive video timings

[Eke]

Also, what are the steps in voltage in those charts? And in reference, how does the brightest white compare to say other RGB outputs (like vga, or amiga, etc)? I would like to get a reference as to how close 0xEEE is relative to 24bit 0xFFFFFF.

You have Ultrascope. When you open waveform, you can turn on cursors. They will show you voltage. Then, you must make measurements and get delta from black level. If we assume black level as 0% and white level as 100%, you can calculate percentage of every color level and lay it on byte of computer's truecolor depth. Understand?

I measured this (in V):

Code: Select all

 Shadow   Highli    Normal 
-------- -------- ---------   
(0) 0.0             0.0 (0)
(1) 0.5            
(2) 0.9             0.9 (1)
(3) 1.3 
(4) 1.6             1.6 (2)
(5) 1.9 
(6) 2.2             2.2 (3)
(7) 2.4   (0) 2.4
          (1) 2.7   2.7 (4)
          (2) 2.9
          (3) 3.2   3.2 (5)
          (4) 3.5
          (5) 3.8   3.8 (6)
          (6) 4.2
          (7) 4.7   4.7 (7)

It seems quite symetrical steps to me, considering signal noise..

I'm also wondering, could we assume that pure white would be at +5V (considering AGC and AVC are some kind of reference signals) ?

[tomaitheous]

Eke: thanks, that's what I was looking for. So max white is 4.7v.

If we did assume max white for RGB was 5v, then that would put max white as ~239 if scaled to 24bit values. That seems to fit the steps by 34 setup for scaling.

[TmEE co.(TM)]

Here's small example of the slight differences in the highlight :
RGB:
http://www.fileden.com/files/2008/4/21/ ... arsRGB.jpg
S-video (using TV's sharpness setting to enhance the effect a bit) :
http://www.fileden.com/files/2008/4/21/ ... erence.jpg

And a photo of the newer SHL test results :
http://www.fileden.com/files/2008/4/21/ ... result.jpg

and from a PC LCD :
http://www.fileden.com/files/2008/4/21/ ... romLCD.jpg
with one of these :
http://www.fileden.com/files/2008/4/21/ ... Dsetup.jpg

[notaz]

It's not really visible after it passes through compo(site/shite) conversion:

[Eke]

Here's small example of the slight differences in the highlight :
RGB:
http://www.fileden.com/files/2008/4/21/ ... arsRGB.jpg
S-video (using TV's sharpness setting to enhance the effect a bit) :
http://www.fileden.com/files/2008/4/21/ ... erence.jpg

The thing is that on the signal recording (directly from VDP output, right ?), there isn't any noticeable differences in the voltage level. There is indeed some voltage drop or jump which result in a tiny separation between bars (some time darker, some time lighter) that gives the impression color changes but they are the same. The tiny voltage variation is probably due to the VDP internally switching between highlight & normal.

Now I have an oscilloscope too. I'd made some records of EDCLK singnal. Take a look:



You can take full record here (use UltraScope for viewing).
Now, it's look like EDCLK more complex, than we think. So, what do you think?

I looked into this and it indeed seems that there are 60 EDCLK at MCLK/5 during a line, like Jorge Nuno measured but that these clocks are "interrupted" by MCLK/4 clocks, like this:

15 cycles @MCLK/5
2 cycles @MCLK/4
15 cycles @MCLK/5
2 cycles @MCLK/4
15 cycles @MCLK/5
2 cycles @MCLK/4
15 cycles @MCLK/5


It seems that HSYNC signal is what triggers the changes in EDCLK frequency, with some latency though.

The total is still 840 EDCLKS (420 pixels) with 780 @MCLK/4 and 60@MCLK/5 (total is 3420 MCLCKS).

[HardWareMan]

I think this dots are caused by EDCLK. Just thoughs...

[TmEE co.(TM)]

2 missing ones in every 64 pixel "zone".....

the dots are VDP CRAM writes there, each pixel being in color the color getting into written CRAM... in that case, things are transferred via DMA, each dot represents one "access slot".

[HardWareMan]

I made record of control bus signals of VRAM (SE0, SC, RAS1, CAS1, WE0, OE1) in last scanline interval of gradient ladder of TmEE SHLTEST. But, Ultrascope allows show only 2 signals, and we need to show 7. Maybe, I can load it as RAW sound data in multitrack sound editor? Any thoughs?

[Snake]

TmEE pointed me at this thread, and I read this:

To clear up S/H, normal once and for all? I've heard that S/H fits nicely into a 12 bit palette and that S/H just adjust the step increments. But I've also read that idea/method this doesn't display right for all games.

... and thought 'ah, this old argument again...' - but now I've read the rest of it.

Clearly, S/H mode does indeed basically (almost) double the number of colours, and the 'doesnt display right for all games' thing is just wrong. I tested this way back in the KGen days.

But, on my crappy old TV, through RF no less - it looked like the crossover point from normal to highlight was at 0x888 - and since that made perfect sense, that's what I did. From Hardwaremans posts its clear the crossover point is actually at 0x777. No big deal, but nice to know

TmEE - fix your program so it displays this properly on real hardware please

While I'm here:

I really doubt you can freeze the CPU in the middle of instruction. At least you can't interrupt it like this for sure. There is some delay between writing the command and actual DMA start/CPU freeze anyway

well, according to the 68k user manual, Bus Access is granted between memory cycles so theorically, this could happen.

but you are right, VDP reaction is not immediate

Yes, it is. When you write the final word to the VDP that will trigger a DMA, the 68K is *immediately* locked until the DMA completes - even if it was in the middle of a move.l instruction. Surprising, but true. Another thing I documented a long time ago.

[Graz]

Another thing I documented a long time ago.

Any chance that documentation is available for us to read?

[Snake]

It was all posted on a devchannel set up by (I believe) Charles Macdonald, and I don't know if it's still around. I did keep notes of a lot of this stuff but then lost it However, pretty much every emulator out there since then handles all the stuff I found.

[mickagame]

TmEE pointed me at this thread, and I read this:

To clear up S/H, normal once and for all? I've heard that S/H fits nicely into a 12 bit palette and that S/H just adjust the step increments. But I've also read that idea/method this doesn't display right for all games.

... and thought 'ah, this old argument again...' - but now I've read the rest of it.

Clearly, S/H mode does indeed basically (almost) double the number of colours, and the 'doesnt display right for all games' thing is just wrong. I tested this way back in the KGen days.

But, on my crappy old TV, through RF no less - it looked like the crossover point from normal to highlight was at 0x888 - and since that made perfect sense, that's what I did. From Hardwaremans posts its clear the crossover point is actually at 0x777. No big deal, but nice to know

TmEE - fix your program so it displays this properly on real hardware please

While I'm here:

I really doubt you can freeze the CPU in the middle of instruction. At least you can't interrupt it like this for sure. There is some delay between writing the command and actual DMA start/CPU freeze anyway

well, according to the 68k user manual, Bus Access is granted between memory cycles so theorically, this could happen.

but you are right, VDP reaction is not immediate

Yes, it is. When you write the final word to the VDP that will trigger a DMA, the 68K is *immediately* locked until the DMA completes - even if it was in the middle of a move.l instruction. Surprising, but true. Another thing I documented a long time ago.

Very interesting !!!
It could explain the special effect from mickey mania?

[Eke]

Very interesting !!!
It could explain the special effect from mickey mania?

Not really, this issue is probably more related to the way VDP is processing sprites during HBLANK: when display is disabled, sprite processing is very likely also disabled and the result is that only some of the sprites will be processed for the upcoming line (in Mickeymania, the first 5 sprites with xpos=0 are processed, you could eventually relates this to the number of clocks during which display is disabled).

The only way to know exact timings would be to spy those VRAM bus signals and see how, when and what stuff are fetched from VRAM

[tomaitheous]

TmEE pointed me at this thread, and I read this:

To clear up S/H, normal once and for all? I've heard that S/H fits nicely into a 12 bit palette and that S/H just adjust the step increments. But I've also read that idea/method this doesn't display right for all games.

... and thought 'ah, this old argument again...' - but now I've read the rest of it.

Clearly, S/H mode does indeed basically (almost) double the number of colours, and the 'doesnt display right for all games' thing is just wrong. I tested this way back in the KGen days.

But, on my crappy old TV, through RF no less - it looked like the crossover point from normal to highlight was at 0x888 - and since that made perfect sense, that's what I did. From Hardwaremans posts its clear the crossover point is actually at 0x777. No big deal, but nice to know

TmEE - fix your program so it displays this properly on real hardware please

While I'm here:

I really doubt you can freeze the CPU in the middle of instruction. At least you can't interrupt it like this for sure. There is some delay between writing the command and actual DMA start/CPU freeze anyway

well, according to the 68k user manual, Bus Access is granted between memory cycles so theorically, this could happen.

but you are right, VDP reaction is not immediate

Yes, it is. When you write the final word to the VDP that will trigger a DMA, the 68K is *immediately* locked until the DMA completes - even if it was in the middle of a move.l instruction. Surprising, but true. Another thing I documented a long time ago.

I don't remember the argument details, but something about brighter than white (which is now known doesn't exist). Not a big deal, but it's nice to know. Matters to me so I can get my colors correct for my apps though I'd still like to know what 0xEEE is really is in comparison to 0xffffff (or just 252/252/252). It would be nice to know if steps by 36 are pretty damn close, so I can keep using those

Also, on the interrupt/halt in between cycles of an instruction; are they in segments of 4 cycles or as low as 2 cycles?

[Snake]

Not a big deal, but it's nice to know.

Oh, sure, very nice But a bit different to what people have been implying to me for a long time, i.e. that the whole thing was way off.

It would be nice to know if steps by 36 are pretty damn close, so I can keep using those

From what I can tell the steps are around 34, so you're pretty close. This was done by outputting from PC to TV and adjusting till it looked right.

However be aware that if you're trying to match the colours on a TV, the NTSC or PAL encoding does, in some cases, make a HUGE difference, and certain colours can look completely different.

Also, on the interrupt/halt in between cycles of an instruction; are they in segments of 4 cycles or as low as 2 cycles?

I believe it's immediate, I.E. if you write a long in which the high word triggers a DMA, and the low word does something that'd completely break that DMA, the DMA always works as expected, and the final word is written after. Figuring out at exactly which cycle the write is performed isn't easy, but probably doesn't make any difference. At least for every possible instruction I tested it made no difference.

It's a bit naughty anyway and probably shouldn't be used