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  • Joined: 31 Oct 2007
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SMS video output levels
Post Posted: Thu Oct 22, 2015 11:58 am


And today I finally got the SMS levels checked out. Good chuck went into debugging my flashing hardware which turned out to be faulty in the end.

I got measuremests from SMS1, SMS2, MD1 and MD2.

MD1 and 2 produced same results, SMS1 and 2 I had a bit of different measurement points. The usual point at voltage divider gave huge DC offset on SMS2 and I couldn't do measurements properly because of it so I measured the VDP output instead.

Here's the measurements for MD1 and 2 :

mV, ratio, RGB
0, 0.000, 0
368, 0.387, 99
604, 0.634, 162
952, 1.000, 255

SMS1 :

mV, ratio, RGB
0, 0.000, 0
324, 0.354, 90
620, 0.677, 173
916, 1.000, 255

SMS2 :

mV, ratio, RGB
0, 0.000, 0
364, 0.350, 89
720, 0.684, 174
1052, 1.000, 255

MD1+2 : 0, 99, 162, 255
SMS1 : 0, 90, 173, 255
SMS2 : 0, 89, 174, 255
Linear : 0, 85, 170, 255

Note that the MD measuremests DO NOT match any of the MD native mode output, there is some analog trickery going on or there's separate DACs used for SMS mode output.

Here's the test ROM (it is big because I hacked one game I have been sloooowly working on) : http://www.tmeeco.eu/SMS/COLOR.SMS

MD measurements found here : http://gendev.spritesmind.net/forum/viewtopic.php?f=22&t=2188
MD2SMSmodeRamp.jpg (371.16 KB)
MD2SMSmodeRamp.jpg
COLOR.rar (18.67 KB)

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Post Posted: Thu Oct 22, 2015 12:47 pm
Should you use gamma correction for the voltage to RGB conversion? My feeling is that TV colours are typically brighter than linear, even if the TV brightness and contrast is sensibly reduced from defaults.
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Post Posted: Thu Oct 22, 2015 2:28 pm
When you apply gamma to that you apply it on top of what your video card and/or monitor alerady applies to the video out, making things worse.
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Post Posted: Wed Dec 30, 2020 7:43 am
The MD VDP decap has shown that the video DAC has a long sausage structure with 17 taps at different points that correspond to various output levels. SMS mode levels have their own 2 taps into it for the middle levels, the rest are used by MD mode with normal and shadow+highlight mode.
VDPDAC.png (298.27 KB)
DAC structure
VDPDAC.png

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SMS video output levels
Post Posted: Fri Jan 01, 2021 12:01 pm
@TmEE is this the 315-5313 decap by Fritzchens Fritz?
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Post Posted: Fri Jan 01, 2021 11:28 pm
Yeah, it should be that one.

All MD ASICs I tested behaved exactly same way too, without any variation in the levels in any mode so the DAC and its logic seem to have been kept the same across all the variations.

MD output levels ------------------------------------------------------------

MD VDP DACs are interesting things with 17 possible output levels. Silicon
shots have revealed a sausage structure sitting between AVCC and AGND with
taps at various points over the length of it, each tap going to one of the
17 transistors that are controlled by 17 incoming logic signals which outputs
go to the buffers for final video output.

Shadow/Highlight and SMS mode levels aren't generated in any analog trickery
as previously thought, they're just part of the predefined 17 levels.

N=Normal, S=Shadow, H=Highlight, s=SMS mode output
+-----+-------+------+------+
|  mV | Ratio | 8bit | NSHs |
+-----+-------+------+------+
|   0 | 0.000 |    0 | NS s |
| 108 | 0.113 |   29 |  S   |
| 192 | 0.202 |   52 | NS   |
| 261 | 0.274 |   70 |  S   |
| 324 | 0.341 |   87 | NS   |
| 368 | 0.387 |   99 |    s |
| 378 | 0.397 |  101 |  S   |
| 432 | 0.454 |  116 | NS   |
| 483 | 0.508 |  130 |  SH  |
| 537 | 0.565 |  144 | N H  |
| 588 | 0.618 |  158 |   H  |
| 604 | 0.634 |  162 |    s |
| 642 | 0.675 |  172 | N H  |
| 699 | 0.735 |  187 |   H  |
| 768 | 0.807 |  206 | N H  |
| 849 | 0.893 |  228 |   H  |
| 951 | 1.000 |  255 | N Hs |
+-----+-------+------+------+
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Post Posted: Mon Jan 04, 2021 12:47 pm
so SMS colors on a MD seems to be based on 0, 99, 162, 255 instead of the theoretical 0, 85, 170, 255?
(also, I wonder why they didn't chose the available 87 and 172 values instead of putting two more for the SMS only...)
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