video voltage levels

[totops]

Hi, has anyone managed to scope the colour voltage levels? Something similar to these tables?
http://forums.nesdev.com/viewtopic.php?t=2113
https://wiki.nesdev.com/w/index.php/NTSC_video#Brightness_Levels

It'd be great if we could feed the raw values into an NTSC algorithm comparable to this one
https://github.com/libretro/glsl-shaders/blob/master/nes_raw_palette/shaders/nes-color-decoder.glsl

Thanks!

 

[totops]

This code is rather interesting, because it deviates from normal blargg code.

http://svn.akop.org/psp/tags/atari800/2.1.0.1/atari_ntsc.c

https://github.com/OpenEmu/Atari800-Core/blob/master/atari800-src/colours_ntsc.c

 

Notably it relies on this document for the luma voltages

http://ftp.pigwa.net/stuff/collections/nir_dary_cds/Tech Info/CGIA.PDF

(page 80)

Volts                Multiplier
2.8255 = Black       0.5651
3.4705 = Luma 0      0.6941
3.6205 = Luma 1      0.7241
3.7800 = Luma 2      0.7560
3.9655 = Luma 3      0.7931
4.1300 = Luma 4      0.8260
4.3500 = Luma 5      0.8700
4.5800 = Luma 6      0.9160
4.8450 = Luma 7      0.9690

 

Which if you read about TIA, it mentions D3,D2,D1 just like in CGIA (and GTIA).

https://archive.org/download/Atari_2600_TIA_Technical_Manual/Atari_2600_TIA_Technical_Manual.pdf

 

 

If one wants pure white though, I guess the other half could be used from the table (D0 = 1). Or re-normalize based on voltages. Luma is somewhat surprisingly non-linear though, which was implied here also.

https://sites.google.com/site/atari7800wiki/rgb

 

 

I suppose if someone compared full luma gradient pictures from black - white on 2600 - 5200 - 7800, it could give a clue if the theoretical values in the document are realistic enough and didn't greatly change between generations.

Edited September 20, 2019 by totops

[ChildOfCv]

I'm not sure what you're really asking.

 

Color "voltage" is a 3.58MHz sine wave that ranges between 0 and 40% of the signal p-p voltage based on saturation, and its phase relative to the color burst determines hue.  The sync tip on NTSC is 30% of the signal, and anything 30-100% is luma.  A TV's automatic gain control will look at the strength of the sync tips to decide how much gain the signal needs to fit within the desired 1Vp-p (plus color) range.  Its automatic color control uses the strength of the color burst as a reference.

 

Now, those are the official numbers.  The consoles, of course, don't hit all the levels perfectly.  But they make an effort to.

It's also good to point out that the internal pre-modulation voltages may not have the same amplitude as the input references.  It appears from your referenced post that NES uses twice the required voltage.  But before it sends the output to the TV, it must divide it down as needed.  If going composite, it attempts the 1Vp-p output through a 75 ohm impedance.  If going to the TV antenna, it will be in the microvolt range.

 

As for scaling, it makes sense that it's not a perfect division.  Most consoles use resistors to create a crude DAC output.  The resistors desired may not exist, so they have to choose values that are close.  That's going to leave some wiggle in an otherwise linear progression.