Author Topic: Converting 15kHz analog RGB to digital  (Read 35028 times)

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Offline miguelvp

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Re: Converting 15kHz analog RGB to digital
« Reply #75 on: July 13, 2014, 03:01:47 am »
I think it will help, the color burst part of the composite might be affecting the TV sync, there is a simple lm1881 "circuit" that can probably be hooked up directly. I put circuit in quotes because they just solder the resistor and a couple of caps on the lm1881 and hook it directly to the power on the connector.


Found the link to it:
http://retrorgb.com/syncstripper.html

That should convert from the bottom signal to the top signal removing the color burst and the composite video.



« Last Edit: July 13, 2014, 03:10:23 am by miguelvp »
 

Offline Legion

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Re: Converting 15kHz analog RGB to digital
« Reply #76 on: July 13, 2014, 04:08:56 am »
Found the link to it:
http://retrorgb.com/syncstripper.html

That should convert from the bottom signal to the top signal removing the color burst and the composite video.

If it weren't for the power requirement I'd probably do the sync stripper as in the linked article. So I think I'm going to do a little perf board circuit with some BNC jacks and a DC power jack and have that sit behind the monitor.
 

Offline Legion

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Re: Converting 15kHz analog RGB to digital
« Reply #77 on: July 13, 2014, 04:27:59 pm »
Here is a picture of the composite signal out of many frames with that red green and blue pattern I've been showing.

Here is a picture of a single composite horizontal line (red ramp part) I hope this helps visualize the data a bit. Also I pasted in the picture my trigger setting to lock into the vertical sync so you can see specific lines from your scope by scaling to see one line and scroll with the horizontal position of your scope to the line you want to look at.

Thanks for those explanatory images. Makes things a lot clearer. So it looks like the amplitude of a given color signal governs it's intensity. I'm guessing the combination of the HSync markers and the correct sampling frequency get you your individual pixel R, G and B values? The big gap between the full frame R, G, B bands is the VSync (front porch/back porch)?

Is there any data contained in the initial color burst signal seen before the ramps?
 

Offline miguelvp

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Re: Converting 15kHz analog RGB to digital
« Reply #78 on: July 13, 2014, 06:51:01 pm »
So it looks like the amplitude of a given color signal governs it's intensity. I'm guessing the combination of the HSync markers and the correct sampling frequency get you your individual pixel R, G and B values?

It's a bit more complicated than that, I was displaying 3 color ramp bands first red then green then blue that's why they show nicely on the capture, but if they were combined like any regular pixes normally is, the video signal needs to be split into YIQ or YUV (the difference is just a different coordinate system for the colorspace.
http://en.wikipedia.org/wiki/YIQ
http://en.wikipedia.org/wiki/YUV

Once you have the YIQ or YUV you can convert it to RGB using some simple matrix multiplications.

This article has a more detailed explanation if you want to dig deeper:
http://codeandlife.com/2012/10/09/composite-video-decoding-theory-and-practice/

The big gap between the full frame R, G, B bands is the VSync (front porch/back porch)?

On the signal whole signal it goes this way:

Vertical sync | Vertical Back Porch | Video Lines | Vertical Front Porch (repeat next frame)

The Video lines consist of the following:

Horizontal sync | Horizontal Back Porch (Includes Color Burst) | Visible Video | Horizontal Front Porch (repeat next line)

For completion, during Vertical Sync you can put close captioning on the non visible video lines, but that's another story. In Europe they use it for Teletext I think.

Is there any data contained in the initial color burst signal seen before the ramps?

The Color burst does have information about the intensity of the video signal. it's 40 percent of the maximum value peak to peak, so from zero volts to the top of the color burst signal it represents 20 percent of the video signal value.

So the maximum signal will be past the scope screen where I placed the 100.



On top of that the Color Burst contains the color subcarrier , that will help you to find out the pixel clock (315/88 MHz for NTSC) and be able to mix different signals to level the colors based on the amplitude of the color burst of both signals and adjusting for pixel clock rates.

More info:
http://en.wikipedia.org/wiki/Colorburst

More info on dot clock rates (pixel clock) for different gaming systems:
https://pineight.com/mw/index.php?title=Dot_clock_rates


« Last Edit: July 13, 2014, 07:10:57 pm by miguelvp »
 

Offline Legion

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Re: Converting 15kHz analog RGB to digital
« Reply #79 on: July 15, 2014, 01:38:04 am »
Thanks for the info. I especially liked the codeandlife article.
 

Offline Legion

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Re: Converting 15kHz analog RGB to digital
« Reply #80 on: July 19, 2014, 07:37:12 pm »
I made the sync stripper and it fixed the sync problem on white screens.
 

Offline miguelvp

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Re: Converting 15kHz analog RGB to digital
« Reply #81 on: July 19, 2014, 08:18:32 pm »
Nice,

No progress here, other than while adding a 500GB drive to make space for the Xilinx toolchain for when I get my Papilio Duo, my computer wouldn't boot.

It's all good now, the memory got unseated when moving it around I guess and it wasn't giving me warnings, nor video and being an old system took me a while to resurrect it.

And since I had 2GB laying around I upgraded the memory as well.
 


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