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Forum Index : Microcontroller and PC projects : Generating colour composite video...
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9585 |
Hi all. 
We all know that the Maximite and VT100 can generate composite video, but it is black-and-white luminance video with no chroma signal(colour). What is involved in generating a colour PAL or NTSC composite video signal? My thoughts are along the lines of: "If it was easy, the Maximite or VT100 would already have colour composite." So, my guess is that generating colour composite is more tricky then you might first think, but if you don't ask, you don't learn anything......  Smoke makes things work. When the smoke gets out, it stops! |
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MicroBlocks Guru  Joined: 12/05/2012 Location: ThailandPosts: 2209 |
It is more tricky then you think. :) So tricky that it will not be worth the effort when you can buy specific hardware solutions for it for around 15US$. Microblocks. Build with logic. |
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kiiid Guru Joined: 11/05/2013 Location: United KingdomPosts: 671 |
Two simple answers - memory and SPI resources (unless you use external components) http://rittle.org -------------- |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9585 |
OK. I'm interested now in the technical reasons behind this. Ignore for the moment, that you can't do it with the Micromite - what I want to learn now is HOW colour composite video is generated, and specifically WHY the Micromite or other can't manage it. IE: If there were enough SPI channels, could you do it? Just how many channels, and how much RAM do you need to generate colour composite? I am guessing that the short-short answer to this is: "A-lot.", as with the likes of the Colour Maximite computer, it can generate colour VGA, but it too cannot produce colour composite. I want to understand the technical reasons. Smoke makes things work. When the smoke gets out, it stops! |
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| kiiid Guru Joined: 11/05/2013 Location: United KingdomPosts: 671 |
I don't know who has said it is impossible. Difficult, yes, but not impossible. It mainly depends on what resolution and how many colours you are after, also is it for graphics or text only. You should probably define that first. http://rittle.org -------------- |
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| bigfix Senior Member  Joined: 20/02/2014 Location: AustriaPosts: 129 |
You can try to hook a MC1377 RGB to PAL/NTSC encoder chip to the VGA outputs This could do the job if the VGA timing is made compatible to composite color But requires some additional components like Xtal, Xformer etc... |
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basicchip Newbie  Joined: 25/11/2014 Location: United StatesPosts: 9 |
It is far easier to generate RGB video and use an old VGA monitor, but if you really want composite color that is done by adding color burst reference signal before the start of the line which is a 3.58 MHz burst. Then color is added as a difference in phase between that reference burst and the color information transmitted at that same frequency added to the BW signal. Lots of old TV info on the web describes generating this, while it would be possible to generate the signal, far easier to dig up old chips that do this though some may be becoming obsolete. Most all TVs I've seen these days take in VGA signals, and is the easier way to go and more accurate than composite. |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6266 |
Something like this VGA Converter There have been reports of some that do not work with the maximite but they all 'SHOULD' Used on the VT100 you would end up with one colout of your choice but still not multiple colours at the same time. Jim VK7JH MMedit |
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palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 1982 |
@ Tassy Jim I bought that exact converter and it won't work with my UBW32. I wanted to use my UBW32 with a 4.3" composite monitor. Paul "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9585 |
Thanks for the replies - lots to think about. I forgot about those VGA-TV converters - I have a couple of those sitting around I could use if this proves to be necessary. @ palcal - did you ever sort that problem out? It was only after you posted your comment, and the VGA-TV converter was mentioned again, that the light-bulb in my head went on about your issue a month or two back. I remember now that I tested a Maximite with a couple of VGA-TV converters here, and they worked fine, so that is probably the way to go in any event. Having said that, it made me think about your issue again - any progress there? Smoke makes things work. When the smoke gets out, it stops! |
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| palcal Guru Joined: 12/10/2011 Location: AustraliaPosts: 1982 |
@ Grogster No never found a solution the converter works Ok when my PC is connected to it, I can only assume that the VGA from the UBW32 is different in some way, it's something I know very little about. Paul. "It is better to be ignorant and ask a stupid question than to be plain Stupid and not ask at all" |
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geraldfryjr Regular Member  Joined: 02/03/2014 Location: United StatesPosts: 61 |
Hi there! I think that color can be done very simply. I just ran across a project where I saw how this can be done very easily but I don't have the link or else I would post it. For composite video back in the day color was introduced by causing a delay in the pixel data I think and this introduces a phase shift that is compared with the 3.579Mhz colorburst signal. The project I saw used a selector to introduce a different delay lines using gates with various propagation delays to the pixel stream, and the amount of extra delay used determined what color value the pixel data stream was going to be. I instantly thought about this and applying it to this system as well and thought could this be implemented to the MaxiMite as well so that I could use it on some of my old TV sets rather than having to use a monitor. Some of the systems from yesteryear also just used a simple 2 or 3 bit D/A as well using just a few resistors. I never really understood how the colors came about until I saw the way the various delays were implemented and then it all became clear to me. I will see if I did save the link in my archives, if not I guess I will be doing some searching for it again myself!! jer :) Keep on DIYin' !!!  |
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| geraldfryjr Regular Member Joined: 02/03/2014 Location: United StatesPosts: 61 |
There are many examples of color bar generators using a PIC some use a similar method I described and some use the old MC1377 modulator here, https://www.google.com/search?num=100&newwindow=1&safe=off&s ite=&source=hp&q=pic+color+bar+generator&oq=PIC+colorbar+&gs _l=hp.1.0.0i22i30.2391.7791.0.10375.13.13.0.0.0.0.82.800.13. 13.0.msedr...0...1c.1.60.hp..2.11.667.UL6LPHkm51Y jer :) Keep on DIYin' !!! |
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| MicroBlocks Guru Joined: 12/05/2012 Location: ThailandPosts: 2209 |
I think the need for such a solution gets smaller and smaller. The reason is that old TV's are not that common anymore and tft/LCD screens can be bought for next to nothing and have SPI/i2C etc interfaces that are made for being controlled digitally. It was interesting because a lot of rearview camera/monitors were available cheaply. Sometimes things are really better then 'in the old days'. Take advantage of it.:) Microblocks. Build with logic. |
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| geraldfryjr Regular Member Joined: 02/03/2014 Location: United StatesPosts: 61 |
In one of the examples one used a 16F84 to control a AD724 that can be still be had from Analog devices, http://www.qsl.net/zl1wtt/page7.html http://www.analog.com/en/audiovideo-products/video-encoders/ ad724/products/product.html#product-samples I think I will order some of samples of these and see how they work. jer :) Keep on DIYin' !!! |
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