Some Notes on Composite NTSC video.


8 posts
by DavidS » Wed Jan 09, 2013 12:59 pm
I have found the hard way that there are certain limits to configuring RISC OS on the RPi with a NTSC composite display. Of cource when you first boot risc you wll need to reduce the screen resolution. To do this type press the (F12) key and type:
Code: Select all
WimpMode X800 Y600 C16M F60

Then press enter twice.
I do not know how the broadcom chip does it, though 800x600 is quite cleare on a display that can only do 480i.
Next you will want to dissable font antialiasing, do thi in the fonts configuration program. you can get there from tha main configure program. You are stuck with the flickering along the horizontal hard black lines that are socommon in the WIMP, the flickering is not rely noticable anywhere that does not have a hard dark line on a light backgroud or vice versa.
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by DavidS » Wed Jan 09, 2013 1:19 pm
I almost forgot:
you will need to go into the screen settings in the configure program and change the settings there, saving them by pressing Set, in order to keep the screen settings. otherwise next time you boot you will be back to the way to high resolution.

In order to correctly use the Screen configure program to set the low resoltions you will need to select Generic for your monitor type.

And when typing the WimpMode command above you probably will not be able to read what you are typing.

The fallowin resolutions are acceptable on my NTSC TV:
    640x256 60Hz
    640x480 60Hz
    640x512 60Hz
    720x480 60Hz
    720x512 60Hz
    800x480 60Hz
    800x600 60Hz
    1024x600 60Hz This is a little blurry.
    1024x768 60Hz This is less blurry than 1024x600
    1280x768 60Hz This is about as blury as 1024x600
    1280x960 60Hz This is very blury though you can still strain and read it.
    1280x1024 60Hz This is slightly less blurry than 1280x960 though still quite blurry.
And of course YMMV. Different TV's/Monitor's are different.
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by Unhban » Wed Jan 09, 2013 2:29 pm
Thanks for the info David. Can I just pin you down on what you mean by 'NTSC TV'?

o do you mean an old analogue NTSC 525 lines, 60Hz TV that has a composite input?
o or an 'NTSC' computer monitor? (that perhaps has an NTSC tuner in that used to allow reception of NTSC signals, but you're using the composite input)
o or a HD-Ready or Full-HD TV with composite input? (this would not be NTSC, nor PAL nor anything, as in the digital domain there are no such things :) )

Here in the UK a HD-Ready TV has a native screen resolution of 720 lines. You can feed 625 lines into it (now actually termed 575 lines in the industry) and it will upscale that to 720 lines but still be standard definition. Or, if you feed 1080 lines (i.e. full high definition) into it that will be downscaled to 720 lines, so you will lose some of the full HD quality.

A Full-HD TV has a native screen resolution of 1080 lines. You can feed 625 lines into it and it will upscale that to 1080 lines but still be standard definition. Or, if you feed 1080 lines (i.e. full high definition) into it then you'll get full HD quality.

Some of these HDTVs can accept 525 lines too.

Now.... from your experiments it would seem to me that your NTSC TV is either the computer monitor or HD-Ready/Full-HD TV. I say this, as being in the industry, with respect I find it difficult to believe that an old analogue TV would lock to many of the resolutions you mention. But it'll be interesting to hear your reply to this.
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by plasticrabbit » Wed Jan 09, 2013 3:44 pm
i usually call a 60hz compatible pal tv 'ntsc' as for the most part in the uk we only really use ntsc for computers that want to put out 60hz.
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by plasticrabbit » Wed Jan 09, 2013 3:47 pm
p.s. the info in the original post was good to know. not sure how i got the picture on my 60hz lcd tv down to a resolution low enough for me to see what i was doing. :lol: next time there will be a lot less squinting and clicking indistinguishable icons hoping for the best.
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by DavidS » Wed Jan 09, 2013 4:33 pm
By NTSC TV I am refering to an old 525 line interlaced analog tube telivision with a standard NTSC Composit input. Ordinarily it should not be possible to display more than 480 lines interlaced, though the brodcom video seems to perform some kind of trickery to allow higher resolutions to be clear. Do not ask me how, as I can not figure out any way to do this (is it using some kind of weird 3 frame interlace or something??).
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by DavidS » Wed Jan 09, 2013 4:38 pm
I wold note though from some of my microcontroler projects; It is possible to display extreamly high horizontal resolutions (1280 at least [though stil only 480 lines])on a standard NTSC set.
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by DavidS » Wed Jan 09, 2013 7:39 pm
Fortunately I am no longer using NTSC now. i needed to get a HDMI to DVI cable to use my monitor, and I just found one localy.
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