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Revision as of 21:55, 27 January 2008 by Fishter (talk | contribs) (How do I get wide-screen TV working on my mythtv box?)

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Adapting the frequencies

To use a TV screen as monitor, the 'Monitor' section in xorg.conf (usually in /etc/X11) should be adapted to allow refresh rates that can be understand by the TV encoder chip of your videocard. Normally this will be 60.0 Hz, or 50Hz in UK/Europe for the PAL TV-standard (although PAL-M is 60Hz)!

I repeat, you need the value for the TV-encoder on the videocard. (in fact, the TV-encoder chip has internal instructions how to handle the rates for your TV. This is what TV-standards like PAL and NTSC are designed for ;-)

As sample I provide you with the 'Monitor' section of my xorg.conf:

Section "Monitor"
        Identifier   "Generic Monitor"
        HorizSync    30 - 50
        VertRefresh  60.0   # Use 50.0 in UK / Europe (PAL-x, except PAL-M)

This will result in all common resolutions (1024x768, 800x600, 720x576 and 640x480) to work at the common frequency of 60Hz. Please note that TV standards understand resolutions at a 4x3 Aspect_ratio; For a discussion about wide-screen see below!

If your output is PAL or SECAM video (e.g. for a UK / European television), you should generally use 50Hz instead, although PAL-M (perhaps others too) uses 60Hz, and some TVs will accept both. Using the correct frequency will allow smoother vblank synchronisation when watching TV and playing DVDs, and minimize image distortion.

For still images and games, you might want to use 60Hz even on PAL systems (you'll get less flicker, but a lower vertical resolution). But be warned: not all PAL televisions will sync at 60Hz: you may get a rolling screen, black and white image, a distorted image, or the TV may just show a blank screen.

An expensive modern LCD/Plasma TV will probably handle everything you throw at it, even multiple video standards and maybe even monitor frequency ranges; cheaper/older TV's (especially CRTs) will usually support only 50 or 60Hz video and nothing else. If you get any strange artifacts at a particular frequency on a CRT television, do not continue to use it (or you risk damaging the TV)!

If you have a video card with dual-output, you might want to leave a VGA monitor plugged in while you experiment, so you an easily edit your config files if your TV rejects the input.

Wide-screen Television

Nowadays, many people have a wide-screen television set.

Compared to a computer monitor, where a wide-screen monitor also has a wide-screen resolution, the wide-screen TV uses always the same resolution. In fact, that resolution is always a resolution what is in fact a 4x3 Aspect_ratio resolution. The wide-screen TV set uses a technique called 'anamorphic imaging' to 'stretch' the actual image to a wide-screen view.

What is 'anamorphic imaging'?

For people who want to know how this technique works, I will try to explain this with some examples:

Just power-on the wide-screen television, and look for a 4x3 Aspect_ratio football/soccer game with a round ball. Press the buttons on your Remote Control to change the picture to wide-screen (real 16x9 Aspect_ratio!) format, and the ball becomes somewhat stretched; in fact it looks like if the ball is oval, or more like a rugby ball.

Compared to that, a real 16x9 wide-screen broadcast with rugby will present you with a compressed (or 'sandwiched') ball if watched in the 4x3 Aspect_ratio setting of the television. That ball will look like a football/soccer ball instead...

How do I get wide-screen TV working on my mythtv box?

In fact it does already work :-)

The TV encoder chip on the video-card always encodes an actual 4x3 Aspect_ratio resolution. If you want to enable a wide-screen view, just keep the 4x3 Aspect_ratio resolution setting, like 800x600 or one of the others mentioned above! Then select a theme which was designed for wide-screen use, normally a theme with a name which ends up in '-wide'.

When viewed in the real 4x3 Aspect ratio setting on the television, the theme will look like 'sandwiched'; it is smaller than it should be. Again as with the explaination from above, please use your Remote Control to switch to the real 16x9 Aspect_ratio, and the theme will appear as if your videocard natively supports a wide-screen resolution; thanks to the anamorphic technique which is used to stretch the compressed/sandwiched images back to their real size...

But, in short, don't try to get a wide-screen resolution working with TV out; just keep the standard 4x3 Aspect_ratio resolutions, and use a wide-screen theme and the wide-screen button on the Remote :-)