EDID's and TV resolutions

Date: 2012-01-03 09:31 pm (UTC)
From: (Anonymous)
In my job I spend a lot of time reading TV EDIDs and swearing.

I only see the 1366x768 on older TV's that marketed at HD-Ready. What they meant, and what they state in the fine print, is that can accept an HD broadcast signal. Then the TV internally downscales to normal NTSC or PAL. I haven't seen one of the TV's that you describe where it actually has a reasonable resolution.

Another thing that is very common are 1080i TV's. Most people get the impression that they are getting 1080 rows of resolutions, when these TV's are also designed to accept 1080i resolution, they also downscale (usually to 720i).

In terms of oversampling and active resolutions, you do get a full 1920x1080 of viewable area. Instead the full resolution is much larger, and consequently the pixel clock frequency of your HDMI/DVI cable it much higher than it needs to be for 1920x1080.

Oversampling is just one small dumb point, the whole backwards compatibility thing really goes way to far. As far as I can tell DVI cables are designed for analog signals, but can also carry digital on separate data lines, even though no one ever used the analog part. The fact that both analog and digital went through the cable together, I guess it made sense at the time that the digital part would just be the analog equivalent. IE nothing more than ADC channel. They could then use the same clock freqency, same synchronization pulse times (to let the gun move from the right edge of the screen back to the left). WTF!!! It's digital why are we sending pulse signals to wait for a CRT gun? HMDI was just DVI, but with audio encoded in the dead time while synchronization pulse was been sent.

Where I think video signals really went wrong is with DisplayPort. Since DVI/HDMI were purely analog signals encoded in digital, DisplayPort would give us the chance to drop the analog oddities like sync pulses. No they didn't drop that, as they thought if they kept them, it would be easy for someone to make a DisplayPort to HDMI/DVI/VGA adaptor. Now in 2012 we still have to carry forward all of this CRT specific crud.
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Matthew Garrett

About Matthew

Power management, mobile and firmware developer on Linux. Security developer at Google. Ex-biologist. @mjg59 on Twitter. Content here should not be interpreted as the opinion of my employer.

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