TV sets - when going from SD to HD (What happens?)

MarkHastings

Senior HTF Member
Joined
Jan 27, 2003
Messages
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Ok, first off, I understand video in the computer world - SD video being at 720x480 and HD being either 1280x720 or 1920x1080....

Now in the computer world, all of these videos are 72 dpi. I realize that this is kind of a non issue in the video world (in regard to TV sets) since the bigger the set is, the bigger the pixels are, you really can't equate DPI because an inch of video footage on different screens is going to equate to varying sizes.

But the fact is, the video sources are at 72 dpi - the only reason HD has more resolution is because the video frame is larger. Unlike print where more resolution means higher DPI.

So being that, when a TV switches from SD to HD content, what exactly is going on? I assume the pixels are getting smaller to allow the size of the HD image to fit into the same area that the SD previously filled (just like changing your computers montior into a higher resolution). So is that what happens? the pixels get smaller?

If not, then what exactly are they displaying? and if so, what causes the pixels to shrink?

I guess I'd have to ask the same of computer monitors. If a monitor can display 640x480 at 72 dpi as well as 1024x768 at 72 dpi, since the physical screen doesn't get larger, then I assume the main difference has to be pixel size?
 

Allan Jayne

Senior HTF Member
Joined
Nov 1, 1998
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CRT computer monitors and CRT TV's "make the pixels smaller" by spacing more scan lines closer together on the same screen.

Non-CRT monitors cannot do that. Instead every so often two scan lines of source material have to be blended together by the supporting electronics producing one scan line that can occupy a given row of pixels. Or vice versa when one scan line is repeated to occupy two rows of pixels. (More sophisticated procedures do more elaborate blending to make diagonal lines appear more crisp and reduce the jagged effect of some scan lines having a row of pixels all to themselves while others have to share.

72 dpi is a suggested setting when editing pictures for display on the Internet or on computer monitors. The actual pixel count, if measured by a ruler, will of course be different depending on the screen size and the selected resolution (640x480, etc.)

Some CRT HDTV's display 1080i all the time, scaling up (by repeating scan lines here and there or using other more sophisticated blending) SDTV material to make use of all 1080i scan lines on the screen. ALthough 1080i is 1920x1080, the supporting electronics are often not good enough to change the electron beam fast enough to make spots as narrow as 1/1920'th the screen width, or the electron beam may be a little too fat.

Video hints:
http://members.aol.com/ajaynejr/video.htm
 

MarkHastings

Senior HTF Member
Joined
Jan 27, 2003
Messages
12,013
Thanks for the link Allan, I'll look through it this weekend.Ah, the SD content is actually 'projected' into this 1080i video space? So it's not like the TV gets thrown into 480 mode, it stays in 1080 mode and just processes the 480 material into the 1080 space?
 

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