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Scan rates and resolution??????

Discussion in 'Projectors, Screens & Video Processors' started by schmidtsm, Jul 7, 2005.

  1. schmidtsm

    schmidtsm
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    I have seen alot of projectors on ebay that state they are fully HDTV (720p, 1080i) compatible. But the resolution of the projector are 1280X1028 with a scan rate of 15-65. I understand that this projector will support 720P, but will it support 1080i? My thinking is no.

    What does the scan rate mean? I know 15.75 is for sdtv 480i? And how does the scanrate work with HDTV. I have a HT cabel, a CS-1 scaler, DVD Upscaling LG, and NEC PG-6 CRT. I would like to know what is the best resolution/scan rate for my set up without pushing my PG to hard?

    Thanks,
     
  2. cosaw

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    Hmm... I wouldn't know. However a few things are worth investigating in order to get the optimum res for your NEC. Don't know much about the NECs so forgive me if some things don't apply.

    Are you talking widescreen resolutions or 4:3?
    Is it Data grade or Graphics grade - what does it scan up to at max bandwidth?
    What tubes does it have do you know? Serial number? Or 7,8 or 9"?

    These things can affect what max res you can achieve. Your pj must have a max scan rate or bandwidth greater than or at the very least equal to the scan rate of the resolution you want to achieve.

    Overall scan rate for a particular resolution is mainly a product of the vertical res and the horizontal res and the refresh rate. It is also related to whether you want to output interlaced or progressive. Lets assume basic progressive at least for which you'll need a pj capable of very roughly 30Mhz ish upwards. This would be on the lower side in resolution and refresh I'm roughly guessing something like 800x600 @ 60Hz. Simplifying the maths (800x600x60)/1000000 = 28.8Mhz. Which isn't far off the 30Mhz - the numbers don't quite add up cos in actuality the maths is probably a little more involved. Its near enough - insert your own numbers and try a few but remember this is only a rough guide.

    You can see that by just upping the refresh at the same res uses more bandwidth. Your example was 1280x1028 but you'll need a refresh as well so lets say at min 50 Hz. (1280x1028x50)/1000000 = 65.792Mhz. Remember this is very rough - you wouldn't want to over-run your pjs bandwidth as it can damage some pjs.

    Thing is it would seem that even though your pjs bandwidth may be able to cope with such a resolution it doesn't mean your tubes will be able to resolve that resolution correctly. Draw the lines too close together on your tube faces and your image will become soft. There is no point aiming too high - optimum is whats best obviously. Usually the larger the tubes the more res they can take physically and resolve. As an example 720p 16:9 would seem an optimum sort of res for most 8" tubes. However I'm sure 720p 16:9 aint gonna resolve on my sd-187 7" tubes.

    Hope this simplified explanation has given you some more insight.

    Simon
     
  3. Welwynnick

    Welwynnick
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    The scan rate is the important do-not-exceed parameter, though bandwidth becomes more of a limitation with high horizontal resolutions. Both parameters are rather higher than resolution x refresh because of the H & V sync and front and back porches that are not displayed. This is quite significant with the HDTV spec for 720p in particular, which requires about 75MHz BW, rather more than you would expect.

    The NEC 6PG is a good 7" CRT with a horizontal refresh rate of 60 kHz. It's may be suitable for 720p if the bandwidth is high enough (not sure), but no more. Fully HDTV compatible is quite a tall order for any display, and it's usually the better 8" sets that measure up.

    Nick
     
  4. cosaw

    cosaw
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    75Mhz for HDTV 720p - ouch! What refresh rate is that at?

    Good job I'm only really interested in dvd at the moment then and can tweak in powerstrip on my pc. I run 720x576p @ 50 Hz 16:9 anamorphic for dvd and seem to rember in pstrip the pixelclock coming out at just over 30Mhz wich is about half the bandwidth of my pj but near optimum for my tubes it would seem.

    Simon
     

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