Limited Horizontal Resolution

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Sep 1, 2001
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Hi all,

I would appreciate it if some body could explain me what the would happen to a projected image of a CRT projector fed with more horizontal resolution than it can cope with. Imagine the case of a Sony VPH-G70Q with 1700x1200 capability, fed with 1920x1080i or 1920x720p.
The projector does have sufficient horizontal scane rate to cope with the signal from 1080i or 720p.

What does happen to the extra 220 points in each line? Are all of them (1920) displayed but overlaped by some amount?
Would it be a problem, i.e. noticeable or not?

Many thanks.

A G70 can't do 1700x1200 anyway. 1920x1080i (720p) should look good on a G70 but it won't be resolving the full horizontal res.
Hi Jeff,

thanks for replying. When you say it will not resolve all the resolution, what do you mean with that, that it will show as much as its own horizontal resolution and crop the rest?

By the way, Sony's manual says its resolution is 1700x1200. Are you sure it is not? And I found that number in several other places. I understand this number is the maximum value, and the image probaly would not look that good, but it is the value Sony puts in the manual.

The g70 is a 8" CRT projector, as such it will only be able to resolve about 800 horizontal lines. The figure in the Sony manual is the maximum res the projector can receive as in input but what the pojector displays is another thing. Its not really an issue though, 1920x1080i (or 720p) will look great on a G70.
Basically a CRT projector is an analog device, in that there is no pixelation or sampling done within the unit, so there isn't really a 'resolution' point that is a true limit as you would find with pixelated units such as LCD and DLP.

With the 1920 'pixels' of the HD signal going into a CRT projector that cannot theoretically resolve those individually (as you have with the G70) then there would be a slight smearing of adjacent pixels into one another. In reality this would be un-noticable except with a true graphic signal source (e.g. a PC screen) as video (even HDTV) is filtered (anti-aliased) to prevent hard edges appearing on single pixels or lines.

What this really means is that numbers in specifications can mean different things under different conditions, and in this case a G70 should produce an excellent picture from a 1080i source.

Hope this helps.
Thank you all. Understood better the effect of limited "virtual" resolution of CRT projector. It works more or less as I thought.

Other question. Richard, you said:
"In reality this would be un-noticable except with a true graphic signal source (e.g. a PC screen) as video (even HDTV) is filtered (anti-aliased) to prevent hard edges appearing on single pixels or lines."

I understand that hi-def video sources are anti-aliased, therefore pixel smearing would not be noticeable.
What about PAL or NTSC video signal scalled to high resolutions?
That is, take 480i and make it 1920 x 1080p using a de-interlacer + scaller. Would it be noticeable with the G70?
I am not asking because I think it is the way to go. Probably in this case 1280 x 720p could be better. Just asking in order to understand what would happens visually if displayed using a CRT with these technical characteristics.

Thank you.


Scaling a 480i signal to 1080i - in this case you are asking about the horizontal pixels, so it's scaling 720 pixels to 1920 pixels.

With H scaling of any video signal there is little, if anything, to be gained by changing H resolution at all for a CRT projector as the resulting signal is still only an analog (non-pixelated) scan.

This of course assumes a correctly designed (and filtered) output satge from a scaler - if an output stage does not adequately filter the signal there will be excessive high frequency spikes between output pixels which can cause even more smearing in the projector.

For LCD, DLP, D-ILA and Plasma then the rules change again. Here it is important to get the same number of pixels in the output as there are pixels in the display as the sampling 'window' used for each display pixel's value can be very narrow and will show any errors as moire fringing (light and dark bands across the screen.

So for the G70, using an up converted PAL or NTSC signal the 1920 sample bit really makes no difference to the image quality unless the scaler used is not outputting a clean signal

Hope this helps

How can I check to see if high frequency spikes are a problem in my system?, I have tried 720x960 and I like but I'd like to make sure I'm not introducing another issue. Will it show on any of the Avia test patterns?

Hi Jeff,

I guess you would need to look at an image with fine detail on a flat background, so a crosshatch with dots would be ideal.

What you would see would be a fine smear (blur) or ringing to the edges - it will look pretty much like ringing from poor cables but at a higher frequency (It may also appear in flat mid tones as a fine vertical pattern)

Using a 'scope to analyse the signal would be a more objective test though!

I have to admit to not using AVIA very often so I can't remember what patterns are available from the top of my head. Though I do have a copy at home I rarely use it as I personally don't trust the calibrations as much as I do VE (obviously I may be biased as S&W helped supervise the technical quality of VE!)

Hope this helps,

Sorry if I'm going slightly off topic ....... but has S&W been involved with the 'new' version of Digital Video Essentials as well ?

Originally posted by RichardA

Using a 'scope to analyse the signal would be a more objective test though!

Where am I going to find a video expert with a scope in Basingstoke? ;)

I you and Jeff don't manage to hook up would it be possible to bring a scope to the Event 2, I'm sure you'd get a lot of people who want to see the results of what comes out of various devices and that would be one of the few times would could get all the different devices in one (very sweaty) place.

Just a thought.


Any S&W involvement in VE 2 ?

Sorry, but I can't answer that one - that's all I can say for now!
Jeff, John.

Definatley something to look at for Event 3 (as I guess we are up to now!)

I don't have a suitable scope to hand now as I work out of the sales office - I really ought to invest in one for all my little home projects I guess ;)
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