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Discussion Starter · #1 ·
I'm going to be in the used CRT front projection market soon, and I'm a bit confused about scanning rates. In the ISF seminar, they taught us that 2:3 pulldown can be achieved by running at refresh rates that are a multiple of 24. The first question is are refresh rates the same as scanning rates?


If they are the same, to achieve 2:3 pulldown and line doubling a rate of 48 khz would have to be used. I tend to see flicker quite easily. All my computer monitors run around 85. TV is fine unless I'm looking at test patterns and then I blink like mad. I'm assuming I will need to run at a scan rate of 72 khz.


I would like to purchase an 8" used CRT FP, that could show a 7' wide screen of at least 720 (1280x720) and meeting the criteria above. I'm not sure what the 4:3 size would be using the full raster, assuming I have a masking system to compensate. I would also like a quiet projector.


1)Please correct errors in my knowledge of scanning rates/refresh rates. And what impact does that have on the next question?


2)Please advise me on CRT FPs that will meet my criteria. I've done some research and until I heard that they were very loud was leaning toward NEC XG 110 or higher. I don't mind tweaking, as long as I can do more watching than tweaking. Sony G70 is under consideration and now Barco, since I heard they were quiet.
 

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3:2 pull-down means that successive frames of film become 3 and 2 fields of video respectively. Which frames get repeated on projection is a separate issue.


By using a multiple of 24Hz, each frame gets repeated the same number of times which makes for smoother motion. 72Hz is the lowest multiple that works for most people, and what you get with a 3-bladed shutter in the cinema;
 

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FYI - don't use what you may be used to with regular computer monitor flicker as the basis for passing judgement on CRT projector. The persistence of the phosphors is completely different between computer monitors and regular TV's or CRT projectors. Example: While many people notice lots of flicker on a computer monitor running at 60Hz, the same people will rarely notice flicker on their 'regular' direct-view CRT TV which is displaying a regular NTSC signal running at 60Hz (and interlaced to boot!).


72Hz is a good starting point. 48Hz is the another option that many people seem to like. Try it and see if you like it.


Kal
 

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Discussion Starter · #4 ·
1) Are refresh rates the same as scanning rates?


I think I need the answer to this question before I continue.
 

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Refresh and scan rates are different but related.


The refresh rate is the number of frames displayed per second.


The scan rate is the total number of horizontal lines scanned (including some padding and vertical retrace) per second. It's the refresh rate times the total number of lines (visible and otherwise).
 

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Discussion Starter · #6 ·
Thank you.


So how do I figure out what bandwidth a projector needs to achieve both line doubling and a 72 hz refresh rate? Do you set refresh rate on the projector or from the scaler or htpc?
 

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how'd this thread get lost? :D


refresh rate and scan rate are both set by the video source (from the projector's perspective). So they will be set by your HTPC or your scaler (which could be the HTPC).


An external source feeding to the scaler (or HTPC capture card) may send in a different refresh rate and scan rate than what the scaler is going to output to the projector.


This tool was mentioned here before:

http://www.myhometheater.homestead.c...alculator.html


it's just one of the ways to compute bandwidth.
 

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Discussion Starter · #9 ·
Thanks to all.


So in order to scan 1280x720 at 72hz I need 99.5 mhz bandwidth? I bet a lot of projectors can't do that, yet I read a lot of threads about people doing HDTV. They must be making some kinds of compromises.


Given the above and that I need a quiet projector. I don't want to have to build a hush box. Please tell me which ones will meet my needs. Barco? Sony G70?
 

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Well, what happens when the bandwidth of the signal exceeds the available bandwidth of the parts its running through is the signal is softened up some. Some detail gets lost.


A good video cable will give you around 400 MHz bandwidth. A high quality video switcher will get you at least 300 MHz.


A Barco Grapics 808 has at least 130 MHz internally I think. So do many of the upper level 8" and 9" projectors should handle a 99 MHz bandwidth just fine, with room to spare.


The other issue is nobody really entirely agrees on how to compute bandwidth for a signal. Its not like horizontal scan rate, which everyone agrees on how to compute properly. I've seen at least 3 different methods for bandwidth already, and I haven't searched very hard.


I would also try 48 Hz. The fiicker really isn't visible during film and video sources. I can easily see (and am very bothered by) 60 Hz on a computer monitor. But 48 Hz isn't even visible to me at 2 feet from my screen, let alone at my seating position. Plus, at 48 Hz the projector has less stress on it (then compared to 72 Hz).


The Barco 808 is a pretty quiet machine in my opinion, and many other here share that opinion. I have nothing to compare it to however. :D Actually, the HTPC driving my 808 is louder than the 808. I can sit right next to my 808 and not really notice it very much. During a quiet scene you do hear a low fan sound, but not enough that I've ever had to turn up the volume for very quiet parts of a movie.


A ceiling mounted 808 would be good without a hushbox, but I'm sure others will chime in with their much more extensive knowledge.
 

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For HDTV, wouldn't you only need 60Hz, not 72?
 

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Discussion Starter · #12 ·
Thanks again to everybody.


Briands, I don't know. I know normal TV is 60. I've never had the pleasure of experiencing an HDTV display.


Any other suggestions on projectors that can achieve a 100MHz scanning rate and that is quiet?
 

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1440 x 960 x 72hz x 1.15 = 114.5 Mhz minimum

NEC XG-110 = 135Mhz

Sony G70Q = 120 Mhz

Barco Graphics 808 = 120Mhz

all with 8" tubes
 

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Of course, if you use a more superior algorithm than 3:2 pulldown, what you end up with is this:

Code:
Code:
Reverse Telecine
-------------------------
[1]     72      72      72      72      72      72      72      72      72      
[2]     24      |       |       24      |       |       24      |       |       
        |       |       |       |       |       |       |       |       |       
[3]     A       |       |       B       |       |       C       |       |       
[4]     A       A       A       B       B       B       C       C       C
Die 3:2 pulldown die!
 

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Most likely. An 808 is pretty quiet. Don't know about the other two, but I know the 808 is rated pretty well as being "quiet".
 
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