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A problem with my calcs

Discussion in 'Projectors, Screens & Video Processors' started by Nic Rhodes, Oct 14, 2001.

  1. Nic Rhodes

    Nic Rhodes
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    Someone over at AVS posted saying they were displaying 1272 at 1600 x 900 @ 72Hz on a Sony 1272. When I questioned how they managed to achieve that no one was able to give me a complete answer. Can some of our local experts have a look at what I was saying and show me my error please. It is driving me nuts.

    Well this is how I see it. The Sony 1272 is a capable and high spec 7 inch unit. 700 peak Lumens, max 93kHz horizontal scan frequency, 150 Hz vertical scan frequency, resolution 1500 x 1200 and an RGB Bandwidth of 85Mhz.

    It was it's ability to show such a high resolution signal on such a device. I suggested that this 85MHz projector wouldn't be capable of showing a 156Mhz signal. My thinking:

    A useful link on Bandwidth calculations is.
    http://www.extron.com/technology/archive.asp?id=vidband

    To calculate Bandwidth, the following equation is usually used.

    SF = [(TP x Vt)/2] x 3

    Where:

    SF = Bandwidth
    TP = The total number of displayable pixels. (When resolution is listed (1600 x 900), multiply these two numbers.)
    Vt = The vertical scanning frequency, or refresh rate.

    If we calculate 1600 x 900 @ 72Hz
    TP = (1600 x 900) or 1440000
    Vt = 72 Hz

    Therefore:

    SF = [(14400000 x 72)/2] x 3
    SF / Bandwidth = 156 MHz

    The vertical refresh rate is well with the 150 Hz max at 72 Hz so no problem there. The horizontal refresh rate is well within the 93/4 Khz max as well at 900 x 72 = 65Khz. The max resolution is just about okay. The problem occurs when you put all these together for the Bandwidth of the 1272. Bandwidth is different from refresh rates. The Sony unit has a high quoted 85 MHz figure but that is still miles of from my calculated 156 MHz for 1600 x 900 @ 72Hz. I would expect some leeway in the quote Sony figure but any signal way beyond the bandwidth of the projector will either not be displayed at all or will severely degraded in quality.

    A projector will have a certain max bandwidth. This I think will be dictated by tubes, electronics etc. This will be quoted as RGB Bandwidth. I think this is measured by the manufacturer and quoted in the spec, a few examples I have dug up

    Sony 1272 85 MHz
    Sony G70 120 MHz
    Sony G90 135 MHz
    NEC 9PG / 10PG 100 MHz
    NEC 110 120 MHz
    NEC 135 150 MHz
    Barco Cine7 75 MHz
    Barco Cine8 120 MHz
    Barco Cinemax 180 MHz

    Any given signal will need a certain bandwidth in order to be transmitted without serious loss. These increases, depending on the resolution used and refresh rate used. For normal TV / DVD then the bandwidth is relatively low. Easily accomplished in a 20 to 50 MHz device.

    Take a DVD resolutions

    720 x 480i (240) 16 MHz (normal interlaced DVD)
    720 x 480p 31 MHz (progressive scan / line doubled DVD)
    720 x 720p 47 MHz (line tripled DVD)
    720 x 960p 62 MHz (line quadrupled DVD)

    These are well within the capability of many projectors (all at USA standard 60 Hz refresh). Therefore the good basic line doubled 7 inch CRT only needs 31 MHz for Line doubling DVD.

    However with ‘computer' resolutions these numbers increase rapidly as both resolution and refresh rates escalate.

    If the required resolution is below the max quoted, the horizontal and vertical are also below the max AND the bandwidth is below the RGB Max bandwidth then everything should be okay, however 1600x900 at 72Hz gives is a big bandwidth number. In fact only the mighty Barco Cinemax has a high enough resolution from the figures quoted above. (Surely a G90 should be able to do this?).

    No device works well at any of these maximums anyway. There will be an optimum according to the electronic and tubes. The sweet spot of the projector. The scan lines need to touch but not overlap. Accuracy of focus is important here. A high resolution like the Sony 1272 and NEC 9PG will show scan lines if just run at line doubled. The NEC is probably best at something like 720 / 768, close to line tripled. Line tripled at DVD resolution at 60 Hz gives 47 MHz signal at about 47 kHz horizontal frequency. This is where the 9PG main board is working at it's optimum. This is half the potential bandwidth of the unit and half the max horizontal frequency. The vertical refresh rate and the resolution are also well within the maximum obtainable from the 9PG. It is the optimum for the unit however and where it will give the best picture.

    The situation with the 1272 is likely to be similar or a bit lower again as it is a slightly lower spec projector. Hence my suggestion of running at 600 vertical resolution.

    I think my calculation on the bandwidth of the signal seems fine. I have taken the bandwidth of the projectors from the manuals. However other have calculated projector bandwidths by using all the max values and come up with a 405MHz bandwidth for the Sony 1272. This just seems wrong to me. Am I missing some simple here?

    Is my thinking wrong on this one?

    [ 14-10-2001: Message edited by: Nic Rhodes ]
     
  2. Zone

    Zone
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    Jesus Nic,just read your post and I've given myself a headache,glad I've just got a TV.

    Si [​IMG]
     
  3. Orbital

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    Errr yes....yes I can confirm... my head has just exploded!!!

    RIP

    Dave

    P.S. Could you put a few more numbers in your next post Nic :D
     
  4. Nic Rhodes

    Nic Rhodes
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    Think yourself lucky guys, I gave you the edited version. :D :D

    I know I am doing something wrong but just can't work out what.
     
  5. Nic Rhodes

    Nic Rhodes
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    PS Do I get this years award for the most numbers in a post and most boring post?

    :)
     
  6. tryingtimes

    tryingtimes
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    Firstly, I'm not one of the aforementioned experts, but from all I have learnt so far your calculations seem spot on to me.

    All the line doubled/tripled/quad figures seem correct.

    I know this doesn't help you but I would just assume that the guy wasn't really managing to do it.

    Even the guys with cinemax/1209 projectors find the sweet spot to be somewhere below this resolution.

    I'm sure someone more knowledgable will clear this up for us!
    Alex
     
  7. Zone

    Zone
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    More knowledgable!!! Thank God, that rules me out, just dying to know the answer though, I'm holding my breath, please someone, GIVE US THE ANSWER GODDAMMIT!!

    Si :D
     
  8. squid

    squid
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  9. Roland @ B4

    Roland @ B4
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    Nic

    Band width is not a 'hard wall', of work or won't work.
    It is a measurement of the ability, of a piece of equipment, to pass information.
    The Extron site does a good explanation.
    Sadly most manufactures quote their bandwidth figures across the bit of equipment that shows them off best (usually the RGB amplifiers).

    The sorts off computer resolutions you have mentioned here for a home cinema situation are going to cause display problems and the vast majority are over kill.

    I'm going to try to use an analagergy which I hope explains this better.
    You have mentioned three of the things that govern a pj's ability to draw this information.
    1. Horizontal resolution
    2. Vertical resolution
    3. Refresh rate


    1. Horizontal resolution matters very little. Your PJ is scanning a spot of electrons from left to right. When it gets to the far right it switches off and sweeps back to the left to draw the next line. As it scans across, the spot is switching on/off and anything in between. The more pixels it has the more transitions are required. Horizontal pixels are not round, as they should be, but dashes.

    2. Vertical resolution for CRT's is more important. It forms out perceived level of sharpness (scan lines). The single most important bit is not to get them to overlap (blooming).
    Often left out on this subject is the aspect ratio. In your Sony 1271 question, 900 lines is possible but only in 4x3. In 16x9the vertical height is CRUSHED not cropped so the lines tend to overlap on a 7" tube at about 700 lines.

    3. Refresh rate. The more pixels either horizontal or vertical you use, the faster a projector has to react. Add to this a desired refresh rate (i.e. the number of times per second you want the whole screen refreshed) and everything has to work faster.

    I look at it this way.
    Take a sheet of absorbent paper like blotting paper, and a really fresh inky felt tip pen. Draw a line from left to right. if you draw it too quickly you won't leave enough ink (light) to make a mark. Too slow and it will start to blotch (bleed).
    Vertical resolution is the number of lines you can draw without the lines touching.

    After a few practices an ideal number of horizontal lines which we could draw without overlap would come out.
    This is our sweet spot.
    We have just drawn the equivalent of a white screen.

    To add the picture information as we draw our lines, we have to lift the pen and press in differing levels to add horizontal pixels.
    The speed at which the pen travels is set by the refresh rate (we have to draw all the lines and get back to the start in a certain time) At lower resolutions we only have to draw a low number of dots as we travel across the paper, lifting the pen is easy.

    Let us take this fantasy one more stage and rather than lift the pen we switch the ink on and off. (bit like a beam of electrons). As the pen travels we create dots, up the horizontal resolution and more dots have to be created. Speed every thing up with a higher refresh rate and after a point it gets impossible to switch the ink on and off quickly enough and the dots start joining up

    THIS........ IS BANDWIDTH.......... THE ABILITY TO SWITCH ON AND OFF AT A GIVEN RATE.

    So back to your question "1600 x 900 @ 72Hz on a Sony 1272."
    Yes physically the projector will display it.
    But if this was a 16x9 screen then it would look very very soft.
    The vertical lines would overlap,
    The projector is working too fast to display the horizontal resolution.

    Oh and by the way all the cables, plugs and other equipment would have to be at least as good as the bandwidth of the projector.

    (I know lets use RCA connectors for video that should bugger everything up!)
     
  10. Nic Rhodes

    Nic Rhodes
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    Roland

    A brilliant explanation just what I was after. Thank you. I think this will provide much useful info for non CRT owners who are considering a CRT purchase as to what is important and what isn’t important.

    I think I had realized that Bandwidth isn’t a brick wall hence my comment

    “I would expect some leeway in the quote Sony figure but any signal way beyond the bandwidth of the projector will either not be displayed at all or will severely degraded in quality. “

    I wasn’t sure whether there was a stage that the signal was so poor that the projector refused to lock on, like say a SPDIF digital signal. I see so many people claiming they run their projectors at these mega resoltion I was questioning my understanding of the whole situation. You seem to indicate I am basically on the right lines re my thinking.

    I find your explanation of the Horizontal resolution, Vertical resolution and Refresh rate very informative.

    I viewed Horizontal resolution as largely irrelevant and therefore planned to run my projector at line tripling, following advice from Gordon.

    Re Vertical resolution we picked the optimum for my projector (720 / 768) rather than the highest possible.

    Refresh rate was dictated by the signal in my case, whether PAL, NTSC or HCPC.

    My thinking seems to have been correct.

    I take it Projector manufacturers measure this RGB bandwidth rather than calculate anything?

    It does beg the question why so many people run the projectors at these ‘silly’ resolutions. Is it ignorance on their part?

    Again, many thanks Roland, I can rest now
    :)
     
  11. Orbital

    Orbital
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    Cheers roland, even to a CRT noobie like myself your explanantion made complete sense.

    Cheers

    Dave :)
     
  12. Zone

    Zone
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    Roland,I have no experience what so ever with Projectors/CRT and everything that goes with them and it even made sense to me!!

    Si

    :D
     
  13. Roland @ B4

    Roland @ B4
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    Measure! no, that would mean it has to do it. No most of them take the lowest value component and quote that. Band width is almost imposlible to truley measure.

    For some reason us chaps seem to have to have the longest, biggest, fastest, longest lasting than anyone else.

    Mines a 9 incher and even my wife agrees that its better than a 7 inch ;)

    [ 16-10-2001: Message edited by: Roland@B4 ]
     
  14. Paul D

    Paul D
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    Mine is only 4 inches :(
    .
    .
    .
    .
    .
    .
    .
    But not all women like them that WIDE!!! :D
     
  15. Guest

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    I dont understand people that want to expand a picture way beyond the native resolution of the source material-you just wind up with a very soft picture.
    I must express puzzlement with Powerdvd and why it wants to switch resolution on dvd viewing to 1024 X 768?
    Surely 800 X 600(4;3) would be the best resolution for Pal?
    Then svga panels in lcd/dlp projectors are enough-why would you need to go for xga resolutions?
    Surely a progressive scan output from p.c.at 800 X 600 from a dvd source would be optimum for any projector with that as native(..at a certain distance..)-Is this the best scenario?
    Doug
     
  16. squid

    squid
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    no not realy . my sweet spot is around 1024x768 in 4:3 . mine is only a 7inch crt get a pj with 8 or 9inch crts and you need a res a lot higher than this or you get scan lines .

    to feed some of the big boys 800x600 just would not do it justice at all if you ask me.

    you just need high res for high res pjs if you want to get the best out of them .

    if you are talking about lcd dlp then a great deal of them are probaly best at 800x600 . crt is just a diferent beast and you need to get the sweet spot of the tubes to see them in there best light . it's only when you go too far and push the pj beyond this that you get a soft pic .

    i do think it a little silly quoting resolutions that it is imposable for the tubes to resolve . some pjs you pay more for becouse the electronics car accept higher band signals but have the same tubes as the lower res pj. why bother if the tubes can't resolve the higher res pic anyway
     

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