1. Join Now

    AVForums.com uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Phosphor decay times?

Discussion in 'Projectors, Screens & Video Processors' started by JohnAd, Dec 10, 2001.

  1. JohnAd

    JohnAd
    Active Member

    Joined:
    Mar 14, 2001
    Messages:
    362
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Location:
    London
    Ratings:
    +4
    CRT heads, one more question in my search for enlightenment.

    This one has been triggered by the AVS thead on 48Hz with CRT's

    http://www.avsforum.com/avs-vb/showthread.php?s=&threadid=97975

    What is the Phosphor decay time (half life or however it is quoted) for various CRT devices and how does this effect the best refresh rate to watch at.

    Looking at a standard interlaced TV you can sometimes see the effect of the long decay time when white objects move across a black background. Presumably these devices need slow phosphors to help mask interlacing. On computer monitors the phoshors seem very fast and need to be refreshed much more often to get stable picture.

    Since it seems that slow phosphors give out more light there must be a tradeoff.

    Where do the different CRT projectors sit on this spectrum and is there a difference between the video ones and the data ones in terms of phosphor speed.

    Also what does all this mean for people like me wanting to avoid 60Hz. From Rolands answer to my question about scanning he says you want the scaline to start with clean and fresh phosphors on each scan line, presumably this is impossible but you want it to be below say 25% of previous brightness before you hit it again.

    John
     
  2. Gordon @ Convergent AV

    Gordon @ Convergent AV
    Distinguished Member AVForums Sponsor

    Joined:
    Jul 13, 2000
    Messages:
    14,013
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    166
    Location:
    Living in Surrey, covering UK!
    Ratings:
    +2,809
    John,

    There is a trade off.....You will notice on the forums that the SonyG90 gets slagged for not having an accurate enough colour gammut for film replay. One of the reasons is they go for MORE light rather than correct colour. They do this by using a non-standard green phosphor. (I don't want to get in to a debate about whether you will notice this anomoly I only mention it to explain Johns point)

    Those of you with crt's can try this trick to see what John is talking about. Put up a cross hatch pattern on your crt, leave it for a minute then turn off the projector. Look in to the tubes as soon as it is off and you'll see the green tube will almost definately still have a cross hatch on it's surface as Green phosphor seems to have the longest decay time.

    I suspect, although I have no way of confirming this, that the G90 has a green phosphor with a longer decay than standard.

    I've no idea where you would get this info..sorry!

    Gordon
     
  3. JohnAd

    JohnAd
    Active Member

    Joined:
    Mar 14, 2001
    Messages:
    362
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Location:
    London
    Ratings:
    +4
    Gordon

    It hadn't even occured to me that the different colors might have different properties :(

    Here's what I think might be happenining anyway

    Imagine a signal coming in with intenisty 0 being replace by an intensity of 5 then back to 0.

    Code:
    
    Input @ 120 Hz     0   0   5   5   5   5   0   0  
    Residual @ 120 Hz  0 0 0 0 0 3 1 3 2 3 2 3 2 1 0
    (calculated from what you see line)
    
    What you See       0 0 0 0 5 3 6 3 7 1 7 3 2 1 0  
             
    
    
    Input @ 60 Hz      0       5       5       0           
    Residual @ 60 Hz   0 0 0 0 0 3 1 0 0 3 1 0 0 0 0
    (calculated from what you see line)
    
    What you See       0 0 0 0 5 3 1 0 5 3 1 0 0 0 0
    
    Here we have a imaginary deive with a decay time which is just a bit too fast for 60Hz and too slow for 120Hz


    John
     
  4. Gordon @ Convergent AV

    Gordon @ Convergent AV
    Distinguished Member AVForums Sponsor

    Joined:
    Jul 13, 2000
    Messages:
    14,013
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    166
    Location:
    Living in Surrey, covering UK!
    Ratings:
    +2,809
    Ever seen a Blue or Red oscilloscope? I think they use green phosphor with a slow decay intentionally in these things.....

    Over at AVS there's a thread about 48Hz v 72Hz and the Terranex and HTPC's. Chris Stevens (Terranex fan and crt modder) favours 48Hz over 72 citing it gives more black level detail and greater percieved dynamic range.

    I was at another forum members last night looking at HTPC and one of the very last things we did was a very (and I mean very..it was early morning) short test of 48 v 72. We didn't look at judder to much as timings need to be set more accurately to fix that one but we did look at detail and colour fidelity. The test was inconclusive. There may have been something to it. Interestingly enough the desktop was VERY flickery at 48Hz but the picture from film was very watchable and you wouldn't notice flicker, at least not the small clips we watched.

    I have no doubt that decay is different for every tube and probably also varies with how well G2 is set etc.. It's a tricky business this CRT malarkey!!!hahaha but it's SO worth it when you see the image attainable.

    Gordon
     
  5. Roland @ B4

    Roland @ B4
    Active Member

    Joined:
    Apr 11, 2001
    Messages:
    2,238
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    51
    Location:
    Reading.
    Ratings:
    +32
    Every colour and every type of tube has a different decay time.

    There are several types of phosphors specified for projection. they are in fact very similar to those used in TV's.

    Then these are special long persistant phosphoers as used in radar and Ossiloscopes and very low resolotion monitors.

    Fast phosphers are used for 3 dimensional work and stereo scopic work where the vertical frequency is high and you need the image to decay quickly.

    The phosper used in a 7" tube glows longer than an 8" EMF . A 9" EMF is even shorter so you need to excite it more by upping the scan rate.

    Like all glow in the dark toys. The longer you shine light it it the more it will glow.
     
  6. uncle eric

    uncle eric
    Guest

    Products Owned:
    0
    Products Wanted:
    0
    Ratings:
    +0
    John,
    Not only that the decay time also varies with age, the tubes not yours. Many other variables some that have already been mentioned by Gordon and Roland, that add up to something like, picking this weeks lottery numbers.
    An interesting point several made in the AVS teranex thread was not the superiority of CRT v Single lense device but the actual margin of that superiority. It was apparently huge.

    Eric
     
  7. JohnAd

    JohnAd
    Active Member

    Joined:
    Mar 14, 2001
    Messages:
    362
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Location:
    London
    Ratings:
    +4
    Gordon

    My take on Chris Steven's take on 48 is mostly beacuse the Teranex doesn't do 72Hz so it's the only rate that does smooth film from that device.

    You're right that this CRT stuff gets more complicated the more you learn. But it's better than work :)


    Roland

    Thanks for the great info, So the smaller tubes have to use more effecient but slower phosphors to make the picture brighter, that makes sense. So if the 7 inchers have the slowest phosphors they should look better at lower frequencies than the big guns and they are probably not worth driving too hard.

    Would you expect to see much flicker on a 7 inch driven at standard 50Hz PAL?

    Sounds like you might on an 9 inch pj which is why the whole 48Hz thing on a 9 inch sounds weird.

    Eric

    I think this may be the one area where the display and hold devices do win, they don't have this decay problem and motion can appear more crisp on them compared to a CRT (this is an observation based on monitors rather than pjs)

    John
     
  8. Roland @ B4

    Roland @ B4
    Active Member

    Joined:
    Apr 11, 2001
    Messages:
    2,238
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    51
    Location:
    Reading.
    Ratings:
    +32
    Certainly better than propper work.

    the electrostatic tubes ie 708,Cine 7, Barco 801, Sony 1271 etc all have a better tollerance of flicker at the standard refresh rates. I don't think I would go as far as to say they were flicker free.

    the higher you go in projector specifications the more it become noticable as well.

    Don't forget you have a retrace time as well which is how long it takes for the line to blank and go back and start the next line. On the bigger projectors this can be adjustable.

    The projection boys at Bafta prefered a slower refresh rate because the image was more like film with the induse flicker.
     
  9. JohnAd

    JohnAd
    Active Member

    Joined:
    Mar 14, 2001
    Messages:
    362
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Location:
    London
    Ratings:
    +4
    Roland

    Looks like my plan of working out the perfect rates to have set up and then leave things alone looks to have backfired somewhat :(

    It now seems that I'll be running out of projector memories trying to fit in all the different rates I'll want to play with.

    Oh well, that was inevitable I suppose.

    The retrace time you mentioned reminded me of the problems setting up Kryton with 72Hz at the event. What restructions are there on the timings that a BD708 will sync happily to, presumambly there is a minumum retrace time and a maximum scanning rate is there anything else that will catch me out?

    Cheers

    John
     
  10. Roland @ B4

    Roland @ B4
    Active Member

    Joined:
    Apr 11, 2001
    Messages:
    2,238
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    51
    Location:
    Reading.
    Ratings:
    +32
    From Memeory The Kryton was not a limitation on the retrace of the PJ.

    There comes a problem with what the projector will do and what it will resolve. I know the problems you have regards size of an 808.

    But for the record these are the absolute maximums (i guess some are minimums too) for the BarcoData 708

    Horzontal scan Rate 15 to 69Khz
    Vertical refresh rate 37 to 210Hz

    Min retrace time Horz 3.3 microseconds
    Min Retrace Vertical 450 microseconds

    The avove assumes that the projector is in exactly the correct possition ie 10.5 degress to screen centre and the correct distance. Any different an the above will be reduce significantly.

    the last drawing I sent you gave an extra 3.5 degrees of tilt. do not expect the above performance.
     
  11. JohnAd

    JohnAd
    Active Member

    Joined:
    Mar 14, 2001
    Messages:
    362
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Location:
    London
    Ratings:
    +4
    Roland

    Ta, I'll be off playing with PowerStrip then.

    John
     
  12. Nic Rhodes

    Nic Rhodes
    Well-known Member

    Joined:
    Mar 23, 2001
    Messages:
    17,133
    Products Owned:
    0
    Products Wanted:
    1
    Trophy Points:
    133
    Location:
    Cumbria
    Ratings:
    +1,277
    Has anyone tried 48 / 50hz over here guys?

    Gordon, how are you and the Rock plus getting on this weekend?
     
  13. Gordon @ Convergent AV

    Gordon @ Convergent AV
    Distinguished Member AVForums Sponsor

    Joined:
    Jul 13, 2000
    Messages:
    14,013
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    166
    Location:
    Living in Surrey, covering UK!
    Ratings:
    +2,809
    Nic,

    I had a bit of anightmare with the Rock the first night I had it. It's a long story that I'll write out later..... I have tried 48Hz....I guess you noticed my post about that at AVS.

    I've now set up 72Hz. Been out crimbo shopping but will be watching Corry and Buffy tonight through it then some DVD's tomorrow. Region1 and 2. Don't know if I'm going to get enough time to do direct 72 v 48 dems. More concerned with checking out functionality and robustness of Rock. I know it's performance is pretty great already.

    Expect post about it from me soon.

    Gordon,
     
  14. JohnAd

    JohnAd
    Active Member

    Joined:
    Mar 14, 2001
    Messages:
    362
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Location:
    London
    Ratings:
    +4
    Did have a go with setting up 48Hz just to see if I could get rid of the judder at all. I found that on a CRT monitor the colours did seem slightly richer but the flickering was awful and motion wasn't anywhere near as smooth as at 72Hz.

    I also played around breifly with using 1440 width and found that I did prefer it, this width seemed to reduce the effect of mosqito noise but it was all pretty marginal.

    John
     

Share This Page

Loading...