50 HZ v75HZ

[008]

Has anyone done any direct comparisons between 50 and 75 hz ?
Id be interested to know if there are any trade offs.

[cosaw]

Yep I'd also like some more input on this. Suppose it could be tube specific at the end of the day. I used to run at 75 but as I hardly notice 50 hz flicker (in film that is) on my machine I switched down to 50 to give my pj an easier time. Some say 50 can look crisper.

Haven't really compared enough to tell the difference yet. Suppose the reasoning behind a crisper image at lower refresh can be related to what I seem to remember is called the "persistence" of the phosphor. The persistence is to do with the chemical properties of the phosphor - it controls how long the particular mix of phosphor continues to emit light after its been hit by an electron.

Presumably then low persistance describes a phosphor which dies (stops emitting light) quickly after being hit by an electron and high persistence the opposite (anyone with more knowledge feel free to correct me if I've got it back to front). Low persistence would be good for high refresh in order to stop succesive beam scans from blurring together at phospor level. Higher persistence would be better for low refresh as it would mean less apparent flicker. Presume they take this into account when building 100Hz tellys and 50Hz tellys.

Id be interested to know if there are any trade offs.

Thing is with a pj there is only one type of phosphor in your machine with a set persistence yet we display at differing frequencies. On this basis it would seem that 50Hz would have the pottential to be crisper all be it with more flicker and 75Hz (or anything higher than 50) would pottentially be less crisp but more flicker free. These then would be the "trade-offs". Suppose its finding the frequency suitable for your particular tubes.

A point to consider also which I've always wondered about in getting this balance - is it really neccesary to run at a factor of the frame rate - i.e. 25fps = 50hz or 75hz? I can see the logic behind it but does it hurt to run PAL at say something like 60 or 65Hz?

Simon

[008]

Ive done a little research on this and in general there are two types of phosphor, p22 (slow decay) and p43 (quick decay) P 43s are used in simulators but can be used in HT

I cant see any reason not to refresh PAL at 60/65 hz except that the timings of coincidence with a PAL signal will not be as frequent and may cause judder. It would be interesting to find out if anyone has experimented

Incedently, the Cine 9 uses LUG tubes, and these only come with a p43 phosphor on the green tube.

[cosaw]

Interesting.

BTW - what's LUG stand for?

The timing issue with PAL is the only one I could think of but wouldn't know if it makes a perceivable difference. Again an area where testing would need to be done.

About the phosphors you mention. If you could get hold of the decay times then you could set an optimum refresh. They must be floating around somewhere.

Si

[008]

No idea what LUG stands for but they are mainly used in Barcos whilst LCPs are used in Marquees. Apparantly the LUG isa superior tube. I spoke to the chap that makes them. LCPs use a p22 (slow) phosphor. But what you say is interesting that there must be an optimum refresh rate for different phosphors, hmmmm...

[welwynnick]

Originally Posted by cosaw

A point to consider also which I've always wondered about in getting this balance - is it really neccesary to run at a factor of the frame rate - i.e. 25fps = 50hz or 75hz? I can see the logic behind it but does it hurt to run PAL at say something like 60 or 65Hz?

Simon

I believe there is a great advantage in running a frame rate that is simply a multiple of the source. It is only necessary to repeat each fram once or twice to upconvert. For other frequencies, frame rate conversion (FRC) gets very messy and you end up with jerky motion.

I've never heard phosphor persistence being mentioned by those in the know as a factor in chosing frame rate - I simply don't think it's a factor. Probably more important is the scanning rate and bandwidth capabilities of your projector (and source). If you push it too high - close to the spec - you will progresively lose fine detail as video gain rolls off at the higher frequencies.

Nick

[Nic Rhodes]

Did try yesterday but the bandwidth kills most applications I think, okay for you Cine 9, G90 owners 'perhaps'. My biggest problem was the scaler and what it was outputting. Idea has some merit (3:3) but I think there are more advantages Genlocking signals or something like 48Hz

[008]

Nic
What exactly did you try out ?

[Jonesthegas]

I have a 1209s and run it at 72Hz for NTSC and 75Hz for PAL. Very stable image with genlock on my Lumagen.

If I reduce the frequency to 48 and 50 flicker is very noticeable for me. However, that apart, the image does look different. The colours seem more vibrant and has a slightly improved depth of image. If I could stand the flicker, I wouldn't hesitate to run at the lower frequencies (my Lumagen has been calibrated by Gordon so differences must just be down to the refresh rates).

BTW, PAL at 60 or 65Hz would be jerky. I've accidentally done 60Hz with my Lumagen and a PAL source and its not an option.

Martin

[Gordon @ Convergent AV]

Good work Martin. Not got the Cine9 yet? Thought you were upgrading!

Gordon

[Nic Rhodes]

I have been playing a bit with resolutions for my scaler supplier at the moment. Took basic output and tried 72 / 75. It shows some interesting results, however my baby CRT won't do the high freq with the resolutions I need (720 / 768p), or at least it is getting to the wrong side of the sweet spot. However there are a few issues with my scaler at these frequencies but HCPC wasn't greatly different. If I had a blank sheet of paper I would see if I could live with 48Hz first (many NTSC people struggle with this), else I would try an use a Genlock device (which mine isn't). 72/75 would be next on the list if I had a big CRT, particularly if I had Genlock. So foir now I am optimising 1366x768/50 before I jump one higher again

There does seem to be a solidity, brightness to the image which is enticing however but I feel I am limited by my equipment here.

[cosaw]

Seems like 60 or 65 or anything not a factor is bad for PAL on the evidence of others. Never put the theory into practice but may do sometime.

A couple of people with scalers mention "Genlock". I'm running via htpc and powerstrip. I sorta understand what a Genlocked pattern is when it comes to geometry and convergence setup but what do you guys mean by it? Seems something different.

Simon

[Gordon @ Convergent AV]

Go to the last question in this FAQ. It explains what genlock is

http://www.lumagen.com/support_faq.htm

[cosaw]

Thanks Gordon - will have a read.

Simon

[Jonesthegas]

Hi Gordon,

Roland is due to set a date for install in September. I'll get you round after that to re do the Lumagen. Should be special.

Martin