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Samsung CRT-HDTV!

MikeK

Novice Member
You'd also have to take account of where on the screen that ruler is sitting - it's very common for the dot/stripe pitch on TV sets to be larger near the edges than it is in the centre.

Dot-pitch does not refer to the inter-dot gap alone - it refers to the distance (usually measured diagonally) between the centre of one dot, and the centre of the same coloured dot in the adjacent triplet. Hence it includes BOTH the diameter of the dot itself AND the gap!
Strip pitch (for aperture grille CRTs) is similar, but is measured horizontally.
Depending on the screen, typically to compare a dot and stripe pitch, you'd take around 10% off the dot-pitch figure - so a .23mm stripe pitch would be about equal to a .25mm dot-pitch - roughly speaking!
 

Nick_UK

Banned
welwynnick said:
.......... So a little less shouting down of other people when you don't know what you are talking about, if you please! :nono:
Ahh, like last week, when you were completely convinced that plasma screens were scanned like CRT's ? (Until a few people put you straight, of course). :oops:
 

silkyandy x

Novice Member
Interesting discussion guys. My questions is if the crt tv can do 1080I shouldnt it be able to do 720p anyway? I thhought that CRT's did not have any fixed pixels and was capable of displaying pratically any resolution upto the maximum resolution?
 

Welwynnick

Distinguished Member
Nick_UK said:
Ahh, like last week, when you were completely convinced that plasma screens were scanned like CRT's ? (Until a few people put you straight, of course).
No. Not , not completely convinced, and not like CRTs. Just curious, and hoping to contribute to a discussion.

What I said was: "Well I distinctly recall reading a very authoritative source saying that they DO scan the frame progressively, a line at a time, just not a pixel at a time like CRTs."

And in my second post I had the good grace (!) to say that I had learned something new. I then asked how a plasma could control all of it's pixels independantly and simultaneously when every pixel in a row shares the same scan and sustain electrodes, and every pixel in a column shares the same address electrode. There were no specific replies, and I would genuinely still love to know the answer. Anyone?

Best regards, Nick :)
 

Welwynnick

Distinguished Member
silkyandy x said:
Interesting discussion guys. My questions is if the crt tv can do 1080I shouldnt it be able to do 720p anyway? I thhought that CRT's did not have any fixed pixels and was capable of displaying pratically any resolution upto the maximum resolution?
Not necessarily. Scan lines can go anywhere they like with a CRT (unlike digital displays), but there are other constraints on what a CRT can display: screen resolution, input bandwidth, vertical refresh rate and horizontal refresh rate. The latter is a restriction for 720p, because the line scanning frequency is relatively high - approx 45kHz: around half again as high as 480p or 1080i. That's a bit more expensive to engineer properly, so manufacturers avoid it if they can get away with it. That's unfortunate, because cross-conversion from 720p to 1080i doesn't usually work very well. The newer Samsungs appear to accept and display 720p properly, which is quite a big step forwards.

Nick
 

cerebros

Active Member
silkyandy x said:
Interesting discussion guys. My questions is if the crt tv can do 1080I shouldnt it be able to do 720p anyway? I thhought that CRT's did not have any fixed pixels and was capable of displaying pratically any resolution upto the maximum resolution?
Unfortunately the resolution that can be drawn will depend on the scanning frequency the set electronics support.

Based on 50Hz refresh, 1080i requires a frequency of 27KHz (540 lines x 50 fields), whereas 720p requires a frequency of 36KHz (720 lines x 50 frames). So just becuase a set can draw 1080i, doesn't mean its electronics support the higher scan rate required for 720p
 

Welwynnick

Distinguished Member
Bang on.
But the frequencies are a bit higher than that because of the lines that are broadcast or stored, but not displayed - teletext, frame sync, etc.
 

Stephen Neal

Well-known Member
cerebros said:
Unfortunately the resolution that can be drawn will depend on the scanning frequency the set electronics support.

Based on 50Hz refresh, 1080i requires a frequency of 27KHz (540 lines x 50 fields), whereas 720p requires a frequency of 36KHz (720 lines x 50 frames). So just becuase a set can draw 1080i, doesn't mean its electronics support the higher scan rate required for 720p
Correct logic - wrong maths.

CRT TVs actually operate at the full, not just the active, line rate. Therefore a standard def TV set runs at 625 lines, not 576, so the scanning rate is 625x25, or 312.5x50.

For 1080i, the total number of lines is 1125 (remember that Japanese standard?!) and for 720p the total number of lines is 750.

The numbers are thus (ignoring 59.94 vs 60 in NTSC) :

480/60i = 525/60i = 262.5 x 60 = 15.75 kHz
480/60p = 525/60p = 525 x 60 = 31.5 kHz
1080/60i = 1125/60i = 562.5 x 60 = 33.75 kHz
720/60p = 750/60p = 750 x 60 = 45kHz

576/50i = 625/50i = 312.5 x 50 = 15.625 kHz
576/50p = 625/50p = 625 x 50 = 31.25 kHz
1080/50i = 1125/50i = 562.5 x 50 = 28.125 kHz
720/50p = 750/50p = 750 x 50 = 37.5 kHz

As you can see, 480/60i and 576/50i are very close - which is why it is easy to make TVs that can cope with both PAL and NTSC (or 480/60i and 576/50i component/RGB) signals.

Similarly 480/60p, 576/50p, 1080/60i and 1080/60p are all pretty close - between 28.125 kHz and 33.75 kHz (though 1080/50i is further away than the other three) so again it is easy to make displays that can cope with all of these signals natively. (480/60i and 576/50i are usually converted to either 480p/576p or 1080i prior to display)

720/50p and 720/60p are significantly different to 1080/50i and 1080/60p (as well as 480p and 576p) so it requires more engineering to create a display that copes with both natively - it is kind of like multi-sync vs fixed sync monitors.
 

Kalos Geros

Active Member
Stephen Neal said:
For 1080i, the total number of lines is 1125 (remember that Japanese standard?!) and for 720p the total number of lines is 750.
Everything you said is spot on except: I can't understand why for example 1080 mode needs additional lines (1125 cumulative) when it actually doesn't need to carry any additional analog information beside picture. Dvd players can do DA conversion but they also need NO extra data to be sent to the display. Or is it a left-over from times when they thought HD would initially be all-analog and additional data would be transmitted via the additional lines...????

About dot pitch: it is the distance between same-colored dots in two neighbouring RGB clusters...the dot is NOT each of R, G, or B elements, but a cluster made up of all three of them...
 

Stephen Neal

Well-known Member
Kalos Geros said:
Everything you said is spot on except: I can't understand why for example 1080 mode needs additional lines (1125 cumulative) when it actually doesn't need to carry any additional analog information beside picture.
You have to separate in your mind the digital storage requirements, using frame stores, from the analogue display systems that CRT displays are still based around.

The CRT is based around a flying spot travelling across a tube.

In the 625/50 video system each TV line lasts 64us. 52us of this time it is tracing active picture, but there is 12us between the end of one 52us active line and the start of the next. This is used in the video signal as a sync pulse (and for chroma burst in composite PAL 50) but it is also a requirement to allow the electron beam to quickly "fly back" from the right hand side of the screen to the left. The magnetic deflection coils and drive systems can't move the electron beam instantly, they need time to do this - and they have 12us to do it.

Similarly when tracing from the top of the screen to the bottom of the screen, they have to trace 287.5 (rounded to 288 these days) active lines in the field, and then the beam has to fly from the bottom of the screen to the top, and it is allowed 24.5 lines to do this.

Sure it would be possible to not deliver the signal from the source to the display using an 1125 or a 750 line standard, and in fact some DVI-D systems exploit this by reducing the amount of horizontal and vertical blanking to increase the proportion of active video carried. This is possible as DVI displays are less likely to be CRT based these days, and LCDs don't have the same flyback issues.

Given that we were discussing why the actual scanning systems that drive the electron beam deflection in a tube are easy to use for both 480p and 1080i but not for 720p - it was pertinent to mention the actual frequencies involved in the tube scanning.

Sure you could use a different line-scan rate for the video interconnects, and clock a 1080 line total signal into a 1080 line frame store, which you then clocked out as an 1125 line signal for CRT scan purposes - but why make life more difficult? Such interconnects would also mandate every CRT display contained a 1080i frame-store, whereas a 480p or 1080i only CRT doesn't currently.

Dvd players can do DA conversion but they also need NO extra data to be sent to the display. Or is it a left-over from times when they thought HD would initially be all-analog and additional data would be transmitted via the additional lines...????
CRTs still require time to fly back from right to left and from bottom to top...

DVDs still output 480i as 525i, and 576i as 625i on their analogue video outputs for this reason, and set top boxes etc. output 1080i as 1125i and 720p as 750p - at least in analogue component HD terms.

It is important to remember that the 1080/60i standard is only a slight tweak of the original Japanese HiVision 1125i/1030-1050i system that has been with us for well over 15 years. (The BBC shot a drama co-produced with NHK using it in 1989, and there is footage of the 1984 LA Olympics using it...) As such it was originally an entirely analogue system - from camera to VTR to TV display.
 

Nick_UK

Banned
An excellent description Stephen :smashin:

It's easy to spot those who gained their knowledge from years of practical experience, and those who merely repeat stuff they have read from text books and dubious articles on the internet.
 

Welwynnick

Distinguished Member
Nick_UK said:
An excellent description Stephen :smashin:
Absolutely. I have long thought Stephen to be the most authoritative contributor to this forum. Those posts ought to go in a sticky.

Do you remember, though, who was the only other person who was questioning the scanning method of plasmas, who said: "I'm not clear on how the sub-field pixels are illuminated. There is obviously a scanning process, as the system is still matrix based." and: "I would be interested to know exactly how the pixels are addressed and "fired", and also how the sub-fields operate."

I don't think anyone on the forum really knows the answer. It would be very good to find that out from someone who does.

Nick :confused:
 

Nick_UK

Banned
This seems to be the best "simple" description. Of course, any display with X and Y addressing is going to end up being addressed in a sequence which emulates CRT scanning. However, the big feature of neon discharge tubes which you are overlooking is that a neon discharge tube remains lit after being "struck" as long as a lower sustaining voltage is maintained. So the information to tell the pixel to light up or stay un-lit is passed to the pixel using a technique similar to scanning, but (unlike the CRT), pixels remain lit after being energised. You then apply PCM encoding to change the brightness of the pixels. It's explained very clearly in the article.
 

Stephen Neal

Well-known Member
Nick_UK said:
This seems to be the best "simple" description. Of course, any display with X and Y addressing is going to end up being addressed in a sequence which emulates CRT scanning. However, the big feature of neon discharge tubes which you are overlooking is that a neon discharge tube remains lit after being "struck" as long as a lower sustaining voltage is maintained. So the information to tell the pixel to light up or stay un-lit is passed to the pixel using a technique similar to scanning, but (unlike the CRT), pixels remain lit after being energised. You then apply PCM encoding to change the brightness of the pixels. It's explained very clearly in the article.
Will read that when I get a chance. The interesting feature of some frame-store based matrix displays is that you can split the display into smaller panels, and address/refresh them separately and simultaneously - whereas with conventional CRTs there is only a single/triple flying spot so you only scan/address one bit of the screen area (or the three phosphor dots) at a time.

This forum is great for kicking round ideas with other curious minds - none of us come close to knowing everything about anything - but it is always useful to share what we do know, or what our informed speculation on a subject is. I've certainly had some mistaken assumptions (often long-held) corrected politely by others - long may it continue.
 

cerebros

Active Member
welwynnick said:
Bang on.
But the frequencies are a bit higher than that because of the lines that are broadcast or stored, but not displayed - teletext, frame sync, etc.
yeah but i couldn't recall the figures off-hand and even the ones i gave give an idea of the differences involved (I also actually posted several minutes before you but it seems to have taken an age to upload to the site for some reason - I wouldn't have bothered posting if your post had been there when I came back to this thread)
 

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