[Solved] CRT TVs

Thread Starter

biferi

Joined Apr 14, 2017
162
I know this is Old Technology but I wanted to know something?

In the USA the CRT TV would Draw 29.97 Frames Per Second.
But ever Frame was Made of 2. Fields a Top Field and a Bottom Field.

So the CRT TV would Draw every Bottom Fields First.
After all the Bottom Fields are Drawn they would start to Fade.

As the Bottom Fields would start to Fade then the CRT TV would Draw all the Top Fields.

So this way the Top Field and Bottom Field would be Interlaced.

Do I have this Right or am I Off on some things?
 

MrChips

Joined Oct 2, 2009
24,233
I would not call it Top Field and Bottom Field.
I would call it Interlaced Scan, or Odd Lines and Even Lines.
In NTSC format there are 525 horizontal lines drawn.
262 lines are first drawn to fill the entire screen in 16.6ms.
In the next 16.6ms the remaining 263 lines are drawn in between the first set, i.e. interlaced.
 

Dodgydave

Joined Jun 22, 2012
10,050
Three systems were, NTSC,( never twice the same colour, lol) PAL, SECAM, ..

These used Interlaced luminane scanning, with separate sideband for the colour signal all modulated together.
 

MisterBill2

Joined Jan 23, 2018
9,305
Post #3 from MrChips is exactly right. The reasoning behind it goes back to te early days of TV, but yes, odd and even fields.just as described.
 

crutschow

Joined Mar 14, 2008
27,755
The reason for interlacing the picture into two fields was, of course, to save bandwidth while minimizing flicker.
Both require the same bandwidth, but a 25-30Hz frame-rate progressive scan picture would have noticeable flicker, whereas a 25-30Hz frame rate picture generated by a 50-60Hz interlace scan has only a small amount of noticeable flicker.
 

DickCappels

Joined Aug 21, 2008
7,743
In case it is not apparent, in the interlaced scan the odd-numbered lines were drawn in the first field then on the next field the even-numbered lines were drawn. Each filed consisted of 262.5 lines. It took me quite a while to understand how the half-lines were made.
 

MrChips

Joined Oct 2, 2009
24,233
In case it is not apparent, in the interlaced scan the odd-numbered lines were drawn in the first field then on the next field the even-numbered lines were drawn. Each filed consisted of 262.5 lines. It took me quite a while to understand how the half-lines were made.
Yes, I intentionally left out the details about the vertical flyback.

1627521253639.png
 

MisterBill2

Joined Jan 23, 2018
9,305
The same video format was also used, and still is used, for NTSC video. That included quite a few solid state TVs, and a lot of flat screen ones as well.
 

ronsimpson

Joined Oct 7, 2019
1,474
to save bandwidth
Certainly true.

Many decades ago, when I was young, I worked with several engineers that worked on the first TVs. I was pretty proud when I built a computer monitor that ran at a horizontal rate 2x that of TV. Then 3x and by the time I was going 4x I complained about how slow 15750hz is. Carol told me that when they picked 15750 for the horizontal rate they could only make 9 to 10khz with out milting the glass in the tubes. By the time the committee and government agreed to 15750 "I hoped we could make it work". He said in that 6 months they just got there.
There is lots of math in the TV frequencies system. Part of the numbers are based on, we can't do it now, but soon.

RonS.
 

MisterBill2

Joined Jan 23, 2018
9,305
Magnetic deflection of the CRT electron beam was certainly complex, and then for shadow mask CRTs the convergence adjustments were amazing. The fact that color sets could be mass produced tells us how good the engineering was.
 

Ramussons

Joined May 3, 2013
1,062
Magnetic deflection of the CRT electron beam was certainly complex, and then for shadow mask CRTs the convergence adjustments were amazing. The fact that color sets could be mass produced tells us how good the engineering was.
True. Those were the times of original creativity.
 

DickCappels

Joined Aug 21, 2008
7,743
In the late 1970’s I was given the project of making B&W monitors run at 100 kHz horizontal scan frequency. Somebody else modified a line of cameras to run the same line rate. They were bought by Los Alamos labs along with a film camera to record underground A-bomb tests. The image was still racing up the cable after the camera melted. We never had a warranty return from them :)
 

Lo_volt

Joined Apr 3, 2014
219
I recall in the late '80s that High Definition TV was in its concept stages. VGA monitors had become standard at 800x600 screen resolution and specialty monitors could more than double that. At the time, though, the big question was how to bring higher resolutions to an already installed base of CRT TVs that certainly couldn't handle the higher resolutions and the requisite bandwidth needed. The problem sort of solved itself with the move to higher resolution LCD screens.

It's always cool, too, to see movies made before 2000 that show futuristic TVs and monitors as CRTs.
 

MisterBill2

Joined Jan 23, 2018
9,305
The big deal with the higher resolution was that in one move it made every single TV obsolete. Prior to that every set post WW2 was still able to display all broadcast video. So the change certainly boosted the TV set market a whole lot. The rather disturbing thing is that the newer sets do not seem to be any more reliable than the old tube-type TV sets, despite no longer having those high power deflection circuits.
 

Yaakov

Joined Jan 27, 2019
3,521
I recall in the late '80s that High Definition TV was in its concept stages. VGA monitors had become standard at 800x600 screen resolution and specialty monitors could more than double that. At the time, though, the big question was how to bring higher resolutions to an already installed base of CRT TVs that certainly couldn't handle the higher resolutions and the requisite bandwidth needed. The problem sort of solved itself with the move to higher resolution LCD screens.

It's always cool, too, to see movies made before 2000 that show futuristic TVs and monitors as CRTs.
I enjoy seeing the classic sci-fi “viewscreens” which were flat panels. I don’t think the authors had any idea what the technology would be, but they had the idea it wouldn’t be a CRT, and that it would be flat.
 
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