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post #1 of 19 Old 09-26-2001, 08:41 AM - Thread Starter
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the HDTV FAQ says analog tv is 480 lines so I had to rummage around to question this. I found this article but I still do not get 480 lines. Can some technical expert please explain. http://www.ee.washington.edu/consele.../ntsc/95x4.htm

plus I have never thought that visually it delivered this. I think VHS is 280 and SVHS is 480 but... what's going on.

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post #2 of 19 Old 09-26-2001, 08:52 AM - Thread Starter
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Ok, still confused. after reading this article it seams to say that the problem is Resolution. http://www.howstuffworks.com/dtv2.htm

Well 525 lines is more than 480 yet the picture still sucks.

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post #3 of 19 Old 09-26-2001, 10:02 AM
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NTSC has 525 interlaced scan lines. However, only 480 of those lines have picture data. Thus the maximum NTSC picture vertical resolution is 480 lines. Of course, if you are using a CRT to view the picture, you see less than 480 lines because of the "over scan", which means the top and bottom lines are not shown because they occur off of the CRT surface; so you can see picture on every square inch of tube surface, unlike a computer monitor.
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post #4 of 19 Old 09-26-2001, 10:41 AM
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yea
what ken said

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post #5 of 19 Old 09-26-2001, 02:33 PM
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Tryg,

There are the two different directions of resolution:
vertical (the number of visible scan lines mentioned by Ken and woowoo) and horizontal (the number quoted when people compare standard VHS with SuperVHS with DVD). Since the vertical resolution is essentially fixed, marketing folks only bother to compare the horizontal resolution.

Of course, with HDTV both vertical and horizontal resolutions vary. And if you use a scaler and appropriate display device you can change them both to whatever is best for your HT.


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post #6 of 19 Old 09-26-2001, 09:15 PM - Thread Starter
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and this...huh?


"Unlike the horizontal direction, the vertical direction of television picture is discrete and not continuous. Still, on the picture tube the lines overlap and are not normally perceived as separate. For this reason (and others, like the Kell factor, but I've digressed enough already from the original point of this document), the vertical resolution of TV is not such a big deal. If it were, people would have abandoned 525 line NTSC long time ago. As all of the analogue formats record picture lines totally independently of each other (with some exceptions, see below), there is no need to state the vertical resolution. It is always the same as in the video format itself -- 575 visible lines in PAL, 485 in NTSC."

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post #7 of 19 Old 09-27-2001, 06:16 AM
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A while back I did some number crunching on the <u>resolutions</u> measured by a test committee when they okayed the current ATSC <u>scan formats</u>. I converted their chart data to a more easily understood form, shown here . Notice that, for example, while the scan format for 1080i HDTV is 1080 <u>active</u> lines by 1920 pixels, the measured resolution is only 400 lines (moving test pattern) and 800 lines (static test pattern). The <u>measured</u> horizontal resolutions are also shown. If you took the sets used for these tests to some remote location where no HDTV signals could reach, they'd display a blank 1080-line raster (with CRTs) and perhaps a potential to display 1920 horizontal pixels per line. Filters in consumer sets reduce horizontal resolution by about 20%.

Similarly, a standard NTSC CRT set taken to a no-signal location would always display a ~480-line raster. The NTSC system is actually ~525 lines but only ~480 lines are visible (active). If you measure broadcast (6-Mhz-channel) NTSC horizontal resolution, you'll see about 330 lines per picture height, or 1.33 X 330 = ~440 lines across the width of a 4:3 screen. Vertical static resolution will be about 336 lines (a Kell factor of about 0.7 X 480). -- John



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post #8 of 19 Old 09-27-2001, 07:11 AM
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Quote:
Originally posted by John Mason:
If you measure broadcast (6-Mhz-channel) NTSC horizontal resolution, you'll see about 330 lines per picture height, or 1.33 X 330 = ~440 lines across the width of a 4:3 screen. Vertical static resolution will be about 336 lines (a Kell factor of about 0.7 X 480).
Does anyone know why broadcasters use a lower resolution than do the networks for their affiliate feeds? An issue that I have with some of the analog-to-digital upconversion going on is that they use the same lower resolution feed for both providing a less detailed image than if they used the 480 signal from satellite. I can see where this simplifies their operation to do the two together, but I have never been sure why they use the lower resolution at all. Is it a economics thing (i.e., hardware costs)? Historical?
Thanks.


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post #9 of 19 Old 09-27-2001, 03:10 PM
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NTSC has 525 horizontally oriented scanning lines, 483 of which are dedicated to the active picture area. Overscan in consumer TV sets chops off between 3 and 10 or so percent. The current state of the art in NTSC decoder technology limits horizontal resolution to about 336 lines for OTA or cable analog reception, and that's just for luminance (the black and white part of the signal.) Chroma resolution (color) doesn't fair nearly so well, but we can't see detail as well within highly chroma-saturated regions of the picture anyway. VHS tape is worse (about 250 horizontal lines of resolution) and DVD much better (up to 500 or so for luminance.) But for all standard-definition NTSC sources the numbr of horizontal scanning lines which contribute to vertical resolution is always 483. "480" is a shorthand approximation.

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post #10 of 19 Old 09-28-2001, 11:30 AM
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Hi Man E. Didn't take a stab at your query in hopes someone familiar with broadcasting practices might answer. That is, someone that knows the different signal paths from NTSC broadcasts versus ATSC 480p digital broadcasts. Could be the same program, of course, but from separate transmitters. The digital ATSC signal may be ~480 X ~700. (BTW, I use 480, as is standard generally here, and ~480 sometimes to indicate that is only the approximate number of horizontal scan lines.) Think you have to be cautious mixing NTSC and 480p ATSC discussion since they can have greatly different bandwidths, even though they both must fit within 6-MHz broadcast channels. The former is standard analog NTSC, while the latter is compressed digital (MPEG-2). I'd want to know if the signal used for the 480p ATSC is a very high bandwidth studio signal, perhaps with a horizontal resolution of 700+ full screen width.

On the 500 lines versus 480. Suspect you know that the 480 is the number of horizontal scan lines, and he's no doubt referring to horizontal resolution with the 500 figure. DVDs, hypothetically, are 480 X 720. But that 720 resolution is prefiltered to a lower number, and set filtering reduces it further. 500 might be typical. My Avia test disc on my HDTV displays higher resolution than that with test patterns.

P.S. I'm getting digital-cable movies via Time Warner here in NYC that I'm convinced are much higher horizontal resolution than OTA channels relayed by Time Warner. Saw a film last night, processed by the Genesis deinterlacer in my Philips 64PH9905, that easily matched the dazzle of some of the best DVDs I've seen. But haven't come across much technical discussion of what digital cable formats are delivering these days. -- John

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post #11 of 19 Old 09-28-2001, 09:02 PM
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Quote:
Originally posted by David McRoy:
The current state of the art in NTSC decoder technology limits horizontal resolution to about 336 lines for OTA or cable analog reception, and that's just for luminance (the black and white part of the signal.) Chroma resolution (color) doesn't fair nearly so well...
So are the broadcasters sending an OTA signal that is (or is nearly) identical to that which I would receive if I could lock their satellite transmission? Stated another way, the limitation is in a component of all consumer devices which is capable of much higher resolution which it obtains from non-tuner methods? I am, of course, referring only to analog here. If either of these questions is true, I'm curious why that was never exploited in a "premium" consumer analog tuner?

Quote:
...and DVD much better (up to 500 or so for luminance.)
But isn't that irrelevant since only 480 are usable within the guidelines of the NTSC standard? For instance, I have an NTSC RPTV that the marketeers presented as capable of 800 lines of resolution. It clearly does not double anything.

Quote:
But for all standard-definition NTSC sources the numbr of horizontal scanning lines which contribute to vertical resolution is always 483. "480" is a shorthand approximation.
Geek alert. Interesting comment. Why is it 483? 480 is divisible by 60Hz, but not 525. Is it because the power is actually just under 60Hz?

Thanks!


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post #12 of 19 Old 09-29-2001, 01:10 PM
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There was a ton of info listed above and it might confuse people so I am going to try to boil it down "for the layman."

NTSC:

As stated, it is composed of 525 veritcal scanning line of which approx. 480 are used to create the actual image. The other 45 are used to do "housekeeping" for the image. This 480 figure is called the VERTICAL RESOLUTION of NTSC. Using a 4x3 AR display with the entire AR being used, the VR can never be higher than 480. 480 is the maximum VR for NTSC.

It can be lower when showing a letterbox image as not all of the scanning lines are used to create the image. In the case of a Panavision type film with a 2.35 AR we use only about 240 of the 480 available scanning lines.

The other resolution is the HORTIZONTIAL RESOLUTION and to make it simplified, it is the number of pixels (picture elements) that make up EACH scanning line. This number can be anywhere from 200 all the way up to 800.

The low end of the HR number is made up by formats like VHS and VCD. In the "middle" we have formats like LD, Hi8, DVD, SVHS which run from 400 to about 480. In the "upper middle" we see formats like ED-Beta and Digital 8 which run 500 to 550.

At the upper end we have the professional NTSC formats like Beta-Cam and such which can hit the 800 number.

Hope this helps.

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post #13 of 19 Old 09-29-2001, 02:13 PM
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Lee, thanks for the post. So the horizontal resolution may not always be AR "correct"? I realize that the AR of the image will be presented correctly by the display device, but what I thought that I just read was that, for example, professional systems may have an 800x480 grid (whereas 800x600 would be 4:3).



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post #14 of 19 Old 09-29-2001, 04:04 PM
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Man,

The hortizontial resolution will not change when you letterbox a movie. The vertical resolution WILL as you are using less scanning lines to create an image.

under the rules of NTSC we quote the H. res. first and the V. res. second. The following will be for a full screen (1.33 AR) presentation:

VHS - 240x480
OTA - 330x480
LD - 425x480
DVD - 480x480

See how the V. res never changes but the H. res. does?

Again for NTSC except this time we are looking at LTBX movies that have a 2.35 AR and are shown on a 4x3 AR TV set.

VHS - 240x240
OTA - 330x240
LD - 425x240

Now lets look at the highest NTSC - Digital Beta-Cam, again on a 4x3 TV:

800x480

The number that you quoted in your post; 800x600 is not NTSC. That number combination is a computer format called SVGA because NTSC cannot have more than 480 for the V. res.

When we shift to HDTV there are two different resolutions:

1920x1080
1280x720P

But these are not the "true" resolutions of HDTV because again we need to add the "housekeeping" lines back into the formulas. The numbers quoted above like all the NTSC are the numbers that the image is made up of. But the "real" resolutions are:

1920x1125
1280x765P

Again like with NTSC 45 Vertical Scanning lines are not used for image generation but are used for the "housekeeping" functions that ANY TV display, be it Direct View, LCD, DLP, D'ila, RPTV, FPTV, Plasma...etc, uses.

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post #15 of 19 Old 09-29-2001, 04:35 PM
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Additional discussion of this topic always helps bring things into focus for newcomers and those who don't follow this geek stuff that closely.

In my initial post I tried to make a distinction between scan formats and resolutions by underlining them. IMO, with the NTSC format, ~480 is the number of active lines in the NTSC scan format, not the resolution. Vertical resolution, expressed as a number, is what you can read when you feed a test pattern signal into a display with 480 active scan lines. Typical measured resolutions have been discussed above. -- John
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post #16 of 19 Old 09-30-2001, 01:46 AM
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The "Housekeeping" stuff that is mentioned here are things like teletext/captions as well as test signals that the station might use to compare a received signal against a pre-broadcast one. It is called VITS (Vision Inserted Test Signal). If you're interested.

Also I believe (although am not sure) that DigiBeta has a res of 720 lines, not 800. Additionally, although everyone agrees that the picture area of a PAL signal is 576 lines, the NTSC resolution seems to vary from 476 to 486 depending on to whom you talk. 480 is certainly the resolution for digital NTSC, however it seems that NTSC Beta SP is (was) 486 lines.

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post #17 of 19 Old 09-30-2001, 07:43 AM
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One of the questions that Man E had was why there are an odd number of visible scanlines (483). I didn't see an answer posted, although I might have overlooked it.

My impression is that a lot of people are responding in terms of digital video displays (which are designed to be convenient for binary computer arithmetic, thus 640x480) and neglecting the original analog NTSC requirements.

There are an extra 21 scanlines in each field because the signal needs to allow for the time it takes the oscillators controlling the crt to deflect the electron beam back up to the top of the screen. Being able to tranmit other information during this time is a bonus.

There are an odd number of scanlines because the extra half-cycle per field is just enough time for the electron beam to be enough further down the screen so that it illuminates the phosphor in between the scanlines of its previous pass.

This description is greatly simplified, of course.
Video display standards have gathered a lot of features since the National Television Standards Committee (NTSC) came up with one designed for black-and-white TVs that used vacuum tubes.

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post #18 of 19 Old 09-30-2001, 11:13 AM
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It turns out, for 4:3 standard television, that static resolutions are about equal: ~330 X 330. The 330 horizontal resolution is per picture height as I mentioned earlier, or 1.33 X ~330 across the screen width. The dynamic vertical resolution (moving images) drops to about 50% of 480 scan lines.

The resolution ratios for HDTV, which I summarized here , are for the 16:9 ratios of HDTV. Vertical resolution of 1080i images varies constantly from ~400 to ~800 (dynamic to static) as parts of the image change. Don't see any problem with that myself. The density of pixels along each horizontal line can vary all the way up to the number sampled by HDTV cameras (1920), although there's about 20% filtration in sets. The more detail the merrier. Images or the aspect ratio won't become unbalanced, just more detailed (within ATSC parameters). As movement occurs, the human eye is insensitive to details within that movement. As resolution drops, as with VHS tapes, images just appear softer because the tape system doesn't have the bandwidth to carry higher frequencies that create finer image details.

What happens with some of the new printers that print relatively few horizontal lines, but with extremely high dot densities on each line? Just more-detailed photo-like images. (Yes, photos have more actual detail.) -- John



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post #19 of 19 Old 09-30-2001, 09:41 PM - Thread Starter
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hold the boat!
this is somthing that has been troubling me.

If you maintain the same abount of verticle resolution (~480) and change the horizontal resolution anywhere from 240 - 480 or more. How do get the picture to maintain any sense of visable accuracy? What I mean is I seems like changing one axis for resolution, but not the other, would often totally screw the AR/Balance/accuracy of the whole picture.

Digital is easy to think about in these terms but Analog still has me a little confused. Why have one axis constant and the other radically variable?
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