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This topic comprises 2 pages: 1 2
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Topic: HD-DVD/Blu Ray - Your current HDTV may be obsolete
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Evans A Criswell
Phenomenal Film Handler
Posts: 1579
From: Huntsville, AL, USA
Registered: Mar 2000
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posted 04-07-2005 02:12 PM
quote: Greg Mueller
Question: How many times better is the resolution of DTV compared to analog TV?
Answer: It is felt that most analog television broadcast stations transmit a picture that contains 480 vertical interlaced lines with approximately 340 horizontal pixels per line. Compare this with DTV which is capable of sending pictures which are 1080 vertical interlaced lines with 1920 horizontal pixels per line.
Whoever answered that question doesn't know how to compare digital resolutions with analog bandwidths. The 480 interlaced lines is correct, but you need an equivalent of around 635 pixels per scan line to be roughly equivalent to a 4.2 MHz luminance bandwith for standard television. The color bandwidth is 1/7 of that (0.6 MHz equiband for broadcast), so there are only roughly the equivalent of 91 pixels of coloring information per horizontal scan line.
A system with 340 pixels per horizontal scan line would be the equivalent of 119 line pairs per picture width (using the commonly-accepted Kell value of 0.7), which is equivalent to a bandwidth of 2.25 MHz, which is only 75 percent of the luminance resolution of VHS (3 MHz).
The person writing that answer did not understand the difference in "TV Lines" (analog resolution along a scan line per picture height), Lines of resolution per picture width (analog), and digital pixel resolution. The commonly quoted value for horizontal resolution on TVs is "TV Lines". That must be multiplied by 1.3333... to get Lines per picture width. That must be divided by the Kell factor (commonly accepted to be 0.7) to get how many digital pixels would be required to be equivalent to that resolution.
Let's figure the digital equivalent resolution to broadcast TV the sane way:
4.2 MHz is the bandwidth for standard broadcast TV. OK. The frame rate is 29.97 and there are 525 lines per frame. That equates to 266.937 Hz per scan line. But wait, the amount of each scan line used for actual picture information is 83.22 percent (52 8/9 over 63 5/9). Multiplying that in leaves 222.1 Hz per scan line for picture. So, 222.1 is the number of line pairs. Multiply that by 2 and you get 444.3 lines of resolution per screen width (I'm keeping more precision in the calculator than I'm typing), regardless of the aspect ratio. The commonly quoted value for "TV Lines" for broadcast comes from multiplying that by 0.75 (3/4) since "TV Lines" is per screen height, not width. That gives 333.2 "TV Lines" for a broadcast. Now that's just the luminance part, and it doesn't mention that some of the luminance is corrupted by the color carrier. So how many digital pixels are needed for a screen width of information at this bandwidth? Divide the 444.3 by 0.7, the commonly-acccepted kell factor, and you get 634.6. Of course, that 0.7 is not an exact value. 640 is a natural number to use, and it would be a convenient one, since 640 by 480 gives square pixels at 4:3, but we all know 720 is the value commonly used nowadays. That gives roughly 13.4 percent more luminance bandwidth than broadcast. This paragraph says nothing about color resolutions, though. :-)
The 1920 digital horizontal resolution would be roughly equivalent to 1344 analog lines of resolution (1920 * 0.7). 1344 divided by 444.3 gives 3.02, so HDTV has just over 3 times the horizontal resolution of standard TV, without corrupted luminance due to the color carrier in broadcast. The 1080 lines can be directly compared to the 480 of standard TV, since they're both discrete. It's 2.25 times as much, so 2.25 for vertical and 3.02 for horizontal -- that says 1080i has 6.80 times the luminance information (and says nothing about color resolution).
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Evans A Criswell
Phenomenal Film Handler
Posts: 1579
From: Huntsville, AL, USA
Registered: Mar 2000
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posted 04-10-2005 04:07 AM
Joe, "TV Lines" are lines of resolution across a scan line in a part of the scan line equal to the picture height. I was not talking about pixels or digital TV at all when I said "That must be multiplied by 1.3333..."
If a broadcast TV picture has a luminance resolution of 333.2 "TV Lines", that means it has 333.2 lines of resolution (in the luminance portion) across three-fourths (assuming 4:3 aspect) of the TV picture, meaning it had 333.2 * 1.333333 = 444.3 lines of resolution across the width of the picture. This says nothing about vertical resolution, which is 480 scan lines (a totally different measurement unit than the 444.3 lines of resolution across the picture). The 480 vertical is equivalent to 480 pixels in height, while the 444.3 lines of resolution across the width of the screen (or equivalently, 333.2 "TV Lines" of resolution for a 4:3 image) are not equivalent to 444.3 pixels. As I explained above, 444.3 lines of resolution across the width of the picture, or equivalently, 333.2 "TV Lines" of resolution, is equivelent to roughly 635 pixels worth of information, using the commonly accepted Kell value of 0.7.
Whether pixels are square or not have no bearing on what I was talking about.
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