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Because (16/9 * 960) doesn't come out to an even number of pixels. Whereas (16/9 * 1080) = 1920 exact number of horizontal pixels.

480p is not square pixels (720x480) whereas the HD formats all use square pixels.
 

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Because 1706 is not divisible by 16... A big no-no in the world of MPEG2 encoding/decoding..


(Actually, don't recall if it's 8 or 16...)



Seriously, It's much more complicated than that... 960 isn't really a standard anyway either.... Hey it's an extra 120 lines of vertical resolution!!! What are you complaining for!!!


:)


- Mike
 

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I sometimes think that 1536x960 in a 16:10 ratio would have made more sense. The 16:10 ratio is very close to the "Golden" ratio of 1.618:1 usually considered to be aesthetic, the 960 lines might have been more compatible with 480 line electronics, and it is probably about the current limit for HD telecine and video recording.


IIRC, the 1920x1080 had something to do with the existing Japanese HD system.


- Tom
 

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NTSC has a total of 525 lines but only 480 of them are active picture. The missing 45 lines are within vertical blanking which is the horizontal black bar that you see if your picture rolls. For 1080i HDTV 1080 is the active line count. The total line count is 1125. For 720p HDTV 720 is again the active line count. The total line count is 750.
 

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The starting point may have been a target for a 37MHz video signal, and something that can MPEG2 compress somewhat effectively into a 6MHz bandwidth transmission signal...


=======================================================

(from http://www.htoc.net/archive/archive06-30-2002.html )


Examples of Bandwidth Calculations


1. NTSC Broadcast Video Horizontal Resolution.


Because the sound is modulated at a subcarrier at 4.5 MHz, the video information must remain below this point and 4.2 MHz was defined as the maximum video frequency.


Although NTSC has about 480 (usually 483) scan lines containing picture information, all 525 scan lines per video frame must be considered when calculating bandwidth.


For NTSC broadcasts the 525 scan lines are repeated 29.97 times per second, 30 frames per second is close enough for our calculations. 30 times 525 is 15750 so you need a frequency of 15750 Hz just to paint the scan lines.


Then, to make pcture details horizontally, we subdivide each scan line. Limited to 4.2 MHz, the video signal can put at most 266-2/3 cycles (4.2 million divided by 15750), each containing one black and one white pixel, on any one scan line, for a total of 533 pixels. In a simple waveform one cycle has one "up" and one "down" and in video "up" means dark and "down" means light, therefore one simple cycle represents two pixels, one dark and one light.


To give the electron beam time to get back to the left side of the screen, and also to provide a place to put synchronizing pulses, NTSC discounts 17% of the scan line. This leaves 83% of the scan line to hold picture information, which spans 442 of the smallest possible details.


So the effective pixel dimensions of NTSC broadcasting are about 442 horizontally by about 480 vertically. In the horizontal direction, the maximum pixel count may vary, for example some systems may "roll off" (start losing) the higher frequencies by more than 50 percent at 3.9 or 4.0 MHz to make sure that there is no video signal above 4.4 Mhz where it may interfere with the audio.


The commonly published figure of 330 lines of resolution for broadcast video comes from the fact that the largest circle fitting in a 4:3 screen spans about 330 of the 442 possible details across one scan line.


Progressive scan needs twice the bandwidth of the corresponding interlaced scan video.


Example 2: 1080 HDTV


We will work this example in the opposite direction, starting off with the pixel count.


1080i HDTV has 1080 scan lines containing picture information (and 45 scan lines for retrace and synchroniziing for a total of 1125 scan lines). Horizontally there are 1920 visible pixels occupying the first 87.3% of the scan line and we must pretend that there are 140 similarly sized sync. pulses with 140 gaps in between, filling the rest of the scan line. (The total pixel count is 2200 per scan line.)


The scan rate is 30 (29.97) full video frames per second.


Multiply 1125 x 2200 x 30 seconds and we get about 74 million pixels per second. Since one cycle consists of one black and one white pixel, the bandwidth needed to display the smallest picture details is 37 MHz.


Coincidentally, 720p also requires 37 MHz.. 720p has has 1280 visible pixels and 1650 total pixels per scan line. It has 720 visible and 750 total scan lines repainted 60 times a second. 750 x 1650 x 60 is about 74 million black/white pixel pairs or about 37 million cycles per second.


Laserdisk allows video frequencies of up to 5.3 MHz recorded as composite video. The approximate total maximum pixels is 680, the visible pixels is 565, the EIA horizontal resolution specification is 425 (TV lines per picture height @ 4:3).


The 480i and 480p DTV formats and DVD provide for 858 total pixels per scan line, 704 to 720 of them visible, with an EIA horizontal resolution specification of about 540 for a 4:3 aspect ratio.


We can get away with equipment having around 25 MHz (a little over half the requirement) for 1080i HDTV because almost no TV set can produce a spot smaller than 1/1000'th the screen width (two pixels). At normal viewing distances the human eye can't distinguish adjacent details that small. Insufficient bandwidth does not make diagonal lines more jagged; picture details can still be put in any of the 1920 x 1080 pixel positions.
 

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Wow, great post, PVR. It's the occasional gem like this one that keeps me addicted to AVSF.


-T.
 

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Quote:
1080i HDTV has 1080 scan lines containing picture information (and 45 scan lines for retrace and synchroniziing for a total of 1125 scan lines). Horizontally there are 1920 visible pixels occupying the first 87.3% of the scan line and we must pretend that there are 140 similarly sized sync. pulses with 140 gaps in between, filling the rest of the scan line. (The total pixel count is 2200 per scan line.)
PVR -


Good post, and I guess your math is flawless. Yet it's funny also because in the world of digital TV only the active area is actually transmitted, so there are no considerations of 1125 lines, etc. until you get to the analog outputs. And the all digital displays like LCD, DLP, plasma etc. also do not need those extra lines. It is just the dumb scanning CRTs that use them because the video timing has to come from the STB.


But the first Japenese HD system was analog, and targeted at the CRT technology of the time so I think that is the legacy that gave us our current numbers like you described.


- Tom
 
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