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Old 11-27-2009, 05:56 PM   #10  
Sony KD34XS955

Join Date: Jul 2005
Location: Pointe-Claire QC
Posts: 2,865

A few points.

The actual frame size for 1080i/p is 1920x1088. The extra 8 scan lines are blank and are discarded by the receiver before the images are sent to the display. They are needed because of the requirements of MPEG compression which require the "dimensions" in both directions to be divisible by 16.

The "divisible by 16" requirement applies to each field in interlaced scanning. Consequently the number of scan lines in an interlaced scan frame must be divisible by 32. 1088 is divisible by 32 making the 1920x1088 frame size compatible with both interlaced and progressive scan.

720i does not exist. 720 is divisible by 16 but is not divisible by 32. Consequently the frame size 1280x720 is compatible with progressive scan but incompatible with interlaced scan.

All interlaced formats in the ATSC 30/60 Hz standard are broadcast at 30 frames per second (or 29.97) OTOH, the progressive scan formats can be broadcast at 24 frames/sec (or 23.976), 30 frames/sec (or 29.97) or 60 frames/sec (59.94), subject only to the overall bandwidth restriction.

The 29.97 frame rate and 59.94 field rate are a throwback to the analogue NTSC colour system. The 0.1% reduction from the integral values based on the AC power transmission rate was needed to eliminate visible anomolies caused by interaction between the colour and audio carriers. This 0.1% reduction is not needed in the ATSC/8VSB standard but broadcasters are free to use it, should they so desire. For all intents and puposes, including bandwidth calculations, the exact frame rates of 24, 30 and 60 are all that need to be considered.

The overall bandwidth restriction eliminates 1080p at 60 frames per second from the ATSC standard but 1080p at 24 fps and 30 fps are valid ATSC OTA broadcast formats that have been part of the standard since Day 1. Disregarding the fact that progrssive scan has been found to be a little easier to compress using MPEG-2, 1080p/30 requires the same bandwidth that 1080i requires and 1080p/24 takes less bandwidth than 720p/60. The oft quoted factoid that 1080p requires too much bandwidth for OTA broadcasting is nothing more than a myth that won't die, propagated by people who do not understand that progressive scan involves multiple frame rates.
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