By Randy Hoffner, December 16, 2008
We in the television technology field have been increasingly hearing about these two topics: "1080p" and "3 Gbps." What is all this about, anyway?
The scanning format 1920x1080 progressively scanned has in fact been with us for awhile, in the form of 1080p/24 frames per second (fps). The appearance on the market of so-called "1080p TV sets" which have 1080p displays, but many of which cannot accept 1080p input signals, has raised the awareness of 1080p in consumers. Add to this 1080p Blu-ray DVDs (which are also 1080p/24 fps) and 1080p video game material, and you have the two sources of 1080p material available to the consumer.
FILM FRAMES AND VIDEO FRAMES
Several years ago, video mastering and archiving of film-based material for the various, and proliferating, video applications, migrated to 1080p/24 (well, really 1080p/23.98). This was a good thing, for several reasons. 24p video establishes a one-to-one relationship between film frames and video frames, and its 1920 pixels per line horizontally by 1080 lines vertically afford the maximum spatial resolution available in the video world, as opposed to the file world. It is, in fact, nearly 2K resolution. Further, by adding the appropriate pulldown (2/3 in the 60 Hz world or 2/2 in the 50 Hz world); subsampling as required; and, for the 50 Hz television world, speeding up to 25 fps, it may be readily converted to any required output format, with very high quality results. Along with 24p video acquisition, 24p editing and post production of 24 fps film-sourced material has been the industry standard for some time.
It is at least theoretically possible to broadcast 1080p/24 material using the ATSC system for terrestrial digital broadcast that we employ in the United States. Along with 1080i at 30 fps and 720p at 24, 30, or 60 fps, 1080p, in the 24 and 30 fps frame rates plus their 1/1.001 variants, are listed ATSC formats
. It is, therefore, technically feasible to broadcast both 1080p/24 (or 23.98) fps, and 720p/24 (23.98) fps, and ATSC receivers operating in film mode should at least theoretically be able to decode it, add 2/3 pulldown, and display it at 60 (59.94) fps. Parenthetically, the same may be said for 1080p/30 fps and 720p/30 fps.
We now know, however, that for all the once much-discussed 18 (or was it 36, or 360?) possible ATSC scanning formats, HDTV has only ever been transmitted in the United States as either 1080i/29.97 fps or 720p/59.94 fps. And for all the talk about megapixels and such, the uncompressed data rates of the three HD scanning formats that are actually used: 1920x1080i/29.97; 1280x720p/59.94; and 1920x1080p/23.98; are, by design, comparable.
The data rates for these scanning formats in 4:2:2 color difference component form (Y, R-Y, B-Y), at a bit depth of 10 bits, are as follows: For 1080i/29.97: 1920 horizontal pixels × 1080 lines × 2 components (R-Y and B-Y are each 1/2 Y) x 29.97 fps × 10 bits = approximately 1.243 Gbps. For 720p/59.94: 1280 horizontal pixels × 720 lines × 2 components × 59.94 fps × 10 bits = approximately 1.105 Gbps. For 1080p/23.98: 1920 horizontal pixels × 1080 lines × 2 components × 23.98 fps × 10 bits = approximately 0.995 Gbps. Each of these signals fits nicely into the SMPTE 292M HD-SDI, which, with overhead, has a nominal data rate of 1.485 Gbps. For terrestrial transmission, these data rates are usually reduced by being subsampled as 4:2:0, typically at 8 bits, and significantly compressed to fit into the ATSC transport stream, but we see that they all start out at relatively comparable data rates.
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