You may be confused by the phrase "high definition" because it has been used in the past to describe many forms of TV, and has several meaning even today.
The best way to understand is to compare the many forms of TV and computer monitors you may have seen.
The following section compares various formats, where the total number of pixels (picture cells, or the dots that make up a digital picture) is compared to a standardanalogue TV picture
Before the 1950s, the shape of televisions was 5:4, and the single BBC Television Service was broadcast. The picture was monochrome and the resolution was less than a third of the standard TV resolution today.
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Peter Harris Monday 23 April 2007 9:34AM
Can I ask where the 414 x 311 you quote for 405 TV comes from ?
IIRR, 405 had a bandwidth limit of 3MHz and an active line of 80 microseconds, making 0.166 microsec per pixel, and 485 pixels per line.
Field blanking was, here the memory fades! about 18 lines, giving 369 active lines per frame, i.e. 485 by 369 resolution and approximately "square-pixels", as the kell factor was not recognised until later on.
Second point with 625 video is that with SD digital TV, the signal source, the camera etc, originates video that carries no more than 572 pixels per line. (5.5MHz bandwidth limit)
The fact that it is then digitised with more samples per active line, under Rec 601 component sampling, does not magically give 720 pixels, or more accurately 702 pixels, of usable video resolutuion per line.
It just means that the sampling process doesn't reduce the natural resolution of the picture source.
And of course with a 2:1 interlace scan, 576 lines per picture height only carries about 400 lines of usable vertical resolution.
Peter Harris: Erm, yeah.. the 405 line examples should read "377 line" not "311" .. typo sorry, and it's carried over into the horizontal resolution.
The 625-lines TV is called "576i" and has in the digital domain 720x576 pixels, and even when interlaced provides 576 lines (of the 625) of visible picture.. I have no idea where you get the idea of "400 lines of vertical resolution" from, as interlacing does not reduce the resolution, it is simply the order that the lines are broadcast in! In the UK the broadcasters of digital TV use the "main profile at main level" which is defined as 720x576.
Briantist: When ( or if) DVB-T2 comes along and bandwidth is squeezed into a 128 Quam. stream, do you think that it will still be possible to transmit a 1920x1080 HDTV picture, in 16:9 format, 1.97 megapixels (at a 1:1 ratio)to reach the same "1080" quality standard on a TV screen as was initially envisaged ? Doesn't something have to give somewhere or is this still possible ?