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| On 25/02/2012, at 3:59 AM, Chris Meyer wrote: > I suspect it's an analog hangover. Yes, this is it. It's all a historical artefact. Prior to digital and HD, TV standards around the world could be categorised as either 525 or 625 lines, usually (but not always) corresponding to NTSC and PAL. However the horizontal resolution would vary depending on the analogue bandwidth that was allocated in each country - usually between 5 and 8 mhz. In the truest sense, analogue TV did not have a horizontal resolution in pixels, it had a bandwidth. If you look at character generators and TV video cards from that period, they didn't actually specify a "resolution" in pixels that we would recognise today- they would specify a timing resolution in nanoseconds. So a 6ns character generator would have a higher resolution than an 8ns character generator (because 6ns is faster). But they'd all work at either 525 or 625 lines. It's a bit like looking at the difference between VHS and Beta SP. Decks in the same country will record the same signal with the same number of lines - they're either 525 or 625 line decks. But VHS has appalling bandwidth compared with Beta SP, so the image quality is much lower. But you can't measure the difference between them in pixels, as they don't have pixels... just bandwidth. But they record either PAL or NTSC, regardless of how well they do it. While everyone on this list is more likely to recognise terms like ITU-R 601, REC 601, D1 rather than something like '6ns', the significance of those letters is often lost. Basically they represent an agreed industry standard on how to take a 'squirt' of analogue bandwidth and convert it into a digital format. The reason we got stuck with 720 horizontal pixels and rectangular pixel aspect ratios lies in the maths behind that standard. But it doesn't mean that analogue PAL and NTSC actually have 720 horizontal pixels, only that 720 is the agreed standard. Early non-linear systems such as the Media 100 digitised video with square pixels - 640x480 - and I remember reading an article that argued it provided a higher quality image than 720x480, as the same amount of bandwidth was distributed amongst fewer pixels, so the image suffered from less compression (hence higher quality). The point is that the number of lines was always constant, regardless of the image quality and analogue bandwidth of a particular device, so that's how they're described. FWIW I found an old copy of 'broadcast engineer' from the late 90s which looked at the proposed standards for ATSC/HD. One of the engineers interviewed said "I hope they choose 1035 lines. Anything else would be stupid..." It made me smile. -Chris |
