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On 24/02/2012 23:22, Chris Zwar wrote: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.The reason it was chosen as 720 is that it was decided to have this common line sample size for both sides of the pond. It was a compromise between the 4:3 square pixel sizes of 640(648*) and 768. It may vaguely interest some of you that in fact the 720 samples cover more of the picture width than did the analog signals, the respective 'active picture' width being approximately 712 pixels for NTSC and 702 for PAL. It is only approximate because the analog timing specifications have a quite wide tolerance. *note the true picture height for NTSC is 486 lines but many formats crop this to 480. As intimated by Chris - analog TV (i.e. NTSC and PAL) don't have pixels. What is confusing for many is that both camera CCDs and display LCD/Plasma screens are all analog devices, but have an inherant digital sampling in the spacial domain. Perry Mitchell |
