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DVD Resolution question (RichardA?)

Discussion in 'Blu-ray & DVD Players & Recorders' started by Plump, May 28, 2003.

  1. Plump

    Standard Member

    Feb 22, 2002
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    under the bridge
    Richard Ansel wrote somewhere :
    I would like to know backgrounds for the decission to make DVD resolution like this?
    (Wondering why 16/9 pixel ratio was not choosen)

  2. RichardA


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    Hi Plump,

    The reasons for those resolutions are from the dim and distant past of television!

    Broadcast TV has never really used square pixels, so a direct relationship of pixels to aspect ratio has never occured (apart from very early NTSC digital signals that were 640x480)

    Early Broadcast equipment (i.e. before SDI technology) would digitise the analog signals using a sample rate related to subcarrier frequency (as most analog signals were composite, even in TV studios) this was usually 4x subcarrier (abreviated to 4fsc). This clock gave 768 pixels per line in PAL and fewer in NTSC (I can't remember the numbers now)

    When standardisation of digital formats began, it was decided that a common sample frequency for PAL and NTSC would be a good thing (i.e. it makes design a heck of a lot easier) and a sample clock of 13.5MHz was chosen - this could be easily derived from PAL and NTSC mathematically - and it was also decided to have equal samples per line for both standards as well - 720 active samples per line.

    This still causes some fun in Broadcast circles where older analog tape recordings have shorter active lines than the current 'digital' lines, causing black bars on both sides of the picture.

    When the PAL or NTSC signal is M-PEG encoded, the macro-blocks (the compressed elements of the picture) are based on 8 pixel wide areas, and the nearest sensible boundary ends up at 702 pixels.

    So, the reason for the numbers is that it is the result of years of standardisation in the Broadcast field to ensure that signals are compatible - to avoid some of the issues that have surrounded PAL progressive compatibility, for example.

    There is a digital standard for 16x9 over SDI which uses a 18MHz clock. This gives the same resolution horizontally in 16x9 as 13.5MHz gives in 4x3 but it doesn't really seem to have been widely adopted by broadcasters.

    I hope this helps to give some background at least!

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