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  1. #1

    Luminance Sampling

    Hi - I am entering the digital video world from the audio realm. My question is about the sampling rate of DV. My camera has a spec of 8-bit, 13.5 MHz, 4:1:1 copponent recording. I understand that the 13.5 MHz is the sampling frequency of luminance in SD digital television. Is this value at all related to the way audio is sampled (48 kHz, 32kHz, etc.)? Is the luminance actually sampled 13.5 million times a second? The tech stuff I have read points to a sampling of 720:180:180 pixels per scan line for luma and chroma. (Does this mean the 180 pixels is streched to complete the line, or there is only chroma data for every fouth pixel)? The math to get to the value of 13.5 million eludes me. - Help!

  2. #2
    Administrator Dave's Avatar
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    I really wanted Scratch to answer this question but he's been moving house and he's lazy at the best of times. Here's my effort instead:

    Digital sampling of video (chroma and luma) is conceptually similar to audio sampling, i.e. it records samples at set intervals.

    The three numbers (e.g. 4:1:1) refer to different parts of the video signal. The first is luma (which is more important) and the second two are chroma.

    In SD digital television the sampling frequency 13.5 MHz which does indeed mean that luminance is sampled that many times per second. The chroma will be sampled a lesser amount according to the 4:x:x ratio (unless it's 4:4:4, in which case chroma has the same sample rate).

    The redued chroma information has to spread out to fill the gaps as you suggest, so it has a lower resolution than the luminance.

    As I understand it, the 13.5MHz sampling frequency and the 720 pixel width are simply standards which have been decided by the powers above. I don't think they have any special relationship.
    Dave Owen
    MediaCollege.com

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