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What exactly is a four symbol pulse?

Discussion in 'Electronic Basics' started by MRW, Nov 19, 2007.

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  1. MRW

    MRW Guest

    Hi! I'm reading this article:
    http://www.broadcast.net/~sbe1/8vsb/8vsb.htm

    I'm trying to get myself familiarized with Digital TV broadcasting. On
    that page, there is this sentence:

    "The ATSC segment sync is a four symbol pulse that is added to the
    front of each data segment and replaces the missing first byte (packet
    sync byte) of the original MPEG-II data packet."

    I'm not quite sure what a four symbol pulse is. If a symbol is
    represented by various voltage levels (+7, +5, +3, -3, etc.) and a
    pulse can be looked at on paper as a square wave with a one max
    amplitude, then how does this fit into a "four symbol pulse"
    description? Maybe my assumption for symbols and pulse is wrong?

    Thanks!

    I'm trying my best to teach myself this stuff. I can ask my teachers
    only so many questions before my time with them is up.
     
  2. Randy Day

    Randy Day Guest

    If it's a reference to Quadrature Amplitude Modulation,
    QAM is a combination of signal amplitude and phase
    representing a two-bit segment of the transmission.

    If that's not what they're referring to, then I dunno...

    HTH
     
  3. Guest

    The 4 symbol pulse is described as a 'sync' pulse 4 symbol times long
    for the receiver to identify start of a data block . Not the same as
    analog sync which indicates start of line or start of field. Look for
    the paragraph "VSB modulator" near the end but I recommend reading all
    of it.

    http://www.hdtvprimer.com/ISSUES/what_is_ATSC.html

    Keep in mind the data rate is 19.34 megabits/second but the
    broadcaster can set the bit rate for the programming including sub-
    channels. Therefore the 4 symbol sync has no reference to the video,
    only the data blocks.

    BTW the computer I'm writing this on is recording an HD ATSC stream
    right now.

    GG
     
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