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What exactly is a four symbol pulse?

Hi! I'm reading this article:

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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.

Reply to
MRW
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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

Reply to
Randy Day

On Nov 18, 11:30 pm, MRW wrote: > Hi! I'm reading this article:

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8vsb.htm >

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.

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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

Reply to
stratus46

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