# What exactly is a four symbol pulse?

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

1. ### MRWGuest

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 DayGuest

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