Re: How does digital TV broadcast prevent ghosting effects?

On Nov 21, 2:12 pm, "Joel Koltner" <JKolstad71HatesS...@xxxxxxxxx>
wrote:
"MRW" <mr.whate...@xxxxxxxxx> wrote in message

news:caa438d0-efc3-495c-926e-e44ffe1b3293@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

How do these standards eliminate ghosting effects?

The basic idea is that, since they're transmitting a digital signal, if the
reflections (ghosts) aren't strong enough to start "flipping bits" (or only
flip a few), the resultant signal is still "digitally perfect" (at least once
error correction is performed) and thus the picture displayed is exactly what
the transmitter started with.

An improvement can be had with a so-called "rake receiver" (see:http://en.wikipedia.org/wiki/Rake_receiver):Assuming the ghosting is
relatively constant, you're just getting delayed copies of the original signal
at various points in time. If you start sampling at those various points in
time and summing up the result, you can faitfully reconstruct the original
signal. (Of course, finding the correct "various points" is not so
trivial...) Hence you're you're "raking in" all the copies of the original
signals to build up the result.

Supposedly first-generation ATSC receiver chipsets didn't do any of this
active ghost cancellation, whereas second- and (the current) third-generation
chips do.

I also read a similar scheme for analog TV using Ghost Canceling
Reference (GCR) signals from both transmitter and receiver. Is this
pretty much the same idea?

Somewhat, yes, although with a rake receiver you don't (necessarily) need a
reference signal -- you just try out the various sample points until you get
one that produces the "best" result. Unlike an analog system (where all you
really have to measure are signal to noise ratios) in a digital system usually
there are plenty of synchronization and test data patterns at known locations
within the signal, so it's usually easy to determine how well the system's
performing.

---Joel


Hi Joel, Thanks for the response.

I'm assuming that the error correction portion is dependent on the
modulation scheme. So, am I right in assuming that the COFDM technique
being describe as more robust to multipath effects than 8-VSB plays a
part in implementing the error correction scheme?

You know, it's a bit tricky for me to patch up all the stuff I've read
from various references. My main surprise is the fact that DVB-T and
ISDB-T are optimized for single frequency networks (only one carrier
frequency for many transmitters) compared to 8-VSB. Although, they are
supposed to be testing 8-VSB in single frequency topology.
.

Relevant Pages
Loading