Re: How does digital TV broadcast prevent ghosting effects?

Jim Thompson wrote:
On Wed, 21 Nov 2007 11:42:14 -0800, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

Joel Koltner wrote:
"MRW" <mr.whatever@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.

Oh, forgot one thing: This new set has an anti-ghosting comb filter. Finally! I've used that technique since the 80's and I wonder what took them so long.

What brand?


Vizio VX37L, which is a 37 incher, at Costco.

--
Regards, Joerg

http://www.analogconsultants.com/
.

Relevant Pages