I've been reading about the new digital TV standards (8-VSB, DVB-T, ISDB-T) and they all mention that the standards are more robust compared to the analog standard. For example, all state that practically ghosting effects are eliminated with DVB-T and ISDB-T providing more robust response to mulitpath effects than 8-VSB.
How do these standards eliminate ghosting effects?
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?
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:
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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.
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.
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.
I just bought a digital TV but I have to head to a client so I can't post any experiences until later. The lab here is in rather extreme multipath conditions. So it'll be interesting. Bottomline is we'll see whether or not we will still have TV when they turn off analog in 2009. It'll be used as a monitor anyhow and for TV we could probably use the web. The new sets usually have VGA and HDMI inputs.
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.
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An audio CD (analog WORM style track) can have scratches and still play as the ECC allows the playback hardware to reconstruct the analog audio stream, but it has no rules that it all be there, and if it isn't, it will even make pieces up.
In digital realm, ie MP3, etc the ENTIRE datagram has to be there, so the ECC must PROPERLY reconstruct all of it in order for the decoder to pass it on to the D/A analog amps.
An missing portions result in a drop out of playback.
So, for digital TV broadcasts, the FEC which is included in the signal ensures that the datagram is all there and 100% correct so the MPEG decoder can call it good data and "render" it. If the bit-error-rate gets much above 10% one will see picture artifacts, lost frames, or the picture and sound completely drop out until the decoder retrains on the stream.
Since it is digital, you will NEVER see any multipath, because each datagram must be completed by the FEC segment before it gets passed to the MPEG decoder. Any incomplete segments get tossed, and the decoder picks up on the stream at the next correct datagram.
So what you see is either a 100% complete and perfect picture, or a frame or two with SOME artifacts (bit blt blocks) OR frames that are skipped completely, which are usually replaced by blank frames that carry the color the hardware was programmed to display during missing frame segments, which is usually black.
On a sunny day (Wed, 21 Nov 2007 20:01:01 GMT) it happened Joerg wrote in :
1366x768?
Ar you kidding me? I was under the impression HDTV was 1920x1080 these days, and 50 and 60 Hz capable too (for Euope the 50). The cannot even sell it here as HDTV with that few pixels. Even my monitor has more.
Digital TV is 100% corrected by the FEC which is also sent in the stream. It isn't that it ignores all but the strongest signal, it IS that it constructs perfect datagrams ALL the time, and ANY missing data is reconstructed from the included FEC. If it cannot reconstruct a perfect datagram, the frame or frames carried in that broken datagram get skipped.
If you don't have a really high quality signal and don't get too close to it, it doesn't matter much. We just bought a little 23" Toshiba TV/DVD with that resolution for the bedroom. It showed a great deal of dislike for the DVD I put in there- I ejected it and then shut off the power. The drive sucked in the DVD as it was shutting down, and then
*flung* it across the room (at least 4'), as its final act (presumably some kind of spring post broke inside). Hopefully the replacement unit will fare better.
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One thing that really ticks me off is that our cable supplier now encrypts the digital TV signals so you can only use their 'official' box rather than the QAM digital tuner built into the TV. I guess unless they're forced to stop it by legislation they'll continue to do so. They do leave a few music only and on-demand channels (hey, I can watch what the neighbors are watching, starting from whenever they started) unencrypted.
Best regards, Spehro Pefhany
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On a sunny day (Wed, 21 Nov 2007 15:29:35 -0500) it happened Spehro Pefhany wrote in :
We have quite a few HDTV transmissions now via satellite, in Europe. Indeed much of the material simply is not in high resolution, not even in their demo channel. But there is a screenshot from BBC HD that comes close: ftp://panteltje.com/pub/00000300.gif It comes close because it is made at a point where the camera stops panning (almost), and I disabled de-interlace. When using deinterlace on LCD the resolution drops to less then half indeed.
I had a DVD player like that, others bought the same after they saw me with it, the mechanism went bad, they had it returned, I put it with the trash. DVDs are, in my view, best played from the PC, as you can select any decoder / processing, then connect to the monitor with DVI or whatever.
I no longer watch encrypted channels, in the past I did lots of decoding stuff, but now it is illegal, death penalty I think even ;-) ), so *if they do not want me to see their commercials, then it is their problem with the advertisers*. There are really hundreds of free to air channels on satellite here, from all over the world, from China to Russia, to South America (relayed), except of course US (no way). But we have CNN Europe free.
I dunno how many radio channels ere on satellite, but many hundreds....
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Just type a name...
Cable was here, dug a hole in my garden, although I told them I wanted no cable. I protested, they filled up the hole again, and I have not missed it ever.
I had cable in Amsterdam, late at night the guys went home and turned of the cable.... Whole political fights what should be on cable and what not, satellite is freedom.
I know you prefer the latest and greatest stuff. However, this one's good enough for here. Of course I could have sunk >$2k into a more expensive one but 37" for around $750 is a pretty good deal.
At 37" your minimum requisite should have at least been 1080p.
1368x768 looks pretty grainy.
You essentially bought three year old technology at a closeout price.
That is aside from the fact that Vizio doesn't use the best panel vendors around, so the end product ain't too hot. Hell, you should have researched a bit. You can get a Viewsonic at that size and spec for the same price, since you are buying the old, marked down stuff.
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