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All HDTV not equal...
Source: Pittsburgh Post-Gazette, September 11, 2003
By Don Lindich

Q: I am hearing that a DirecTV HDTV signal is far superior to a HDTV signal from my cable company. Is this true? My cable company is now offering HDTV through its digital service but I understand that this is sometimes a very soft signal. Garbage in and garbage out really has never reached such levels because I know that a bad signal on a great HDTV is only magnified like never before. Can you help? What do you know about this? ...Mike Freer, Columbus, Ohio

A: The quality of the HDTV signal transmitted over cable systems is wholly dependent on the particular cable company. The quality of the signal will vary depending on the capacity of its system and its philosophy when it comes to image quality and signal transmission. Cable companies have capacity limitations over their cable systems, and an HDTV signal transmitted at full quality takes up a lot of available space. There have been reports that some cable companies offering "HDTV" service are not transmitting HDTV channels at full quality in order to conserve this space for additional pay-per-view offerings, video-on-demand and other similar channels and services.


Digital television signals are composed of millions of digital bits transmitted every second. The more bits, the better the picture. To be considered true HDTV, the signal must be at least 19.4 million bits per second. I recently spoke with a technician installing new capabilities for one of the country's largest cable providers. I asked him what bit rate it was using for HDTV, and he told me it was 10 million bits per second. While such a picture may look good compared with a regular digital channel, it is still well below what a full-quality HDTV signal should be.


A few months ago I received a question similar to yours regarding HDTV via cable. I contacted three cable companies and asked the bit rate being used for their "HDTV." All three said they would get back to me. None of them ever did.


In terms of personal experience, I recently visited a friend who is getting HDTV via cable. She has a Mitsubishi HDTV similar to the one I own. The PBS HDTV demo channel did not have nearly the clarity, vibrancy or visual impact I experience with my Mitsubishi HDTV receiving the same channel via an antenna. While she thought her picture was very good, I imagine she will be disappointed with it when she sees the same channel on my television.


Again, cable companies differ. If image quality is of prime concern to you, I would recommend you stick with HDTV via satellite or via an antenna over-the-air as it is a safe bet. If you would like to give HDTV via cable a try, call and ask a technician what bit rate the company is using for HDTV transmissions. If its HDTV is being broadcast at 19 million bits per second (Mb/s) or greater, you should have picture quality that rivals satellite and over-the-air HDTV transmissions.
 

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This is really a horrible answer as it is more pushing the idea that cable companies are adding compression, rather than not. I'm pretty sure my cable company is passing through their signals at 100% (mcuban confirmed it with HDnet), and I think we all know that DirectTV is recompressing their HDnet and HBO transponder. So the answer for me would be completely the opposite. It's a pretty well known fact that Cable systems are not under the major bandwidth contraints that Satellite companies are, it's odd that he failed to mention that.
 

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It's taken me 5 minutes of typing and erasing my thoughts on the distortions in this article before I could come up with something I wouldn't be embarrassed for my mother to read. Arrgh...


First, for a technician to have any clue as to what bit rate the engineers are sending down the pipe is ludicrous. Secondly, nearly every agreement cable providers have with programmers prohibit alteration, compressing, or bit rate reduction of the signal.


Thirdly, the 19.4mbs is required to send the full 8VSB signal over the air. As cable is an enclosed pipe, the robust error correction bits required in 8VSB are not required to be sent, nor is some of the other "administrative" data required solely for a receiver to be able to tune 8VSB.
 

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Most of the conclusions this author reached are misguided.


To reinforce what gmclaughlin says, I have it from one of my reliable sources that Comcast corporate policy prohibits altering of HDTV signals in anyway that could affect signal integrity. In other words, they pass HDTV in the same quality they receive it.
 

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Yeah I dont get it. Sure, the quality of the HD signal transmitted over any particular cable co is wholly dependent on the particular cable co. So what? So is the HD signal quality transmitted over a particular DBS co wholly dependent on the particular DBS co. (theoretically cramming too much on a transponder). So is the HD signal quality of any particular OTA station wholly dependent on that particular station (perhaps allocating too much bandwidth to a sub-channel).
 

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This information comes directly from Concast:

Quote:
To be clear, Comcast has not compressed or rate-shaped any HD broadcast signals. Most HDTV signals are transmitted at a 19.4 Mb/s 1080i....Comcast demodulates the Mpeg2 signal and then we multiplex it in with other Mpeg2 signals and modulate onto a 256QAM which is transmitted over the cable system. This is a complete pass through.


In short, Comcast does not permit the compression of HDTV signals.
Although it's possible there are some exceptions, by far this is the overwhelming situation for cable HDTV.
 

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As cable systems have 'gone digital' there seems to be a growing variety of ways to manipulate digital video. Here's a recent article , authored by folks at a MPEG hardware firm, that covers digital-cable signal processing in some detail. I found the sections on rate shaping and statistical multiplexing very informative. For more detail on 'statmuxing' there's a nice pdf-format paper , with good diagrams, from Scientific Atlanta.


Also, here's a TV Technology article quoting a Comcast VP about cable companies stripping away unnecessary forward-error-correction and PSIP data.


In NYC, Time Warner recently installed new Terayon hardware for rate shaping and maximizing bandwidth. -- John
 

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John,


Is this leading us back to our "QAM isn't compressing / Yes it is" argument?
 

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Propaganda for the satellite industry! They don't recompress? We know they do!


The idiot also gives no thought to the fact that the difference he sees between his OTA and his friends cable could easily be caused by set calibration and/or STB design/manufacturing issues.


People who don't know what the hell they are talking about shouldn't write columns.


Fool.
 

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I live on Staten Island. Although it is not official Time Warner will on request provide a Scientific Atlanta HD box to decode and display HDTV.


On my calibrated Toshibia RPTV there is frequently very obvious pixilation in the background and blues are often neon like in intensity almost as if the saturation is way to high. I don't think this is an issue with the box because sometimes the image is perfect.


Something is going on locally with what's being put out on the Staten Island network.


Richard Smith
 
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