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Discussion Starter · #1 ·
Last night I did my first real detailed comparison of the local HD channels between the OTA and Comcast cable's channels. The OTA picture was very nice yet the cable channels had a lot of noticable compression artifacting and the contrast was overblown! When they showed medium shot (distance wise) you could see the grass shimmering very noticably yet the OTA was very solid.


Has anyone else compared their cable to OTA local HD channels and do you get similar results?
 

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Interesting, Frank. I've gotten what you might call overblown contrast with cable, but it was very subtle and apparently caused by the cable converter's component-out circuit. Several have reported this faint fogging effect with SA2000HD/3100HD converters.


Didn't detect compression artifacts when I compared CBS OTA and via Time Warner Cable here. (Two years back I noticed the worst fogging effect when comparing HBO delivered as an OTA-like 8-VSB signal via Time Warner to my set's HD tuner and another HBO signal delivered to a SA2000HD from TWC with 256 QAM. The first signal avoided the 2000Hd's component outputs and had better higher-rez low-contrast detail--such as faint blue stripes in an actor's white shirt.)


Here's a post referencing some articles about 'rate shaping' being used by some cable headends to trim the bit rates of cable signals. My cable company, as the sublink notes, uses rate shaping. But some recent posts here say Comcast isn't using rate shaping or other compression changes. With distinct, repeatable artifact differences, I'd try to have my local broadcast and cable engineers account for the change in images. -- John
 

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Are you seeing compression, or just the result of inferior electronics on the output of the cable box?


The only fair way to do the comparison would be to use an same 8VSB/QAM tuner for both.


Ken H has reported that Comcast passes through the full 19.3Mbps HD signal unchanged, FWIW.


I've never heard of compression causing overblown contrast...
 

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I think it is the 5100 box from comcast. I also have comcast, expressvu and directv. When comparing Sunday or Monday night football I notice the PQ out of the comcast 5100 box was darker throughout compared to directv (mits hd5 or expressvu (6000) That slight darkness always is the case from the comcast box or signal, and directv and expressvu have a cleaner, brighter PQ, but I think it is the motorolla 5100 that is inferior as I know directv HD and expressvu HD should not have a constistantly better PQ wth the way they stat mux...
 

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Based on my eyes, I agree. Every MNF game (OTA with no multicasting) so far has looked just a little better than every SNF game (DirecTV) to me. It's not a huge picture quality change, but it makes the difference between a 720p picture that is on par with the best 1080i stuff versus one that makes 720p look inferior to 1080i (which it is not).


-Reagan
 

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conversely, comparing ABC OTA vs ABC on DISH I have to comclude that when they are not identical that the edge goes to the DISH feed.
 

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Here in Tampa fl I can not see a difference at all between OTA via HD5 and Cable via Pace hd550. I have the output of the HD5 ran through the Pace hd550 pass through, for a quick comparison. Also have compared the integrated OTA tuner in my mitsu ws55711 vs my Cable. My ABC affiliate does not multicast.
 

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Receiving and converting a digital signal from 8VSB to QAM should change anything, unless, as these chicago viewer seem to be indicating, a decompression/recompression cycle is being performed.


Is there a delay between the OTA and cable broadcasts? That might yield some clue.
 

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Discussion Starter · #11 ·
Quote:
Originally posted by Azanon
OTA is going to look better because you're directly receiving the incoming signals yourself with your antenna, instead of the cable company using thier antenna, THEN transmitting that signal to you. The signal is likely manipulated (unintentionally) in some way in that transfer process.
There is more then a slight difference between the two and to much to put if off as unintentional manipulation.


Quote:
Originally posted by jckessler

Is there a delay between the OTA and cable broadcasts? That might yield some clue.

Actually there is a 2 to 3 second delay between the two which does seem to indicate them doing something! Comcast get all the networks via fiber feed so there should be no human noticable delay from their receiving the streams from the networks.


Quote:
Originally posted by jckessler

Are you seeing compression, or just the result of inferior electronics on the output of the cable box?

I think it is probably a little of both but my main reason for starting this thread is to warn people considering cable over satellite/OTA that thye may want to consider the quality difference between the two options. The artifacting is definately from compression and was quite noticable monday night.

Quote:
Originally posted by jckessler

Ken H has reported that Comcast passes through the full 19.3Mbps HD signal unchanged, FWIW.

I have always heard Comcast (and most other cable companies) wanted to do 14Mb instead of 19.2Mb which is why CBS has been avoiding Comcast for the most part.
 

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I have not compared ABC, but I did some comparisons between the local PBS-HD OTA and Comcast signals, and the picture quality looked identical. Also, there was no delay between the two different broadcasts. Components used included a Samsung SIR-T151 OTA tuner, the Motorola 5100 cable HD tuner, an ISF-calibrated Toshiba 56H80 display, and a Pioneer Elite 47TX receiver performing component video switching between the two HD tuners.
 

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The cable companies don't have the gear to recompress the signal. Nor do they have the need, with the bandwidth available on a modern cable system.


Take a look at gear from Wegener, for instance (I'm not allowed to post URLs but it's obvious) to see what they're using. This receiver takes an 8VSB input and outputs a signal ready to go into the cable modulator.


Differences you're seeing probably just have to do with the set-top box.


Incidentally, a 256QAM cable channel runs at 38.8 Mbps. You can see that two 8VSB terrestrial signals fit nicely within one channel. Converting to 14 Mbps wouldn't help you any as you would still only be able to get two HDTV's within one cable channel.


JT
 

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Quote:
Originally posted by FrankS


Actually there is a 2 to 3 second delay between the two which does seem to indicate them doing something! Comcast get all the networks via fiber feed so there should be no human noticable delay from their receiving the streams from the networks.
Was the Comcast signal later in time ? I would expect that if they were

rate shaping agressively.
 

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MNF through Comcast looked fine. No artifacts at all. SNF looked great the night before too. This is on a SA 3100.


No shimmering or contrast problems for me.
 

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I would imagine that the reason comcast's feed is later is because they have to convert it to QAM then the box has to decode it.
 

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Quote:
Originally posted by 4runner4
conversely, comparing ABC OTA vs ABC on DISH I have to comclude that when they are not identical that the edge goes to the DISH feed.
To what ABC on DISH are you referring to ? Do you mean DIRECT TV ?
 

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This is an excerpt from an e-mail that I received from Comcast.


I recently posted this in another thread, but it appears that the rumors and speculation about compression continue.


It addressed CBS/KYW-HD carriage in Philadelphia and the rumor of rate compression:


"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 which Comcast multiplexes into a 6 Mhz channel. No compression is done to any of our HD signals.


The CBS KYW feed is transmitted 19.4 Mb/s via 8VSB modulation over the air, 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."



I would imagine that this is a company-wide policy.
 

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Quote:
Originally posted by FrankS
Actually there is a 2 to 3 second delay between the two which does seem to indicate them doing something! Comcast get all the networks via fiber feed so there should be no human noticable delay from their receiving the streams from the networks.
This is suspicious. I don't know for sure, but converting 8VSB to QAM I would think should be virtually instantaneous. Recompressing, rate shaping, whatever would obviously require a buffer, plus some processing time.

Quote:
but I did some comparisons between the local PBS-HD OTA and Comcast signals, and the picture quality looked identical. Also, there was no delay between the two different broadcasts. Components used included a Samsung SIR-T151 OTA tuner, the Motorola 5100 cable HD tuner
Hmmmmm. No delay, and no observed difference. vs. delay with observed difference. Something sounds fishy here.


More reports like this would be nice.
 

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Discussion Starter · #20 ·
Quote:
Originally posted by 4runner4
conversely, comparing ABC OTA vs ABC on DISH I have to comclude that when they are not identical that the edge goes to the DISH feed.
Dish does NOT have ABC in HD nor does DirecTV. We are talking about the HD channels NOT SD channels which you are obviously referring to.

Quote:
Originally posted by the q
This is an excerpt from an e-mail that I received from Comcast.


I recently posted this in another thread, but it appears that the rumors and speculation about compression continue.


It addressed CBS/KYW-HD carriage in Philadelphia and the rumor of rate compression:


"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 which Comcast multiplexes into a 6 Mhz channel. No compression is done to any of our HD signals.


The CBS KYW feed is transmitted 19.4 Mb/s via 8VSB modulation over the air, 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."



I would imagine that this is a company-wide policy.
I would hardly think Comcast would voluntarily admit to compression and would put out dis-information unless someone could catch them in the act where they can not deny it. If it's not compression it is definately something they are doing to make a less quality picture.

Quote:
Originally posted by samiam95124
Was the Comcast signal later in time ? I would expect that if they were rate shaping agressively.
Yes, Comcast's signal was the delayed one.
 
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