ATSC to QAM - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 10 Old 12-03-2009, 09:57 AM - Thread Starter
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Just a question for curiosity's sake. When the cable company remodulates an ATSC OTA signal to QAM are they using the exact Video stream from the ATSC or do they re-encode the stream when they remodulate it?

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post #2 of 10 Old 12-03-2009, 10:41 AM
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Short answer Yes.
After demodulating the off air signal the same signal is just remodulated to a QAM channel.
This only applies to OTA.
All other signals may be reencoded.
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post #3 of 10 Old 12-03-2009, 11:55 AM
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Quote:
Originally Posted by RCbridge View Post

Short answer Yes.
After demodulating the off air signal the same signal is just remodulated to a QAM channel.

So why does cable lose image quality? In my area, my cable picture is equal to my OTA picture as long as the image is static. As the degree of motion increases, the quality of the OTA picture is visibly superior to the cable picture.

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post #4 of 10 Old 12-03-2009, 12:13 PM
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Quote:
Originally Posted by WS65711 View Post

So why does cable lose image quality? In my area, my cable picture is equal to my OTA picture as long as the image is static. As the degree of motion increases, the quality of the OTA picture is visibly superior to the cable picture.

Because they reduce HD image quality in a number of ways.

It varies, but many cable systems use 3 per QAM while others do that and real time rate shaping, and/or resolution reduction. I've even been told some cableco's pack 4 HD channels into a QAM.....

2 HD channels per QAM is considered a pass through that if done properly does not negatively affect HD image quality. Going to 3 per QAM is the rough ATSC equivalent of 12.7Mbps per channel, compared to a full ATSC payload of 19Mbps.

Comcast for example, passes local HD and some channels like ESPN, TNT, Versus at 2/QAM, and others come from Comcast Media Center (CMC) at 3/QAM; to my knowledge, they do not additionally compress HD with resolution reduction or rate shaping. CMC is a sophisticated system that uses state of the art technology from a central point to distribute channels via satellite and fiber across the nation. It's not ideal, but it compares very favorably to most other options, except Verizon FiOS, which is the only national HD provider that does not additionally compress or otherwise reduce HD image quality when received from the source.

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post #5 of 10 Old 12-03-2009, 12:29 PM
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Quote:
Originally Posted by olyteddy View Post

Just a question for curiosity's sake. When the cable company remodulates an ATSC OTA signal to QAM are they using the exact Video stream from the ATSC or do they re-encode the stream when they remodulate it?

Depends on your cable provider, they can do it either way.
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post #6 of 10 Old 12-03-2009, 01:51 PM
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Quote:
Originally Posted by WS65711 View Post

So why does cable lose image quality? In my area, my cable picture is equal to my OTA picture as long as the image is static. As the degree of motion increases, the quality of the OTA picture is visibly superior to the cable picture.

Quote:
Originally Posted by Ken H View Post

Because they reduce HD image quality in a number of ways.

It varies, but many cable systems use 3 per QAM while others do that and real time rate shaping, and/or resolution reduction. I've even been told some cableco's pack 4 HD channels into a QAM...........

Charter Northshore, Louisiana does exactly this. They have HDNET, HDNETMOVIES & HD Theatre on one QAM carrier, a travesty of biblical proportions. I can see the degradation constantly when viewing the channels.

They also carry three local networks on one clear QAM carrier. There are two other QAM carriers with 3 HD channels each. In fact there is one carrier with four HD channels, though they are of questionable quality to begin with. The local engineer claimed that I was the only complaint concerning this issue.

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post #7 of 10 Old 12-03-2009, 03:44 PM
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Bright House of Bakersfield has KGET NBC HD, KERO ABC HD, CW24 SD and V-Me SD all on RF channel 79. KGET and KERO OTA have subchannels, so the bandwidth on cable is probably not compressed anymore than the OTA signal, leaving plenty of room to put the SD channels on the same RF channel with them.

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post #8 of 10 Old 12-04-2009, 03:04 AM
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Quote:
Originally Posted by raouliii View Post

The local engineer claimed that I was the only complaint concerning this issue.

In the "good old days" you could write a complaint letter to them and they had to keep it in their "Public File" for anyone to be able to view during business hours. Don't know if that's still the FCC reg. If so, you could always go there and look to see if anyone else has complained by snail mail.

A quick google found this, but I'm too lazy to keep googling to see if complaint letters are still part of "The specific documents required to be maintained in the public inspection file".
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post #9 of 10 Old 12-04-2009, 11:06 AM
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Originally Posted by Phantom Gremlin View Post

In the "good old days" you could write a complaint letter to them and they had to keep it in their "Public File" for anyone to be able to view during business hours.

And here is the "modern day Public Complaint File":

http://www.buzzle.com/img/articleImages/211224-4med.jpg
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post #10 of 10 Old 12-04-2009, 12:10 PM
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TWC and Charter reencode locals in most areas. Comcast, Cox, and Verizon generally pass through. Other providers are a real crapshoot. There's a tradeoff between channel space and expense for the encoders, so it really depends on how badly they need the extra space.
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