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PostPosted: Tue Feb 24, 2009 6:36 pm 
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I just got off the phone with my cable provider (comcast). The woman I spoke with tried telling me that the "HDTV" that's being broadcast over the air (ATSC) is not the same qualitywise as the QAM signals that comcast can provide to me via their box for a monthly fee of an additional $7 per TV. Is this statement accurate?

Isn't "720p" whether it's OTA or QAM-encrypted from the cable company the same 1280x720 progressive mpeg2 stream?

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PostPosted: Tue Feb 24, 2009 6:42 pm 
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graysky wrote:
I just got off the phone with my cable provider (comcast). The woman I spoke with tried telling me that the "HDTV" that's being broadcast over the air (ATSC) is not the same qualitywise as the QAM signals that comcast can provide to me via their box for a monthly fee of an additional $7 per TV. Is this statement accurate?


Ha! Yeah, assuming you get a decent signal in your area, the OTA is better quality . . . and it does not cost $7/mo. The cable co's have to do at least a little processing of the signal to get it on their system. As far as I know, they are not sending mpeg2 down the pipe. Most likely they're sending mpeg4.

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PostPosted: Tue Feb 24, 2009 6:49 pm 
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That's what I thought... she was either misinformed or knowingly attempted to deceive me.

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PostPosted: Wed Feb 25, 2009 10:10 am 
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graysky wrote:
That's what I thought... she was either misinformed or knowingly attempted to deceive me.


Nah -- she was absolutely 100% correct. OTA is not the same quality-wise as QAM... it's better. Did she say otherwise? :D


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PostPosted: Wed Feb 25, 2009 10:19 am 
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graysky, as long as you are getting good signal strength, they are the same. For a while Comcast hidef was not as good as OTA as they were reducing the bitrate on their hidef channels in order to fit more channels on the pipe. In my area they are dropping some of the analog channels (33+), so I suspect this will give them the bandwidth to restore full bitrate on the hidef channels -- time will tell.

Another thing they don't tell you: If you subscribe to the "basic" package, the box they give you won't tune any hidef channels and they leave you with the impression that you need to pay for a more expensive package to get any hidef channels. In my area (SF / bay area) the OTA hidef channels are available to you even with the most basic package as long as you have your own clear-QAM tuner.

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PostPosted: Wed Feb 25, 2009 3:26 pm 
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It is true that not all HD signals are the same. The MPEG2 encoder and the bit rate have profound effects on the resultant MPEG2 stream.

For example my local NBC station broadcasts their main channel at 1080i and also has two sub-channels at 480p. Their bit rate on the 1080i main channel is lower (and therefore shows more blocking and mpeg artifacts) than the local CBS station which also broadcasts in 1080i but only has one 480p sub-channel. The CBS affiliate is able to run a higher bit rate on the main 1080i channel than the NBC affiliate because of only having the one sub-channel. Also the CBS affiliate seems to have a newer and probably better mpeg2 encoder.

So why did I just tell you all that? Broadcast ATSC channels are limited to ~19.2 Mbps whereas cable operators using QAM can have bit rates up to I believe 39.4 Mbps per channel. (Which is why you sometimes see up to a dozen or more sub-channels on one QAM main channel.)

So theoretically cable operators (using QAM) can broadcast at higher bit rates than OTA which is probably why they make the statement that cable HD is "better" than OTA. The problem is that in this case Comcast is receiving the source from the local stations and I'm certain it is at whatever resolution and bit rate those stations broadcast OTA with. Comcast certainly isn't able to make the signal better than what they receive from your local station. (Of course they can certainly make it worse if they re-encode it prior to broadcast!) Technically they could re-encode the source from the local station to a higher bit rate prior to broadcast but all that would accomplish is re-encoding the "noise" that's already there plus adding more - it's still a net lose. It's like dubbing a cassette tape to a CD - sure it's now digital but it isn't going to sound any better than the tape you dubbed it from.

Taking the discussion even further, I think you will find that most cable providers are actually receiving the local stations OTA and then rebroadcasting them. I know both Comcast and Verizon in my area do. I've seen the antennas on the side of their buildings and it's been confirmed by local station engineers. Eventually the signals will come to the Cable companies over fiber but most areas don't have that infrastructure in place yet.

How's that for making a short answer long?

Martian

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PostPosted: Wed Feb 25, 2009 3:48 pm 
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Thanks for the replies, all!

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PostPosted: Thu May 14, 2009 11:58 am 
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graysky wrote:
that comcast can provide to me via their box for a monthly fee of an additional $7 per TV.


I'm pretty sure that you'll find at the very least the local channels in the clear in HD/QAM from comcast with no extra $/month (as mandated by the FCC), in fact, I'm getting close to 100 digital channels from them right now! Yep, all of expanded basic, in the clear, sweet.

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PostPosted: Thu Jun 04, 2009 1:58 pm 
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Martian wrote:
How's that for making a short answer long?

Martian

I love getting edja-macation on all that that stuff.... :lol:


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PostPosted: Tue Jan 19, 2010 3:25 pm 
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Just to supplement Martin's explanation.

There are two ways your cableco might have a higher quality than OTA.

First is simply the cableco signal should have a very high SNR, while OTA is subject to the vagaries of RF reception via an antenna. So just like in the analog days where cable was better because it didn't have snow etc, HD cable is better because it doesn't have audio drop-outs and picture breakup.

Second is where the cableco is receiving the feed before the station compresses and muxes it for transmission instead of receiving the feed via their own antenna farm. This also assumes the cableco then compresses the feed at a comparable or higher bitrate with comparable or higher quality encoders.

Otherwise, the best that your cableco can do is pass the OTA bitstream along to you unaltered. However, this is not always the case and they may be "rate shaping" (i.e. recompressing to a lower bitrate) in order to fit more channels into a single QAM.


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PostPosted: Wed Jan 20, 2010 2:13 pm 
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poflynn wrote:
graysky wrote:
I'm pretty sure that you'll find at the very least the local channels in the clear in HD/QAM from comcast with no extra $/month (as mandated by the FCC), in fact, I'm getting close to 100 digital channels from them right now!



I tried scanning my local comcast line (I'm unfiltered) and before wrecking my OTA HD list, I tried the pcHDTV's dtvscancommand - with weird results.

I get a lot of

Code:
channel = 111 QAM_256
Signal: 93
look for sync
got to here
PAT found ckets
PAT found ckets
PAT found ckets
PAT found ckets
PAT found ckets
PAT found ckets
PAT found ckets
PAT found ckets
Unable to recieve valid packets
did find pat
`ERROR vct item ptr null couldn't get channel info


With one channel spitting out:

Code:
Signal: 95
look for sync
got to here
PAT found ckets
demux_ts: demux error! invalid payload size -26
demux_ts: demux error! invalid payload size -59
demux_ts: demux error! invalid payload size -8
demux_ts: demux error! invalid payload size -72
demux_ts: demux error! invalid payload size -69
PAT found ckets
PAT found ckets
PAT found ckets
PAT found ckets
PAT found ckets
PAT found ckets
PAT found ckets
Unable to recieve valid packets
did find pat
`ERROR vct item ptr null couldn't get channel info


and

Code:
Signal: 89
look for sync
got to here
PAT found ckets
demux_ts: demux error! PAT with invalid CRC32: packet_crc32: 00000000 calc_crc32: 9316d875
PAT found ckets
demux_ts: FIXME: (unsupported) PAT consists of multiple (255) sections
PAT found ckets
demux_ts: demux error! PAT with invalid CRC32: packet_crc32: 00000000 calc_crc32: 9316d875
PAT found ckets
demux_ts: FIXME: (unsupported) PAT consists of multiple (255) sections
PAT found ckets
demux_ts: demux error! PAT with invalid CRC32: packet_crc32: 00000000 calc_crc32: 9316d875
PAT found ckets
demux_ts: FIXME: (unsupported) PAT consists of multiple (255) sections
PAT found ckets
demux_ts: demux error! PAT with invalid CRC32: packet_crc32: 00000000 calc_crc32: 9316d875
PAT found ckets
demux_ts: FIXME: (unsupported) PAT consists of multiple (255) sections
PAT found ckets
demux_ts: demux error! PAT with invalid CRC32: packet_crc32: 00000000 calc_crc32: 9316d875
PAT found ckets
demux_ts: FIXME: (unsupported) PAT consists of multiple (255) sections
PAT found ckets
demux_ts: demux error! PAT with invalid CRC32: packet_crc32: 00000000 calc_crc32: 9316d875
PAT found ckets
demux_ts: FIXME: (unsupported) PAT consists of multiple (255) sections
PAT found ckets
demux_ts: demux error! PAT with invalid CRC32: packet_crc32: 00000000 calc_crc32: 9316d875
PAT found ckets
demux_ts: FIXME: (unsupported) PAT consists of multiple (255) sections
PAT found ckets
demux_ts: demux error! PAT with invalid CRC32: packet_crc32: 00000000 calc_crc32: 9316d875
PAT found ckets
demux_ts: FIXME: (unsupported) PAT consists of multiple (255) sections
Unable to recieve valid packets
no pid or atsc tables found


So I guess my only question is: Should I be scanning with MythTV's scanner? or is something else going on?

Thanks,

-Ben

EDIT: turns out, that scan came up with this output file:

Code:
[root@MythTV ~]# cat comcast_scan.txt
      :      :14.1 :14: 2:1
      :      :16.1 :16: 2:1
      :      :21.1 :21: 2:1
      :      :22.1 :22: 2:1
      :      :61.1 :61: 2:1
      :      :64.1 :64: 2:1
      :      :65.1 :65: 2:1
      :      :67.1 :67: 2:1
      :      :68.1 :68: 2:1
      :      :71.1 :71: 2:1
      :      :72.1 :72: 2:1
      :      :73.1 :73: 2:1
      :      :74.1 :74: 2:1
      :      :75.1 :75: 2:1
      :      :76.1 :76: 2:1
      :      :77.1 :77: 2:1
      :      :78.1 :78: 2:1
      :      :79.1 :79: 2:1
      :      :80.1 :80: 2:1
      :      :81.1 :81: 2:1
      :      :82.1 :82: 2:1
      :      :83.1 :83: 2:1
      :      :84.1 :84: 2:1
      :      :85.1 :85: 2:1
      :      :86.1 :86: 2:1
      :      :87.1 :87: 2:1
      :      :88.1 :88: 2:1
      :      :89.1 :89: 2:1
      :      :90.1 :90: 2:1
      :      :91.1 :91: 2:1
      :      :92.1 :92: 2:1
      :      :93.1 :93: 2:1
      :      :94.1 :94: 2:1
      :      :98.1 :98: 2:1
      :      :99.1 :99: 2:1
      :      :100.1 :100: 2:1
      :      :101.1 :101: 2:1
      :      :102.1 :102: 2:1
      :      :103.1 :103: 2:1
      :      :104.1 :104: 2:1
      :      :105.1 :105: 2:1
      :      :106.1 :106: 2:1
      :      :107.1 :107: 2:1
      :      :108.1 :108: 2:1
      :      :109.1 :109: 2:1
      :      :110.1 :110: 2:1
      :      :111.1 :111: 2:1
      :      :112.1 :112: 2:1
      :      :113.1 :113: 2:1
      :      :114.1 :114: 2:1
      :      :115.1 :115: 2:1
      :      :116.1 :116: 2:1
      :      :117.1 :117: 2:1


So maybe it is working.. Now I just need to figure out the scte stuff mentioned here http://www.mythtv.org/wiki/Comcast_Users_And_scte65scan. Anyone have the VCT_ID for Champaign, IL?


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