Comcast Begins Converting 1080i Channels to 720p to "deliver superior video."

[dubious]

Originally Posted by Naiera

And this change included the source material going from 1080i to 720p?

Yes, the transition was Comcast moving their channels to MPEG-4 as well as 720p.

Basically, based from what I read on the TiVo forums regarding this issue, is that Comcast was transmitting 4x MPEG-2 1080i30 channels per QAM channel in the prior configuration. Moving to MPEG-4 would essentially allow them to cut the bitrate per channel in half, allowing 8x MPEG-4 1080i30 channels to be stuffed into a single QAM. However, if you change the broadcast stream from 1080i30 to 720p60, you can stuff 9x MPEG-4 720p60 streams into a single QAM.

Comcast has cheaped out on their infrastructure upgrades over the years, so in order to expand channel offerings and broadband speeds, they need to free up QAMs. By stuffing more channels into each QAM they can offer "more" service over "better" service. A QAM is a constant bitrate modulation format with a capacity of 38.8 Mbits/sec.

Comcast also uses a variable channel bitrate system, so it can allot more bitrate to a channel showing an action movie over a news channel with static overlays, etc. This shift of bitrate allotment can only be shared by channels in the same QAM though, and all channels together are encoded together in the 38.8 Mbits/sec QAM.

So bascially, in the older MPEG-2 1080i30 QAM configuration, each HD channel used 25% of a QAM on average. If the transition had been to MPEG-4 and retaining 1080i30, then each HD channel would have used 12.5% of a QAM on average. In the new configuration, each of the MPEG-4 720p60 channels uses 11.11% of a QAM on average.

So Comcast was essentially giving MPEG-2 1080i30 channels 9.7 Mbits/sec.

If they had migrated to MPEG-4 1080i30 and doubled the number of channels in the QAM, this would equate to 4.85 Mbits/sec.

... instead they migrated to MPEG-4 720p60 with 9 channels per QAM, which is 4.31 Mbits/sec.

Don't quote me for accuracy, I'm relaying and simplifying information here from other forums I've read, but those numbers are the breakdown as far as I understand it at the moment.

1920*1080 = 2073600; * 30 FPS = 62208000; * 8 Channels = 497664000
1280*720 = 921600; * 60 FPS = 55296000; * 9 Channels = 497664000