Comcast Media Center gets creative with compression
by Darren Murph 
posted Dec 14th 2007 at 8:01AM
Make no mistake about it, as the bandwidth belt tightens around cable, carriers are grasping for ways to cram more channels into an increasingly small space. Reportedly, Comcast Media Center has devised a method to stuff three HD signals into a single 6MHz carrier, which is typically just enough to handle two HD channels without picture quality taking a dive. Though the process sounds quite technical, the long and short of it is that a "second-pass MPEG-2 encoding system from startup Imagine Communications" is reportedly being used to "stack together three signals at variable bit rates into one 6MHz QAM channel." Of course, it's hard to say if this clever methodology results in noticeably poorer picture quality, but unless something drastic happens in the world of coax, you can count on seeing a whole lot more where this came from.
Reader Comments (Page 1 of 1)
Jim Mallory @ Dec 14th 2007 11:20AM
This article is a little inaccurate. Three HD channels (when properly placed) can be placed on single QAM256 carrier at 39 mbps. No HD channel runs at a full 19 mbps constantly...the bit rate is dependent on the type of programming being ran (example CNNHD will use less bandwidth than a CBS football broadcast) or if it is 720p or 1080i (720p tends to max out at 15 mbps). So if you are careful on where you place things, you can statistical mutiplex 3 HD channels together and not take a hit in picture quality. Now there are cases either due to a.) bad planning or b.) an unusually high bit-rate on all the stat mux'ed channels that you could run out of bandwidth. The problem with a stat mux is that it will start to drop bits on the floor which rapidly degrades picture quality. the one advantage of this "2nd look MPEG2 encoder" is that it may be a bit more graceful in these conditions. Maybe it even has a way of prioritizing what channels get "recompressed" first. Maybe recompress CNNHD before NFL Network? Definitely interesting that you can do that pretty much in real-time now.
Diceburna @ Dec 14th 2007 1:05PM
It would be nice if someone at Imagine Communications can explain how this 2nd look MPEG2 encoder works. I dont think they'll cram channels like CBS, Fox, ESPN, etc on that type of bandwidth. Maybe channels like DiscoveryHD, TBSHD, NatGeoHD, etc will be better suited for this like you've stated in your comment.
looseinthedeuce @ Dec 17th 2007 1:39PM
Bright House Networks in Michigan does this 3:1 compression on a few HD channels. The non-technical response to Jim Mallory, is that this compression looks bad.
I noticed poor picture quality on a few stations, and found out these channels were being over-compressed. Let's not kid ourselves, compression is compression. ANY compression will produce artifacting, even a 25mb/s signal (although this would look quite good). Compressing HD channels down to ~12mb/s from 16mb/s is indeed noticable.
OTA HD signals look the best, but I'll at least agree to 2:1 with 256 QAM just so it doesn't ever get worse than this. NO MORE 3:1, PLEASE!!