HD 101: What is ATSC, PSIP, QAM, and 8-VSB?

qam posts


Comcast is stepping up its busy HD rollout (and channel squeezing) schedule, with several new channels spotted today in the Detroit and Chicago areas. Science HD, Disney HD and ABC Family HD are probably in your local channel guide -- Discovery HD is now listed DSCHD, while Disney is DISHD, confused yet? -- and word is they should hit many other areas Wednesday. Posters at AVSForum indicate more of the 3 channels-on-one-QAM compression tactics are afoot, take a look and let us know if having High School Musical 2 in HD still satisfies.
It's been a while since we checked in on the MythTV project, just updated to version 0.21, most notably adding QAM recording for HDHomeRun owners. If merely recording unencrypted HDTV via cable isn't enough, upgraded users may notice lowered memory consumption, the ability to record main and/or secondary audio channels, a couple new official plugins and a bunch of other tweaks. Check the wiki for the full list and get your Linux DVR going.
In the linked interview, Bresnan Communications' VP of strategic engineering, Pragash Pillai, doubles the company's earlier HD channel plans for 2008, moving the mark up to 50. He also indicates that figure does not include the couple hundred hours of HD VOD (video on demand) that's also planned for the year. The tech plans detailed by Pillai are interesting: Besnan has about 96-percent of its systems at 750 MHz and above, and will be moving to 256 QAM (quadrature amplitude modulation) to get the most out of its bandwidth. In the end, he expects 2.5 HD channels per 6 MHz QAM channel. Looking to 2009, he also expects to see rollout of MPEG4 to homes as necessary. This kind of long-term planning is encouraging to see, especially from a smaller player -- infrastructure improvements are costly, and only companies that see the HD future will be prepared to keep growing their offerings and feeding our appetite.
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.







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