Originally Posted by posg
Pure speculation, but they have been testing HD on a couple of channels (123 & 124) in Raleigh. Each had one 720p and three 1080i channels running (duplicates of current channels). How'd they do that ???
Someone in my local NYC TWC forum mapped the frequencies of all the HD channels, with one frequency for 3 HD channels excerpted here
. To squeeze in 4 HDs, even if one is in the more easily compressible 720p format, likely involves some heavy rate shaping (see link) with requantization
, although rate-shaping hardware/software firms have long claimed they can fit 4 HDs where only ~2 went once (256-QAM, ~39-Mbps cable slots). Suspect the only way to judge how much PQ may have been been lost in the 'squeeze' is with direct A-B comparisons and rapid switching with 'untinkered' sources--unless someone has access to instruments such as spectrum analyzers and attempts to measure differences, or makes easier bit rate comparisons.
Just to add to the optimistic speculation, TWC, after testing switched video (SV) in a few states, supposedly plans on rolling it out nationwide, either this year or within several years, depending on what you're reading. SV, only delivering programming when it's requested--somewhat akin the video on demand--can save a lot of bandwidth on cable systems for more HD channels and other services. -- John