Join Date: Mar 2003
Location: State College, Pennsylvania
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Digital cable is different than analog in that received signal level doesn't directly impact picture quality, that is to say a signal many dB lower than another may appear identical to the viewer. Of course, there are limitations.
Our HD channels are delivered over a channel using QAM256 modulation. Included in this bit stream are Forward Error Correction bits, basically, parity bits to allow the receiver to rebuild corrupted data in the bit stream to provide the same picture you would see as though you captured 100% of the data perfectly right up front.
With no FEC the 6 Mhz QAM256 channel would provide around 41.6 Mbps, the FEC takes around 2-3 Mbps so payload ends up in the 38-39 Mbps range. Because the signal-to-noise ratio (SNR) on cable can be managed better than OTA a more robust coding scheme can be utilized with fewer FEC bits.
If your signal drops below the required SNR for the channel type you're watching you'll either see outright breakups, corruption of the image, or dropouts, depending on how gracefully your receiver handles it. On analog you'd see this as fuzziness, static, loss of sync, etc, but with digital it really is nearly ALL or NOTHING.
For those looking at WJAC-DT as a comparo to others, keep in mind they likely are providing high motion 1080i in something on the order of 12-13 Mbps, perhaps less. This results in much macroblocking and poor image during movement. NBC, as a network, is also known to have some trouble with image quality. Games presented on the Mouse's family of networks are all 720p which is less bandwidth intensive and is delivered at an appropriate rate, as best as I can tell, by our local providers.
As to drop off on other channels, I can't really speak to that. My impression is that Adelphia passes through the stream as they receive it without any rate shaping or re-encoding. Certainly the picture from Adelphia is better than DirecTV.
As to the DCT6412, this box is brain dead. I don't know if it was rushed or Motorola expects us to believe this crap is the best they can do. It gets quite wonky when you expect it to do any normal. I hope the Comcast/Tivo software for the box isn't too far out.