I read a very enlightening story on /. yesterday. The Slate.com story was about watching Video on Demand (VOD) which other people nearby had selected. It turns out that comcast is not nearly as sinster as I would have made them out to be. I’ll summerize the important and non voyeristic results of the story and my testing below.
Before yesterday, I had always assumed that because they could cables companies would intentially obfuscate all signals carried over thier wires with the exception of the 50 or so channels which must be carried in over the air (OTA) format for all those legacy cable ready TV sets. Since they say that you need a cable box to recieve any other channels, I assumed that they were technically correct. It would not be difficult to slightly modify any existing transmistion protocol to the point were only “in the know” cable boxes could decipher the signal and display it. You don’t even have to eyncrypt the content to prevent its display; although, that is certainly a more fulproof method. Given how much cable and content companies are spending on DRM these days I took it as a mater of course that I was out of luck getting HD (or any digital) signal off of my cable wire without the appropriate $5-10 a month box.
But I was wrong! Comcast, at least in the DC (see above story) and Boston areas, is much more lazy than they are evil. They transmit HD signals for your normal OTA channels in the clear along side the the analog OTA channels, over their cable in a format called QAM. Now, it just so happens that my new HDTV has a built in QAM tunner. At the sugestion of the Slate.com story I tired it out, hooked a cable straight from the wall to my TV, and told my TV to search out the channels.
It found ~120 channels. Now, that includes the 50 or so analog channels as it also has a normal analog NTSC tunner in it. (To round out the info the tv also has a ATSC tunner which is the format for OTA HDTV). That number does not inlcude the sub, dot, or hyphen channels depending on what you want to call them. A digital tv singal can contain some number of sub channels it appears at least 14. The available bandwidth is split between these channels so you can say run 1 subchannel at 1080i resoultion, but you could run say 4 at 720p resoultion, or even more if you intended to only display standard 480i content you could run 14 or something nuts like that.
Most these subchannels above channel 80 were broadcasting black, not no signal — actually broadcasting black. Three of these were actually broadcasting video. One of them was playing V for Vendetta, another some unkown movie, and third one was playing something with an MTV on Demmand logo in the lower right — confirming that these are the on demand channels as described in the story. There is little reason why we would want to watch this stuff, but its interesting that we can. I am paying for digital cable so non of this should be content for which I am not already paying. Anyways, I am quite excited about the OTA HD channels coming on the cable because it means I won’t have to futz around with an antenna.