Up until last month, we only had extended basic cable TV at my house. Having cable ready TVs, that meant we didn’t have to worry about cable boxes. Just plug the cable into the TV and bingo, 70 channels of varying levels of shlock.
Well, the cable company lured us in with promises of wonders (ok, how about promising big discounts on our phone bill) and we now are reluctant subscribers to digital cable. Instead of 70 channels, we’re now approaching 200.
To access these wonderful additional channels, we now have a digital cable box. The cable goes in, the cable comes out, and POOF more homeshopping channels then you can shake a credit card at. What does that box actually do? Why can’t I still just plug my TV into the cable coming out of the wall? The TV thinks it can be tuned to channel 198, but without the magic box, all I get is snow. Is it actually just a translater, allowing me to view a signal that is really being sent to all my TVs anyway, but they just don’t speak the same language?
It’s really a computer that’s specialized to the task of taking in an MPEG-4 digital datastream and converting the appropriate bits into an NTSC video signal and audio.
There’s no comparing it to analog cable - ie: there’s no “channel 57” datastream next to the “channel 58” data - it’s one big deluge of bits all intertwined, and the cable box sorts it out.
Can I piggyback on this thread and ask if it sees HDTV as different? 'Cause it’s anamorphic on my TV, for one, and my boyfriend says he can’t make his homemade Tivo get it but then he explained a bunch of letters and I tuned out. My cable box is also a DVR - does that handle the HDTV differently too?
That’s not how it works. The bandwidth of the cable system is divided up into 6 MHz channels. Each channel can carry one analog TV signal or one digital signal. The digital signal can contain many streams of digital TV, along with data for user cable modems. The same bandwidth that carries one analog TV channel can instead carry 8 digital TV “channels”, or a gazillion music channels. The cable operator chooses which channels to operate in analog mode and which channels to operate in digital mode, depending on the needs of the system.
The digital cable box includes a cable modem, microcomputer, MPEG decoder, and NTSC encoder. When you tune it to digital channel 666, it looks up the corresponding 6 MHz channel, say 53, tunes the cable modem to that channel, and starts processing any packets that are labeled for digital channel 666. It extracts the MPEG encoded digital video from those packets, decodes it back into analog video, and sends the analog video to your TV. The uplink side of the cable modem is used for things like pay-per-view, video-on-demand and other interactive applications. The microcomputer is also responsible for maintaining the channel guide. Some information, like the mapping of cable box channels to real channels, is broadcast periodically. More detailed information, like the description of the movie currently playing on channel 88, can be requested from the cable company’s servers.
The digital cable box is also responsible for security. It knows what channels you are authorized to receive, and it has the keys needed to decrypt any encrypted video stream that you are authorized to watch.
As long as we’re on the subject of cable boxes and how it filters the signal, I have a question. About a week or so ago I stopped getting three of the local HD channels on my box. It just shows a message saying “This channel should be available shortly” but it’s been that way for days. And it’s only three of the six or seven local HD channels. Everything else is fine. Anybody know why just a couple of random channels would drop? The not-so-helpful tech support guy I talked to at the cable company suggested I unplug the box and plug it back in, but all that did was make me lose my program guide for about a half hour. The channels are still not there.