<<Is it conceivable, in the future, that the "cable TV" signal can be sent thru ADSL ? I enjoy TV and I enjoy ADSL for my internet access. If the two can be bundled by one supplier, I would not mind at all. However, at this small city, there is only separated services - cable tv and ADSL.>>
Hello Q-sand, VoD (video on demand) was one of the first intended uses for DSL. In terms of conceivable, sure. You'll hear arguments that try to persuade you it's not even theoretically possible because the available bandwidth wouldn't even approach the bandwidth (and therefore the total number of channels) available on your cable system's cable.
That bandwidth comparison alone is true, but that doesn't preclude the possibility of virtually unlimited DSL-based TV channels given a main feed that has zillions of channels. Switching and Multiplexing are the keys.
This is kind of hard to explain in just a few short paragraphs so I hope this gets the concept across: The main feed contains potentially zillions of channels packed into many 6 MHz channels. Each one of those 6 MHz channels itself contains several channels.
Based on the user channel selection on the TV, the upstream DSL path selects the appropriate one of these 6 MHz channels (this channel includes the one you selected on screen, as well as some others) from the main 50-550 MHz feed (or whatever), and demultiplexes it. Then the specific digital channel (the one you chose on your TV) is stripped out of this 6 MHz channel, and sent to your DSL modem (I skipped some details to simplify).
The modem then (demodulates it and) sends it to your TV (again skipping some details). So any TV channel within the 50-550 MHz band can be selected by referencing 2 channels: the 6 MHz channel within the 50-500 MHz band, and the specific channel you selected from within the 6 MHz channel. This is hidden from the user. The user just selects a channel the same old way.
In this way, the DSL connection could support any number of channels. If you throw picture-in-picture into the mix, things change because then the bandwidth needed of the DSL connection increases, but for single channel viewing it's perfectly capable for today's TV.
This, by the way, is similar to the hierarchical way a DBS (digital broadcast satellite) channel selection happens (select transponder #, then specific channel), except all this selecting, switching, and multiplexing happens at the service provider in the DSL case.
dh
To my knowledge, no such system has been implemented--but you only asked about "conceivable." |
