In article ,
Dave M wrote:

I forget what they call it here but to save bandwidth the cable "box"
only streams TV to your set on the channel you are watching. That way
your converter is not being sent bandwidth from every channel. Does
that make sense? It really doesn't to me since you can only watch one
channel at a time.

That can't be correct. How does the cable company know which channel you
want to watch? That's the only way bandwidth into the box could be reduced.
The only bandwidth reduction would be between the cable box and your TV set,
which is only getting channel 3 instead of the entire TV RF spectrum.

I think there are two different issues being conflated here.

The biggest gain, in switching to a digital distribution system, is
that it's possible to carry several digital programs within one 6 MHz
channel allocation (which is only enough to carry one analog NTSC
channel). Cable operators love the idea of being able to make more
channels available through their existing cable plant, and they could
carry the equivalent of 3 or 4 standard-definition programs (with
decent video quality) in one channel slice.

That's the first part of the "bandwidth savings". All of these
channels are available simultaneously to the subscriber (although any
given set-top box or TV is only able to tune to one of them at a time,
unless it has picture-in-picture capability).

This is the way that almost all digital cable-TV set-top boxes work,
or have worked up until recently.

There's another technique coming into use on some cable systems -
"switched digital". In this system, some of the less-frequency-viewed
channels are not being sent down into any particular part of the cable
plant at all times... they're only transmitted when somebody wants to
watch them. This *does* require "upstream" signalling from the cable
set-top box to the cable head-end. If a viewer tries to tune to an
obscure channel (say, the latest Bollywood film translated into
Polish), the set-top box sends a message over the cable net to the
head-end, saying "I want to see station #87265.35, please". The cable
head-end, finding that it's not currently sending that station,
locates a free frequency on the cable system, starts downlinking that
digital station on that frequency, and sends a message back to the
set-top box saying "OK, tune to channel 78 and decode digital
substream 4". The system keeps transmitting that obscure station
until the last viewer watching it changes to a different channel...
and at that point the head-end can stop transmission and reclaim the
frequency.

This approach yields "bandwidth savings" by allowing a cable operator
to offer a vast number of different channels (including numerous
video-on-demand / pay-per-view selections) without having to reserve
cable bandwidth for all of them, all of the time.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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