On Wed, 06 Jul 2011 18:27:50 -0700, Dave Platt wrote:

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.

Thanks for the clarification. I'm not up enough with the technology
other than to read the service menus on my DVR and remember words like SDV
, Carousel read erros etc... and know the TV cards operate via tcp/ip.
I also did a little reading and the SDV does operate here as you
describe. Only sending out more frequent watched content from the headend
via fiber then to the wire nodes hence saving bandwidth.



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