Re: Head-end Technicalities

On Thu, 24 Apr 2008 09:34:01 -0400 Cinder Lane <Cinderlane@xxxxxxxxx> wrote:

| What are the steps at a CATV head-end facility to convert 8VSB from an
| off-air receiver to QAM for remodulation?

Buy a box that does it, and configure it according to the system channel
scheme.


| Must each subchannel be broken out and processed separately before being
| recombined into its new channel?

Since QAM256 has twice the bandwidth as 8VSB (per 6 MHz), it would not make
sense not to break them out. They could pass the whole transport stream of
an off-air signal as-is to a cable channel. And maybe there is an easy way
to merge two of them together into one channel space on the cable. But the
approach to breaking them apart and arranging them in various ways on the
cable channel scheme provides the most flexibility.


| Does each modulator have a finite number of inputs (for the
| subchannels), or are the subchannels combined in a separate device
| before being fed into the modulator?

Now days, there's a big box doing lots of this at once, with plug in "blades"
for various modulators and demodulators. I don't know how many channels one
big box can do.


| Is the PSID encoder a part of each QAM modulator, or must PSID
| information be inserted into each feed *prior* to being fed into the
| modulator?

It is part of the bit stream that is assembled before modulation. A box that
primarily serves as a modulator may also be able to assemble the bit stream,
so I can't say just how the duties are divided up when separate boxes are being
used. It may vary depending on what boxes are used.

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