Contemporary Research QMOD-SDI Integration Guide - Manual (Page 12)

Contemporary Research 12 QMOD-HD Integration
Combining Channels
Somewhere at the cable company head end, the company combined all the different cable channel
modulators in to one RF delivery system.
A Modulator receives an analog or digital video signal on one end, and converts it to single
channel on the other. Each modulator can be set to a different channel.
Remember those little TV splitters you’ve used to drive one to four TV sets from the same antenna
or cable feed? A combiner is the same idea – you just use it the other way. The four channels go
into the four inputs on the combiner, sent out as one cable at the other end. The four channels can
mix together because they are set to different frequencies, so they don’t interfere with each other.
Your RF equipment supplier can suggest a number of commercial combiners to use for your
application.
Inserting Channels
Inserting a QMOD HD channel into the system above is a simple matter. Use a Channel Filter to
block out QCTV (which nobody watches in our company) and insert the QMOD on the same
frequency.
There are several variations on this design. If a cable feed has a mix of analog and digital channels,
you want to block the analog channels – a digital channel can carry 12 programs, so you usually
don’t want to block those.
Many cable feeds don’t go over channel 100, so it’s often easier to go to 105 or where the cutoff is.
Note that some in-house cable systems can’t handle higher channels, so the right answer depends
on several factors. We’ll discuss those options more on the Measuring RF section.