RF Distribution: A Refresher Course

Many installation companies consider the baseband signal distribution system to be the only respectable way to distribute video around the house.

The equipment (made by Crestron, Elan, Niles and many others) provides numerous baseband inputs, such as composite, component, DVI and even HDMI, from source devices. It provides matrix switching functions to route the signals to locations around the house in a star topology (home-run cables to each outlet/device location). The less sexy approach of simply modulating A/V sources onto the cable system to be viewed on any TV seems to have gone out of style.

Some installers associate the word “modulator” with “crummy picture” because, so often in the field, modulators are installed incorrectly. Levels can be incorrectly measured. The equipment can be connected to a poorly designed coax infrastructure — there are a number of common mistakes made in modulating A/V sources this way. Even CEDIA has pared down its RF distribution course offerings.

Perhaps a contributing factor to the demise of correct modulation was the confusion over the analog broadcast shutdown. It's important to note that this was not the end of analog transmissions, just broadcast analog. Cable operators will still be carrying analog for many years to come.

The signal generated by a modulator (and tuned, or demodulated, at a receiver) can generate as good of a picture as that fed to a TV directly via composite or S-video. Tuners in modern television displays are very good. If the modulator generates a “clean” signal, the TV will generate a great picture.

Using a coax infrastructure to distribute video has many advantages:

• No matrix-type switching function required
• All sources available everywhere simultaneously
• Sources can located be anywhere
• Supports long cable runs with minimum loss
• One cable for everything
• Uses TVs' built-in tuners for source selection
• Very low-cost distribution infrastructure (cable, amps, splitters, etc.)
• Used to distribute CTV anyway
• Easy to distribute over-the-air broadcasts (still the best source of HD programming)

Of course, there are also disadvantages, such as:

• The trickiness of combining in-home modulated channels with existing CTV channels
• A lack of digital (8VSB or QAM) modulators
• The need for a demodulator (tuner) on the other end
• A lack of single-source dealer support

The following is not intended as a short course on coax distribution, but a quick review to help you in your next job.

Comparing RF Distribution Infrastructures

There are two basic cable distribution architectures: a single-cable system and a dual-cable system.

A single-cable, downstream-only system is typically used to distribute cable TV (CTV). A dual-cable system, originally developed as part of the CEBus standard, supports an upstream cable used to attach in-home modulated video sources. The dual-cable architecture combines in-home and CTV signals so that they can be viewed at any downstream outlet (not to be confused with just running two coax cables to an outlet — the second cable being a spare or used for satellite signals to a set-top box).