Installer/designers seem to associate the word “modulator” with “crummy picture”. This is undoubtedly due to a bad experience in the past.

RF Distribution TIPS

Grayson Evans | The Training Department

RF Distribution TIPS

Author: Grayson Evans, Training Reels - Copyright 2008

Installer/designers seem to associate the word “modulator” with “crummy picture”. This is undoubtedly due to a bad experience in the past. Someone installed a modulator incorrectly, didn’t know how to measure levels, or connected it to a poorly designed coax infrastructure and the whole thing got a bad name. This is understandable since the knowledge on how to do it correctly is becoming a lot art. Perhaps a contributing factor is the confusion over the Feb. 2009 date to shut down analog broadcasts. This is not the end of analog transmissions, just broadcast analog. Cable operators will still be carrying analog for many years to come.

Most custom installation companies I’m familiar with consider the only respectable way to distribute video around the house is to use a baseband signal distribution system. The equipment—made by Crestron, Elan, Niles, and many others—provides numerous baseband inputs (composite, component, DVI, even HDMI) from source devices and 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 sources onto the cable system to be viewed on any TV seems to have gone out of style.

Installer/designers seem to associate the word “modulator” with “crummy picture”. This is undoubtedly due to a bad experience in the past. Someone installed a modulator incorrectly, didn’t know how to measure levels, or connected it to a poorly designed coax infrastructure and the whole thing got a bad name. This is understandable since the knowledge on how to do it correctly is becoming a lot art. Perhaps a contributing factor is the confusion over the Feb. 2009 date to shut down analog broadcasts. This is not the end of analog transmissions, just broadcast analog. Cable operators will still be carrying analog for many years to come.

This lack of respect is a shame since the signal generated by a modulator and tuned (demodulated) at a receiver can be just as good as picture fed to the TV directly via composite or S-video (we’ll talk more about higher def. sources shortly). 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
  • Supports long cable runs with minimum loss
  • One cable for everything
  • Uses TV’s built in tuner for source selection
  • Very low cost distribution infrastructure (cable, amps, splitters, etc.)
  • Used to distribute CTV anyway
  • Easy to distribute off-air broadcasts-still the best source of HD programming

Of course there are disadvantages.

  • Combining in-home modulated channels with existing CTV channels can be tricky
  • Current lack of digital (8VSB or QAM) modulators

This TIP is not intended as a short course on coax distribution, but a quick review is probably worth it and will hopefully give you some ideas you can incorporate in your next job.

There are two basic cable distribution architectures. A single cable, downsteam only system typically used to distribute Cable TV (CTV), and a dual cable system—originally developed as part of the CEBus standard—that supports an upstream cable used to attach in-home modulated video sources. This architecture combines in-home and CTV signals so 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.

 

 

The single cable architecture, above, is straightforward but there are several things to keep in mind when you design this system.

  • I don’t recommend combining the amplifier and splitter. Using a separate amplifier and splitter gives you the ability to pick the best parts and measure how each performs.
  • ALWAYS use top quality parts from companies in the Cable/CATV industry. This is
    NOT the place to save money.
  • The gain in the amp will need to be equal to the loss in the splitter + the loss of the longest cable run. Your goal is provide a signal between 0 and +10 dBmV at EVERY wall jack.
  • Use attenuators on the splitter outputs for short cable runs that result in a signal over+15 dBmV at the outlet.
  • You WILL need a signal level meter capable of measuring the video or digital carrier on each channel from 2-125 (for cable). These are available from several vendors for less than $1000.

The typical dual cable architecture has been popular since the mid 1990’s and is SUPPOSED to provide an internal cable path back to the structured cable enclosure where signals are combined, amplified and mixed with external signals for redistribution on the external cables (below). In-home modulated signals have to be placed on a channel that is not used by the cable company. Even “high-end” modulators require two channels (the channel it is tuned to plus the channel below). If there are many modulated sources (dvd players, satellite receivers, cameras, etc.) it could take up a lot of channels. Modulated channels are typically placed above the highest used cable operator channel.

 

Unfortunately, over the years, most cable operators have “eaten up” nearly all of the available 124 cable channels. Even when they haven’t, it’s almost impossible to tell which channels are not used without a spectrum analyzer. This has lead many an installation company to abandon this solution.

Things to watch out for in this design:

  • The signal level of the modulated sources should be about the same at the combiner.
  • Base everything on the modulator with the lowest output level (assuming it’s not adjustable) and attenuate the remaining modulators, using in-line attenuators, to make their level the same within +/- 5 dBmV.
  • Combined signals (CTV and in-home) should be approximately the same level (within+/- 5 dB)
  • Assume 5 dB loss per 100 ft. of RG6 as a rule of thumb
  • If you use filters to eliminate channels from the CTV feed, eliminate only analog channels (public access channels are good candidates). Each digital channel contains from 6-10 encoded channels”.

It IS possible to modulate sources above the highest cable channel (125), avoiding the cable operator all together, but that’s a story for another TIP.

 

 

An alternative dual cable architecture, above, avoids the channel sharing problem by simply separating the downstream channels into two physical cables. One cable is dedicated to CTV service and one is dedicated to in-home generated channels. Since in most custom installs the sources are located in a central location such as a rack or cabinet, modulators can be located in the same place and their output combined in one cable to the structured cable enclosure. There, the combined signals are amplified as necessary and split just like the single cable architecture. This is a nice application for a 2,3, or 4 channel modulator. The best one I know of is still the Channel Plus SVM-24. It takes stereo S-video in and is frequency agile on each channel.

The only disadvantage of this design is the TV either needs two “Cable” or RF inputs or, for older TV’s, an external A/B switch. Note that modulators can be set to output either CTV channels or off-air channels so either the “cable” or “Antenna” inputs on receivers can be used.

 

Digital Modulating

You might want to check out a new product by ZeeVee Inc. (are all the normal company names taken?) www.zeevee.com, called the ZvBox. It’s designed to take the VGA output from a PC (only), digitally modulate it, using QAM modulation—the same as used by the cable operators—and inject it onto the home’s cable wiring (this definitely takes customer support guts). It’s the only box I know of that contains an inexpensive frequency agile QAM modulator that takes a“regular” video source as it’s input. There are a bunch of digital modulators out there that expect to take a serial MPEG encoded video source. Not too handy. The fact that the box is under $500 is nice enough, but the box contains something more interesting to me. It auto scans and selects unused channels so the customer doesn’t have to. Supposedly, you can use the connected PC to find out what channels are unused. Cool. That’s worth the $500 to use the box as a piece of test equipment.

 

Other TIPS for the Best Picture

  • Keep those outputs terminated! Use self-terminating barrel connectors everywhere you can.
  • Make sure F connectors are screwed on TIGHT.
  • It is perfectly OK to use dual-shield RG6 - copper or plated copper braid with 80% coverage is best. Just make sure the braid isn’t a few strands of aluminum wire that looks like someone added it as an afterthought.
  • Make sure the power supplies for amplifiers and modulators are adequately filtered and are not under rated (the main source of hum noise).

If you have any questions about anything in this TIP or a RF distribution tip, drop me an e-mail at grayson@trainingdept.com I will actually answer it!

For other Training Reels TIPS, visit www.trainingdept.com/html/support.html


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