/ The Importance Of Balance In RF Modulation
To deliver whole-house video, installers are using modulator devices. The trick is to maintain uniform picture quality throughout the system.
By Fred Harding, Capitol Sales Company
Today's systems use plenty of high-test video products. At the same time, more and more customers are requesting the ability to watch those video signals on multiple televisions throughout the house. Several different manufacturers make modulator devices, which allow installers to "inject" a video source onto a customer's antenna or cable system.
When you modulate signals throughout an RF system, major considerations include balancing the system, adjacent channel spacing, and the remote control.
Of course, you need to explain any technical product to your clients in terms they can clearly understand. If you tell them you plan to modulate a source onto an unused channel, odds are their eyes will start searching for the nearest exit.
But if you describe this option as a way of making the video source its own television channel, your suggestion will probably get a better reception. Your clients will understand if you explain that the satellite will be on Channel 48, the VCR on Channel 50, the security camera on Channel 52 and DVD player on Channel 54.
The Balancing Act
The first issue to consider is balancing the system. Modulators are essentially miniature television broadcast systems. Like cable and rooftop antenna signals, modulators have signal strength measured in decibels.
The product literature for most high-quality modulators lists a signal strength specification. These units commonly have a value in the 25 to 30dB range.
Your objective is to maintain uniform picture quality throughout the system. You want to keep the signal level from a modulator at about the same value as the incoming signal strength. If the incoming signal is weak, you need a large enough amplifier to bring the levels to within a couple decibels of each other.
Further, if you modulate onto a cable system, you need to put an amplifier in place to keep the modulated signal from flowing back upstream into the other cable users' systems in the neighborhood.
Modulation occurs when you connect the audio and video outputs from your device into the inputs of the modulator. The output is converted to an unused TV channel. An agile modulator allows you to set the channel being used.
One popular feature that installers like is a set of pass-throughs for audio and video on the modulator. Essentially, this allows you to put a single audio and video output set into the modulator, and you still have a set of audio/video outputs for local use.
Once the signal is modulated, you need to combine it with those amplified off-air or with cable signals.
Combiners and Splitters
If you're modulating from a single output device, use a combiner with two inputs and a single output. A high-quality, two-way splitter will work. In fact, many of the better brands label their product as splitter/combiners. Make sure that the signal paths go in the appropriate legs of the splitter.
If you have a two-way splitter, put the modulated signal and the antenna/cable signal into the legs marked "out". From there, the signal will go out the "in" port. You'll get the idea when you see this.
The signal will need to go to the RF distribution board you've laid out. Remember, a backwards splitter takes the same amount of signal that a forward one does, when you figure your strength calculations.
If you try to feed a total of eight television outlets with this system, the amplification from the modulators and the signal amp will be sufficient to deliver 5 to 9dB of signal, depending on cable length.
If you modulate from several different modulators, install a multi-way splitter. You can use a four-way splitter/combiner if you have three modulators and a local source.
But what if you need to go further than that, or you have more TV sets on your distribution system? You can put an amplifier between the backwards splitter and the rest of the outward splitters. Make sure the amplifier you spec has the ability to handle an incoming signal level of at least the output level of the modulator or initial amp, whichever is larger.
If the amplifier doesn't have the necessary amplitude for that, you'll need to attenuate the signal down from the combining network to allow reamplification without distortion.
Make sure you keep the modulated signals apart from the other channels. Modulators have a tendency to beat up on the adjacent TV channels when they're spaced next to each other. If you keep at least one empty channel between each modulated channel, the system's quality will be noticeably better.
In some areas, you need to trap out channels that your customers don't necessarily want to see. For example, the local cable system may have foreign-language channels in the frequency range where you wish to play, and your clients may have no interest in this programming.
A low-pass filter that can pass only channels up to a designated number, inserted before combining, will give you a clean slate to work with. It's a good practice to incorporate this when in doubt.
Once you've modulated the signal, you'll need to control the device you've modulated from the remote TV locations. In some cases, that won't be necessary. Some satellite receivers have UHF remotes, and security cameras tend to be set-and-forget.
But VCRs, DSS systems, laserdisc players, and DVD players require a remote. You can use some universal remotes that translate IR to RF and then back again. These sometime work great, but not all the time. A more reliable system uses dedicated control wiring for the infrared signals