For the past twenty years people have been using modulators to distribute video sources such as security cameras, throughout their homes on unused TV channels. It is a simple concept that can be described in a few basic steps:
connect the camera(s) to the modulator
join the output of the modulator with your incoming cable feed
program the modulator to an unused TV channel.
Those who have attempted to do this recently might say that these steps aren?t so easy. Many people who have tried to create a modulated video distribution system since the advent of digital cable have noticed that when they program their modulator to, what appears to be an unused channel, the signals look ?snowy? unless the main cable feed is removed. This frustration has lead many to claim that modulating video signals with digital cable just isn?t possible. On the contrary, modulating with digital cable is possible, it just requires the information you are about to read.
There are two fundamental challenges encountered when trying to modulate on a digital cable system:
The modulated channel being mixed with the cable company must be inserted on to an empty channel. Filtering is generally required to accomplish this goal.
The modulated channel will generally not pass through the digital box, and therefore an alternative route must be provided to get around the cable box.
These challenges are independent of one another and must be solved separately. This article will address these two problems in the order stated above.
First of all, it is important to understand that digital cable is really a hybrid system ? part analog and part digital. The analog channels are typically 2-78 and 95-99 , they are broadcast in the frequency range between 55 MHz and 550 MHz. These channels are transmitted in standard NTSC format and can be viewed on any cable-ready television without additional equipment. The transmission of digital cable channels is not so easy to understand. The cable company is using the frequency range that would normally carry analog channels 79-94 and 100-116 (550 ? 750 MHz) to broadcast their compressed digital signals that contain hundreds of TV channels (see Figure 1). When these signals enter the digital set-top box, they are decompressed and routed to an arbitrary channel number on the box.
So why does this matter? In order to modulate a channel and successfully join it with a cable feed, one must program the modulator to an unused channel. Without the luxury of the information contained in the previous paragraph, one might mistake channel 85 for an unused channel when in fact it contains digital cable information. When the modulator is programmed to a channel that already contains a digital cable signal the result will be a very ?snowy? picture. To verify that cable interference is the problem, disconnect the main cable feed and see if the picture clears up. If it does, then the snowy picture is caused by cable interference. To solve the problem, install a low pass filter that removes signals above 750 MHz (Channel Vision part number 3102-118) and reprogram the modulator to channel 123 or higher. This will preserve the digital cable signals while still allowing the modulated signals to pass without interference.
Now that we have conquered our first challenge, lets address the second one. We have to get the modulated channel into the RF input of the TV set without looping it through the cable box. There is a Channel Vision product designed for this purpose (model 3101). The 3101 allows the cable box to feed the TV set on channel 3 while the modulator delivers its signal on a higher channel such as 123. The system will operate in the following way: when viewing the cable box, the TV will be tuned to channel 3 and when viewing the modulated signal the TV will be tuned to channel 123. Figure 2 shows the system that has been described above.
Using these basic ideas as guide, a good designer can create a system that works well and is simple to operate. Even though digital cable may present interesting challenges, it is still possible to distribute video sources around the house using a video modulator and the existing cable TV wiring. For more information about modulators, please visit www.channelvision.com .