How to Modulate an

Internal Camera Signal on a Digital Cable System

As digital services are being offered by more and more CATV companies, cable subscribers who have modulators installed or new installation are experiencing problems that they don't understand. In order to be successful, one must first understand some basic facts about digital cable. First of all, it is actually a hybrid system. Channels 2-78 and 95-99 are usually transmitted in standard NTSC (analog) format, which means that you can receive them on a standard cable TV set without a digital box. The cable company is using the frequency range that would normally carry analog channels 79-94 and 100-116 (550-750MHz) to broadcast their digital carriers. (Some cable companies may be broadcasting their digital carriers as high as 860MHz, but this is not very common.) All cable subscribers are receiving those digital signals even if they are not subscribers to digital service. These digital carriers look like snow (or an empty channel) on a standard cable TV set. Even the digital box is misleading in this regard, because channels 79-94 will commonly appear blank. In reality the digital carriers that are broadcast in this frequency range are decoded by the digital box, which then re-maps (or reassigns) the signal to an arbitrary channel number that has no correlation to the originally broadcast frequency that delivered the information into the viewers home.

So what does this all mean? In the past you would look for what appears to be an empty channel and then program the modulator to that channel. Using this approach, typically the modulators would be programmed between channels 79-94 because these often appear to be unused. As stated above, there are actually digital carriers in this range. The result will usually be an extremely snowy picture, which clears up as soon as the main cable feed is disconnected from the system. In order for the modulated signals and digital cable signals to coexist successfully, you will need to do some strategic filtering.

Filter Guide

Option 1 – Digital Cable Subscriber

The digital cable subscriber would be required to place a filter (ECF-2282) at the entry point of the system which will eliminate a small amount of the CATV signal that is coming in from the cable company. This filter will allow analog channels 2-116 to pass through, it is unlikely that any of the digital carriers will be blocked, thus no digital cable channels will be lost. This will enable the installer to program the modulator to channel 118 or higher, allowing it to be seen on all the TV sets that are not using a digital box. The TVs that are using a digital box will require an additional step. Due to the re-mapping done by the digital box; the modulated signal will not be able to simply loop through digital box. To solve this problem, a cable bypass converter kit (model CB-CK) can be used at each TV that has a cable box. See Installation Diagram for Digital Cable Subscriber.

Option 2 – Non-Digital Cable Subscriber

This subscriber would place a filter (ECF-2405) at the entry point which will delete almost all digital cable service. This filter allows analog cable channels 2-78, 95-99 to pass. This will enable the installer to program the modulator to analog channel 82 and above. These channels now can be viewed directly on the television. See Installation Diagram for Non-Digital Cable Subscriber.