Does it mean that CATV systems can stay on the sideline and wait until the price of the average HDTV set drops from $7,000 to $1,000, so the transition to DTV can pick-up speed? The answer is: Definitely not! Every month dozens of new DTV carriers appear in the 470 to 800 MHz spectrum, and ultimately 1,500 DTV stations must be squeezed into the UHF band.
Observe the two Polaroid pictures taken from the screen of the spectrum
analyzer, demonstrating why CATV systems should start upgrading their UHF
antenna systems now, rather than waiting until disaster strikes.
|The Channel 65 video carrier is in the center of the screen. Observe the high amplitude DTV spectrum below the NTSC channel.||This is a "free-standing" DTV carrier. No analog adjacent carriers above or below.|
Despite the fact that the FCC's DTV frequency allocation was prepared with great care, more and more CATV systems are exposed to DTV adjacent channel interference problems while carrying NTSC (analog) UHF stations.
Installing a UHF trap? The narrow bandwidth of the trap can cut out only a very limited segment of the 5.38 MHz wide interference signal.
Applying a bandpass filter: Bandpass filers in the UHF frequency range are prone to inefficien-cies. The application would not solve the interference problem either.
The DTV spectrum also exhibits a lower and upper sideband splatter, which, passing through the signal processor's IF amplifier, will create random noise or snow in the NTSC picture. Should the amplitude of the sideband splatter exceed a critical threshold,the picture impairment may appear in the form of impulse noise. The white (black) streaks show up randomly, just like poor quality satellite pictures.
The objective should be to reduce the signal level of the DTV signal, while maximizing the video carrier of the desired NTSC UHF station. The solution: Install a phased-array, forcing a radia-tion pattern null into the direction of the undesired DTV transmission. The array design should be based on a Computer Aided Adjacent Channel Interference Study, providing exact, computer cal-culated relative angle, great circle distance, and horizontal spacing information. Conditions are presented in an easy to understand format on the DTV Signal Direction Sheet.*
The age of DTV interference is here. However, there is no assurance
that it will hit only bon one channel. It is the CATV system's best
interest to seek professional help before DTV interference strikes.
* This is a proprietary product of Biro Engineering.