reducing adjacent channel interference
Adjacent channel interference can be drastically reduced if stations, operating on adjacent ENG and STL channels, limit unusable high frequencies.
Many years ago, when hollow state devices were still popular, it was difficult to maintain video bandwidth out to 5 Mcs. Some of today's DAs have responses beyond 100 Mhz. With these modern video DAs, it is desirable to limit video frequencies above 4.5 Mhz.
The primary reason is that they serve no purpose, and the information is unusable in present NTSC systems. More important is the fact that these frequencies contribute to unwanted sidebands, making your ENG or STL channel wider than necessary. This wastes precious fade margin and signal to noise ratio, by taking power from audio subcarriers and useable video information. In addition to causing indirect degradation to audio subcarriers, some high video frequencies fall right at audio subcarrier frequencies, and are not totally removed by low-pass video filters. This is especially true in systems using 4.83 Mhz audio subccarriers.
Unnecessary high frequencies contribute to modulation bandwidth, which subtracts from your video headroom, which limits your allowable video levels before clipping, and audio subcarrier cut-off (sync buzz in audio).
But this is only part of the story. In some cases, COMREX and other IFB systems that operate at 26 Mhz, have ridden into "remotes" on video cables, and were re-transmitted as sidebands, 26 Mhz above, and 26 Mhz below the channel that the ENG transmitter was on. Again, when you transmit any signal, it takes power, and transmitting unwanted or unusable frequencies takes away power from your useable video signal and audio subcarriers.
But worst of all, unusable high frequencies cause valuable power to spill over into the adjacent channels!
The purpose of all this is to point out the necessity to use a "brick wall" video filter at the input to the ENG or STL transmitter. This low pass filter should pass video up to 4.5 Mhz, and MUST be phase equalized.
Matthey makes an ideal filter for this application, model TBW446, which is priced around $625 (as of 2/18/'98). If stations operating on adjacent ENG channels use these filters, it will be to their advantage.
Now that the transmitters are clean, it is necessary to upgrade your receiver. Adjacent channel selectivity in most ENG receivers is only about 10 to 13 db. To increase it to beyond 70 db, give us a call at 1-.
Example: A station sends a news van out to cover a story. The location is 20 miles away. Another station sends it's van to do a story only 2 miles away. Both are on adjacent 2 Ghz channels, close to the same compass bearing, and shoot back to a common receive site on a downtown building. In most cases, the van 2 miles away, will cause interference to the other receiver. Our SAW Filter mod prevents this.