Imagine, for a moment, the difficulty of identifying the source of an offending co-channel DTV signal that's interfering with your station's digital broadcast.
It isn't as if there was a ghosted signal that could be identified when a station ID is broadcast. When digital television signals interfere with one another, receivers respond in one of two ways. To a point, they tolerate interference and ignore the unwanted signals. However, if the interfering signals are less than 15 - 20 dB below the desired signals, the receivers suffer from co-channel interference and the "cliff effect," that is, they go dark.
In that case, the only way to reliably determine the identity of the interfering signal would be to turn off your transmitter to let the receiver pick up the offending signal for identification — not an attractive option.
However, thanks to work underway to develop distributed transmission (DTX) as a viable alternative for broadcasters transitioning to digital television, a much more benign solution may be at hand.
Known as an RF watermark, a unique signal that identifies a transmitter in a DTX system can be buried far enough under a standard ATSC signal that it does not impact signal reception by consumer receivers but is still detectable with a special receiver.
A DTX system, which relies upon multiple digital transmitters emitting trellis-coded 8-VSB signals, will require engineers to be able to identify individual transmitters so they can optimize the system by adjusting the timing of RF emissions and the power levels of the transmitters. Embedding an RF watermark 30dB down provides a solution for transmitter identification that doesn't interfere with reception of a transmitted DTV signal. Moreover, the RF watermark can be reliably detected when the signal carrying it is itself 20 dB below the level of the signal from another transmitter (i.e., the RF watermark is 50 dB below the stronger signal).
According to consultant Merrill Weiss of the Merrill Weiss Group, who has worked with a group of engineers to develop the RF watermark concept as part of the DTX standard, the RF watermark will offer television engineers responsible for every type of digital transmission a solution to a problem they may not yet know they have.
"In the digital environment, co-channel interference will be very difficult to sort out," he said. "You won't know for sure whose signal is causing yours to stop working.
"The RF watermark offers a method of transmitter identification and interference determination that's much more acceptable than the alternative, (namely) turning off your own transmitter."
