Let's look at the problem of getting the signal from the studio to the transmitter. Like most things in television, it sounds like a simple, little task. It isn't.
Join the band
Most stations prefer to use the 7GHz auxiliary broadcast band for the STL. These paths are less fussy than those at 12GHz or higher, because the equipment is a bit easier to troubleshoot and dish alignment is simple. Also, the channels in that band are nice, big 25MHz channels. So manufacturers have designed sytems to squeeze both the analog and digital television signals into one microwave channel, along with data, with mixed results.
The system du jour is a 16QAM system with an 8-VSB modulator. This appears to provide the most dependable service. The digital side of these systems transports the ATSC 19.39Mb/s datastream along with at least a T1, two RS-232 channels and some additional logic circuits. The analog side provides a channel for standard video with several FM audio subcarriers.
Separate and unequal
The scheme usually includes separate power supplies and amplifiers for the two channels. At some future date, when analog video goes away, the facility can change that channel to a second digital channel with just the purchase of a bunch of electronics. Sounds good. And it is — for the most part.
There is a large school of thought that recommends using a separate link — completely separate transmitters and receivers — for analog and digital signals. Separate antennas can avoid a lot of loss in the combining/splitting/filtering system, resulting in a better fade margin for the same antenna size. Equipment for that simpler function is readily available for those locations where its use is feasible. Where band occupancy won't allow such single-channel equipment, the dual systems may be necessary.
Another scheme is to combine all the signals, NTSC, ATSC, logic, audio, etc. into one big digital datastream, then use one of the big digital-microwave systems to carry the signal to the transmitter site. There, the signals are decoded as necessary to feed the separate transmitters. It works, but it is a bit expensive. And it's certainly too complex for most station staff members. So, back to the simpler world of broadcast STL systems.
Using a combined system isn't necessarily bad, as long as you do the right calculations. First, base the numbers for transmitter power output and receiver threshold on the actual input/output connectors of the equipment, not on the whole system. When joining the two systems into a common antenna, the dual-stream system adds loss in a combiner. There is another loss at the receiving end when the signal splits for the two receivers. If hot standby is involved, split the signals again at the receive site. You have to include those losses in the overall path loss or the fade margin isn't going to be what you expected.
Drive a hard bargain
This is a new technology and, as with most new technologies, it has its share of problems. So, when purchasing such equipment, be a tough negotiator. First, don't pay that last payment until the system is operating — properly. And the warranty shouldn't start until then, either. The manufacturer might not prefer such terms, but they will deal.
When calculating the reliability of the system, the old numbers still apply. A good reliability rate would still be 99.999 percent (the classic five nines) for an STL system. A design fade margin of 40dB is still desirable. The output power is somewhat lower for the digital systems. It's basically the same power amplifier, but the tuning is a bit different. Actually, the amplifiers must have a wider flat bandwidth for the digital signal. If the bandwidth goes up, the overall gain will usually go down (remember the old gain-bandwidth product?). At the same time, the receiver sensitivity is not quite as good as analog systems (again, the same bandwidth considerations). The result of all these issues is that the path loss should be a bit less than for a simple analog system. Usually, you can accomplish this by increasing the dish size, if possible. If all else fails, space diversity may be necessary. That can pick up several dB, resulting in the desired fade margin.
The short version
In short, the technology here is a bit different, as are all things digital. Some of the early channel-sharing systems were terrible. But that seems to be straightened out. Still, watch your back when dealing with any of these systems. A good fade margin with high reliability is still a must, whether you're dealing with analog or digital signals.
Don Markley is president of D.L. Markley and Associates, Peoria, IL.
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