Russell Brown /
11.16.2008 09:12 AM
Converting to digital

As any station manager or chief engineer will tell you, keeping the station on the air is a prime concern, in addition to the station’s budget. These two concerns converge when the subject of DTV transmitter redundancy is considered, and with the upcoming transition, this should be a concern of all stations with only one DTV transmitter.

San Francisco’s KMTP-TV32 is such a station, but being an independent public station, it does not have a backup to its one and only UHF DTV transmitter; fortunately, it does have a companion UHF analog transmitter from the same manufacturer. With the analog shutoff on the horizon, the station is looking forward to lowering its electric bill by shutting off the analog transmitter, but it is also concerned about redundancy for the digital transmitter. From the beginning, the station planned to convert the analog transmitter to digital after the transition, and although the two are not exactly alike, the challenges the conversion presents will be an interesting project.

Goals

This type of conversion project is really two different projects in one. The first is to convert the analog transmitter to digital, and the second is to modify the RF system to handle a second transmitter with all the required high-power filters. There are two ways to use two transmitters for redundancy; KMTP is choosing one is based on the need to save on construction and operating costs. (See Figure 1.)

The best way is to make sure the signal is never dropped, even for a few seconds, by the loss of either transmitter. To increase reliability, the two transmitters are combined in a “magic-T” combiner, with each producing half the needed power. If one goes down, the other can still provide half power by reconfiguring the magic-T, which is really a switchless combiner and a 180-degree input hybrid system. The problem for KMPT is that this would not reduce the electric bill. With both transmitters running 24/7 at the needed power level, each transmitter would produce about 10kW, which is too low for an inductive output tube (IOT) to produce efficiently.

The alternative is to have the two DTV transmitters back each other up by running each to an RF switch that can direct either one to the antenna. In this way, if one transmitter goes down, the other can be started up and switched on the air. Although this would mean being off the air for a number of minutes while the other transmitter warms up, it’s the lowest overall cost alternative. (See Figure 2.)

KMTP will try to keep costs down by performing as much of the work as possible in house and to reuse as much equipment and parts as possible. KMTP’s goal is to have two DTV transmitters, each capable of transmitting in full power with an RF switch, so either one can drive the antenna to full power.








Transmitter power

One major issue is whether the analog transmitter can produce the needed power for DTV. DTV power is measured as an average, whereas NTSC power is measured at its peak during the sync pulse. DTV (8-VSB) does have peaks, but they do not occur on a regular basis and, therefore, are not as meaningful (but still very important) as they are with NTSC. When thinking about DTV power, whether for transmitters or transmission lines, both the average and peak power must be considered. Peak power for DTV is calculated to be 7dB above average, which can quickly be computed by multiplying the average power by five. This is the power level the transmitter must be able to produce and the transmission line must be able to handle. As an example; a 10kW average power DTV transmitter must be able to produce a 50kW peak, and a 35kW average power transmitter must produce 175kW peak. These peaks only last a short amount of time; in fact, the 7dB factor only occurs about 0.10 percent of the time. Although it is short in duration, it is still important to be able to produce this power level to correctly transmit the symbols that are used in 8-VSB and, thus, the transport stream. (See Figure 3.)

KMTP’s analog transmitter produces a 30kW peak transmitter power output (TPO) using a beam voltage of 30.5kV. To meet full DTV power, it will need to hit a 108.6kW peak power (a 21.7kW average), and to do that, its beam voltage will need to be raised to 35kV. These power levels will yield 500kW effective radiated power at the antenna. (See Figure 4.)

One of the challenges during any analog to digital conversion will be that IOTs can exhibit certain problems when required to produce more power and/or work at a higher beam voltage after they have settled in at a lower one. The beam voltage will need to be gradually raised over time to avoid, or at least reduce, arching.

Even though KMTP’s IOTs will be almost 5 years old with about 39,000 hours on them at the time of this conversion, Michael A. Boyle, engineering manager at L-3 Communications, says the analog IOT should be able to handle the changeover, but he cautioned that the change in beam voltage (31kV for analog and 35kV for digital) will need to be addressed by slowly increasing it over time, thereby letting the tube adjust.

Next Time

The next “Transition to Digital” tutorial will continue with the digital conversion of a transmitter.

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