A quick review of typical UHF transmitter construction explains why.

UHF analog transmitters commonly use two or three klystrons to generate the visual and aural signals separately- one or two klystrons for visual, and one for aural. This design can be modified as needed so as to allow the aural portion of a UHF analog transmitter to be “multiplexed” or combined with the visual klystron. Multiplex operation is commonly used to allow continued broadcasting due to some sort of failure within the transmitter. The “multiplexed” klystron then amplifies both the visual and aural signals to much higher power levels for delivery to the antenna. Multiplex operation is attractive because it can be done with little compromise in transmitter output power. With a little creativity, instead of using the aural klystron as it was originally intended, the aural klystron can be dedicated as a high power DTV amplifier with the aural portion of the analog signal being permanently multiplexed into the visual klystron. The original analog transmitter design has provided the broadcaster with an inexpensive DTV power upgrade option available from existing equipment.

Specifically, here’s why you may want to consider saving that UHF analog transmitter from a trip to the scrap yard:

1. Klystrons can be used for DTV. All klystrons are suitable for DTV amplification whether integral cavity, external cavity, or MSDC’s.

2. An upgrade path that makes sense. Your current UHF klystron transmitter can provide high power analog visual and aural signals as well as high power DTV simultaneously.

3. Implementation cost. According to Harold Stanton, President of American Broadcast Services, a typical analog klystron transmitter can be modified with the addition of some extra components to provide high power DTV at about one third the cost of a new transmitter with comparable output power. The actual cost will vary somewhat from station to station depending on their current analog transmitter, its condition, and their allocated DTV channel. An existing low power DTV transmitter, or parts of it, may also be used to “excite” the klystron and this will further lower the DTV high power upgrade cost by eliminating the cost for an expensive DTV exciter and this will preserve at least part of the investment in the low power DTV transmitter.

4. Operational cost. Since the analog transmitter originated as a two or three klystron device, the only increase in power cost after the conversion would be the difference in power that the aural klystron would have drawn compared to the same tube operated at a higher beam current in DTV service. The aural power cost would actually get folded into the power required for DTV. In fact, depending on your DTV power, you may not notice a significant increase in your power bill because of the elimination of the aural klystron. It is very likely that the power cost will be lower with this scenario than if a separate high power DTV transmitter were purchased, regardless of efficiency, and operated along side your existing analog transmitter.

5. Back-up. The analog transmitter, once outfitted for digital, can be used as a high power back-up for an already existing DTV transmission system.

6. Future upgrades. If or when analog television is discontinued, the transmitter could be further upgraded to provide either additional redundancy or even more DTV power output. Either of these options makes the klystron transmitter useful for many more years. And don’t forget…klystrons can be re-built.

7. Efficiency. Since the DTV peak to average ratio is fixed to a factor of about 4:1, and because there are no horizontal or vertical synchronizing pulses contained in the 8VSB RF envelope, the klystron will operate at a higher average efficiency compared to analog.

Broadcasters are currently viewing this high power DTV option as a sensible upgrade path and may be especially important in protecting your DTV coverage contours in a timely manner without spending a ton of money. This path may make sense for your station. If you feel this may apply to your needs and circumstances, please feel free to contact Randy Davis at Freeland Products, Inc. (800) 624-7626, or Harold Stanton at American Broadcast Services (601)758-9283.

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