WPSX-DT’s 1kW booster in Altoona, PA, is part of the world’s first Distributed Transmission (DTx) network.
Since DTV's infancy, many operators of translator systems or LPTV stations have worried about their ability to continue operating. Western state operators who routinely depend on large networks of translators to serve small valley markets are particularly worried.
Timing is everything
New ATV operation requirements have exacerbated translator operators' problems. David Hershberger of Axcera, LLC has discussed these new problems at length in technical papers presented at NAB conferences and other meetings. Visit www.axcera.com or contact Mr. Hershberger directly at firstname.lastname@example.org. The company has done a significant amount of work in this area and offers numerous solutions.
The problem with single- or multiple-frequency translator networks lies with the timing signals. ATV modulators require the synchronization information that is included in the trellis code. But facility transmitters remove the trellis-code data before transmitting the signal, depriving translators of the timing data. The solution is to modify the SMPTE310 data stream. The translators are designed to cadence sync using information contained in the distributed transmission packets to synchronize their symbols, timing and pilot frequencies. GPS receivers at each translator location obtain the timing reference. Basically, for two-channel systems, the main transmitter sends out both the main signal and the SMPTE310M signal to the translators. The translators then operate sharing the same channel. The ability of standard consumer receivers to handle multipath makes the whole thing work. When a receiver obtains more than one signal, it simply treats them as multipath. Fifth-generation decoders announced last spring can handle multipath signals equal in strength to the primary signal.
In plain English
The FCC finally came out with a Report and Order establishing the new rules for translators and LPTV stations. The entire document is available on the commission's Web site. Search for MB Docket No. 03-185, and you'll get all 120+ pages.
For a great overview of many of the main issues covered by this FCC document, the law firm Fletcher, Heald and Hildreth, P.L.C. publishes a Memorandum to Clients. You can obtain that document by visiting the firm's Web site at www.fhhlaw.com. The document is unusually clear and easy to understand. Even engineers can understand it. What's more, the authors have sneaked some humor into it — most unlawyer-like. The October issue (No 04-10) is quite helpful for deciphering some of the LPTV issues. Most importantly, it reveals that existing analog translators can rebroadcast converted DTV signals in an analog format. Conversely, digital translators can receive analog signals and transmit them in a digital format. It seems that the commission feels that this policy will get the most signals to the greatest number of people.
Coping with changing rules
The new rules also contain all the normal stuff like power levels, interference prediction and masks. Here, Fletcher, Heald and Hildreth accept the fact that they, as lawyers, don't understand this technical stuff — a rare and gracious admission.
Because many translators operate outside of the core channels, many operators have expressed concern about their ability to continue operating. But the new FCC rules make it quite clear that they can continue, at least for the time being. The commission's main requirement is that translators must not interfere with primary facilities. Many of those channels have already been sold, and the commission expects all LPTV stations now on channels 60 through 69 to move to other channels by the time full DTV conversion takes place. That conversion is scheduled to occur between 2006 and 2009.
The commission has stated that a digital transmitter can retransmit over the same channel on which it receives. This brings up the distributed-network problems discussed earlier. Without discussing the legal problems involved in authorizations, the on-channel DTV operation does seem to be the same as an on-channel booster. But, as before, the timing signals rear their ugly heads. The question here is whether the primary station will agree to transmit the 310M signal to give everyone the necessary timing signals. The commission hasn't completed the full rules for DTS operation, but it's working on them. I've been told that this isn't a real problem and can be resolved in a reasonable fashion.
To allay the fears of the Class A crowd, the commission will allow existing Class A LPTV stations to “flash cut” to DTV without losing their protected status. An interesting term, “flash cut.” It gives you the impression that a giant arc of electricity will appear, a deep voice will shout “Shazam!” and your station will be operating in DTV. The giant arc does sometimes occur, but it's usually just a transmission-line center conductor vaporizing.
We'll talk a bit more about the actual technical rules in a future issue.
Don Markley is president of D.L. Markley and Associates, Peoria, IL.
Send questions and comments to:email@example.com
Thank you for signing up to TV Tech. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.