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Changing Over to Digital Transmitters

October 28, 2007

The FCC adopted proposed rules last spring aimed at motivating broadcasters to convert their facilities to digital in time for the end of the transition (as detailed in my May 27 story). Broadcasters submitted piles of comments, mostly all asking for flexibility. A few broadcasters have since visited the Commission in person to make their cases, laying out their specific plans. I will describe two of those below.

The FCC has not yet issued its final rules in this matter. On October 18 there was a big meeting between FCC staff (15 participants) and broadcast engineering people (23 of them) to discuss the issues. Must've been a big room.

Broadcasters are now transmitting two signals: an analog signal on their old channel and a digital signal on an interim channel. At the end of the transition, they will cease broadcasting on their analog channel when their only broadcast signal will be transmitted on their final assigned digital channel.

For many stations, the channel they are now using for their interim digital signal will also be their final digital channel. For them, the switchover should be relatively simple.

But for many stations their final digital channel will be different than their interim digital channel. They might be switching their digital transmission over to the channel they had been using for their analog broadcasts. Or their final digital channel might be different from either of their analog or interim digital channels. In most cases, this last alternative is technically the most challenging.

This TV channel musical chairs situation was necessary because double the number of channels was needed during the transition--one analog and one digital per station, meaning everyone could not get the channel they wanted. Either someone else was using it, or it was next to another occupied channel and interference between the signals would be too great.

The other complication was that TV channels 52 through 69 are being lost to other users. After the transition, broadcast TV channels will only go up to 51. Stations that have been transmitting in those "out-of-core" channels 52 - 69 have to move to an "in-core" channel.

So many stations are now confronted with a musical chairs challenge with their transmitters and antennas, which is why the transition is not going to be as nice and neat as some are saying. To wit: here are examples of what two stations are looking at, one west coast and one east coast ---


One west coast station is now broadcasting analog on channel 40, and digitally on out-of-core channel 55. At the end of the transition, it will move its digital transmissions to its current analog channel (40).

The station's analog signal now uses an omni-directional antenna mounted at the top of its tower. Top-mounted antennas are preferred because their signal is not obstructed by the tower itself. If the transmitting antenna is mounted lower on the tower (a "side-mount" antenna), it has to be split in two or more parts in order to send out a good signal 360 degrees around the tower.

So, anyway, this west coast TV station is planning to place its order for their final digital top-mount antenna this fall. This antenna will replace the analog antenna that is now at the top of their tower. In some cases, stations might elect to use the same antenna if their final digital channel assignment is the same as their current analog channel, but there are other considerations that would call for a new antenna.

Transmission antennas are really big (as much as 66 feet in length and 15,000 pounds), and the towers are a couple of thousand feet high, so switching them out is no trivial task.

Stations can't just turn off the analog TV broadcasts for a couple of months to do the work, so this broadcaster is planning to install a borrowed side-mount analog antenna next summer (2008). This temporary antenna would be lower down on the tower, and at the same power levels, will therefore not reach as far out as the top-mount antenna. It will, in fact, provide service to just 72% of that station's current analog viewers.

So right away 28% of the people who are looking at their analog transmissions will lose their picture, or the picture will be degraded. This is summer 2008.

As soon as the temporary side-mount analog antenna is installed and working, the station will begin to remove their current top-mount analog antenna and install the new digital antenna (ordered this fall) in its place.

Then they will route their analog signal from the borrowed side-mount antenna to this new top-mount digital antenna, but because this particular digital antenna will have power input limitations, it will provide service to just 86% of the station's current analog viewers.

Then, during the late fall of 2008, they will reconfigure their analog transmitter, with half converted for their digital channel and half for the analog channel. At this stage, they will reach 84% of their nominal viewers.

On Feb 18, 2009, they will begin using their final digital antenna with their half-digital transmitter, reaching 93% of their viewers.

During the spring and summer of 2009 they will convert the other half of their transmitter to digital operation and be able to provide full signal strength to 100% of their viewers.


Back on the east coast another station will be moving its digital transmissions from an out-of core channel (53) to an in-core channel (48). In this case, its analog channel (5) is being transmitted via a separate transmitter, tower, and antenna, and so will not be affected. On September 17, 2009, their analog equipment will simply be turned off.

This station's interim digital channel's antenna is located on a 2000' tower shared with four other DTV stations. This adds some complications, because while any of these other stations is doing work on the tower, all of the others will have to power down for safety reasons. Obviously, during these periods service to viewers will be affected. Doing the work during the wee hours of the night is not an option due to safety concerns for the tower crew.

Like the west coast station, this eastern station plans to install a temporary antenna lower down on the tower (in this case, at 1300'). This temporary antenna will be in place by mid-2008, and will be compromise-designed to work with two different channel frequencies--possible in this case because the interim digital channel and final channel are not too far apart (53 and 48, respectively).

It will be far enough down from the top of the tower that it can be transmitting while workers are at the top of the tower, likely at a reduced power level. Transmissions will have to shut off while crews are transiting up and down the tower, interrupting service to digital viewers.

During January 2009 half of the digital transmitter will be reconfigured for the new channel -- that will take four weeks. During that period, the station's interim digital channel will be operated a half-power.

At the end of February 17, 2009, the lower side-mount antenna will start tranmitting at full power. Work will begin to replace the existing top-mount digital antenna (designed for the interim digital channel) with a new one designed for the final channel. That work should be completed by early March when full digital signal strength will begin.


These examples illustrate the complexity of switching over to a new digital channel. Both of these stations are in temperate climates. In northern parts of the country, tower work is impossible during the winter, compounding the difficulty.


Until next week, when the House of Representatives will hold another DTV transition hearing--this time TV industry witnesses will testify (Wednesday), and the FCC's Consumer Advisory Committee meets again on Friday.