This month I decided to take a break from talking about TV transmission to focus on TV reception. I receive emails from non-engineers with questions about TV transmission and reception who happen to stumble upon my articles.
People are starting to realize the incentive auction and repack will impact viewers’ TV reception. This month I’ll explain the impact of the incentive auction, channel sharing and the repack on over-the-air TV reception.
Stations may sell their spectrum and go away, meaning the programming they carried may no longer be available. Stations may sell their spectrum and “channel share” with another station. These stations’ primary channels will be available on another channel, perhaps at reduced quality, but some multicast channels on either of the stations sharing the channel may disappear.
Viewers who watch TV via translators or over LPTV stations may find they are no longer available. Finally, during repacking, stations’ coverage may change, temporarily or permanently, making reception more difficult in some areas.
Will your favorite DTV program channel disappear after the incentive auction? In part, that will depend on that channel’s popularity and viewership. When talking about “channels,” it is easy to get confused. A station has only one RF channel (their spectrum), but may run several virtual program channels on it—28.1, 28.2, 28.3, 28.4. These are transmitted in the same spectrum, but each takes some of the data bandwidth.
While stations can’t discuss their auction plans during the FCC’s quiet period, the value of the spectrum versus the value of the programming will likely determine whether or not the program channel survives the auction.
CHANNEL SHARING MISUNDERSTOOD
Stations can sell their spectrum, but retain their license, through channel sharing. This is probably the most misunderstood aspect of the incentive auction. First, stations are not, strictly speaking, sharing spectrum. The “sharee” station has no spectrum; the sharer has 6 MHz, one full channel. An ATSC DTV channel cannot be less than 6 MHz. It cannot be divided. What can be divided is the data rate transmitted on that channel—19.392 Mbps.
Viewers watching two or more stations sharing a channel are likely to notice some reduction in picture quality. How noticeable it is will depend on the total number of program streams sharing one RF channel; how many of the program streams are in HD; the way the bits are allocated between program streams; and the content on the channel.
In the early days of legacy ATSC, expert OTA viewers used TSReader software to see what bit rate stations were using to transmit their HDTV programs. Stations that sacrificed HDTV bit rates to add additional program streams were criticized. Now that some stations are transmitting two HDTV program streams and one or two SD program streams in the same 19.392 Mbps, most stations have at least one SD multicast, but there don’t seem to be as many complaints. Why?
|Fig. 1: Statistical multiplexing bandwidth allotment
The answer is statistical multiplexing (“statmux”). Each program is given the bit rate it needs for the video on the air right now. If the scene isn’t changing—a talking head, for example, or slide without motion—the bandwidth can be used by other programs.
Statistical multiplexing allows multiple video streams to efficiently share limited bandwidth. Each HDTV stream can hit peak bit rates up to the amount left after all the minimum bit rate streams are assigned. Data is also required for audio and program information, but this is a fixed data rate. I’ll cover this in more detail in a future column. Fig. 1 shows the basic concept.
One key point—“statmux” and MPEG- 2 encoding technology has improved significantly since DTV broadcasting began. Stations that are channel sharing will have to spend some of their auction money to upgrade their encoding systems to the latest technology to minimize the impact channel sharing will have on video quality.
As I outlined in my earlier article on the incentive auction and repacking timeline, channel sharing will take place early in the process, within the first year of the 39-month deadline for completing the repack. Your favorite station may give up its spectrum and channel share, but if the station it shares with has good coverage, you won’t lose your programs, at least until the repacking begins.
There are multiple ways viewers could lose access to OTA programs during the repack. The first and worst impacts viewers that receive stations by translators or whose favorite programming happens to be available only on a low-power TV station. These stations are not protected during the repacking, and if they have a channel in the wireless band, they may not be able to find a replacement channel in the more densely packed TV band after the repack.
WHERE’S THE MONEY?
While replacement channels may be available in rural areas, Congress and the FCC have allocated zero dollars to pay for the channel change and any new equipment that might be required. Local governments, civic organizations and public broadcasting stations may have trouble finding the money to make the channel change. I’m worried because all of my TV viewing when I’m home is from a PBS TV translator on Channel 50!
Viewers in areas served by full-power stations may find they encounter reception problems during the repack. The reason is many, if not most, stations will have to use transmission facilities with less coverage than they have now while changing channels.
Stations will construct interim facilities, which may later be used for backup, with lower antennas and less power so they can remove and replace the main antenna with one that will work on the new channel.
In some markets, stations may be able to share a broadband antenna, which might result in less loss than a low-height, low-power antenna, but still not be as good as their original antenna.
As I explained in my last column, FCC flexibility in allowing coverage increases in some areas could help stations with directional antenna patterns avoid major coverage loss if they need to share an antenna.
If a large number of stations have to be repacked, given the limits of tower crew availability and weather, some stations may have to live with reduced coverage until late or even after the 39-month repack period ends. Stay tuned!