One of the main obstacles to effective mobile TV service is the fact that VHF stations are harder to receive on devices with small antennas like cell phones. Broadcast engineers and the Open Mobile Video Coalition are hard at work on solutions.
You can look it up: Higher frequencies with shorter wavelengths are more easily received by smaller antennas.
This means that when TV broadcasters start delivering mobile digital TV, those with UHF channels will find they have an advantage over their VHF rivals when trying to reach cell phones and other handheld devices with tiny antennas.
“VHF is at a disadvantage over UHF, which is all about the laws of physics.” says Jay Adrick, vice president of broadcast technology at Harris, which is among the leading transmission companies building products for mobile digital TV. “A handheld device with a built-in antenna or a little pull-out antenna performs much better at higher frequencies than it does at lower frequencies.”
In fact, some believe effective mobile broadcasting in the VHF band, especially in the low V band between chs. 2 and 6, may not even be possible.
“Sticking a three-foot antenna onto a three-inch mobile phone [to receive VHF signals] is going to be really tough and it just isn’t going to work,” says Perry Priestley, vice president of Linear Industries, a mobile transmission equipment vendor.
“Walk anywhere in the country with a good network and your cell phone will work,” he says. “Do that with a TV; go down highways, go anywhere outside of a general coverage and you have no coverage, no TV station there.”
But others believe that as the transmission and receiver technology evolves, VHF mobile will eventually overcome some its inherent disadvantages and close the gap with the UHF variety.
“You’re not going to have a two-foot-tall antenna on a mobile DTV device so it’s going to require some further work to get low V,” says LG corporate spokesman John Taylor. “Our initial implementation has an antenna that’s designed for high V and UHF with a VHF-UHF receiver on the chip.”
The long wavelengths of VHF are not the band’s only problem. It’s also likely to encounter more trouble from impulse noise and electromagnetic interference from consumer devices. Of course, those problems only crop up if the devices can receive the signals.
After the cut off of analog simulcasts last June, many consumers reported trouble receiving VHF signals on indoor antennas. In response, some stations have asked the FCC if they can move or revert to UHF channels.
Approximately 500 of the 1,700 full-power stations are still broadcasting in the VHF band.
The digital mobile DTV standard is designed to overcome all the reception problems that arise when you put a receiver in motion through continually changing environments.
“By lowering the data rates for mobile, they put in more error correction and that means you have better sensitivity and that’s what makes mobile go,” says Gary Sgrignoli, a consulting engineer with Meintel, Sgrignoli & Wallace.
“And if you’re talking about handhelds, unless you’re going to carry the boomboxes of the 1970s, you lose a lot with the small antenna,” he adds.
But broadcast engineers who have worked with mobile DTV signals feel that they are rugged enough for the job, even for reception by handhelds.
“Normal DTV is finicky if you don’t get that antenna positioned just right,” says Brett Jenkins, vice president of technology for ION Media Networks. “Mobile transmission is so robust that you are able to use a small antenna like you have on your cellphone and not think about where it is pointed or where you are holding the phone or any of that,” he says. “It really works regardless of that.
“We will not see the same types of problems that consumer experienced [in receiving conventional digital broadcasts] with VHF in Philly and Chicago and other markets that had problems.”
Another early experimenter agrees.
“I would probably prefer UHF over VHF, but VHF will work and we are investing in mobile in VHF stations,” says James Ocon, vice president of Gray Television.
Reception on handheld devices is keeping the Open Mobile Video Coalition busy these days.
Executive Director Anne Schelle is another believer in the progress of technology. Mobile digital TV-enabled receivers will follow the same 18-month development/deployment curve as other products introduced by cell phone companies, she says.
Broadcasters and mobile handset vendors will eventually come up with the right antennas to make mobile V and mobile U practicable, she predictss. “Antenna implementations and improvements in receivers … can be done.”
Broadcasters have a number of other options to improve signal delivery, including installing extra beam tilt to strengthen signals within urban areas and possibly even using channel repeaters.
But getting VHF signals into pocket-size devices will challenge the industry for the next 18 months and perhaps longer. The initial equipment, no matter what, “won’t be optimal,” Sgrignoli says, but then again, what new technology ever is?
“I expect mobile will come out; it will work, but it won’t work as well as the year after that and the year after that. If it’s pursued properly in the right amount of time, they’ll make it work.”