HAVE you ever wondered, if you are of an age with your correspondent, about those missing channels on old television sets? Apart from channel two, the rest of the original VHF channels on the dial were usually just the odd numbers from three to 13. That was because, in over-the-air VHF broadcasting, the channel between two analogue stations had to be left unused so that it would not interfere with adjacent ones. When UHF broadcasting came along, empty “guard bands” were added to each channel for the same reason. In some places, this so-called “white space” of unused frequencies separating working channels amounted to as much as 70% of the total bandwidth available for television broadcasting.

Mobile phone companies and other would-be users of wireless spectrum have long lusted after television's empty airwaves. This week, after two years of haggling and testing, the Federal Communications Commission (FCC) in Washington, DC, finally gave the go-ahead for others in America to use them.

In November 2008, the FCC voted to reallocate the various segments of white space and unused channels between 54 megahertz and 806 megahertz (channels two to 69) that would no longer be needed when the last of the country's analogue television transmitters switched to digital broadcasting in June 2009 (see “Wireless at warp speed”, November 7th, 2008). Unlike analogue transmissions, digital signals do not “bleed” into one another and can therefore be packed closer together.

As a consequence, television broadcasters now need little more than half the spectrum they hogged before switching to digital transmission. That has not stopped them fighting tooth and claw to hang onto their unused white space. Most had grand plans for using such frequencies, after going digital, to sell information services to the public.

That is not to be. Instead, the FCC has used the switch to digital television as an opportunity to free huge swathes of bandwidth for others to use. The most valuable frequencies of all—those in the 700 megahertz band (channels 52 to 69)—have been auctioned off to mobile phone companies. Between them, Verizon, AT&T and others paid close on $20 billion to clinch this prime spectrum.

The reason these channels are so valuable—and why they were chosen for terrestrial television in the first place—is because their signals travel for kilometres, can carry a lot of information, are unaffected by weather and foliage, go through walls and penetrate all the nooks and crannies within the bowels of buildings. They will allow mobile carriers to cover, from a single tower, up to ten times the area possible from a tower using existing frequencies. Dropped calls should then become a thing of the past.

By contrast, the white space freed up below 700 megahertz is to be made available for unlicensed use by the public. Unlicensed does not mean free. Network infrastructure will still have to be built. But a new breed of wireless internet service providers using white-space frequencies will not have to pay for their spectrum. They should therefore be able to offer high-speed broadband at far lower rates than today. It also means that start-up firms lacking the deep pockets of incumbents should be able to get a foot in the door.

Indeed, by opening up television's white space to the public, the FCC hopes to trigger another wireless revolution—one potentially bigger than the wave of innovation unleashed a decade or so ago when Wi-Fi, Bluetooth and other wireless technologies embraced the unlicensed 2.4 gigahertz band reserved previously for microwave ovens, baby alarms and remote openers for garage doors.

The difference this time is that the frequencies being released will allow larger chunks of data to be moved further and faster. The latest version of Wi-Fi (802.11n) shuttles data at 160 to 300 megabits a second. White-space devices are expected to be able to zip data along at 400 to 800 megabits a second once they start using the same tricks as the latest forms of WiFi. And while Wi-Fi signals peter out after 100 metres (330 feet) or so, their white-space equivalents could have ranges measured in kilometres.

Microsoft, an active proponent of white-space wireless, is using just two of its experimental “White-Fi” transmitters to blanket the company's entire 200-hectare campus at Redmond, Washington, in place of the thousands of Wi-Fi routers that would otherwise have been needed. No wonder white-space is being referred to as “Wi-Fi on steroids”.

Enthusiasts talk about white-space devices offering a “third pipe” for access to the internet, to rival cable and telephone broadband. Others see white-space as providing an alternative to mobile telephony. When wireless hotspots cover entire neighborhoods rather than mere coffee shops, who needs a mobile-phone contract? A smart phone running Skype or something similar would be essentially free of usage charges and unfettered by all the restrictions that carriers impose.

Such thoughts have doubtless crossed many a mind at Google. Not having to put up with carrier-required compromises that hobble functions and features, owners of Android smart phones would be free to use the full power of their devices to surf the web for information, social networking and entertainment—and, in so doing, rake in billions more advertising dollars for Google. That, after all, was why the search company developed its free Android operating system in the first place. It is not impossible to envisage Google—in partnership with, say, Spectrum Bridge, a company in Florida that has installed several demonstration systems which use white-space technology to blanket whole communities—emerging as a mobile operator in its own right one day.

Before any of that can happen, however, a lot of technical problems have to be licked. For one thing, white-space transmitters not only have to avoid interfering with local television stations, but also with the wireless microphones used in conference halls, sports arenas, karaoke bars, theatres and churches. As a white-space gizmo moves around a city, the channels it can use will change depending on how close it gets to various television stations. The central access tower it communicates with may then have to hop from one channel to another—and check with all the other client devices using it to see if they can follow suit. If a newcomer then joins the network (client devices are expected to be joining and leaving continuously) and happens to be near a transmitter, the tower and its various clients will have to scramble to find yet another channel they can all use without causing interference. The computational problem is not exactly insignificant.

Under pressure from television broadcasters, the FCC originally proposed that all white-space devices used during the trial period would have to adopt, as an added precaution, a redundant belt-and-braces approach to avoiding interference. One method involved using electronic detectors to sense other transmitters in the area; the other relied on using GPS signals to determine the device's exact location, and then to query a central database to find out what channels were free in that area. Microsoft has created one such geolocation database. Google has offered to build another.

To date, the white-space prototypes used in the feasibility trials have had little trouble sensing occupied television channels. In general, they have picked them up at signal strengths less than a thousandth of that needed to display an image on a TV screen. In other words, they would be able to hop off an occupied channel and onto a vacant one before causing so much as a blip on television sets in the area. Even so, the equipment makers argue that, while doable, all this sensing palaver makes white-space devices needlessly complicated and expensive.

The FCC seems to agree. At its meeting on September 23rd, the commissioners voted unanimously to ditch the spectrum-sensing requirement and let device makers rely solely on interrogating the online databases to find vacant channels. Meanwhile, wireless microphones are to be allocated two separate channels of their own.

Given the go-ahead they were hoping for, equipment makers now expect that the chip-sets needed to make the system work in phones, laptops, tablets, e-readers and other gadgets will start trickling out over the next year or so. White-space consumer products could then hit the retail market by late 2012. If the introduction of Wi-Fi is anything to go by, white-space will change the way people live, work and play more than anyone can currently imagine.