HD Radio – What is it?

HD Radio is the radio industry’s attempt to convert “over the air” to digital broadcasting, in the same sense that TV (in 2009) is finally about to complete its conversion from Analog to Digital broadcasting.  The reasons for wanting to get to digital signals are similar for TV and Radio.  The primary motivation for over-the-air radio rolling out HD seems to have been competition from XM and Sirius satellite radio.   Satellite radio offers hundreds of high quality “stations”.  Even though they do charge a subscription fee, millions of Americans have demonstrated they are willing to pay for (mostly) commercial free radio with many choices of programs and music.

Like Television, once an FM radio station has added Digital capability, FM stations have the ability to send up to 3 “HD” channels over a single station, with a channel designator at the end, usually written as -HD2, HD3 – so when Wxxx-FM “goes digital’, it then has 3 channels of content –  Wxxx-FM/HD, Wxxx-2 and Wxxx-3.

In addition to having up to 3 audio channels, the HD signal allows for transmitting non-radio digital data, like the names of the songs and artists being played, real time traffic alerts, stock prices, etc.  It also makes possible “surround sound” with more than two audio channels.

AM/HD is also digital AM, but has had very limited adoption.  AM radio has only 10 khz (10,000 bits/second) of separation,  so even with digital compression, digital AM is still very limited in what is possible.   AM/HD is limited to sending one “FM quality” signal from an AM station.  Since most AM stations stopped broadcasting music 30 years ago, listening to Rush Limbaugh in FM quality is not necessarily worth spending the money to make possible.

Unlike Europe, the United States (FCC) has not allocated any radio spectrum for a new FM all-digital radio band, but rather forced FM/HD to run in a “hybrid” mode, where the analog signal continues to be broadcast, and the digital signals “hide” in the fringe areas between stations.   FM stations have 200 kHz of separation, which far exceeds what was needed to broadcast a high quality audio signal, even in stereo.  Normal Analog FM radio doesn’t hear the digital signals because they are just white noise that doesn’t interfere with hearing the normal audio.

There is no plan as of 2009 to force radio to ever cut over to all-digital.   Doing so would require people to replace all their existing radios, including those in their automobiles.  New cars generally come with HD capable radios, but it will be 10 or 15 years before most cars have an HD capable radio.    

There has been a little bit of talk about perhaps using TV channel 5 and 6 after the end of analog TV (in June 2009?) to create a new all-HD FM band, but no real decisions.  The Obama administration’s incoming chairman may have some opinions on what to do about this issue.

More detailed information can be found at the HD Radio Alliance Web Site

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