Radio Shack has brought digital FM to the iPhone through an external adapter. But is it necessary? The $79.99 iPhone adapter, called - deep breath - the Gigaware In-Line Control with HD Radio for iPhone, tunes in HD Radio, known generically as "in-band on-channel" (IBOC). Digital radio is broadcast alongside analog signals by about 2,000 U.S. stations, largely major commercial chains and public radio.

Pairing an HD Radio receiver with the iPhone makes for strange company, because the iPhone has so many options for listening to streaming and downloaded audio of all forms through free and commercial apps. It seems redundant with an unlimited data subscription to pay for and manage an accessory that streams a signal you can just tune in via an iPhone app. Or, you might simply start playing a podcast, which would likely have the same or higher quality as HD Radio, with the advantage of controlling when you listen.

I suppose this adapter makes HD Radio portable; most people who listen to digital FM tune in using a car stereo receiver. But you're still reliant on receiving a strong broadcast signal and wanting to listen to what's on the limited number of digital stations in your area - if you live in a major market with any digital signals at all. (For more on listening to HD Radio with a car receiver, see "Tag Radio Songs for Later Purchase While You Drive," 19 June 2009.)

Apple offers built-in analog FM reception in just a single device: the revised iPod nano. The iPhone and iPod touch lack radio features, but you can use a variety of free and commercial Internet radio apps over cell and Wi-Fi connections, or simply download and then listen to podcasts offline. (For more background on the nano's radio features, see "iPod nano Delivers Static in Radio Interface and Features," 28 September 2009).

The Gigaware device description makes no mention of AM tuning or analog AM or FM reception. Only a handful of AM stations offer digital broadcasts for a variety of technical and regulatory reasons.

The only portable media device that has HD Radio tuning built in is Microsoft's Zune HD. I tested the Zune HD after its release and found its tuning features poor - just as bad as Apple's tuning features for the iPod nano - but it was capable of picking up Seattle's digital FM stations.

The Gigaware adapter supports iTunes Tagging, which lets you mark songs that you're listening to for later purchase via the iTunes Store. But only Clear Channel currently supports the over-the-air tag format required by Apple to identify songs, so it's of little utility at the moment.

A separate, free application is needed to operate the Gigaware adapter, which works like an external radio in many ways. The adapter has external controls for volume, play/pause, rewind, and fast forward, as well as a Tag button. Details aren't available about how its pause feature works.