It's a huge hit on the iPhone, and though we experienced a few quirks during testing, the Shazam music-ID service seems to have gained nothing but goodness by adding 2 million tracks to its knowledge base. We do, however, wish it took a broader view of music history.

The idea is appealing -- let your phone help identify a song you're hearing by transmitting it to a database that can ID it for you. It's a tricky task and there are a lot of services that think they have an angle on it: Midomi lets you sing or hum your track, the Open Music Encyclopedia has a variety of searches for the musically adept, Unknown Track awaits for those quick enough to record a sample, and SongTapper keeps time for those who can only manage percussion.

But those are Web-based services; Shazam sits in your phone -- or the friendly front end does, anyway. Offered in the iPhone Apps store since July (and currently topping the free-music-app chart there) and available in the Android Market as well as in various Samsung music-capable phones, Shazam has gained fair notoriety as the star of one of Apple's most current iPhone advertisements.

The problem -- with Shazam or any other music-ID service -- is that there's an awful lot of music in the world. This week, Shazam announced that it was boosting the number of songs known to its database to eight million -- more than Mufin, more than iTunes carries, more than anyone. Our testing revealed that while it's very, very good, there remains more music in heaven and on earth than any one service can tackle.

Not trusting our own somewhat dusty and obscure musical tastes, we asked various users of the Android and iPhone applications to do their worst, as we did the same. Combined testing indicated that on the whole, Shazam works when the phone is in either an open area (outside near a speaker worked fairly well), or indoors in a room with not too much echo. Echo, distance from the speaker, and loud ambient noise resulted in poor or no matches for individual songs, though accuracy rarely dipped below 50% in any case.

And what does a database of eight million tracks know? We had fair results (correct identification 80% or more of the time) with old and new pop hits, most relatively mainstream Western music genres, and even dance remixes from here and Europe. We had a few pleasant surprises as Shazam picked out more obscure artists, though even some tracks available from iTunes didn't register with the ID engine.

The system was on the other hand fairly useless on pre-1950s tracks, and it had some trouble with mashups by artists such as Girl Talk -- where we discovered that the software tended to identify the vocal track, not whatever sample was underpinning it. Our results with non-Western selections were reasonably good, though we admit our familiarity with those genres is limited; as the database expands, rapid improvement can be expected.

(There is, by the way, fun to be had on the Shazam Web site if you check to see what other people are IDing with the service in more or less real time. Your reviewer was flabbergasted at various points to see ID requests for both Jason Mraz's "I'm Yours," which has those two words as lyrics and not much more, and The Beatles' "A Day In The Life," which she was pretty sure could be identified by everyone in this galactic quadrant by now.)

Shazam for iPhone and for Android is free for download and use for the foreseeable future; the company does charge for its ID-by-SMS offerings, currently available in the UK. Fans of obsolete genres need not apply, but it'll soothe that ID itch next time you're hearing something you like at the mall or elsewhere in the 21st century.