You can file doubleTwist's $10 AirPlay Recorder under Controversial. The tool, originally developed for Android and now available for OS X (10.7.3 and later), was designed for one purpose only: capturing iTunes audio streams for later playback.
You mean like tracks from your iTunes library?
Not really. After all, you already have copies of those tracks, so why make new recordings of them?
You mean like audio previews from the iTunes Store?
I suppose you could, but I'm not sure why you'd want to, since they're not full tracks.
Oh, you mean like grabbing songs that play over iTunes Radio?
So you're saying that the only legitimate use for this utility is stealing music?
"Stealing" is such a loaded word. I believe technologists refer to this as "time shifting" and justify it by saying that there already exist plenty of utilities for capturing audio being played by your Mac. AirPlay Recorder just happens to be easier to use (as well as more limited, as it can capture audio only from iTunes). The fact remains, however, that you use the app to make a copy of music that you didn't purchase.
Some might suggest that a more legitimate use is to record an iTunes Radio station at home so you can listen to it on the go without using your cellular data, or listen to it on an iPod touch or Wi-Fi-only iPad while out of range of a Wi-Fi network. But rights holders could more legitimately reply that your license does not grant such liberties. How you feel about this is entirely up to the direction your moral compass points.
With that out of the way, let's turn to how AirPlay Recorder actually works. Using the app couldn't be much easier. You launch it, and it registers itself with iTunes as an AirPlay destination. Then you launch iTunes and choose dt Recorder as the destination from iTunes's AirPlay menu.
Now, just play a track or an iTunes Radio station, and AirPlay Recorder automatically captures that track, or the tracks in that stream, as it plays, also capturing each track's metadata: title, artist, and album art. (If a track is shorter than 45 seconds--meaning it's likely just an ad--it won't be recorded.) Your recorded tracks are encoded in Apple Lossless format and saved in a folder named Recorder in your home folder's Music folder (~/Music/Recorder).
Because iTunes thinks you're sending its audio to a remote AirPlay destination, if you want to listen along, you must click Multiple in iTunes's AirPlay menu and enable both Computer and dt Recorder. Even then, AirPlay Recorder doesn't give you a way to monitor tracks as they play and choose which tracks to record. Rather, you just fire up AirPlay Recorder, play an iTunes Radio playlist or station, and walk away for an hour or so while the app captures the stream and parses tracks into individual files. You can later listen to the tracks (as well as choose exactly which tracks you'd like to keep or copy to your other devices).
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