When it comes to digital music, can portability and high quality sound coexist?Since 1975, Aphex has been manufacturing signal processing products for professional audio, broadcast, fixed installation, musical instrument, touring-sound, and home-recording markets. The company says its technology has been used at the Oscars and Grammys. Paul McCartney, Bruce Springsteen, and Led Zeppelin have each paid $30 per minute to apply Aphex during their own audio mastering.On Monday, leading audio enhancement technologists  Aphex joined the fight against subpar sound on mobile devices with the release of Audio Xciter for iOS (with Android to come next month). They have done a fantastic job.Grandiose claims of heavy rockstar use aside, it became pretty clear that Aphex knows sound after just a few minutes with the Audio Xciter app (price varies — see below). It works pretty much like your typical music player, with standard screens and playback controls. The difference: During playback, you’ll notice an “X” in the middle of the screen. When lit, this X indicates that the app is using the Xciter digital signal processor (DSP) to analyze and enhance the audio signal, which it does in real time. You can adjust the DSP settings by choosing from low, medium, or high.Even if you have a piece of an earbud lodged in your ear canal, you should notice the difference in sound quality immediately – it’s that impressive. Audio Xciter opens up heavily compressed music files restoring the detail, richness, and three-dimensionality of your favorite music. Skeptics can turn the Xciter off by tapping the X to check the difference.This app comes in three versions: Free, which limits you to fifteen minutes of Xciter enhanced playback per day; Basic ($3 until July 31 or $5 thereafter), which removes the time restriction; or Studio ($8 until July 31; $10 after), which allows you to further customize the DSP settings with more precision than just low, medium, and high. Fine-tuning the DSP settings definitely made a difference between individual songs, which is why you’re going to end up springing for the $8 version if you like the way the free one sounds. However, throwing on one of the pre-programmed settings worked too, which is good for those of us who want good sound but don’t want the burden of adjusting it for each track.In our testing, Audio Xciter consistently delivered improved sound quality (DRM-free tracks only), regardless of music genre. Details from Joanna Newsom’s complex musical arrangements became more audible. The Wu-Tang Clan sounded like they were swarming around their recording studio. Lemmy from Motörhead might as well have been shouting at me from a foot away.If you’re looking for high quality sound on your iOS device, we heartily suggest springing for Audio Xciter Basic or Studio, depending on how much control you think you want to have. The asking prices are steep, to be sure, but this is truly an impressive app — and rockstars paid way more for it than you will.Besides, you can rest easy in the knowledge that a portion of the proceeds go to Respect The Music, a nonprofit organization that supports musical education and immersion. Of course, you can also give the free version a whirl before deciding whether or not to shell out any money.Consider these bonuses if you’re still on the fence, because yes, eight bucks is a lot to pay for an app: 1) Audio Xciter’s enhancement technology works on all audio, including podcasts and audiobooks; 2) You can make playlists within the app; 3) Apps are really cheap compared to their hardware equivalents.