Accessory maker Radiopaq has begun selling the Sound Jacket, an inline add-on for the fourth-generation iPod nano. The device performs automatic hardware equalization of a Nano's output, in theory compensating for distortion, as well as quality lost during file compression. Radiopaq claims that in some circumstances, quality should be improved by as much as 60 percent.

The device also contains its own amplifier, which boosts newly-adjusted output. The volume of some frequencies can be doubled over those of an unassisted iPod, the company says. Radiopaq is selling the Sound Jacket for £70, or approximately $113 US.