Apple didn't do much new when it introduced the fourth generation of the iPod, but it didn't need to. No one has beaten the company at the portable-audio-player game yet, but with 75 percent market share, the only way to go is down. The fourth-generation iPod performs pretty much the same as the third-generation player, with some detail improvements. But since we last looked at the player, we've tweaked our audio performance evaluation measures. As a result, we've identified some minor audio issues with the iPod's equalizer (EQ) presets. The iPod, however, is not alone in this, as we also noticed similar problems in other large hard drive players, such as the Archos Gmini XS200, Dell DJ 30, and Toshiba gigabeat MEG-F60.

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Like the Archos Gmini XS200 and Samsung YH-925, the iPod distorts, sometimes heavily, when you use the EQ settings. Harmonic distortion is minimal with no EQ, but at higher volume levels most of the EQ settings showed audible distortion. When we couldn't hear it, we could see it plainly on our audio spectrum analyzer. Bass response is about 5 dB down at 40 Hz, the practical lower limit for most music.