To measure the iPod's technical performance, I used Bias Peak 3.0 running on my Macintosh PowerBook to prepare uncompressed AIFF files of the 16-bit test signals I use to assess CD players, and prepared a playlist using Apple's iTunes program. Plugging the iPod into the laptop with its FireWire connection automatically updated the contents of its hard disk; from then on, I merely selected the appropriate track with the iPod's menu button..

The iPod's frequency response was flat (fig.1). Channel separation was fundamentally good in both directions (fig.2), but slightly if inconsequentially compromised at low frequencies by the battery power supply's rising impedance in this region.

Fig.1 Apple iPod, frequency response at -12dBFS into 100k ohms (right channel dashed, 0.5dB/vertical div.).

Fig.2 Apple iPod, channel separation (10dB/vertical div.).

Fig.3 shows a 1/3-octave analysis of the iPod's output while it decoded uncompressed data representing a dithered 1kHz tone at -90dBFS. A very small amount of second-harmonic content can be seen, but the fundamental peaks at exactly -90dB, as it should. The noise floor below 1kHz is above that of the dither used to encode the signal, and is presumably analog noise emanating from the output circuitry. This can also be seen in fig.4, a similar but wider-band spectral analysis of the player's output while it decoded 16-bit "digital black" data. The rise in the noise floor above the audioband is presumably due to the noise-shaping used by the DAC.

Fig.3 Apple iPod, 1/3-octave spectrum of dithered 1kHz tone at -90dBFS, with noise and spuriae, 16-bit AIF data (right channel dashed).

Fig.4 Apple iPod, 1/3-octave spectrum of digital black, with noise and spuriae, 16-bit AIF data (right channel dashed).

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