Today I needed to feed a preamp with a 1kHz sine wave. I have an audio CD full of sine waves (and other torture devices) so rather than build a cheesy little signal generator out of an op-amp and a light bulb I thought I'd be ever-so-21st-century about it and upload the CD to my iPod to turn it into a signal generator on steroids. And lo...
This is what a 1kHz sine wave looks like, playing on my iPod with its volume control set at one notch less than maximum:
Pretty, isn't it?
This sine wave is recorded at '0 dBFS' (which is geek speak for saying that it is at, but not beyond, the maximum amplitude that digital audio should correctly reproduce). When fed with a 0 dBFS sine wave a well-behaved system produces exactly the waveform above, and will do so regardless of where you set the volume control (above zero). So far so good.
Here's the same 1kHz sine wave played with the iPod volume increased (by one notch) to maximum:
The built-in amplifier in the iPod can't cope: the waveform is horribly clipped, with the peaks and troughs severely flattened. This might not have been disastrous fifteen years ago (when there was still some dynamic range left in many recordings) but these days, with the loudness wars keeping most recordings at 0 dBFS as much as humanly possible, if you listen to your iPod turned all the way up then this is going to cause noticeable distortion (of the nastiest kind) on a lot of material.
When I did listen to my iPod a lot I almost always used an external headphone amplifier (to take the strain off the built-in amplifier with low-impedance headphones, or to get a reasonable level with high-impedance ones). The headphone amp had a volume control of its own that I used while leaving the iPod's volume alone. I never liked the sound with the iPod turned up full and now I know why: Apple designed the iPod's volume control to go all the way to '11', even though they probably knew that '10' was the limit.