By Bill Machrone
Apple’s new iPod shuffle has stellar audio performance. In the bass registers, it blows away the competition, including its bigger siblings. I mentioned this in my review of the shuffle, saying that Apple’s hard drive players lag behind some newer players in bass performance, but that the shuffle was way better than anything else out there. An Apple exec has basically challenged me to prove it.
Although the most important thing with any audio device is how it sounds, we use common audio test techniques to quantify such factors as frequency response, harmonic distortion, and maximum loudness. Along with an iPod shuffle, I gathered four other players and ranked them for perceived audio performance, concentrating on the bass. I used a pair of Apple earbuds and my Sennheiser Pro headphones for all the listening tests. The five players, from best to worst, were the iPod shuffle, a Creative Zen Micro, a Dell DJ 20, a 15GB Apple iPod (third generation), and an Apple iPod mini.
Good bass performance requires realistic reproduction of low piano notes, string bass, electric bass, cello, and kick drum or bass drum—and the players’ abilities were noticeably different. The difference clearly was not in the codecs or digital electronics but in the output stage, which drives the headphones.
Apart from a slight low-end falloff on some of my 20-Hz to 20-KHz swept sine wave tests, I couldn’t quantify the difference I was hearing. I tried something different: a 40-Hz square wave. Roughly, 40 Hz is low E, the lowest note produced by a standard upright bass or an electric bass guitar. The square wave contains the fundamental frequency and a theoretically infinite number of harmonics. The harmonics give instruments their distinctive timbre; sine waves are never found in musical instruments. Square waves are also the most demanding kind of wave for an amplifier to reproduce accurately, as the output voltage must rise instantly to the full amplitude, remain at that voltage for a period of time (at 40 Hz, it’s 1.3 milliseconds), then drop through zero to the full negative amplitude. Then it has to hold the negative voltage for another 1.3 ms, and the process starts over again.
I tested the players with the 40-Hz square wave, with and without the load of a standard pair of Apple earbuds, and judged them on their ability to form a good square wave and sustain the voltage. Without the load, all but the iPod mini were able to form a good square wave. With the load, all but the iPod shuffle failed, in varying degrees, to sustain it. The speed with which the square wave sinks back toward zero indicates how long the player can sustain bass notes and their harmonics under load. All of the players except the iPod shuffle showed deterioration of the wave, but the ones that showed the least deterioration sounded best.
I then played pink noise through the players to verify my findings. Pink noise is random but has the same amount of power at every frequency. To the player’s electronics, it resembles music more than white noise or sine wave sweeps do. With the earbud load, the players showed different levels of falloff in the low bass, as predicted by the 40-Hz square wave test. You can see the results displayed at http://home.comcast.net./~machrone/playertest/playertest.htm.
The iPod shuffle’s near-perfect rendering of the square wave means that it uses push-pull output instead of the single-ended, capacitor-coupled output found in just about every other player. You just can’t get this kind of audio performance from a single-ended circuit. I find Apple’s audiophile approach exciting on several different levels. You can hear the improvement; will Apple incorporate the same technology in future hard drive players? And technologically, it’s fascinating. My inner geek wants answers to half a dozen questions, including how they’re generating the negative power supply voltage and whether they’ve gone with a capacitorless design. I’ve asked Apple, but so far the company is mum.
I believe I proved that my ears were right: Several other hard drive players edge out older Apple players, but the iPod shuffle does them all one better. I think I also proved conclusively that the iPod mini’s output capacitors are woefully undersized, as some audiophiles have been saying since Apple introduced the device. I also found that the iPod mini has lots of harmonic distortion—everywhere but at the industry-standard 1-KHz measuring point.
You’d think that Apple would be touting the breakthrough performance of the iPod shuffle, but again, not a word. Does the company think we don’t care about fidelity? And if so, why did Apple bother with such a radical redesign and audio improvement?