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Quantifying Digital Audio Player Performance by Bill Machrone Copyright 2005 Bill Machrone This Web page is a companion to my column, "Shuffle's Got a Secret" in PC Magazine. It's an attempt to quantify the performance differences among several dig ital audio players. While all have essentially flat (or flat-enough) fre quency response with sine wave sweep tones and less than 01% THD at 1KH z, there are audible differences when driving professional headphones or self-powered studio monitors. All can drive a standard set of earbuds ( in this case, the standard Apple earbuds) to 100+dB in-canal loudness us ing a rock test track that fills virtually the entire audible spectrum w ith a constant din. Most of the difference is in the bass response, but no differences are di scernible from one unit to the next with a 40Hz sine wave (roughly low E on the electric bass). After performing many different tests, I came up with the following test tracks, which I loaded onto each of the players : Pink noise, first with no load, then the same pink noise file with stan dard Apple earbuds plugged into the unit a 40Hz square wave, with and without the earbuds. The 40Hz square wave is an extreme test, since it asks the output stage to act like a servo a mplifier, driving to a full voltage, holding it at that level, then driv ing back the opposite way. This is really a test of the output stage and possibly the power supply, but it's been noted many times that an amplifier is nothing more than a modulated power supply. All of the digital audio players do a decent job of reproducing the squar e wave without a load. With the earbuds plugged in, they generally have difficulty sustaining the voltage, and it collapses back towards zero. One in particular sustains remarkably we ll and subjectively has noticeably better bass response. The five players I tested, in order the quality of their perceived bass p erformance in listening tests were: iPod Shuffle Zen Micro Dell DJ 20GB 15GB iPod (3rd Generation) iPod Mini All but one of these players use single-ended, capacitively coupled outpu t stages. It's an inexpensive and effective way to deliver acceptable, i f not superb performance, but the size of the coupling capacitor and the impedance of the headphones have a significant effect on the player's a bility to sustain a complex bass tone. The screen shots below are just a small portion of the testing I did, but should give you an idea of the differences among players.
I used an M-Audio Transit USB, an external USB sound card, as the audio i nterface. It has a high-impedance input that doesn't load the circuit un der test, and it has no external adjustments for gain--no knob settings to screw up; It also has ab out 120dB signal to noise ratio and can sample at 96KHz, sufficient head room for tests like these.
Apart from some ove rshoot, player 1 does a pretty good job of generating an unloaded square wave. Unloaded square wave per formance doesn't really tell you all that much--unless the player can't form a decent square wave, as one below couldn't.
The top line i s the response without the headphone load; the bottom line is the respon se with the headphones plugged in. The load of the headphones has little effect on the upper frequency response, but the drop is noticeable at l ower frequencies.
and falls down badly when it comes to sustaining it u nder load. In my testing, players that were dragged down to zero by the load of the earbuds didn't sound as full as the ones that had a little l eft in in the tank at the end of the cycle.
My son took the 15GB iPod back to college with him befor e I could run the pink noise tests. While the iPod is generally we ll-regarded, bass performance is not stellar. This sweep was run with th e headphone load, and you can see the fall-off in the lower octave. The rest of the frequency response curve is flat, however.
Once I plugged in the earbuds, the waveform of the iPod Mini really deteriorated. Audiophiles and high-end headphone manufacturers agree that Apple undersi zed the output capacitors on the Mini, whether for space, cost, or power reasons.
With the pink noise test, this Player 3 shows a lar ger variance between the load and no-load lines than any other player in the test. The divergence, as expected, is greatest at low frequencies.
With earbuds, the iPod Shuffle's signal looks darn near identical to the no-load signal. I checked and rechecked this resul t because I couldn't believe my eyes. The iPod Shuffle sounds great, wit h a solid low end, and no need for bass boost. The reason for this sterling performance is that the left and right chann els each have two transistors, one pushing, one pulling, and no capacito r that gets discharged over time.
The pink noise performance of the iPod Shuffle i s, as you might expect, exemplary. The load/no load performance is very close, even at the deepest bass frequencies. You might be wondering why these pink noise charts seem to sink down as f requency goes up. The reason is that you're looking at a logarithmic cha rt, and the intensity of pink nose tapers off as frequency goes up. This is to avoid the problem of white noise, which has the same amount of po wer at every frequency. That sounds like a good thing, but in fact would mean that fully half of the sonic power would be above 10KHz, and sound just isn't like that in the real world.
Update Since I wrote the column, Apple introduced the iPod Mini 6GB. I subjected it to some of the same tests as the other players, and as you can see f rom the screen captures, Apple has obviously improved the audio amplifie r The 6GB Mini sounds noticeably better than the original Mini.
The curve gets pulled down to zero with standard Apple 32 ohm earbuds. We could have hoped for more capacity, but it's still be tter than the original iPod Mini.
I didn't get a chance to run the load/no load sweep tes t on the 6GB Mini, but I did run a 1/3 octave sweep with the earbud load . You can seen from the graph that the lowest frequencies are still some what deficient, but there's definitely more bass from 30Hz on up.
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