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May 26, 2010

4th-generation iPod

What can you say about the world's most popular digital audio player? How about, "It could be better."

The 4th-generation iPod is the player that all others are compared to for ease of use, style, and substance. It's more than just an audio player; it's a platform, with hundreds of accessories and add-ons to tailor it to your needs. While other manufacturers have added FM radios, recording, and other features to grab just a bit of the iPod's market share, Apple has resolutely held to its music-only stance. (The iPod photo is a different model, aimed at a different user.)

The iPod's audio performance should be as iconic as everything else about it. Unfortunately, it's not as consistently wonderful as the rest of the player.

If you don't use EQ on the 4th Generation iPod, everything's generally fine. When you turn EQ on, things are not so fine. But even without EQ, the iPod could--and should--have more bass.

Here's the frequency response with the standard white earbuds plugged in:

It has the typical audio player droop in the bass, mostly caused by headphone loading on the single-ended, capacitor-coupled output circuit. Down 5dB at 40Hz, down 10dB at 20Hz? That's a lot of bottom end to give up.

The harmonic distortion in the iPod 4G is acceptably low with no EQ. You can see a single harmonic in the bass, and it comes back again in the treble. But it's way below audibility:

When you select bass boost, however, it's not really boosting the bass, it's piling on harmonics in the bass range:

When your brain hears more harmonics in the bass, it thinks it's hearing more of the fundamental. It's a nice psychoacoustic trick, but in fact it's blurring the bass notes and lending an unrealistic timbre to the instruments. When you listen to the sweep tones, you can hear the difference--a nasal timbre instead of the purer sound of the sine wave. The stuff over on the right is low-pitched hissy/poppy noise as the sweep tone goes through the mids and trebles.

Now let's try another EQ preset, this time the Jazz setting:

Look what they've done to my song, Ma! We've got a nice stack of harmonics in the midbass to midrange, but complete chaos in the treble. Something inside the iPod has hit the analog or digital wall, and is now spewing subharmonics. When you hear a note like this, say a violin holding a note, you hear the note, EEEEEEEEEE, and behind it you hear sssshhhhrshhhshhhhshhsrrrhsh. It's 30 and 40dB below the note, but it's there. The distortion-plagued sweep tone itself sounds like a catfight played over an old AM radio.

I'm pushing the player pretty hard, however. The earbuds are cranking out 90 to 100dB of in-ear volume during this sweep. So let's back it off to a more comfortable 85dB in-ear:

Ahhh, that's better. There's one lone gothic arch in the treble and a very faint harmonic in the midrange. The question is, is 85dB in-ear loud enough for your listening enjoyment, and how quickly does the distortion get bad when the loud passages come along or when you crank it up a little?

I don't want to vilify the iPod too much. It's a fine-sounding player most of the time. But it's also just a couple of volume clicks away from serious distortion if you use the EQ. Each of the settings behaves a little differently, but they all exhibit this behavior, sprouting hair and growing fangs as the volume goes up.

One final torture test. This is the iPod's unloaded 40Hz square wave, which looks pretty darn clean:

But when you add the earbuds, it's like a kid playing the tuba--not enough air to sustain the note:

The result is that droopy frequency response curve we saw at the outset.

There's a moral to this tale. Several of them, actually. One is that you don't need to be great to be good. The iPod's fidelity is more than satisfactory for the vast majority of users, especially in the noisy commuting, exercise, or travel environments. The second is that 32-ohm earphones really stress the player. It performs better and sounds better into higher-impedance phones. Unfortunately, earbuds and in-canal phones tend to go the other way, to lower impedance, in an attempt to suck more power out of the player. Equally unfortunately, there's not enough power there to satisfy their craving at the low frequencies.

The iPod with my 64-ohm Sennheiser HD280 Pros is sweet--bass is vastly improved, and the efficiency of those big cans means you hardly have to crank the volume. You stay well away from the distortion zone.

If you really want to hear what the iPod can do, use high-bitrate compression or lossless compression and play it into your stereo. The high-impedance load preserves all of the iPod's fidelity, even with the EQ on. And if you want startling fidelity when you're on the go, with headphones of any type, invest in a headphone amplifier. About the size of an Altoids tin, these things can crank up to half a watt into your headphones. Not that you need that much, but they offload the iPod's load-sensitive output stage and do all the heavy lifting.

You don't need a headphone amp when you're jogging or in other noisy environments, but when you want to get down with your music, it elevates your iPod to its fullest potential.


More to come....

Posted by machrone at May 26, 2010 01:36 PM

Comments

Thanks for info on iPod fidelity, as an old hi-fi buff (really old - - - I built my first reciever from a Heath kit in 1959) I've always wanted more info on the fidelity of MP3 players.

Does anyone manufactuer a MP3 player that is made to work only with a hi-fi reciever? You know, with a large display and full size controls (that do everything small MP3 player controls do and hopefully more) and compatible fidelity.

Thanks for any feedback,
Bill

Posted by: Bill Clarke at July 30, 2005 05:20 PM

Ok, I see you did you 'thing' here ... But I have something to tell you ... Not everything is analysed in charts and ratings ... the iPod's are byfar one of the worst MP3 Players made ever ...

Posted by: Mohammed at September 30, 2005 06:56 PM

This is the first other person I've seen to complain about the bass response of the iPods, which I found to be, er... "deeply" unsatisfactory when I borrowed a friend's iPod mini. Even with the bass boosted with the EQ (now, after seeing Bill's graphs, I know why). For me, plugging in a better set of headphone (Sony MDMs) didn't improve things as much as I would have liked - maybe the impedance wasn't that high, though.

I think Bill's "moral" is dead-on, except I'd phrase it more sarcastically: consumers are sheep, and the Apple marketing department are geniuses.

Posted by: Warren Sirota at November 17, 2005 03:12 PM