September 14, 2010

iPod nano

Barely larger than the shuffle, the iPod nano packs the same 4 gigabytes as the original mini, but it's flash, not hard disk. The screen is color, it shows pictures, but you already know all that. The real question: How does it sound?

In terms of overall frequency response and lack of distortion, it's right up there with the very best players. In terms of bass response, it's the second-best player I've ever heard or tested.

The nano takes a back seat to the shuffle in sheer bass performance, but it's still well ahead of any of the large iPods, and has a slight edge over the Zen Micro. It's the equal of the Archos PMA400, which is saying something.

The overall sound is open and spacious, it has that unconstrained sound of flat frequency response and generous amounts of power. That generous power extends down to all but the lowest frequencies, where it bagins to tail off:

This is a logarithmic sine wave sweep, which is an easy test for the player's electronics, but very demanding at the lowest frequencies. As you can see, response tails off. The player would do this dead flat if the earbuds weren't plugged in, but their load is especially severe at the lowest frequencies. Measurements like this are typically taken 10dB below the clipping level, as these were. The nano was very impressive in that once it came up to the -10dB level and stayed within a couple of tenths of a dB all the way through the rest of the sweep, instead of dancing around a fair bit, as most players do. There may not be any audible benefit, but it shows that the amplification is very accurate across the spectrum.

Here's another look at frequency response, with white noise instead of sine waves:

White noise spreads the power around randomly and evenly through the spectrum, so the amount of power at any given frequency is lower than a sine wave would be at a single frequency. So without having changed the volume control, the level drops. As you can see, it's below the "sag" level at the lowest frequencies, so the response is flat. Is it loud enough? Oh yes. but if we were to crank the player's volume up, the curve would begin to look like the previous one, sagging in the bass.

The shuffle, by comparison, remains dead flat.

Here's that most evil of tests, the 40Hz square wave, first unloaded:

As you can see, it's essentially perfect. Now let's plug in the earbuds:

Unlike the shuffle, the output stage is analog, and gets pulled down. But as I've observed elsewhere, any player that doesn't get pulled all the way to zero sounds pretty darn good and has enough power to drive earbuds at reasonable listening levels. This waveform is comparable to the Zen Micro and Archos PMA400, and if anything, the nano sounds a bit livelier.

There's one more way we need to look at the tone--in a full-frequency spectrogram. This is the view that reveals harmonic distortion, and in nano's case, with the EQ turned off/flat, there is none:

The thin, red line is the -10dB signal as it sweeps through the audio spectrum; the yellow and green down low is the lower output at low frequencies. There's not a hint of harmonic distortion. This is very clean.

Now let's turn on one of the EQ settings:

This is bass boost, and you can see that the nano increases the amount of fundamental (the red line fading into orange down low), but also adds several harmonics to sounds below 200Hz to fool your ears into thinking there's even more fundamental frequency. It sounds convincing, but it also adds intermodulation distortion up high. It's less prevalent here than in many other players, but it's still there and audible as a very faint sizzle around bells, triangles, and similar high, clean tones.

Every EQ setting adds its own blend of distortion, meant to fool your ears in different ways. No small players have true tone controls; of the players I've tested, only the Archos PMA400 actually adds and subtracts frequencies in a specific range instead of playing around with psychoacoustics.

I found the nano to be utterly enjoyable with flat EQ, and it appears that with the improved low-end power, the designers were able to take a lighter hand with the psychoacoustic manipulations. So if you decide to use an EQ, you won't hear out-and-out distortion and crappy, artificial-sounding music, as you would on many other players.

The nano is a winner, and a more-than-worthy successor to the mini.

Posted by machrone at 09:34 PM | Comments (7)

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 01:36 PM | Comments (3)

May 20, 2010

iPod mini 6GB

A new hard drive, 50 percent larger. New colors. Better battery life. New audio performance? The original iPod mini caused quite a stir when it was introduced, but the audio performance, especially in the bass, left something to be desired.

Actually, it left a lot to be desired.

Audio enthusiasts complained about the bass response right from the outset, even as people snapped up minis by the millions. Hey, I've got one. And I don't regret it one bit. Weak bass or not, it's been the source of many hours of listening pleasure. The 6GB version of the mini gave Apple the opportunity to right some wrongs, as well as take aluminum anodizing to new heights. The question is, did they?

I breathed a sigh of relief when I ran the first tests on the iPod mini 6GB because Apple had, in fact, addressed the bass performance. It's not stellar, but it's noticeably better than the original mini.

This is the octave-band sweep, snapped while a sweep was in progress. The vertical bars are unimportant; the recorded peaks show the response at each 1/3 octave measuring point.

The amount of droop at the lowest frequencies is typical for players of this class, and is largely caused by the load from the standard iPod earbuds; when the iPod is unloaded or playing into a high-impedance load such as a car stereo or a headphone amplifier, the response is much flatter.

Even with the falloff at the low end, the 6GB mini sounds better than its predecessor. There's more bass there. Switching back and forth between an original 4GB mini and a 6GB with the same earbuds and the volumes set to the same level, there's a subtle, but noticeable improvement.

The 40Hz square wave test is about the harshest thing I could come up with to wring out the bass response. The unloaded response is noticeably better than that of the original mini. Here's the original 4GB mini:

This droopy-looking curve above shows that the original mini can't make a decent square wave even with no load.

The 6GB iPod makes a decent-looking, if not exceptional square wave, a much more promising foundation for bass performance.

The 40Hz curve of the original 4GB mini with the earbuds attached is a complete mess.

The 6GB mini is a big improvement, but the 32-ohm earbuds manage to pull the voltage down to zero by the end of each half-wave. The result: it still runs out of steam in the bass.

more to come....

Posted by machrone at 09:34 PM | Comments (0)