May 20, 2010

Creative Zen Micro

Audio geeks love to hate the Creative Zen Micro.

"It doesn’t look serious enough. White plastic bathtub case? Lollipop face colors—nine of them? Pulsating fiber optic rim light? I mean, come on!"

Get over it.

The Zen Micro is a solid little player with some interesting features. It also has some sonic flaws, but oddly enough, they’re self-healing. I like the touch interface, but it's an acquired taste. Some people hate it. Some people hate the iPod touch wheel, too.

The Zen Micro has the same problem as many other players--very good audio performance when the EQ is off; some nasty distortion when it's on. But when you turn on Smart Volume...

... the distortion magically disappears.

Here's the frequency response of the Zen Micro as I feed it a sweep tone from 20Hz to 20KHz:

It looks pretty good. It's flat, with the usual weakness in the bass, down about 3dB at 40Hz and 7-8dB at 20Hz. It's better than most players. We'll take another look at bass response further along. If there's any harmonic distortion, this view of the player's response doesn't show it, because the loudness of the distortion is lower than the loudness of the fundamental frequency. To see the distortion, we need to switch to another view. Here's the spectral view of the same 20-20KHz sweep tone, with the EQ off:

This is clean, listenable performance. There's not a hint of harmonic distortion. The transition from red to orange at 40Hz and below shows the weakness in the bass that's typical of virtually all digital audio players, but it's tolerable.

Now let's turn on the EQ. This is the Rock EQ setting at a volume level of 23 (out of 25):

Nice, huh? Those bands over the bass side are harmonics. Some audio designers think this is an OK way to boost some frequencies: fool the ear by adding harmonics, but don't make the fundamental frequency any louder. Psychoacoustically, it kind of works. When your brain hears a 120Hz tone added to a 60Hz tone, it perceives that the 60Hz fundamental is louder. But if you listen carefully, you'll also hear that the fundamental is blurred and that the sound is no longer an accurate representation of what the instrument sounds like.

Those cathedral-like traces in the high frequencies are wolf tones. They're the result of some electronic component being driven beyond its capabilities or the product of a harmonic above the test tone combining with the test tone to create a beat frequency that's harmonically unrelated to the fundamental. They sound like you're tuning an old radio between channels--squeals and hisses. As a result, the Rock EQ setting sounds like you've stuffed cotton in your ears, or that there's a blanket over the music. It's literally become smudged and blurred.

Then you turn on Smart Volume, and tranquility reigns again:

What's going on here?

The Zen Micro documentation describes Smart Volume as a volume leveler, which equalizes the volume within a track, making the quiet passages louder and keeping the loud passages from getting too loud. It also equalizes the volume from song to song, so you have less need to reach for the volume control as you move through your playlist. Circuits that do this have been around for 60 or 70 years, and are called limiters or automatic gain controls. If Smart Volume were nothing more than an AGC, it would just introduce more sonic flaws. It wouldn't be interesting--and it wouldn't do what you see on the screen above.

I talked with Dr. Lee Morse, an audio scientist at Creative about Smart Volume, and learned that it's a patented algorithm that Creative uses in a number of its audio devices, including sound cards and the Zen Xtra. Far more than being an AGC, it actually looks a frame ahead of the codec at the music and gives the codec hints of what's coming so it can do a better job decoding it accurately. Moreover, it stores the hints in the metadata tag area of the file so it can do a better job next time. That's right: When Smart Volume is on, the selection will sound better the second time you play it.

Creative came up with Smart Volume because they wanted to maximize audio quality and improve listenability in noisy environments without coloring the music; eliminating distortion in the EQ settings was apparently a surprise. Morse speculated that it may be attributable to switching to 32-bit arithmetic in the DSP when Smart Volume is turned on.

There have been reports that Smart Volume cuts into battery life. Morse agrees that this could be the case because it uses extra CPU cycles. I haven't done rundown tests yet, but it appears that it could be shaving half an hour off the total runtime of the Zen Micro.

So how does it sound? Fine by me. The Zen Micro will still pump 100dB into your ear canals through its stock earbuds at a setting of 23 out of 25 on the volume control, with zero distortion. My test tones measure significantly quieter, but music is only off the no-Smart Volume peak by about 5dB. Here's a picture of the sweep tone with Smart volume on and no EQ:


The volume leveling is evident on my favorite loudness test, Boston's "More Than a Feeling," which is notable for having a remarkably well distributed sonic spectrum, better than most full-on orchestral music because of the electric bass line and the incessant cymbals. The song begins with a fade-in of an acoustic guitar, which is noticeably, but not irritatingly louder than when Smart Volume is off. The peaks in the songs chorus are reined in, and only jump up 2-3dB instead of the more usual 5-6dB. Again, it's not enough to ruin your enjoyment, and it's handy when you're just doing casual listening, especially in a noisy environment.

More to come....

Posted by machrone at 09:34 PM | Comments (4)