Technology: February 2009 Archives

The iTunes 4kHz upgrade

2 February 2009

After cutting a revised deal with a further set of major record labels on top of those signed last year, Apple decided to offer DRM-free files to existing customers as iTunes Plus upgrades.

Charles Arthur posed a question: was it worth going for the upgrade or was it just as effective to just have iTunes make CDs and then re-rip the songs? My first reaction was that it would be worth finding out. Then I had a closer look at the difference in bitrates: protected files use 128kbit/s encoding; the unprotected versions 256kbit/s. Given that MP4/AAC encoding is meant to be more efficient than MP3, the new bitrate seems luxurious.

I quickly updated my thoughts with a "it's probably a no-brainer: the iTunes Plus versions are likely to be better". But, curiosity got the better of me and I picked a couple of tracks to be guinea pigs. I assumed that I would stand a better chance of hearing differences in files that haven't been compressed for maximum loudness. This is signal compression as opposed to data compression, in which a filter jumps up and down on the signal until it's ironed out most of the peaks and troughs. You can spot a heavily compressed file a mile off when looking at its waveform: it looks completely flat. And, if you take into account what happened to Metallica's last album, it sounds it too.

My two victim tracks were A Strange Day, a 1982 song by The Cure from their album Pornography, and Can's Moonshake, taken from 1973's Future Days. There is nothing particularly special about these other than I had DRMed iTunes versions of them, were now 'upgradeable' and I know how they sound pretty well. A Strange Day remains one of my favourite songs.

I had iTunes write the protected version out to CD, downloaded the Plus versions of the albums and did the same. Then, I loaded them into DSP Quattro to make sure the starts lined up and compared them in Sony's Sound Forge. I had to go over to Windows XP because it's really hard to find a Mac audio editor that does sonograms without a lot of messing around. This doesn't tell you which are better but I wanted to see if there were obvious differences before I went any further.

The sonograms show how the frequencies in a sound file change over time and, in this case, were pretty revealing. Both of the DRMed files showed that Apple basically lopped off the top end at around 16kHz. You get 4kHz more on the Plus versions. You can see that in the picture, which is one sonogram subtracted from the first in Photoshop. This is a bit of a cheat but it shows what's going on more clearly than if you use a tool like Spear to do it for real.

moonshake-sonogram-diff.jpg

Initially, my reaction was: you're bound to hear the difference between these. Apple, you've been short-changing us for years. Then I remembered I'm over 40. I kissed goodbye to 20kHz a while ago.

Even someone who is half my age is probably not going to hear the difference between these files either, as the energy in those upper frequencies is fairly low and the peaks come mainly from Jaki Liebezeit's percussion work, which will reduce your chances of hearing that upper detail at all against the main body of the sound. However, I'm not going to argue with anyone much younger than me who feels that the Plus version is a little more airy.

I went onto stage two: actually listening to the differences. I rigged up Ableton Live to crossfade between each pair of tracks, phase matched as close as possible. Interestingly, I couldn't get clean cancellations between them when I inverted the signal on one side, which either indicates that psychoacoustic data compression does nasty stuff to audio or I hadn't got the setup quite right. Even so, when crossfading there was no obvious jump or phasing.

Having seen how much of the audio band Apple had filtered off the protected versions, presumably to make them encode better at 128kbit/s, I was expecting a difference. Maybe not stunning but something. And I got...zip. I reckon there was a subtle improvement on the Plus but if it was there it was really subtle, as in barely audible. I couldn't guarantee that if I did a double-blind test - which this clearly wasn't - that I'd be able to pick one from the other more accurately than tossing a coin.

So, try it out for yourselves but I reckon the burn-and-rip strategy is going to work just as well, if a bit less conveniently, than forking over the extra cash to Apple. And, frankly, it's a bit of cheek on the side of Apple and the record companies to demand a 25 per cent surcharge on material you already own just to release the files from DRM. A service fee maybe. In fact, I reckon it's worth hanging on to the protected versions just to make Apple keep its authorisation servers up and running.