Originally posted by tychay
Since anecdotal evidence is often taken as "rule of law" among Mac users, I'm more suspicious of a claim that the quality of the preview is substandard when Apple has no prima facia reason to have lower-quality previews and a many good reasons to make sure that the quality is the same.
I can confirm that many, but not all, of the previews are indeed lower quality than the purchased tracks. I have a transparent http proxy on my home network, so all music store downloads (including previews) are logged in the proxy logs. I looked through the log and found the actual urls of the preview files, then downloaded a few separately. Opening them up in QuickTime Player and showing file details revealed that most previews I selected were 64kbps AAC. A few were 128kbps but not many of the ones I tried.
Apple really shouldn't have stated that the previews are full quality unless that was actually true.
Now, everyone of us seems to have platinum ears that can tell the difference between 128kbps AAC (they must be platinum because "golden ears" can't tell the difference).
I agree wholeheartedly that many people on many different forums I've read since the music store went live are making claims about quality that they can't substantiate. Most people, when taking a blind test would have trouble consistently identifying the original CD vs. a well encoded AAC or MP3. Instead, it's easier to spout off about how it's compressed and therefore the quality must suck. ;-) Either that or they don't setup a good blind test and like you say, biases creep in to invalidate the results.
However, under the right set of listening conditions (good equipment/speakers/headphones), with the right reference track, many people will be able to hear a difference at 128 kbps AAC, or even 160. Some even higher than that with really good equipment and good ears that know the material intimately (and knowing what to listen for). Sometimes having the right reference track which exposes particular flaws can make all the difference in the world.
But for 99.9% of music, 99.9% of listening environments (your computer, iPod, etc, even with good headphones), and 99.9% of listeners, this is a complete non-issue. I've been re-encoding my CD collection at 160 kbps AAC since I got my new 30 gig iPod (yay!) and I'm extremely pleased with the results. I couldn't reliably identify AIFF vs AAC rips at that bitrate and on my typical listening equipment, so it works for me, bottom line.