One of the things I got for Christmas was the Memoirs of a Geisha soundtrack. As with all my other CDs, I immediately wanted to rip it for my MP3/Ogg collection. This sounded simple, but ended up being rather unpleasant.
The first sign of trouble came when AudioCatalyst couldn't calculate checksums for the tracks. Looking at the manual, the reason this would occur is that there wasn't at least one frame of silence at the beginning and end of each track. One thing this may mean (which is what I initially suspected) is that the tracks are not discrete, but transition directly from the end of one track to the beginning of another, with no inter-track delay (the Gladiator soundtrack did this, just to name one prominent example). This is troublesome because it's difficult to perfectly mimic this in lossy audio formats, as well as that it sounds bad when you play the tracks on shuffle (like I do).
Well, the real reason turned out to be even more annoying. A quick look at the tracks revealed that there was noise at the beginning and end of each track; but even just looking at the waveform, it was oddly shaped noise. The frequency graph confirmed what I'd suspected from looking at the waveform: there was a prominent spike at an upper frequency, embedded in otherwise brown noise; further investigation revealed that this spike was centered at 15,733 hz. At the beginning and end of tracks (it fades out in towards the center), this peak stood about 50 db above that of the surrounding noise.
While this spike was only present at the beginning and end of the tracks, I soon found that a different spike was present throughout most of the track. Some careful isolation revealed that this spike was centered at 15,700 hz. The intensity of this spike varied anywhere from 14 to 27 db, and may be different intensity on the two channels.
The 15,700 hz spike I managed to dispose of, by means of manually performing notch filters on each track. The 15,733 hz spike, however, I left, as it would be a pain to deal with, only at the beginning and end, and it's not present during most of the tracks.
So, now that we know what these spikes (hums) are, why are they there? Unfortunately, I don't know the answer, though I can think of a number of possibilities. It could be the hum of some piece of equipment in the studio, during the recording of the music (though I've never encountered this kind of thing before); I'm told that power supplies could produce hums in this frequency range. It could also be some sort of resonance with a stray instrument or item in the studio, causing it to vibrate and amplify those particular frequencies. Finally, it could be some kind of watermark or rip-protection system; if it was the latter, I could test for it by recording the music directly from my stereo (a significant amount of work, considering that the stereo's in a different room than my computer), but I don't really have any way to test the first.
So, there's Q's unsolved mystery of the week.