Here's the wikipedia's sample white noise ogg vorbis file.

http://en.wikipedia.org/wiki/File:Whitenoisesound.ogg

I opened Audacity and analized its spectrum. To my surprise, it is complete up to 22 kHz (being a 44.1 kHz sampled file). Usually vorbis music files contain frecuencies up to somewhere 16 to 18 kHz. Removing high frecuencies is parto of lossy compression and this vorbis file is not q10 or anything. But I think even q10 files don't have anything above 20 kHz.

For example this vorbis file

http://en.wikipedia.org/wiki/File:Johann_S...02Epre_cmaj.ogg

doesn't have anything above 20 kHz and very little above 18 kHz. And this one:

http://en.wikipedia.org/wiki/File:Bach_-_B..._1._Allegro.ogg

doesn't have anything above 10 kHz.

Maybe one of you with deep vorbis knowlegde know the explanation or can tell me why I'm expecting something that I should not expect. Maybe I'm using Audacity in the wrong way or misinterpreting it someway.

Unless you encoded the files yourself and know EXACTLY what command line was used, I wouldn't pay too much attention to this.

I don't have any "deep knowledge", an I dont use OGG. But, I know these lossy compression schemes are based on psychoacoustics, not visual spectrum analysis.

If both the music and the white noise sound like the uncompressed original, the compression is doing its job!

White noise has relatively more high frequency content than most music. If there is lots of "important" or dominant high-frequency information, I'm not surprised if the encoder tries to preserve the dominant content. The algorithm trys to preserve the most important information and in some cases, with "simple" sounds, the algorithm may be able to compress the sound without actually throwing-away any information. (I assume that when you compress a constant 1kHz tone with lossy compression, nothing is lost!)

In theory, it's possible to "perfectly" compress a pure 20kHz tone (or higher) at very low bitrates.* But, the compression algorithms are optimized for music, not pure tones, and I don't have the "deep knowledge" to know how well the various algorithms handle pure tones.

Just listening to the Bach pieces, the first sounds relatively "high pitched", but I don't hear a lot of high frequency energy. The second one is more balanced, but with less high frequency content. To me, it doesn't sound like "bad recordings" or bad encoding,... It just sounds like the nature of the classical (baroque?) instruments. If there were cymbals or brass, there would be more high-frequency content. I don't know, but it's possible that you wouldn't see any high frequencies in the spectrum of an uncompressed version of the same recording.


* If I say "20kHz, 0dB, 1 hour", that's all the information you need to reproduce that tone, and I've "compressed" that information into 18 bytes!