Re: mp3 audio specs?

I don't have any specs or measurements, but I've done a lot of taste testing over the past few weeks as I work to convert all of my CD's into MP3 format.

I think measured specs would be a bit tricky; they should vary from song to song, depending on the compressibility of the source material.

I'm using the Xing Tech MP3 encoder, latest rev. Brief checks on deja showed this to be the best liked encoder.

My methodology was to use blind tests; I encoded the same song (David Bowie / "Looking for Satellites") at 128, 192, 256, and 320kbps, and also as an uncompressed wav file. I used a soundblaster live for both recording and playback, and played back to a pretty respectable stereo at fairly loud volume. We ran through the song twice at each bitrate, and we were able to tell which was which ("This is the same as the third one").

A friend and I sat and listened, while someone else played the various clips in random order. We made notes and waited until hearing all of the bitrates twice before learning what each of them was. The results were fairly dramatic:

128kbps: "Oh my god, this sound terrible. Ugh. Hats are smeared all over, there is no real bass. Sounds muffled and lifeles. Ugh, ouch, pain. "

192kbps: "Decent hats, but somewhat smeared. Bass feels like it's coming from a different song; weird thumps happen out of time with the rest of the song. Feels dead and somewhat flat."

256kbps: "Pretty good -- some presence, hats are really clean. Bass has some problems; low tones that are supposed to be continuous seem to throb and pulse depending on what's going on elsewhere in the spectrum. Listenable, but not CD quality."

320kbps: "Very clean. Bass is in time and sounds right, hats are perfect. Song has real presence and feels lively and pretty good. Near CD quality."

Uncompressed: "This has to be uncompressed."

So, the bottom line is that even at 320 (the highest the Xing encoder will go), it's not really CD quality. We verified this by playing the actual CD and the 320kbps stream in near-sync with each other and switching back and forth between them. There was a noticeable improvement when we went to the cd, and a noticeable lack at 320kbps. The change was at least a bit subtle, though, and it had more to do with "presence" or "warmth" than a noticeable defect in any particular instrument or sound.

Hope that's educational. I did a lot of work for this, and I would encourage you to go the highest bitrate you can. I'm almost tempted to go uncompressed (*almost*, but that's a lot of storage!).

Your mileage may vary.