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Subject: mp3 encoding quality and bitrates, how it works?
From: Cristian (cguiducci_at_gostec.net)
Date: 2004-04-17


Hi, I was trying to understand the behavior of the mas encoder on the archos with rockbox, I did some sample recordings, but I didn't understand exactly how quality, bitrates, and frequency are related.

I mean: If I record at quality 2, and I change the frequency, I hear les artifacts at a samplerate of 44 than at 32 or 22,5. If quality determins a bitrate, it doesn't make sense, since a cut in frequency should result in a dumpened file, with les trebles, but less artifacts too.

I than noticed for instance, that quality 4 or 3, I don't remember, produces with a 24 khz samplerates, mono, files with a bitrate of 64, that sound quite bad, and quality 2 or 1, produces the file at 48 kbps and the quality is better... Plus, If i have let's say, quality 5 at 44 khz produces a file at 128 kbps, i would expect, that by setting mono, the file bitrate would at least decrease to 80 kbps, if not 64, but this is not the case, the bitrate remains the same...

Can someone explain me how the whole thing works? What I controll with quality, and I also noticed that some quality values make mp3 2.0, where as other values yieald a mpg 1.0 mp3 file.

And, I also noticed that in some cases, winamp reports a bitrate of 192 kbps, isn't it strange for the mas encoder?

and refering to winamp, it never says vbr, although I set the option: calculate average bitrate in vbr files. Is this winamp's fault or some header is missing? Plus, I always read, header found at 4 or 5000 bites within the file. Shouldn't it be at the beginning? at 0 bites?

Thanks to everybody in advance and sorry for being a pain in the neck!

             Cristian

p.s. While recording, I hear a slight hiss, and the hiss increases if I set the sample frequency at 32khz. is this normal?
p.p.s. Is there a way to lower the recording volume or gain to zero while recording? Sometimes it would be nice to be able to fade out a recording, as you can do with archo's firmware. Is this feasible with rockbox? if not, is there a reason for that strange behaviour or is it mee!

          Bye,

           Cristian again

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