mp3 to wav to mp3 to wav same? Options

[gts]

If a file is encoded to mp3, then coverted back to a wav, and then that wav is convered back to mp3, will the both mp3s be the same?

[MugFunky]

wow.. old post. ("holy thread resurrection, batman!!!!11!!1!1")

JPEG isn't the same as mp3, even though they use DCT and mDCT.

for starters, though the mDCT is perfectly reconstructible, the input filterbank of mp3 (where it splits into 32 subbands, mp2 style) is not perfectly reconstructible. Jpeg goes straight for the DCT, and there's no overlapping.

also, JPEG uses a fixed quantizer and a fixed quant matrix for each block. there's no encoder delay or overlapping blocks or anything like that. theoretically you're only going to get rounding error from the repeated DCT and iDCT, and the conversion to 8 bits (even more loss if dither is used in the decoder).

the big, glaring difference, however, is that mp3 uses psychoacoustics and JPEG uses jack-all in the way of psychovisuals (the quant matrix could be seen as a primitive form if it). assuming a perfect, transparent mp3 encode - what it's done is examined the source, and determined specific masking thresholds for the human auditory system. imagine a spectrogram with a curved line under it. the encoder will bring noise up to that line, but if it goes over it, it will be audible.

when you re-encode an mp3, the encoder has no knowledge of the masking decisions the previous encoder made. so it'll draw a line of it's own, and add noise up to but not over it. the big problem is that the total quantization noise in the resulting file is now up to twice the magnitude it was in the original mp3. this guarantees there will be audible artefacts.

the above assumes a perfect psy-model, which of course mp3 doesn't have. things get even worse when the original encode wasn't perfect.

basically, you can't make analogies between image and audio coding, because much of what goes in an audio encoder is completely meaningless in images - that's why we can't swap our eyes for ears and get the same use out of them.