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I have a bunch mono .wav files which I need to convert to stereo in order to run thru lame. Lame can't encode stereo mp3s from mono input so that's what I need to do.

I don't need any modifications to the data made other than duplicating channel into two.

I've tried to use sox and it seems to alter sound data in eerie ways by up-downsampling and changing bit depths in between as well as running filters I don't need.

So please recommend me some tool to batch convert .wav files from mono to stereo. I'm on linux.

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sox input.wav output.wav channels 2
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sox doesn't do verbatim conversion, it applies numerous transforms and filters (in your case, for example, dither - look up -D; but god knows what else) – alamar Nov 30 '13 at 12:00
Uh, no. With only channels it either duplicates or downmixes unless the format has to be changed for some reason. $ sox -V ... sox INFO sox: effects chain: input 48000Hz 1 channels sox INFO sox: effects chain: channels 48000Hz 2 channels sox INFO sox: effects chain: output 48000Hz 2 channels – Ignacio Vazquez-Abrams Nov 30 '13 at 12:02

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