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What is Audacity doing with a compressed format when importing an mp3 or flac into it?

It looks like it is extracting it

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When I open a wav file, it looks different - it just processes for a second and it's there:

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What happens? What is the difference? Is Audacity processing the mp3 or wma after having converted (extracted?) them as wav?

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When you feed it MP3-encoded files, it will decode the files into an uncompressed format that is used for internal representation.

Working on encoded files directly is not easy since audio samples may be interleaved or dependent on others. An audio sample is represented in the percetpual hearing domain – so, what you see in the file is not actual audio: you have to decode it again, using psychoacoustic methods. If you decode the file, you will get an easy to work with waveform with Pulse Code Modulation.

In fact, Audacity will create chunks of 1 MB files as temporary files, and they seem to be just regular PCM-encoded WAV, meaning raw audio. That's also why importing WAVs is so fast – they do not have to be transcoded, they can just be used as-is.

  • "Audacity will create chunks of 1 MB files as temporary files" - are these the files within the Audacity projects, which can be saved as .aup? – user162573 May 23 '14 at 15:38
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    Yes, those are the .au files in the project. They're split so that they are easier to process and skip through. I once had a document that explained the internals of Audacity in more depth… let me find it. – slhck May 23 '14 at 15:38
  • Ah, sorry, I don't have this — it was course material I no longer have access to. But the rationale is, in essence, to have a good performance. – slhck May 23 '14 at 15:41
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It's decompressing and converting them to a raw (or a proprietary) format so that you can easily edit them in a non-linear fashion.

The MP3 and the WMA are more than likely compressed, where as the WAV more than likely isn't, or as least isn't compressed nearly as much.

This decompression and conversion is what you're waiting for when you import them (note it's import them, not just load them).

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    MP3 and WMAs are compressed and stored in a perceptual domain, whereas PCM-based WAV is just plain sample data in the audible domain. Hence easier to process. – slhck May 23 '14 at 15:34
  • @slhck - is 'perceptual' to be put in contrast with 'audible'? – user162573 May 23 '14 at 15:42
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    @cipricus My wording is a bit sloppy here. Basically, PCM gives you actual waveforms that you could feed to a speaker, hence audible. MP3 data is perceptually encoded, so you can't "read" it directly. – slhck May 23 '14 at 15:44
  • @slhck - in the link on audio formats it says: It is important to distinguish between a file format and an audio codec. The codec being the one that has allowed the waveform to be coded and then allows to "read" indirectly the coded format. Correct? – user162573 May 23 '14 at 15:47
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    @cipricus By standard terminology, codecs are stored within formats. The MP3 codec is stored in the MP3 file format (.mp3), and the PCM audio ("codec") is stored in the WAV file format. See this question with an analogy for video codecs and formats. – slhck May 23 '14 at 16:12

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