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I recorded a mono microphone with an MP3 recorder which can only record stereo formats. Now I want to reduce the file size by removing the silent channel.

Is there a lossless way to do this?

This question was also asked here but the answer gives no lossless solution nor does it say whether its technically possible.

Frank Breitling
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  • While you might (albeit unlikely) find someone having written a program specifically for this goal, I suppose technically it's at best possible only if the file wasn't encoded with joint stereo. – Tom Yan Feb 20 '23 at 11:52
  • If it wasn't encoded with joint stereo, how would I do it? – Frank Breitling Feb 20 '23 at 21:45

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TL/DR:

This depends on the format required for the end result.

  1. If the end result must be MP3, then no. There's no escaping that MP3 is a lossy file format.
  2. If the end result can be WAV, AIC, ALAC, FLAC or any other lossless format, then yes, easily. Simply open the file on any audio application, split the stereo to 2 mono channels and save the channel with the audio to a new file. My weapon of choice for this kind of quick work is Ocenaudio (a shameless plug*).

When the source file is opened in an audio application, any encoding disappears, it's simply raw audio data. The application can split the stereo channel to separate mono channels, which can then be saved to 2 new files.

To ensure these files can be post-processed, all audio applications save the files in a uncompressed, lossless format, most commonly encoded as a WAV. The file can usually be encoded also as AIFF (MacOS-only applications tend to default to this) or AU; or it can be saved as a raw PCM data with no encoding.

Files can also be often directly saved is a compressed, lossless format such as FLAC, ATRAC, ALAC (MP4) or WMA Lossless (Windows Media Audio).

If the resulting file must be in MP3 format, it must be converted. The audio can usually be exported directly as MP3 from the application, in which case the conversion happens during the export process, or the saved lossless file can be converted afterwards.

MP3 conversion algorithm passes the audio through psychoacoustic modeling, which utilizes auditory masking to determine what parts of the audio must be kept, which can be discarded to reduce the file size. The discarded information will be permanently removed, causing the loss of quality. Further reduction in the file size is achieved by compression.

The loss can be minimized by converting with highest quality, but not completely avoided. Depending on the complexity of the source material, it's possible that the loss of quality in the end result is humanly undetectable, excluding professional mastering (and some mixing) engineers. For example speech or a live recording of a rock band can survive a lot more quality loss than a recording of a string quartet. Again depending on the source material, variable or fixed bitrate produce different results.

*I'm in no way, shape or form affiliated with Ocenaudio or any of its affiliates, just a very satisfied user for over a decade and a half due to its simplicity and capability :-)

Peregrino69
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  • In my case the source format is already MP3. Do you mean this can be split losslessly? If so, why can't this be copied to a new file? – Frank Breitling Feb 20 '23 at 21:43
  • Thank you @Peregrino69. However, I'm still wondering, whether existing MP3 data of one channel could be used without reencoding. – Frank Breitling Feb 21 '23 at 09:48
  • Now I'm more confused. Are u saying 2 files could be split out of my mp3 losslessly without re-encoding? How? – Frank Breitling Feb 22 '23 at 10:58
  • @FrankBreitling Sorry, I made the classic AssUMe mistake - I assumed the end result *must* be also an MP3 file. I revamped the answer, hope this one is clearer. – Peregrino69 Feb 23 '23 at 08:56
  • Thanks @Peregrino69. The problem is that MP3 re-encoding is not lossless and therefore I was simply asking for a method to just copy the corresponding data to another file. This is for example possible, when you split files by time. – Frank Breitling Feb 23 '23 at 12:35
  • @FrankBreitling Yeah, it is. I've honestly got no clue how that works. Thanks to yourself for not letting me off the hook :-) – Peregrino69 Feb 23 '23 at 15:47