Testing if FLAC is truly lossless

[gs684]

Out of my paranoid curiosity, I decided to test whether FLAC audio is truly lossless or not. Using Adobe Audition, I tested it as follows:

- Open WAV audio file
- Save as FLAC (using same frequency and sampling settings as the original WAV)
- Close & reopen the new FLAC
- Save as a new WAV file (using same frequency/sampling settings)

Then, I run a file comparison on the 2 WAV files, which produces a mismatch.

I am assuming that this has something to do with some metadata in the WAV files which is different each time even if the audio is the same, but I want to be sure. Anyone have any ideas?

[poisondeathray]

Difference could be due to dithering settings

[gs684]

Difference could be due to dithering settings

Dithering was disabled.

[Hoser Rob]

I think it may be the metadata, the note outlined in blue at the top of this mentions preserving it: https://helpx.adobe.com/ca/audition/using/viewing-editing-xmp-metadata.html

[ivanb]

Yes, it's likely that there are some changes in metadata. Or maybe differences in layout, padding or something like that.

You can open your input/output files with this tool: http://media-analyzer.pro and check if your 'data' chunks have the same offset, at least. You may spot some differences in 'fmt ' chunk, as well.

Perhaps the best way to satisfy your paranoid curiosity is to compare not WAV files, but RAW files (similar to WAV, but without additional headers)

[Cornucopia]

Or you can just load your original on 1 track, your flac copy on a 2nd track, invert the (phase/polarity of the) copy, and then export the mix of the 2 (which should be digital silence if an exact match using such a "difference" mix).

This way avoids all the file based, packet order, metadata issues and just tests raw payload.


Scott

[gs684]

Or you can just load your original on 1 track, your flac copy on a 2nd track, invert the (phase/polarity of the) copy, and then export the mix of the 2 (which should be digital silence if an exact match using such a "difference" mix).

This way avoids all the file based, packet order, metadata issues and just tests raw payload.


Scott

This is brilliant!

[hello_hello]

foobar2000 calculates an MD5 when converting to a lossless format. Either that or the codecs do it themselves. The MD5 isn't saved for a wave file, but you can convert from wave to flac to wave to flac and the MD5 shouldn't change. The MD5 is calculated on the uncompressed audio, so you'd need to convert to flac, back to wave and then back to flac to know the audio hasn't changed due to the flac compression.


[Attachment 75826 - Click to enlarge]


[Attachment 75827 - Click to enlarge]

[Hoser Rob]

Or you can just load your original on 1 track, your flac copy on a 2nd track, invert the (phase/polarity of the) copy, and then export the mix of the 2 (which should be digital silence if an exact match using such a "difference" mix).

This way avoids all the file based, packet order, metadata issues and just tests raw payload.


Scott

That's basically a classic analog audio test, digitized. I like it.

[pandy]

You can export only PCM data (so remove metadata uncertainty) and perform ANY sane comparison (assuming that PCM data are not misaligned) even simple "DOS" FC (filecompare 'fc /b') should be fine. Hypothetical misalignment may be related to codec internally aligning data to packet size (usually data are aligned with silence).

[dellsam34]

I've been backing up every CD I've owned throughout the years into FLAC, Don't tell me guys all that effort is gone to waste, Lol. I highly doubt there will be any extra or missing bits from the audio data itself.

[gs684]

You can export only PCM data (so remove metadata uncertainty) and perform ANY sane comparison (assuming that PCM data are not misaligned) even simple "DOS" FC (filecompare 'fc /b') should be fine. Hypothetical misalignment may be related to codec internally aligning data to packet size (usually data are aligned with silence).

How to export as raw PCM?

[pandy]

How to export as raw PCM?

Simplest (from my perspective) is to use SoX to perform WAV to RAW (PCM) - however you need to manually choose proper RAW (PCM) format and of course prevent dither to be used.