Bug: FLAC Multichannel Export seems broken

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Bug: FLAC Multichannel Export seems broken

Permanent link to this post Posted by mr.matt.eastwood » Mon Nov 06, 2017 2:12 pm

Hello, Audacity team and community! I'm really happy about this new version, but I think I found a bug.

Steps to Reproduce:
1. Open any multichannel (e.g. 6 channels aka 5.1) FLAC file in Audacity
2. In Preferences => Import/Export, enable "Use custom mix"
3. Select File => Save Other => Export Audio...
4. In the save dialogue, select "FLAC Files" as format, compression level 8, Bit Depth 24.
5. In "Advanced Mixing Options" dialogue, leave Output Channels at 6, and leave channel assignments as default (see attached channels screenshot).
6. In Metadata dialogue, leave everything as is.
7. Export and play back in e.g. VLC, Parole, SMPlayer

Current Behaviour:
Upon playback in any player (VLC, Parole, SMPlayer), the audio now sounds distorted, extremely loud and badly clipped. Opening the exported file in Audacity reveals that channels 2 and 4 have been compromised; it seems like other tracks have been merged into them.

Expected Behaviour:
The exported file should preserve the original file's channels.

Notes:
While exporting, there's no progress bar dialogue or any indication that the computer is doing any work. The only way to tell that it is happening is to open a file explorer and watch the new file grow in size.
"Before" and "after" screenshots attached.
Test system: Ubuntu 17.10 (Xorg session), Audacity 2.2.0
This issue did not occur in previous versions of Audacity.

before.jpg
before.jpg (1.05 MiB) Viewed 122 times


channels.jpg
channels.jpg (41.97 KiB) Viewed 122 times


after.jpg
after.jpg (985.91 KiB) Viewed 122 times
Last edited by steve on Mon Nov 06, 2017 2:28 pm, edited 1 time in total.
Reason: Place images inline to preserve board layout
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Re: Bug: FLAC Multichannel Export seems broken

Permanent link to this post Posted by steve » Mon Nov 06, 2017 2:35 pm

The problem is due to the track gain sliders on tracks 2 and 4. On export, the additional gain is applied to the audio, which is pushing the level into clipping.
(see: http://manual.audacityteam.org/man/audi ... .html#gain)
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Re: Bug: FLAC Multichannel Export seems broken

Permanent link to this post Posted by mr.matt.eastwood » Mon Nov 06, 2017 2:56 pm

I did some more research of this bug and found:

  • I cannot reproduce this issue with other files, so it seems to be something about this particular file.
  • During export, in the Export dialogue, when selecting "Other uncompressed files" and then "FLAC (Free Lossless Audio Codec)" and "Signed 24-bit PCM", Audacity does show a progress bar while exporting.
  • I tried reproducing the bug with other files and can't. In fact, saving this file as WAV, then converting that WAV to FLAC works around the issue here, too.

It seems like I just hit a problematic file, and there is no bug here after all. Sorry for the false alarm! I hope this thread can still be of some value, concerning the export progress bar dialogue.
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Re: Bug: FLAC Multichannel Export seems broken

Permanent link to this post Posted by mr.matt.eastwood » Mon Nov 06, 2017 2:59 pm

Ahhh, you're right. My bad!! I should have seen this. I guess I accidentally grabbed those sliders while moving tracks around.
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Re: Bug: FLAC Multichannel Export seems broken

Permanent link to this post Posted by waxcylinder » Mon Nov 06, 2017 11:20 pm

mr.matt.eastwood wrote:Ahhh, you're right. My bad!! I should have seen this. I guess I accidentally grabbed those sliders while moving tracks around.

Something that I think is far too easily dome - which is why I wrote this Proposal: http://wiki.audacityteam.org/wiki/Propo ... in_sliders
I too have been "bitten" by this :(

But thus far no developer has been interested in pursuing this.

WC
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