MP3 Export Anomaly - Developer Comments?

[pzkpfwvi]

When exporting WAV files to MP3 files, I have gotten two different output file sizes when exporting the same file and using all the same parameters.

When the "Export File > Options" dialog box opens, you can make choices for Bit Rate, Quality, Speed, and Mode.

If you select "Preset" for Bit Rate, the Mode block is grayed-out and cannot be selected. BUT whatever value it was set to previously effects the size, and quality(?), of the output file for this iteration.

Using the same WAV file as input:
With MP3 Export Options set to: Bit Rate = Preset; Quality = Standard; Speed = Fast; and Mode = Joint Stereo from the last iteration, an output file of size N bytes is created.

With MP3 Export Options set to: Bit Rate = Preset; Quality = Standard; Speed = Fast; and Mode = Stereo (Discrete stereo as opposed to Joint stereo) from the last iteration, an output file of size N+12288 bytes is created.

That's 3 extra 4K blocks on disk from a function that, in theory, cannot be selected. Spacewise, three extra 4K blocks is no big deal, but why does the Mode parameter, which cannot be selected when using the Preset Bit Rate, cause a difference in output file size if it is theoretically not used? Is this a bug?

What Mode is actually employed by Audacity behind the scenes when using the Preset Bit Rate? Just curious.

JP

[Gale Andrews]

This has now been fixed in our source code and was only an issue on Windows. When 1.3.13 Beta is released, the "Joint Stereo"/ "Stereo" radio buttons will now be enabled when encoding with a preset.

"Joint Stereo" or "Stereo" was in fact being used when encoding with a preset, as per the state of the visible but "greyed out" buttons. Stereo normally gives a larger file than Joint Stereo. The buttons were disabled because we had just changed from using Blade to LAME and at the time it was thought better to disable the buttons because Blade did not allow joint stereo/stereo to be chosen when using a preset.



Gale