FLAC Exporting Larger Than the Original

I open a 24-bit FLAC file with a bitrate of 618 Kb/s, change absolutely nothing, and export as 24-bit FLAC (level 5). The exported file is 24-bit with a bitrate of 1280 Kb/s and is about twice the size of the original. Why? Shouldn’t it be roughly the same bitrate and size as the original?


Audacity 3.1.3
macOS 10.15.7

Probably due to “dither” (https://manual.audacityteam.org/man/dither.html)

That solved it. Will it hurt anything to leave Dither set to “None”?


Exported audio in “lossless” formats (such as WAV, AIFF, FLAC,…) may have slightly lower sound quality when dither is set to “None”.

Dither is beneficial when the sample format is reduced. Your test case (original post) is the exception to the rule because all samples in the project have exact integer values: Exporting exact integer values as integers (example: 16-bit values to 16-bit WAV, or 24-bit values to 24-bit WAV) is perfectly exact when dither is off. As soon as the audio has been processed in any way (for example, by amplifying or any other effect) then the audio will have samples that are not exact integers, so the values have to be rounded in some way when exporting as a “lossless” integer format. “Dither” is an alternative to mathematical rounding and has the advantage that it avoids “quantization noise” (which is an unpleasant artefact of mathematical rounding).

The benefit of dither is much less significant for 24-bit audio than for 16-bit, but as seen in your test case, dither may cause FLACs lossless compression to be less effective (bigger file size). In other cases, dither may make no difference to the file size - that depends on how effectively FLAC is able to losslessly compress the data.

Overall, dither is beneficial in real world cases, which is why it is enabled by default. It is best to leave it enabled as the general rule, and only turn it off in special cases where dither does more harm than good. Most users just leave dither enabled as “shaped”, which in most cases is best.

Thanks for the info. Makes sense.