Hello,
I am using Audacity 2.0.0 on Windows 7 64-bit.
I have a multichannel 5.1 AC3 movie track that I am trying to transcode into another format through an external command line encoder. I tried this through Audacity, and it worked fine. Output had a bitrate of 198 kbps (disregard that it's a low bitrate because that's beyond the point). However, when I do the same thing through ffmpeg using the same quality settings, I get a 220 kbps bitrate output stream. Through some tests, I came to the absolute indisputable conclusion that Audacity is applying some filtering to the AC3 input which is leading to this discrepancy, whereas ffmpeg is simply decoding the AC3 untouched. I checked my Audacity options, and I couldn't find anything indicating input filtering of any kind.
Also worth noting:
- Command used in Audacity: external-encoder.exe -quality n -options - "%f"
- Command used in ffmpeg: ffmpeg -i input.ac3 -acodec pcm_s16le -f wav - | external-encoder -quality n -options - output.xxx
- When doing the same experiment using a 2 channels stereo track, the output files from Audacity and ffmpeg were identical.
So my question is what is Audacity doing, and how can I prevent it from doing whatever that is, when loading the AC3 5.1 audio?
Audacity is applying dither by default, check the Quality tab of the preferences.
Also check if the sample rate hasn't been changed.
Audacity is applying dither by default, check the Quality tab of the preferences.
Also check if the sample rate hasn't been changed.
Wow, that was a quicker response then I got in Audacity forums, and yes, it's true: dither was on, turned it off, and outputs now match. Only remaining question is what is dither, and do I need it or should I turn it off? Also, why was it being applied only on 5.1 audio (granted my 2 channels stereo test file is a song, which is much different than a movie track, but still...)?
Also, for future reference, this had nothing to do with AC3 input. I got my hands on a WAV 5.1 sample, and same thing happened: Audacity vs ffmpeg = different outputs with dither on, same outputs with dither off.
OK, I read the Audacity wiki regarding "dither", and I still have some questions:
- How could "dithering" lead to input needing a lower bitrate when encoding in VBR mode if it's harmless? Wouldn't that be the equivalent of smoothing out a picture or video before compressing?
- Wiki states that Audacity does its calculations in 32-bit float, and so when working with 16-bit or 24-bit input, it's best to use dither for better accuracy when rounding. My question is: can I set Audacity to use 16-bit calculations internally when I know beforehand that I will be working on 16-bit input, as to avoid this whole rounding in the first place? If yes, is this what the "Default sample format" option is in the Quality tab??
- Wiki also states that when working on 16-bit audio with simple editing (cut, paste, trim...) rather than processing (amplifying, equalizing, filtering...), then it's better not to use dither on exporting output. My question is: when downmixing 5.1 to stereo, I usually use a -3db gain on the Center and LFE channels. Is this considered "processing" and thus would be needing "dithering"?
(This question is really moot if I can set Audacity to do its internal calculations in the same bit depth as my input material, as I prefer the less accurate calculations then the whole rounding + random noise thing).
I hope this post will help you a bit:
http://forum.audacityteam.org/viewtopic.ph...=164356#p164356 (http://forum.audacityteam.org/viewtopic.php?f=16&t=62355&p=164356#p164356)