如何使用 FFMPEG 将 MP3 转换为恒定比特率

Question

I have found that MP3's encoded with variable bit rate cause the currentTime property to be reported incorrectly , especially when scrubbing. That has wreaked havok on my app and has been a nightmare to debug.

I believe I need to convert all my MP3's to constant bitrate. Can FFMPEG (or something else) help me do that efficiently?

• Props to Terrill Thompson for attempting to pin this down *

Answer 1

I also had issues with HTML5 being inaccurate for large mp3s. Since quality was not a big issue for my audio, I converted to constant bit rate of 8kbps, sample rate 8k, mono and it solved my issues.

You can convert to a contant bit rate for a few files using Audacity (export > save to mp3 > constant bit rate).

Or, using FFMPEG :

ffmpeg -i input.wav -codec:a libmp3lame -b:a 8k output.mp3

If you also want to reduce to mono and a 8k sample rate:

ffmpeg -i input.wav -codec:a libmp3lame -b:a 8k -ac 1 -ar 8000 output.mp3

Using the second compressed an hour of audio to under 5MB.

Answer 2

Something else is going on. currentTime should not be influenced by the fact that you are using variable-bit rate MP3s.

Perhaps the context sampleRate is not the same as the sample rate as the MP3s? That will mess up timing of the audio samples because WebAudio will resample the MP3s to the context sample rate.