How to get the real, actual duration of an MP3 file (VBR or CBR) server-side
I used to calculate the duration of MP3 files server-side using ffmpeg - which seemed to work fine. Today i discovered that some of the calculations were wrong. Somehow, for some reason, ffmpeg will miscalculate the duration and it seems to happen with variable bit rate mp3 files only.
When testing this locally, i noticed that ffmpeg printed two extra lines in green.
Command used:
ffmpeg -i song_9747c077aef8.mp3
ffmpeg says:
[mp3 @ 0x102052600] max_analyze_duration 5000000 reached at 5015510
[mp3 @ 0x102052600] Estimating duration from bitrate, this may be inaccurate
After a nice, warm google session, i discovered some posts on this, but no solution was found.
I then tried to increase the maximum duration:
ffmpeg -analyzeduration 999999999 -i song_9747c077aef8.mp3
After this, ffmpeg returned only the second line:
[mp3 @ 0x102052600] Estimating duration from bitrate, this may be inaccurate
But in either case, the calculated duration was just plain wrong. Comparing it to VLC i noticed that there the duration is correct.
After more research i stumbled over mp3info - which i installed and used.
mp3info -p "%S" song_9747c077aef8.mp3
mp3info then returned the CORRECT duration, but only as an integer, which i cannot use as i need a more accurate number here. The reason for this was explained in a comment below, by user blahdiblah - mp3info is simply pulling ID3 info from the file and not actually performing any calculations.
I also tried using mplayer to retrieve the duration, but just as ffmpeg, mplayer is returning the wrong value.
Answer
I finally found a proper solution to this problem using sox - which returns the correct information.
sox file.mp3 -n stat
Samples read: 19321344
Length (seconds): 219.062857
Scaled by: 2147483647.0
Maximum amplitude: 1.000000
Minimum amplitude: -1.000000
Midline amplitude: -0.000000
Mean norm: 0.141787
Mean amplitude: 0.000060
RMS amplitude: 0.191376
Maximum delta: 0.947598
Minimum delta: 0.000000
Mean delta: 0.086211
RMS delta: 0.115971
Rough frequency: 4253
Volume adjustment: 1.000
Length (seconds): 219.062857