How to get the real, actual duration of an MP3 fil

Question

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 1

Simpler is to use ffmpeg to copy the file from the one with the faulty duration in its ID3 tag. This causes it to write the correct information.

ffmpeg -i "audio.mp3" -acodec copy "audio_fixed.mp3"

Because it uses copy it takes a fraction of the time the original encoding takes. This is hardly noticeable with a song, but you really appreciate it with a 7 hour audiobook. After re-encoding, the ID3 "Duration" tag now has the correct information.

Answer 2

Extending solution from LordNeckbeard. To get only stats you can add flags -v quiet -stats

ffmpeg -v quiet -stats -i input.mp3 -f null - 

Answer 3

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

Answer 4

You can decode the file completely to get the actual duration:

ffmpeg -i input.mp3 -f null -

The second to the last line of the console output will show something like:

size=N/A time=00:03:49.12 bitrate=N/A

Where time is the actual duration. In this example the whole process took about 0.5 seconds.