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I have about 1200 mp3 files (about 45 minutes each) that contain lectures (not music) I used some ffmpeg commands to speed them up by 1.5x and to convert them to Mono instead of Sterio (to reduce file size to fit in a cd).

I used these two commands (in two stages):

ffmpeg -i source.mp3 -filter:a "atempo=1.5,volume=1.2" -q:a 100 first_stage.mp3

ffmpeg -i first_stage.mp3 -ac 1 -b:a 32k final.mp3 

After I performed this in batch, when I put them in my car mp3 player, I noticed that all the files show maximum of 7 minutes duration, but they play all the file with no problem. My car mp3 player shows a countdown , and I used to look for the remaining time of each file to know whether I start a new one or not, etc.

Now, all the files start with 5 to 7 minutes (instead of 30-45 minutes), and it counts down to zero and remains 0 to the end of the file, so I complete the lecture blindly with no timer, which is annoying for me.

What can I do to fix this, i.e., to let the car player read the time correctly?

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    Does it happen in other players? Are the files CBR or VBR? – golimar Dec 5 '17 at 8:54
  • I tested them on my pc (MediaPlayerClassic), no problem. I have not tested in other car players. What are CBR and VBR? – Mohammad ElNesr Dec 5 '17 at 9:10
  • I 've googled CBR and VBR, I think they are CBR, but I am not sure. – Mohammad ElNesr Dec 5 '17 at 9:16
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    @MohammadElNesr You don't need -f. The file name was in the incorrect location: needs to go after -i. See my previous comment and compare it to your command in preceding comment. – llogan Dec 6 '17 at 7:16
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Thanks to the comment of @golimar, I found the solution

One should add the parameter -write_xing 0 before the output file name.

The conversion (in one line) should be like that:

ffmpeg -i source.mp3 -filter:a "atempo=1.5,volume=1.2" -q:a 100  -ac 1 -b:a 32k -write_xing 0 out.mp3

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