ffmpeg change tempo with wrong bitrate

[darekpro]

Hello, I dont know why but ffmpeg change bitrate when I change tempo for audio track from 25 to 23.976.
Original file bitrate is 384 kb/s but ffmpeg converting this bitrate to 448 kb/s why?

Thank for help.

ffmpeg -i "%F" -filter:a "atempo=0.95904" -acodec codec "copy" -vn "%F".ac3

Code: Select all

Input #0, matroska,webm, from 'VTS_01_2.mka':
  Metadata:
    encoder         : libebml v1.4.0 + libmatroska v1.6.2
    creation_time   : 2020-08-25T21:12:14.000000Z
  Duration: 01:24:57.28, start: 0.000000, bitrate: 384 kb/s
    Stream #0:0: Audio: ac3, 48000 Hz, 5.1(side), fltp, 384 kb/s (default)
    Metadata:
      BPS-eng         : 384000
      DURATION-eng    : 01:24:57.280003200
      NUMBER_OF_FRAMES-eng: 159290
      NUMBER_OF_BYTES-eng: 244669440
      _STATISTICS_WRITING_APP-eng: mkvmerge v49.0.0 ('Sick Of Losing Soulmates') 64-bit
      _STATISTICS_WRITING_DATE_UTC-eng: 2020-08-25 21:12:14
      _STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
Stream mapping:
  Stream #0:0 -> #0:0 (ac3 (native) -> ac3 (native))
Press [q] to stop, [?] for help
Output #0, ac3, to 'test.ac3':
  Metadata:
    encoder         : Lavf58.45.100
    Stream #0:0: Audio: ac3, 48000 Hz, 5.1(side), fltp, 448 kb/s (default)
    Metadata:
      BPS-eng         : 384000
      DURATION-eng    : 01:24:57.280003200
      NUMBER_OF_FRAMES-eng: 159290
      NUMBER_OF_BYTES-eng: 244669440
      _STATISTICS_WRITING_APP-eng: mkvmerge v49.0.0 ('Sick Of Losing Soulmates') 64-bit
      _STATISTICS_WRITING_DATE_UTC-eng: 2020-08-25 21:12:14
      _STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
      encoder         : Lavc58.91.100 ac3
size=   19946kB time=00:06:04.73 bitrate= 448.0kbits/s speed=66.9x 

[steve]

Original file bitrate is 384 kb/s but ffmpeg converting this bitrate to 448 kb/s why?

Does it matter? The file size will be a little larger but other than that it shouldn't be a problem. I'd guess that if you look through the FFmpeg documentation there will be an option that you could add for the target bit-rate.

[darekpro]

there will be an option that you could add for the target bit-rate.

I'm sure will be but I use this code in context menu to convert many files with different bitrate.

omg i'm idiot I wrote post on wrong forum hehe