[FFmpeg-trac] #4905(undetermined:open): Splitting a flac file using -ss and -t creates chunks with wrong duration

[FFmpeg]

#4905: Splitting a flac file using -ss and -t creates chunks with wrong duration
-------------------------------------+-------------------------------------
             Reporter:  cdrsgi       |                    Owner:  (none)
                 Type:  defect       |                   Status:  open
             Priority:  normal       |                Component:
                                     |  undetermined
              Version:  git-master   |               Resolution:
             Keywords:  flac         |               Blocked By:
             Blocking:               |  Reproduced by developer:  1
Analyzed by developer:  0            |
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Comment (by martin):

 I am facing this same issue:


 Input flac File: https://file.io/D1EXEgJ0OtgM

 I have a shell script `split_by_silence.sh` which takes an input audio
 file, detects split points where silence (or very low volume audio)
 occurs, and then splits the audio file **using ffmpeg** into the
 individual split point segments.

 So if 10 split points = 11 audio files exported.

 When I run it on an mp3, it produces 11 files (as expected with my input
 file) and each file has correct length metadata.

 **But when ffmpeg splits a flac file, the metadata output for 'length' is
 broken**

 I've tried following the closest question, marked as a duplicate, here:
 https://stackoverflow.com/questions/66732630/cutting-flac-using-ffmpeg-
 does-not-change-timestamps-accordingly

 By adding a step to re-encode my flac file in ffmpeg:

 `ffmpeg -i full.flac -ss 0 -t 2241 fixed.flac`

 Which I'm not sure if is working, but outputs a fixed.flac file. Which
 when I run with my split command:

 `ffmpeg -i "inputFile.flac" -c copy -map 0 -f segment -segment_times
 "22,441,5567,1122,etc..." "%d_output.flac"
 `

 The output files still have broken flac 'length' metadata, like you can
 see in my image, even though when opened in audacity, the file does have a
 real length. ffmpeg just encoded the wrong value?

 [![enter image description here][1]][1]

 This is my current ffmpeg version:
 ```
 $ ffmpeg
 ffmpeg version 4.4.2-0ubuntu0.22.04.1 Copyright (c) 2000-2021 the FFmpeg
 developers
   built with gcc 11 (Ubuntu 11.2.0-19ubuntu1)
 ```


 And here is my .sh file I run with

 `$ ./split_by_silence.sh`

 ```
 # -----------------------
 # SPLIT BY SILENCE
 # Requirements:
 #    ffmpeg
 #    $ apt-get install bc
 # How To Run:
 # $ ./split_by_silence.sh "full_lowq.flac" %03d_output.flac

 # output title format
 OUTPUTTITLE="%d_output.flac"
 # input audio filepath
 IN="/mnt/c/Users/marti/Documents/projects/split_by_silence2/full.flac"
 # output audio filepath
 OUTPUTFILEPATH="/mnt/c/Users/marti/Documents/projects/split_by_silence2"
 # ffmpeg option: split input audio based on this silencedetect value
 SD_PARAMS="-11dB"
 MIN_FRAGMENT_DURATION=120 # split option: minimum fragment duration
 export MIN_FRAGMENT_DURATION

 # -----------------------
 # step: ffmpeg
 # goal: get comma separated list of split points (use ffmpeg to determine
 points where audio is at SD_PARAMS [-18db] )

 echo "_______________________"
 echo "Determining split points..."
 SPLITS=$(
     ffmpeg -v warning -i "$IN" -af
 silencedetect="$SD_PARAMS",ametadata=mode=print:file=-:key=lavfi.silence_start
 -vn -sn  -f s16le  -y /dev/null \
     | grep lavfi.silence_start= \
     | cut -f 2-2 -d= \
     | perl -ne '
         our $prev;
         INIT { $prev = 0.0; }
         chomp;
         if (($_ - $prev) >= $ENV{MIN_FRAGMENT_DURATION}) {
             print "$_,";
             $prev = $_;
         }
     ' \
     | sed 's!,$!!'
 )
 echo "split points list= $SPLITS"

 # add '5.5' to each split\
 IFS=',' read -ra SPLITS_ARRAY <<< "$SPLITS"
 for i in "${!SPLITS_ARRAY[@]}"; do
   SPLITS_ARRAY[i]=$(echo "${SPLITS_ARRAY[i]}+5.5" | bc)
 done

 SPLITS=$(IFS=','; echo "${SPLITS_ARRAY[*]}")
 echo "SPLITS=$SPLITS"

 # using the split points list, calculate how many output audio files will
 be created
 num=0
 res="${SPLITS//[^,]}"
 CHARCOUNT="${#res}"
 num=$((CHARCOUNT + 2))
 echo "_______________________"
 echo "Exporting $num tracks with ffmpeg"

 ffmpeg -i "$IN" -c copy -map 0 -f segment -segment_times "$SPLITS"
 "$OUTPUTFILEPATH/$OUTPUTTITLE"

 echo "Done."

 echo "------------------------------------------------"
 echo "$num TRACKS EXPORTED"
 ```
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/4905#comment:5>
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