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MKV to MP4 Remuxing Script

This is based off thewinchester's script posted in his answer here. I started off with his and it eventually morphed into a rewrite of most of it, but the concept is the same.

The basic function is to take an MKV file containing h.264 video and some audio and convert it to an MP4/M4V file.

My goal is to make the resulting files compatible with the iPad and AppleTV, but it should work with anything that can handle MP4/M4V files. Video is not transcoded, so quality remains the same, audio is converted if necessary, but higher quality tracks are kept if possible.

Changes

A partial list of the changes I've made:

• Handling multiple files or directories passed as arguments
• Uses (and requires) the higher quality NeroAAC to encode AAC audio from AC3 or DTS
• Handling multiple audio tracks in accordance with the AppleTV standard to allow for proper playback and surround sound on AppleTV:
  • 160kB/s stereo AAC
  • (Optional) AC-3 (typically surround sound) as the second track, disabled
• Converting DTS to AC-3
• Handling multiple tracks regardless of order (i.e. AAC, Video, AC3)
• Various coding changes to make it easier to modify later

Requirements

The script requires the aforementioned NeroAAC, as well as Aften (AC3 encode) and libdca/dcadec (DTS decode). As with the original script, ffmpeg is also required.

Known Issues

The script is fairly bulletproof - if it runs into something it can't deal with, it's typically ignored or quits. The original files are left intact, so there's little to no chance of data loss.

I've used it on a lot of my media library and many of the changes have been to fix bugs I've run into.

That said, there are a few things to be aware of, and room for improvement:

• Handling multiple audio tracks of the same type - 1 AC3 and 1 AAC is fine, but 2 AAC tracks won't work. This shouldn't come up often, but keep it in mind.
• No support for non-audio/video tracks. Subtitles, chapters and artwork are all ignored.
• No detection of surround sound AAC. The script copies any AAC track, it won't down mix to stereo. Technically for iPad and AppleTV support, the main audio track should be stereo AAC, but I think in most cases an iPad or AppleTV can handle it gracefully.

Code and use

I'm posting it here hoping that someone will find it useful - please feel free to use and modify as you see fit for non-commercial uses. If you come up with an interesting modification it might be nice if you shared it here or elsewhere.

#!/bin/bash

#Close stdin - avoid accidental keypresses causing problems
exec 0>&-

# Find MKV files
for file in "$@";
do
  find "$file" -type f -not -name ".*" | grep .mkv$ | while read file
  do
    fileProper=$(readlink -f "$file") # full path of file
    pathNoExt=${fileProper%.*} # full path minus extension

    #Check if M4V already exists
    if [ -f "$pathNoExt".m4v ]; then
      echo "M4V already exists, stopping"
    else
      # Get number of tracks
      numberOfTracks=`mkvmerge -i "$fileProper" | grep "Track ID" | wc -l`
      echo "Found $numberOfTracks Tracks"

      # Set base extraction command
      extractCmd+=(mkvextract tracks "$fileProper")

      # Determine type of tracks
      for (( i=1; i<=$numberOfTracks; i++ ))
      do
         trackType=`mkvmerge -i "$fileProper" | grep "Track ID $i" | sed -e 's/^.*: //'`
         if [[ "$trackType" == *video* ]]; then
            echo "Track $i is Video"
            extractCmd+=( $i:"$pathNoExt".264)
            fps=`mkvinfo "$fileProper" | grep duration | sed -e 's/.*(//' -e 's/f.*//' | sed -n ${i}p`
         elif [[ "$trackType" == "audio (A_AAC)" ]]; then
            echo "Track $i is AAC"
            extractCmd+=( $i:"$pathNoExt".aac)
         elif [[ "$trackType" == "audio (A_AC3)" ]]; then
            echo "Track $i is AC3"
            extractCmd+=( $i:"$pathNoExt".ac3)
         elif [[ "$trackType" == "audio (A_DTS)" ]]; then
            echo "Track $i is DTS"
            extractCmd+=( $i:"$pathNoExt".dts)
         fi
         # Insert cases for handling other audio and non-AV tracks here
       done

        "${extractCmd[@]}" # Extract Tracks

        # Check files and encode audio if neccessary
        if [ -f "$pathNoExt".264 ]; then
            # Video file exists
            mp4BoxCmd+=(MP4Box -new "$pathNoExt".m4v -add "$pathNoExt".264 -fps $fps)
            if [ -f "$pathNoExt".aac ]; then
                # AAC exists
                mp4BoxCmd+=( -add "$pathNoExt".aac)
                if [ -f "$pathNoExt".ac3 ]; then
                    mp4BoxCmd+=( -add "$pathNoExt".ac3:disable)
                elif [ -f "$pathNoExt".dts ]; then
                    # Encode DTS to AC3
                    dcadec -o wavall "$pathNoExt".dts | aften -v 0 - "$pathNoExt".ac3
                    mp4BoxCmd+=( -add "$pathNoExt".ac3:disable)
                fi
            else # Encode AAC from AC3 or DTS
                if [ -f "$pathNoExt".ac3 ]; then
                    ffmpeg -i "$pathNoExt".ac3 -acodec pcm_s16le -ac 2 -f wav - | neroAacEnc -lc -br 160000 -ignorelength -if - -of "$pathNoExt".aac
                    mp4BoxCmd+=( -add "$pathNoExt".aac -add "$pathNoExt".ac3:disable)
                elif [ -f "$pathNoExt".dts ]; then
                  ffmpeg -i "$pathNoExt".dts -acodec pcm_s16le -ac 2 -f wav - | neroAacEnc -lc -br 160000 -ignorelength -if - -of "$pathNoExt".aac
                    # Encode DTS to AC3
                    dcadec -o wavall "$pathNoExt".dts | aften -v 0 - "$pathNoExt".ac3
                    mp4BoxCmd+=( -add "$pathNoExt".aac -add "$pathNoExt".ac3:disable)
                else
                    echo "Warning: no audio file found"
                fi
            fi
            # Create m4v
            "${mp4BoxCmd[@]}"
        else
            echo "Error: no video file found"
        fi
  #remove temporary track files
  rm -f "$pathNoExt".aac "$pathNoExt".dts "$pathNoExt".ac3 "$pathNoExt".264 "$pathNoExt".wav
  fi
 done
done

Here are two PowerShell scripts to split long videos into smaller chapters by black scenes .

Save them as Detect_black.ps1 and Cut_black.ps1. Download ffmpeg for Windows and tell the script the path to your ffmpeg.exe and your video folder under the option section.

Both scripts won't touch existing video files, they remain untouched.
However, you will get a couple of new files at the same place where your input videos are

• A logfile per video with the console output for both used ffmpeg commands
• A CSV file per video with all timestamps of black scenes for manual fine tuning
• A couple of new videos depending on how many black scenes are previously detected

First script to run: Detect_black.ps1

 ### Options __________________________________________________________________________________________________________
$ffmpeg = ".\ffmpeg.exe"            # Set path to your ffmpeg.exe; Build Version: git-45581ed (2014-02-16)
$folder = ".\Videos\*"              # Set path to your video folder; '\*' must be appended
$filter = @("*.mov","*.mp4")        # Set which file extensions should be processed
$dur = 4                            # Set the minimum detected black duration (in seconds)
$pic = 0.98                         # Set the threshold for considering a picture as "black" (in percent)
$pix = 0.15                         # Set the threshold for considering a pixel "black" (in luminance)

### Main Program ______________________________________________________________________________________________________

foreach ($video in dir $folder -include $filter -exclude "*_???.*" -r){

  ### Set path to logfile
  $logfile = "$($video.FullName)_ffmpeg.log"
  
  ### analyse each video with ffmpeg and search for black scenes
  & $ffmpeg -i $video -vf blackdetect=d=`"$dur`":pic_th=`"$pic`":pix_th=`"$pix`" -an -f null - 2> $logfile

  ### Use regex to extract timings from logfile
  $report = @()
  Select-String 'black_start:.*black_end:' $logfile | % { 
    $black          = "" | Select  start, end, cut
    
    # extract start time of black scene
    $start_s     = $_.line -match '(?<=black_start:)\S*(?= black_end:)'    | % {$matches[0]}
    $start_ts    = [timespan]::fromseconds($start_s)
    $black.start = "{0:HH:mm:ss.fff}" -f ([datetime]$start_ts.Ticks)
    
    # extract duration of black scene
    $end_s       = $_.line -match '(?<=black_end:)\S*(?= black_duration:)' | % {$matches[0]}
    $end_ts      = [timespan]::fromseconds($end_s)
    $black.end   = "{0:HH:mm:ss.fff}" -f ([datetime]$end_ts.Ticks)    
     
    # calculate cut point: black start time + black duration / 2
    $cut_s       = ([double]$start_s + [double]$end_s) / 2
    $cut_ts      = [timespan]::fromseconds($cut_s)
    $black.cut   = "{0:HH:mm:ss.fff}" -f ([datetime]$cut_ts.Ticks)
    
    $report += $black
  }

  ### Write start time, duration and the cut point for each black scene to a seperate CSV
  $report | Export-Csv -path "$($video.FullName)_cutpoints.csv" –NoTypeInformation
}

How does it work

The first script iterates through all video files which matches a specified extension and doesn't match the pattern *_???.*, since new video chapters were named <filename>_###.<ext> and we want to exclude them.

It searches all black scenes and writes the start timestamp and black scene duration to a new CSV file named <video_name>_cutpoints.txt

It also calculates cut points as shown: cutpoint = black_start + black_duration / 2. Later, the video gets segmented at these timestamps.

The cutpoints.txt file for your sample video would show:

start          end            cut
00:03:56.908   00:04:02.247   00:03:59.578
00:08:02.525   00:08:10.233   00:08:06.379

After a run, you can manipulate the cut points manually if wished. If you run the script again, all old content gets overwritten. Be careful when manually editing and save your work elsewhere.

For the sample video the ffmpeg command to detect black scenes is

$ffmpeg -i "Tape_10_3b.mp4" -vf blackdetect=d=4:pic_th=0.98:pix_th=0.15 -an -f null

There are 3 important numbers which are editable in the script's option section

• d=4 means only black scenes longer than 4 seconds are detected
• pic_th=0.98 is the threshold for considering a picture as "black" (in percent)
• pix=0.15 sets the threshold for considering a pixel as "black" (in luminance). Since you have old VHS videos, you don't have completely black scenes in your videos. The default value 10 won't work and I had to increase the threshold slightly

If anything goes wrong, check the corresponding logfile called <video_name>__ffmpeg.log. If the following lines are missing, increase the numbers mentioned above until you detect all black scenes:

[blackdetect @ 0286ec80]
 black_start:236.908 black_end:242.247 black_duration:5.33877

Second script to run: cut_black.ps1

### Options __________________________________________________________________________________________________________
$ffmpeg = ".\ffmpeg.exe"            # Set path to your ffmpeg.exe; Build Version: git-45581ed (2014-02-16)
$folder = ".\Videos\*"              # Set path to your video folder; '\*' must be appended
$filter = @("*.mov","*.mp4")        # Set which file extensions should be processed

### Main Program ______________________________________________________________________________________________________

foreach ($video in dir $folder -include $filter -exclude "*_???.*" -r){

  ### Set path to logfile
  $logfile = "$($video.FullName)_ffmpeg.log"

  ### Read in all cutpoints from *_cutpoints.csv; concat to string e.g "00:03:23.014,00:06:32.289,..."  
  $cuts = ( Import-Csv "$($video.FullName)_cutpoints.csv" | % {$_.cut} ) -join ","

  ### put together the correct new name, "%03d" is a generic number placeholder for ffmpeg
  $output = $video.directory.Fullname + "\" + $video.basename + "_%03d" + $video.extension

  ### use ffmpeg to split current video in parts according to their cut points
  & $ffmpeg -i $video -f segment -segment_times $cuts -c copy -map 0 $output 2> $logfile        
}

How does it work

The second script iterates over all video files in the same way the first script has done. It reads in only the cut timestamps from the corresponding cutpoints.txt of a video.

Next, it puts together a suitable filename for chapter files and tells ffmpeg to segment the video. Currently the videos are sliced without re-encoding (superfast and lossless). Due to this, there might be 1-2s inaccuracy with cut point timestamps because ffmpeg can only cut at key_frames. Since we just copy and don't re-encode, we cannot insert key_frames on our own.

The command for the sample video would be

$ffmpeg -i "Tape_10_3b.mp4" -f segment -segment_times "00:03:59.578,00:08:06.379" -c copy -map 0 "Tape_10_3b_(%03d).mp4"

If anything goes wrong, have a look at the corresponding ffmpeg.log

References

• FFmpeg download for Windows
• Segment dokumentation: segment_times and segment_frames are options

Todo

• Ask OP if CSV format is better than a text file as cut point file, so you can edit them with Excel a little bit easier
  » Implemented
• Implement a way to format timestamps as [hh]:[mm]:[ss],[milliseconds] rather than only seconds
  » Implemented
• Implement a ffmpeg command to create mosaik png files for each chapter
  » Implemented
• Elaborate if -c copy is enough for OP's scenario or of we need to fully re-encode.
  Seems like Ryan is already on it.