Converting AAC audio track to AC3 (In linux ofcourse)

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#!/bin/bash
# ----------
# This script takes a MP4 or MKV file with an AAC audio track as input and
# outputs an MKV file with an AC3 audio track
# ----------
# Needed tools:
# MKVToolnix (install Ubuntu mkvtoolnix package)
# FAAD2 (install Ubuntu faad package)
# Aften (install Ubuntu aften package)
# ----------

# Usage:
# AAC2AC3 Input_File Output_File [Optional_Working_Directory]
# ----------

# initialize variables
INPUT=$1
OUTPUT=$2
WORKDIR=$3

# make things a bit more pretty
echo

# make sure all programs we need are installed
if ! which mkvmerge mkvextract &> /dev/null
then
echo "MKVToolnix is missing. Please install the mkvtoolnix package."
echo
exit 1
fi
if ! which faad &> /dev/null
then
echo "FAAD2 is missing. Please install the faad package."
echo
exit 1
fi
if ! which aften &> /dev/null
then
echo "Aften is missing. Please install the aften package."
echo
exit 1
fi

# make sure passed variables make sense
if [ ! "$INPUT" ]
then
echo "Usage: AAC2AC3 Input_File Output_File [Optional_Working_Directory]"
echo
echo "No input file specified."
echo
exit 1
fi
if [ ! -f "$INPUT" ]
then
echo "Usage: AAC2AC3 Input_File Output_File [Optional_Working_Directory]"
echo
echo "Specified input file doesn't exist."
echo
exit 1
fi
if [ ! "$OUTPUT" ]
then
echo "Usage: AAC2AC3 Input_File Output_File [Optional_Working_Directory]"
echo
echo "No output file specified."
echo
exit 1
fi
if [ -e "$OUTPUT" ]
then
echo "Usage: AAC2AC3 Input_File Output_File [Optional_Working_Directory]"
echo
echo "Specified output file already exists."
echo
exit 1
fi
if [ ! "$WORKDIR" ]
then
WORKDIR="`pwd`"
fi
if [ ! -d "$WORKDIR" ]
then
echo "Usage: AAC2AC3 Input_File Output_File [Optional_Working_Directory]"
echo
echo "Specified working directory doesn't exist."
echo
exit 1
fi

# check if any temp files are already present
if [ -e "$WORKDIR/AAC2AC3Temp.mkv" ] || [ -e "$WORKDIR/AAC2AC3Temp.aac" ] || \
[ -e "$WORKDIR/AAC2AC3Temp.ac3" ]
then
echo "Files AAC2AC3Temp.mkv and/or AAC2AC3Temp.aac and/or AAC2AC3Temp.ac3"
echo "already exist in the working directory. These will be overwritten."
echo
echo -n "Continue (y/n)?: "
read CONTINUE
echo
if ! ([ "$CONTINUE" == "y" ] || [ "$CONTINUE" == "Y" ])
then
echo
exit 1
fi
fi

# check if the input file is an MP4 file
if [[ "$INPUT" == *.mp4 ]] || [[ "$INPUT" == *.MP4 ]]
then
mkvmerge -o "$WORKDIR/AAC2AC3Temp.mkv" "$INPUT"
INPUT="$WORKDIR/AAC2AC3Temp.mkv"
echo
echo "**********"
echo
fi

# print out details about the file in question
mkvmerge -i "$INPUT"

# ask which track we are converting
echo
echo -n "Please specify track to be converted (look for the (A_AAC) label): "
read TRACK
echo

# check if the track was properly specified
if [ "`expr $TRACK - $TRACK 2> /dev/null`" != "0" ]
then
echo "Specified track has to be a numeric value."
echo
exit 1
fi
if [ $TRACK -lt 1 ]
then
echo "Specified track can not be less than one."
echo
exit 1
fi

# extract the specified track
mkvextract tracks "$INPUT" $TRACK:"$WORKDIR/AAC2AC3Temp.aac"

# make things pretty
echo
echo "**********"
echo

# confirm audio sample rate
echo "Please specify audio track sample rate (press enter for default"
echo -n "of 48000Hz): "
read RATE
echo

# setup default sample rate if needed
if [ ! $RATE ]
then
RATE=48000
fi

# check if the rate was properly specified
if [ "`expr $RATE - $RATE 2> /dev/null`" != "0" ]
then
echo "Specified sample rate has to be a numeric value."
echo
exit 1
fi
if [ $RATE -lt 1 ]
then
echo "Specified sample rate can not be less than one."
echo
exit 1
fi

# confirm number of audio channels
echo "Please specify the number of channels in audio track (press enter"
echo -n "for default of 6 channels): "
read CHANNELS

# setup default number of channels if needed
if [ ! $CHANNELS ]
then
CHANNELS=6
fi

# check if the rate was properly specified
if [ "`expr $CHANNELS - $CHANNELS 2> /dev/null`" != "0" ]
then
echo
echo "Specified number of channels has to be a numeric value."
echo
exit 1
fi
if [ $CHANNELS -lt 1 ]
then
echo
echo "Specified number of channels can not be less than one."
echo
exit 1
fi

# faad outputs a raw 24 bit PCM stream (anything higher than 24 bits seems to
# introduce anomalies) which is then piped to aften and converted to a 640 kbit/s
# AC3 stream (640 kbit/s is max allowed and yields highest possible quality)
faad -b 2 -f 2 -q -w "$WORKDIR/AAC2AC3Temp.aac" | aften -v 1 -b 640 \
-raw_fmt s24_le -raw_sr $RATE -raw_ch $CHANNELS -chmap 0 - "$WORKDIR/AAC2AC3Temp.ac3"

# make things pretty
echo "**********"
echo

# check if we are appending the ac3 track or replacing any existing audio tracks
# with the new ac3 track
echo "The AC3 audio track can be appended (resulting MKV file will contain"
echo "the existing audio tracks plus the new ac3 audio track) or the AC3 audio"
echo "track can replace all existing audio tracks (resulting MKV file will"
echo "contain only the AC3 audio track)."
echo
echo -n "Append or replace (press enter for default of append) (a/r): "
read APPENDREPLACE
echo

# setup default append/replace if needed
if [ ! $APPENDREPLACE ]
then
APPENDREPLACE="a"
fi

# do different things depending on what was specified
if [ "$APPENDREPLACE" == "a" ] || [ "$APPENDREPLACE" == "A" ]
then
# merge everything back together
mkvmerge -o "$OUTPUT" "$INPUT" "$WORKDIR/AAC2AC3Temp.ac3"
elif [ "$APPENDREPLACE" == "r" ] || [ "$APPENDREPLACE" == "R" ]
then
# merge everything back together (except audio tracks from original file)
mkvmerge -o "$OUTPUT" -A "$INPUT" "$WORKDIR/AAC2AC3Temp.ac3"
else
# neither append nor replace was specified
echo "Neither append nor replace was specified."
echo
exit 1
fi

# make things pretty
echo

# clean up temp files
rm -f "$WORKDIR/AAC2AC3Temp.mkv"
rm -f "$WORKDIR/AAC2AC3Temp.aac"
rm -f "$WORKDIR/AAC2AC3Temp.ac3"

# all done
exit 0