#!/bin/bash -e

# Many DVDs released in Australia are sped up from 24fps to 25fps.
# This script reverses the procedure, correcting the audio pitch.
# The video framerate is adjusted without re-encoding. The audio is
# slowed, and encoded as AAC, preserving surround sound.

if [ -z "$1" -o -z "$2" ]; then
    echo "Usage: $0 infile outfile"
    exit 1
fi

set -xe
FORCEFPS="24"
SLOWFILTER="-filter asetrate=46080,aresample=osr=48000:resampler=soxr"

function mux_replace_audio {
    local infile="$1"
    local audiofile="$2"
    local outfile="$3"

    local trackid="$(mkvmerge -i "$infile" | grep 'Track ID.*video' | sed 's/^Track ID \(.\):.*$/\1/')"
    mkvmerge -o "${outfile}" --default-duration "${trackid}:${FORCEFPS}fps" --no-audio "$infile" "$audiofile"
}

function encode_audio {
    ffmpeg \
        -i "$1" \
        -vn \
        $SLOWFILTER \
        -c:a libfdk_aac -vbr 3 \
        "$2"
}

function convert_file {
    local infile="$1"
    local outfile="$2"
    local audiofile="${tmpdir}/audiofile.m4a"

    encode_audio "${infile}" "${audiofile}"
    mux_replace_audio "${infile}" "${audiofile}" "${outfile}"
}


infile="$1"
outfile="$2"
tmpdir="$(mktemp -d "${TMPDIR:-/var/tmp}/pal-XXXXXXXX")"
convert_file "$infile" "$outfile"
rm -rf "$tmpdir"