#!/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 audiodelay="$(get_audio_delay "$infile")"
    local videotrackid="$(get_video_trackid "$infile")"

    mkvmerge \
        -o "${outfile}" \
        --default-duration "${videotrackid}:${FORCEFPS}fps" \
        --no-audio "$infile" \
        --sync "0:$((audiodelay / 1000000))" \
        "$audiofile"
}

function get_video_trackid {
    mkvmerge -i "$1" | sed -n 's/Track ID \([0-9]*\): .*video.*/\1/p'
}

function get_audio_delay {
    mkvmerge -F verbose-text -i "$1" | \
        sed -n 's/Track ID [0-9]*: .*audio.*minimum_timestamp:\([0-9]*\).*/\1/p'
}

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"