surround_to_stereo

#!/bin/bash
# surround_to_stereo

FF=$(which ffmpeg)
[[ ! $FF ]]  && >&2 echo "Can't Find ffmpeg" && exit

if [ $# -lt 2 ]; then
  >&2 echo "surround_to_stereo <filename>"
  exit
fi

# audio rate -- DVD AC3 5.1 is 448k native
A_RATE="320k"
AENC="libfdk_aac"
# AENC="aac"  # default if not there


# A_RATE="448k"

# use single quotes because parameter requires double inside
AF='-ac 2 -af "pan=stereo|FL=FC+0.30*FL+0.30*BL|FR=FC+0.30*FR+0.30*BR"'

# if your hardware does Dolby ProLogic II this will sound better. Mine doesn't.
# AF='aresample=matrix_encoding=dplii -ac 2 -ar 48000 -ab 256k'

# fixes backwards-endian audio bytes or does nothing if it is OK
BSF="aac_adtstoasc"

fname="${1%.*}"
ext="${1##*.}"
outfile="${fname}.2ch.${ext}"



# 6 to 2 channel  reencode with Dolby II matrix

${FF}  -i "${1}"               \
    -fflags +genpts            \
    -c:v copy                  \
    -c:a ${AENC}"              \
    -b:a "${A_RATE}" "${AF}"   \
    -bsf:a "$BSF"              \
    "${outfile}"


#END