I'm trying to use sox to resample a series of tracks that should be played back gaplessly. If I resample each track individually, I can sometimes end up with clicks at the track boundaries due to the individual resampling not quite lining up. The solution seems conceptually simple: concatenate all of the tracks, resample them as a single unit, and split them again. However, I'm not sure how to go about that in an automated fashion. The concatenation step is easy (just pass all the files to a single sox invocation) but how do I split the result again with the same durations as the original tracks?
1 Answer
I ended up making a script to handle this:
#!/usr/bin/env bash
set -o errexit -o pipefail -o nounset
# Can be changed to point to, e.g., a DSD-enabled SoX build.
SOX="sox"
TRACKS=()
while [[ $# -gt 0 ]]; do
case $1 in
-b|--bits)
# Use the specified number of bits-per-sample for the output
BITS="$2"
shift 2
;;
-o|--out)
# Output template for resampled tracks
OUT="$2"
shift 2
;;
-r|--rate)
# The sample rate of the output
RATE="$2"
shift 2
;;
-*|--*)
echo "Unknown option $1" >&2
exit 1
;;
*)
TRACKS+=("$1") # positional arg
shift
;;
esac
done
if [[ -z ${OUT+x} ]]; then
echo "Option --out is required" >&2
exit 1
fi
if [[ -z ${RATE+x} ]]; then
echo "Option --rate is required" >&2
exit 1
fi
if [[ ${#TRACKS[@]} -eq 0 ]]; then
echo "No input files provided" >&2
exit 1
fi
if [[ -n ${BITS+x} ]]; then
BITS_ARG=(-b "$BITS")
else
BITS_ARG=()
fi
if [[ ${#TRACKS[@]} -eq 1 ]]; then
TRIM_ARGS=()
else
# Get lengths of all tracks except the last one
LENGTHS=($("$SOX" --i -D "${TRACKS[@]:0:${#TRACKS[@]}-1}"))
TRIM_ARGS=(trim 0 "${LENGTHS[0]}" : newfile)
for length in "${LENGTHS[@]:1}"; do
TRIM_ARGS+=(: trim 0 "$length" : newfile)
done
fi
# --guard resamples to a temporary file with added headroom, then writes the
# output as close to the original level as possible without clipping.
# Remove to write directly to the output files at the originally level, with the
# possibility of some samples clipping if the input has insufficient headroom.
"$SOX" --guard "${TRACKS[@]}" "${BITS_ARG[@]}" -t wav - rate -v "$RATE" | "$SOX" - "$OUT" "${TRIM_ARGS[@]}"
Sample usage:
Resamples flac files to 48kHz with the same number of bits per sample (tags aren't copied):
$ resample.sh -o output.flac -r 48k ../96kHz/*.flac
Convert DSD audio to 48kHz, 24-bit FLAC (requires DSD enabled SoX):
$ resample.sh -o output.flac -r 48k -b 24 ../DSD/*.dsf
Output files will be named output001.flac, output002.flac, et cetera.
Adding additional options to the script (e.g., the ability to specify dither when creating 16-or-less-bit files) left as an exercise for the reader. 🙂