Gapless resampling with sox

Question

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?

Answer 1

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. 🙂