sample-shrinker.sh

Conditionally batch-convert audio samples into minimal .wav files

• Description
• Usage
  • Basic use cases
  • List samples + preview changes, based on the given options
  • Automatically convert stereo to mono when the content is mono
  • Normalize before downsampling bit-depth
  • Convert a directory without creating backups
• Setup
  • Requirements

Description

This script scans directories for audio samples and batch-converts them into small .wav files, based on target criteria. This is useful to save storage space and reduce the I/O stress during simultaneous real-time streaming of multiple .wav files on devices like the Dirtywave M8 tracker.

If you have directories full of 24/32-bit stereo .wav files or stereo samples with effectively mono content, this script can reclaim wasted storage space and reduce I/O stress on your SD card. It can even detect if the content of a stereo sample is actually mono & convert it for you!

Usage

bash sample-shrinker.sh [options] FILE|DIRECTORY ...

Each DIRECTORY is recursively searched for audio files to process, based on their extension (.wav by default, configure with -x). Any FILE specified directly will be processed (regardless of its extension).

The basics:

• Samples are converted in place
  • The original file is backed up under '_backup/' (change with -d)
  • Backups generate spectrogram .png files to compare old & new files (disable with -S)
• Only samples that DON'T meet the target criteria will ever be changed
• Samples are only converted to SMALLER target bit-depth (-b) or channels (-c)
  • ...unless a minimum bit-depth is specified (-B, disabled by default)
• Stereo samples can be conditionally converted to mono using auto-mono (-a)
  • the threshold for automatic conversion is configurable (-A)

If a sample does not already meet the target BIT_DEPTH or CHANNELS, it will be converted in place and the original will be backed up to a parallel directory structure (default: _backup).

Upon conversion, spectrogram .png files are generated alongside the backed-up original file, to compare the original vs new audio files (disable with -S)

You can review the characteristics of all sample files, and see if/why they would be converted by running with -l. You can preview only samples that would change with -n.

Run sample-shrinker.sh -h to see all the options.

Basic use cases

bash sample-shrinker.sh directory_of_samples/

This uses the default options, where:

• The target bit-depth is 16 (change with -b BITRATE)
• Stereo channels are always left unchanged (change with -a, -A DB, or -c1)
• The original files are backed up to a parallel directory structure under _backup/ (change location with -d)
• Spectrogram .png graphics that compare the old and new files are generated alongside the backed-up file (Use -v to pr

Unless you use -v to increase verbosity, the script will output one line per sample, summarizing its properties and any changes it makes:

bash sample-shrinker.sh -a -A-80 inst/ vocals/

16        st->m+A    -inf      16/16  inst/drums/anvil.wav  [CHANGED]
16        st->m+A    -84.29    16/16  inst/drums/kalimba.wav  [CHANGED]
16        st         -60.21    16/16  inst/drums/closed_hh.wav
8         mono                 8/8    inst/waves/triangle.wav
16        mono                 16/16  inst/waves/monowave.wav
24->16    mono                 23/24  vocals/otr-snippet.wav  [CHANGED]
32->16    st         -22.48    31/32  vocals/accapella.wav  [CHANGED]

• Changed files are followed by [CHANGED]
• Specific changes are denoted with ->
  • If the change has additional was automatically decided, it will be followed with + and a character to describe the reason:
    • +A: decided by auto-mono (-a/-A)
    • +P: pre-normalize before down-converting bit-depth (-p)
    • +M: auto-converted to meet the minimum bit-depth (-B, unset by default)
• The columns are: bit depth, stereo, stereo diff, effective bit-depth, file path

List samples + preview changes, based on the given options

Use -l to scan and list each sample, and summarize what would be changed based on the other options:

bash sample-shrinker.sh -l -a -A-80 inst/ vocals/

16        st->m+A    -inf      16/16  inst/drums/anvil.wav  [CHANGE]
16        st->m+A    -84.29    16/16  inst/drums/kalimba.wav  [CHANGE]
16        st         -60.21    16/16  inst/drums/closed_hh.wav
8         mono                 8/8    inst/waves/triangle.wav
16        mono                 16/16  inst/waves/monowave.wav
24->16    mono                 23/24  vocals/otr-snippet.wav  [CHANGE]
32->16    st         -22.48    31/32  vocals/accapella.wav  [CHANGE]

The output will look identical to conversion, but it will not alter any files

Automatically convert stereo to mono when the content is mono

This detects stereo samples that are effectively mono and converts them to mono:

bash sample-shrinker.sh -a dir1/ dir2/

Stereo content is "effectively mono" if―after summing its first two channels after inverting one of them—the resulting Peak dB level is below -95.5 dB.

To configure the auto-mono threshold to be -80 dB:

# Note: `-A` implies `-a`
bash sample-shrinker.sh -A -80 dir1/ dir2/

Normalize before downsampling bit-depth

bash sample-shrinker.sh -p dir1 dir2

Pre-normalizing before downsampling will preserve as much dynamic range as possible, but it is disabled by default: if the sample is part of a level-balanced collection (like a single hit from a drum kit), normalizing the sample would change its relative volume with the rest of the collection.

Convert a directory without creating backups

bash sample-shrinker.sh -d - .

Specifying - as the backup directory will disable backups. As a side effect, This also disables the before/after spectrograph images (because they are saved under the backups' directory tree).

Setup

Requirements

• Bash
  • Tested with Bash 5.1.16
  • Requires at least Bash 5
    • NOTE Mac/OSX users will need to homebrew install bash, because the default terminal bash is stuck at 3.2(!)
• SoX
  • Tested with 14.4.2
  • Requires sox >= 14.3 to support automatic --no-dither for 8-bit samples