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ADPCM-XQ

Xtreme Quality ADPCM Encoder/Decoder

Copyright (c) 2024 David Bryant.

All Rights Reserved.

Distributed under the BSD Software License.

What is this?

While very popular at the end of the last century, ADPCM is no longer a common audio encoding format, and is certainly not recommended as a general purpose encoder. However, it requires minimal CPU resources for decoding, and so still is ideally suited for certain embedded games and applications that contain canned audio samples.

This encoder combines two different techniques to achieve higher quality than existing ADPCM encoders while remaining fully compatible with standard decoders. The first is dynamic noise shaping, which shifts the quantization noise up or down in frequency based on the spectrum of the source signal. This technique is identical to the algorithm used in WavPack's lossy mode and can make any audible quantization noise much less annoying (or, in some cases, inaudible).

The other technique is "lookahead" in which the encoder exhaustively searches ahead to find the optimum coding sequence based on future samples. This process can reduce the quantization noise from about 1 to 10 dB (depending on the source) and also reduces or eliminates the harmonic content in the noise that sometimes plagues ADPCM. Unfortunately, at its maximum settings this can be very slow, but this should be relatively irrelevant if the encoder is being used to generate canned samples.

Adpcm-xq consists of three standard C files and a header file (adpcm-lib.h). It can be used as a stand-alone command-line program implemented in adpcm-xq.c, or the library, which consists of adpcm-lib.c and adpcm-dns.c, can be built into and utilized by another application. The library portion has been designed with maximum portability in mind and should work correctly even on 16-bit and big-endian architectures.

What's New?

The latest version of adpcm-xq has many enhancements including greatly improved performance of the conversion and the ability to calculate and display the quantization noise introduced in the operation. See all the details here.

Variations

There are several forms and variations of IMA ADPCM encoding. The one handled by adpcm-xq is the canonical one used in Microsoft WAV files. The audio is divided into fixed-sized blocks that include a 4-byte header (or 8-byte for stereo) that includes the first sample. The size of the blocks is stored in the WAV header and the audio nibbles are ordered least-significant temporally first.

The latest version of adpcm-xq also includes 2-bit, 3-bit and 5-bit ADPCM. These are not well-supported at all (and some support is buggy) but they might be useful if this library is decoding them in situations where more compression or higher quality is desired. BTW, Rockbox provides excellent support for them!

Some applications like games and consoles that decode IMA ADPCM in hardware or microcode use modified versions of IMA ADPCM that do not use headers and do not divide the audio into blocks or frames of any kind. The decoding parameters are simply initialized to zero and the audio nibbles continue uninterrupted to the end of the clip. I have created an experimental version that will generate two variations of this data. One is standard nibble order and the other is reversed nibble order (sometimes called Intel/DVI4 or ADP4 and is the format used in AIFF files). These formats are only writable as "raw" by the adpcm-xq command-line program because they are not representable in WAV files, and they cannot be decoded by the program either (for the same reason), however the library itself handles them. The experimental branch is here.

Building

To build the command-line tool (ADPCM-XQ) on Linux:

$ gcc -O3 *.c -lm -o adpcm-xq

on Darwin/Mac:

$ cmake -DCMAKE_OSX_ARCHITECTURES="arm64;x86_64" . ; make

on MS Visual Studio:

C:\cl -O3 adpcm-xq.c adpcm-lib.c adpcm-dns.c

Help

 ADPCM-XQ   Xtreme Quality IMA-ADPCM WAV Encoder / Decoder   Version 0.5
 Copyright (c) 2024 David Bryant. All Rights Reserved.

 Usage:     ADPCM-XQ [-options] infile.wav outfile.wav

 Operation: conversion is performed based on the type of the infile
          (either encode 16-bit PCM to 4-bit IMA-ADPCM or decode back)

 Options:  -[0-16]= encode lookahead samples (default = 3, max = 16)
           -b<n>  = override auto block size, 2^n bytes (n = 8-15)
           -d     = decode only (fail on WAV file already PCM)
           -e     = encode only (fail on WAV file already ADPCM)
           -f     = encode flat noise (no noise shaping, aka -s0.0)
           -h     = display this help message
           -n     = measure and report quantization noise
           -q     = quiet mode (display errors only)
           -r     = raw output (little-endian, no WAV header written)
           -s<n>  = override default noise shaping, (-1.0 < n <= 1.0)
           -v     = verbose (display lots of info)
           -w<n>  = override default 4-bit ADPCM width (2 <= n <= 5)
           -x     = exhaustive search (old behavior, very slow at depth)
           -y     = overwrite outfile if it exists

 Web:       Visit www.github.com/dbry/adpcm-xq for latest version and info

Caveats