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63 lines
2.8 KiB
Text
63 lines
2.8 KiB
Text
# MODIFIED
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this is a modified version of adpcm-xq which fixes memory safety issues.
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////////////////////////////////////////////////////////////////////////////
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// **** ADPCM-XQ **** //
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// Xtreme Quality ADPCM Encoder/Decoder //
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// Copyright (c) 2022 David Bryant. //
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// All Rights Reserved. //
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// Distributed under the BSD Software License (see license.txt) //
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////////////////////////////////////////////////////////////////////////////
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While very popular at the end of the last century, ADPCM is no longer a
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common audio encoding format, and is certainly not recommended as a general
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purpose encoder. However, it requires minimal CPU resources for decoding,
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and so still is ideally suited for certain embedded games and applications
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that contain canned audio samples.
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This encoder combines two different techniques to achieve higher quality
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than existing ADPCM encoders while remaining fully compatible with standard
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decoders. The first is dynamic noise shaping, which shifts the quantization
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noise up or down in frequency based on the spectrum of the source signal.
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This technique is identical to the algorithm used in WavPack's lossy mode
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and can make any audible quantization noise much less annoying (or, in some
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cases, inaudible).
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The other technique is "lookahead" in which the encoder exhaustively
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searches ahead to find the optimum coding sequence based on future samples.
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This process can reduce the quantization noise from about 1 to 6 dB (depending
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on the source) and also reduces or eliminates the harmonic content in the
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noise that sometimes plagues ADPCM. Unfortunately, at its maximum settings
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this can be very slow, but this should be relatively irrelevant if the
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encoder is being used to generate canned samples.
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Adpcm-xq consists of two standard C files and builds with a single command
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on most platforms. It has been designed with maximum portability in mind
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and should work correctly even on 16-bit and big-endian architectures
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(WAV files are of course always little-endian).
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Linux:
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% gcc -O2 *.c -o adpcm-xq
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Darwin/Mac:
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% clang -O2 *.c -o adpcm-xq
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MS Visual Studio:
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cl -O2 adpcm-xq.c adpcm-lib.c
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Bugs:
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1. Unknown RIFF chunk types are correctly parsed on input files, but are not
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passed to the output file.
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2. The lookahead feature does not work for the last samples in an ADPCM
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block (i.e. it doesn't utilize samples in the _next_ block).
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3. In some situations the lookahead can get very slow or seem to be stuck
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because it needs improved trellis pruning. However the default level 3
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should always be fine and then the user can simply try increasing levels
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until the time becomes untenable.
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4. Pipes are not yet supported.
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