[FFmpeg-devel] [PATCH v7 1/3] libavcodec: Added DFPWM1a codec

[Jack Bruienne]

 From the wiki page (https://wiki.vexatos.com/dfpwm):
> DFPWM (Dynamic Filter Pulse Width Modulation) is an audio codec
> created by Ben “GreaseMonkey” Russell in 2012, originally to be used
> as a voice codec for asiekierka's pixmess, a C remake of 64pixels.
> It is a 1-bit-per-sample codec which uses a dynamic-strength one-pole
> low-pass filter as a predictor. Due to the fact that a raw DPFWM decoding
> creates a high-pitched whine, it is often followed by some post-processing
> filters to make the stream more listenable.

It has recently gained popularity through the ComputerCraft mod for
Minecraft, which added support for audio through this codec, as well as
the Computronics expansion which preceeded the official support. These
both implement the slightly adjusted 1a version of the codec, which is
the version I have chosen for this patch.

This patch adds a new codec (with encoding and decoding) for DFPWM1a.
The codec sources are pretty simple: they use the reference codec with
a basic wrapper to connect it to the FFmpeg AVCodec system.

To clarify, the codec does not have a specific sample rate - it is
provided by the container (or user), which is typically 48000, but has
also been known to be 32768. The codec does not specify channel info
either, and it's pretty much always used with one mono channel.
However, since it appears that libavcodec expects both sample rate and
channel count to be handled by either the codec or container, I have
made the decision to allow multiple channels interleaved, which as far
as I know has never been used, but it works fine here nevertheless. The
accompanying raw format has a channels option to set this. (I expect
most users of this will not use multiple channels, but it remains an
option just in case.)

This patch will be highly useful to ComputerCraft developers who are
working with audio, as it is the standard format for audio, and there
are few user-friendly encoders out there, and even fewer decoders. It
will streamline the process for importing and listening to audio,
replacing the need to write code or use tools that require very
specific input formats.

You may use the CraftOS-PC program (https://www.craftos-pc.cc) to test
out DFPWM playback. To use it, run the program and type this command:
"attach left speaker" Then run "speaker play <file.dfpwm>" for each file.
The app runs in a sandbox, so files have to be transferred in first;
the easiest way to do this is to simply drag the file on the window.
(Or copy files to the folder at https://www.craftos-pc.cc/docs/saves.)

Sample DFPWM files can be generated with an online tool at
https://music.madefor.cc. This is the current best way to encode DFPWM
files. Simply drag an audio file onto the page, and it will encode it,
giving a download link on the page.

I've made sure to update all of the docs as per Developer§7, and I've
tested it as per section 8. Test files encoded to DFPWM play correctly
in ComputerCraft, and other files that work in CC are correctly decoded.
I have also verified that corrupt files do not crash the decoder - this
should theoretically not be an issue as the result size is constant with
respect to the input size.

Changes since v5:
Moved channel check to init, and added sample size check in decoder.

Changes since v4:
Fixed missing channel check in decoder.

Changes since v3:
Added support for multiple interleaved channels, and cleaned up the
code a bunch.

Changes since v2:
I've found that the reference encoder has a few errors, and sounds
worse than the Java-based implementation that is used most often. I got
in contact with someone who knows DFPWM much better than I do, and I
worked with them to make a few adjustments that should improve the
audio quality. I also made sure that the output matches the Java
codec exactly, so it should have the exact same quality as other codecs.

Signed-off-by: Jack Bruienne <jackbruienne at gmail.com>
---
  Changelog                 |   1 +
  MAINTAINERS               |   1 +
  doc/general_contents.texi |   1 +
  libavcodec/Makefile       |   2 +
  libavcodec/allcodecs.c    |   2 +
  libavcodec/codec_desc.c   |   7 ++
  libavcodec/codec_id.h     |   1 +
  libavcodec/dfpwmdec.c     | 134 ++++++++++++++++++++++++++++++++++++++
  libavcodec/dfpwmenc.c     | 121 ++++++++++++++++++++++++++++++++++
  libavcodec/utils.c        |   2 +
  10 files changed, 272 insertions(+)
  create mode 100644 libavcodec/dfpwmdec.c
  create mode 100644 libavcodec/dfpwmenc.c

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