[FFmpeg-devel] [PATCH v6 1/2] libavcodec: Added DFPWM1a codec

[Tomas Härdin]

tor 2022-03-03 klockan 10:44 -0500 skrev 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.

This sounds similar to something I wrote for the Atari 2600 a number of
years ago ( https://www.pouet.net/prod.php?which=59283 )

I found an encoder online for DFPWM, and it seems to suffer from the
same "beeping" that my debeeping hack fixes (suppressing 0xAA and 0x55
bytes). Perhaps a similar hack could be useful in dfpwmenc.c

> 
> 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.

>From experience it's usually better to be strict when it comes to stuff
like this. The ComputerCraft people should work out a standard for
this, preferably a container. We've had a similar problem in FreeDV
where which codec was being used was implicit most of the time, which
has been resolved with the .c2 format.

> ---
>   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 +
>   libavcodec/version.h      |   4 +-
>   11 files changed, 274 insertions(+), 2 deletions(-)
>   create mode 100644 libavcodec/dfpwmdec.c
>   create mode 100644 libavcodec/dfpwmenc.c

Patch doesn't apply on current master (e645a1d)

/Tomas