[FFmpeg-devel] PATCH - libmad MP3 decoding support

["David Fletcher" <David@megapico.co.uk>]

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Please find attached a patch adding support for MP3 decoding using
libmad. This is against release ffmpeg-5.0.1, and works for libmad
0.15.1b (the most recent available).

I hope that this is useful. Some context - I found the FFmpeg inbuild
fixed point MP3 decoder produced very distorted audio for low bitrate
streams (56k/s and lower). This was on an ARMv4 CPU for which I needed
to make some adaptation to the fixed point maths routines which
currently target more recent CPUs. I was not able to resolve the
distortion issue and it was easier to integrate libmad decoding which is
already optimised for very fast decoding on the ARMv4 CPU. More info
about this application here:
http://www.megapico.co.uk/sharpfin/mediaserver.html.

Best regards, David.

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diff -Nur ./ffmpeg-5.0.1/configure ./ffmpeg-5.0.1-mad/configure
--- ./ffmpeg-5.0.1/configure	2022-04-04 15:40:22.000000000 +0100
+++ ./ffmpeg-5.0.1-mad/configure	2022-05-01 22:50:01.435432431 +0100
@@ -244,6 +244,7 @@
   --enable-libklvanc       enable Kernel Labs VANC processing [no]
   --enable-libkvazaar      enable HEVC encoding via libkvazaar [no]
   --enable-liblensfun      enable lensfun lens correction [no]
+  --enable-libmad          enable MP3 decoding via libmad [no]
   --enable-libmodplug      enable ModPlug via libmodplug [no]
   --enable-libmp3lame      enable MP3 encoding via libmp3lame [no]
   --enable-libopencore-amrnb enable AMR-NB de/encoding via libopencore-amrnb [no]
@@ -1772,6 +1773,7 @@
     frei0r
     libcdio
     libdavs2
+    libmad
     librubberband
     libvidstab
     libx264
@@ -3334,6 +3336,7 @@
 libilbc_encoder_deps="libilbc"
 libkvazaar_encoder_deps="libkvazaar"
 libmodplug_demuxer_deps="libmodplug"
+libmad_decoder_deps="libmad"
 libmp3lame_encoder_deps="libmp3lame"
 libmp3lame_encoder_select="audio_frame_queue mpegaudioheader"
 libopencore_amrnb_decoder_deps="libopencore_amrnb"
@@ -6569,6 +6572,7 @@
 fi
 
 enabled libmodplug        && require_pkg_config libmodplug libmodplug libmodplug/modplug.h ModPlug_Load
+enabled libmad            && require libmad "mad.h" mad_decoder_init -lmad
 enabled libmp3lame        && require "libmp3lame >= 3.98.3" lame/lame.h lame_set_VBR_quality -lmp3lame $libm_extralibs
 enabled libmysofa         && { check_pkg_config libmysofa libmysofa mysofa.h mysofa_neighborhood_init_withstepdefine ||
                                require libmysofa mysofa.h mysofa_neighborhood_init_withstepdefine -lmysofa $zlib_extralibs; }
diff -Nur ./ffmpeg-5.0.1/libavcodec/Makefile ./ffmpeg-5.0.1-mad/libavcodec/Makefile
--- ./ffmpeg-5.0.1/libavcodec/Makefile	2022-01-14 18:45:39.000000000 +0000
+++ ./ffmpeg-5.0.1-mad/libavcodec/Makefile	2022-04-06 22:56:06.000000000 +0100
@@ -1054,6 +1054,7 @@
 OBJS-$(CONFIG_LIBILBC_DECODER)            += libilbc.o
 OBJS-$(CONFIG_LIBILBC_ENCODER)            += libilbc.o
 OBJS-$(CONFIG_LIBKVAZAAR_ENCODER)         += libkvazaar.o
+OBJS-$(CONFIG_LIBMAD_DECODER)             += libmaddec.o
 OBJS-$(CONFIG_LIBMP3LAME_ENCODER)         += libmp3lame.o
 OBJS-$(CONFIG_LIBOPENCORE_AMRNB_DECODER)  += libopencore-amr.o
 OBJS-$(CONFIG_LIBOPENCORE_AMRNB_ENCODER)  += libopencore-amr.o
diff -Nur ./ffmpeg-5.0.1/libavcodec/allcodecs.c ./ffmpeg-5.0.1-mad/libavcodec/allcodecs.c
--- ./ffmpeg-5.0.1/libavcodec/allcodecs.c	2022-01-14 18:45:39.000000000 +0000
+++ ./ffmpeg-5.0.1-mad/libavcodec/allcodecs.c	2022-04-06 22:54:45.000000000 +0100
@@ -744,6 +744,7 @@
 extern const AVCodec ff_libgsm_ms_decoder;
 extern const AVCodec ff_libilbc_encoder;
 extern const AVCodec ff_libilbc_decoder;
+extern const AVCodec ff_libmad_decoder;
 extern const AVCodec ff_libmp3lame_encoder;
 extern const AVCodec ff_libopencore_amrnb_encoder;
 extern const AVCodec ff_libopencore_amrnb_decoder;
diff -Nur ./ffmpeg-5.0.1/libavcodec/codec_id.h ./ffmpeg-5.0.1-mad/libavcodec/codec_id.h
--- ./ffmpeg-5.0.1/libavcodec/codec_id.h	2022-01-14 18:45:39.000000000 +0000
+++ ./ffmpeg-5.0.1-mad/libavcodec/codec_id.h	2022-04-06 22:52:18.000000000 +0100
@@ -516,6 +516,7 @@
     AV_CODEC_ID_HCA,
     AV_CODEC_ID_FASTAUDIO,
     AV_CODEC_ID_MSNSIREN,
+    AV_CODEC_ID_LIBMAD,
 
     /* subtitle codecs */
     AV_CODEC_ID_FIRST_SUBTITLE = 0x17000,          ///< A dummy ID pointing at the start of subtitle codecs.
diff -Nur ./ffmpeg-5.0.1/libavcodec/libmaddec.c ./ffmpeg-5.0.1-mad/libavcodec/libmaddec.c
--- ./ffmpeg-5.0.1/libavcodec/libmaddec.c	1970-01-01 01:00:00.000000000 +0100
+++ ./ffmpeg-5.0.1-mad/libavcodec/libmaddec.c	2022-04-12 19:00:21.000000000 +0100
@@ -0,0 +1,181 @@
+/*
+ * MP3 decoder using libmad
+ * Copyright (c) 2022 David Fletcher
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <mad.h>
+
+#include "libavutil/channel_layout.h"
+#include "libavutil/common.h"
+#include "avcodec.h"
+#include "internal.h"
+#include "decode.h"
+
+#define MAD_BUFSIZE (32 * 1024)
+#define MIN(a, b) ((a) < (b) ? (a) : (b))
+
+typedef struct libmad_context {
+  uint8_t input_buffer[MAD_BUFSIZE+MAD_BUFFER_GUARD];
+  struct mad_synth  synth; 
+  struct mad_stream stream;
+  struct mad_frame  frame;
+  struct mad_header header;
+  int got_header;
+}libmad_context;		
+
+/* utility to scale and round samples to 16 bits */
+static inline signed int mad_scale(mad_fixed_t sample)
+{
+  /* round */
+  sample += (1L << (MAD_F_FRACBITS - 16));
+  
+  /* clip */
+  if (sample >= MAD_F_ONE)
+    sample = MAD_F_ONE - 1;
+  else if (sample < -MAD_F_ONE)
+    sample = -MAD_F_ONE;
+  
+  /* quantize */
+  return sample >> (MAD_F_FRACBITS + 1 - 16);
+}
+
+static av_cold int libmad_decode_init(AVCodecContext *avc)
+{
+    libmad_context *mad = avc->priv_data;
+
+    mad_synth_init  (&mad->synth);
+    mad_stream_init (&mad->stream);
+    mad_frame_init  (&mad->frame);
+    mad->got_header = 0;
+
+    return 0;
+}
+
+static av_cold int libmad_decode_close(AVCodecContext *avc)
+{
+    libmad_context *mad = avc->priv_data;
+
+    mad_synth_finish(&mad->synth);
+    mad_frame_finish(&mad->frame);
+    mad_stream_finish(&mad->stream);
+
+    mad = NULL;
+    
+    return 0;
+}
+
+static int libmad_decode_frame(AVCodecContext *avc, void *data,
+                          int *got_frame_ptr, AVPacket *pkt)
+{
+  AVFrame *frame = data;
+  libmad_context *mad = avc->priv_data;
+  struct mad_pcm *pcm;
+  mad_fixed_t const *left_ch;
+  mad_fixed_t const *right_ch;
+  int16_t *output;
+  int nsamples;
+  int nchannels;
+  size_t bytes_read = 0;
+  size_t remaining = 0;
+  
+  if (!avc)
+    return 0;
+    
+  if (!mad)
+    return 0;
+      
+  remaining = mad->stream.bufend - mad->stream.next_frame;
+  memmove(mad->input_buffer, mad->stream.next_frame, remaining);
+  bytes_read = MIN(pkt->size, MAD_BUFSIZE - remaining);
+  memcpy(mad->input_buffer+remaining, pkt->data, bytes_read);
+  
+  if (bytes_read == 0){
+    *got_frame_ptr = 0;
+    return 0;
+  }
+  
+  mad_stream_buffer(&mad->stream, mad->input_buffer, remaining + bytes_read);
+  mad->stream.error = 0;
+  
+  if(!mad->got_header){
+    mad_header_decode(&mad->header, &mad->stream);
+    mad->got_header = 1;
+    avc->frame_size = 32 * (mad->header.layer == MAD_LAYER_I ? 12 :	\
+			    ((mad->header.layer == MAD_LAYER_III &&	\
+			      (mad->header.flags & MAD_FLAG_LSF_EXT)) ? 18 : 36));
+    avc->sample_fmt = AV_SAMPLE_FMT_S16;
+    if(mad->header.mode == MAD_MODE_SINGLE_CHANNEL){
+      avc->channel_layout = AV_CH_LAYOUT_MONO;
+      avc->channels = 1;
+    }else{
+      avc->channel_layout = AV_CH_LAYOUT_STEREO;
+      avc->channels = 2;
+    }
+  }
+
+  frame->channel_layout = avc->channel_layout;
+  frame->format = avc->sample_fmt;
+  frame->channels = avc->channels;
+  frame->nb_samples = avc->frame_size; 
+    
+  if ((ff_get_buffer(avc, frame, 0)) < 0)
+    return 0;
+ 
+  if (mad_frame_decode(&mad->frame, &mad->stream) == -1) {
+    *got_frame_ptr = 0;
+    return mad->stream.bufend - mad->stream.next_frame;
+  }
+
+  mad_synth_frame (&mad->synth, &mad->frame);
+  
+  pcm = &mad->synth.pcm;
+  output = (int16_t *)frame->data[0];
+  nsamples = pcm->length;
+  nchannels = pcm->channels;
+  left_ch = pcm->samples[0];
+  right_ch = pcm->samples[1];
+  while (nsamples--) {
+    *output++ = mad_scale(*(left_ch++));
+    if (nchannels == 2) {
+      *output++ = mad_scale(*(right_ch++));
+    }
+    //Players should recognise mono and play through both channels
+    //Writing the same thing to both left and right channels here causes
+    //memory issues as it creates double the number of samples allocated.
+  }
+  
+  *got_frame_ptr = 1;
+ 
+  return mad->stream.bufend - mad->stream.next_frame;
+}
+
+AVCodec ff_libmad_decoder = {
+    .name           = "libmad",
+    .long_name      = NULL_IF_CONFIG_SMALL("libmad MP3 decoder"),
+    .wrapper_name   = "libmad",
+    .type           = AVMEDIA_TYPE_AUDIO,
+    .id             = AV_CODEC_ID_MP3,
+    .sample_fmts    = (const enum AVSampleFormat[]) { AV_SAMPLE_FMT_S16, AV_SAMPLE_FMT_NONE },
+    .capabilities   = AV_CODEC_CAP_DR1 | AV_CODEC_CAP_CHANNEL_CONF,
+    .priv_data_size = sizeof(libmad_context),
+    .init           = libmad_decode_init,
+    .close          = libmad_decode_close,
+    .decode         = libmad_decode_frame
+};
+

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