Re: [FFmpeg-devel] PATCH - libmad MP3 decoding support

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

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Following today's posts about help with submitting patches I realised I
sent the libmad patch yesterday in the wrong format. Apologies, I was
not familiar with the git format patches.

Hopefully the attached version is now in the correct format against the
current master branch.

The bug report about why this exists is at the following link, including
a link to sample distorted audio from decoding an mp3 stream on ARMv4
hardware: https://trac.ffmpeg.org/ticket/9764

Best regards, David.

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From c7012e7743d245bfc737bd921341a462cd5c843c Mon Sep 17 00:00:00 2001
From: David Fletcher <david@megapico.co.uk>
Date: Tue, 3 May 2022 19:20:23 +0100
Subject: [PATCH] Patched to work with libmad MP3 decoding codec

Signed-off-by: David Fletcher <david@megapico.co.uk>
---
 configure              |   4 +
 libavcodec/Makefile    |   1 +
 libavcodec/allcodecs.c |   1 +
 libavcodec/codec_id.h  |   1 +
 libavcodec/libmaddec.c | 181 +++++++++++++++++++++++++++++++++++++++++
 5 files changed, 188 insertions(+)
 create mode 100644 libavcodec/libmaddec.c

diff --git a/configure b/configure
index 196873c4aa..d5f5b79614 100755
--- a/configure
+++ b/configure
@@ -245,6 +245,7 @@ External library support:
   --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]
@@ -1775,6 +1776,7 @@ EXTERNAL_LIBRARY_GPL_LIST="
     frei0r
     libcdio
     libdavs2
+    libmad
     librubberband
     libvidstab
     libx264
@@ -3339,6 +3341,7 @@ libjxl_decoder_deps="libjxl libjxl_threads"
 libjxl_encoder_deps="libjxl libjxl_threads"
 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"
@@ -6572,6 +6575,7 @@ if enabled libmfx; then
 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 $zlib_extralibs; }
diff --git a/libavcodec/Makefile b/libavcodec/Makefile
index cfaa6f196a..1e434222d9 100644
--- a/libavcodec/Makefile
+++ b/libavcodec/Makefile
@@ -1065,6 +1065,7 @@ OBJS-$(CONFIG_LIBILBC_ENCODER)            += libilbc.o
 OBJS-$(CONFIG_LIBJXL_DECODER)             += libjxldec.o libjxl.o
 OBJS-$(CONFIG_LIBJXL_ENCODER)             += libjxlenc.o libjxl.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 --git a/libavcodec/allcodecs.c b/libavcodec/allcodecs.c
index c47133aa18..e3df6178c8 100644
--- a/libavcodec/allcodecs.c
+++ b/libavcodec/allcodecs.c
@@ -744,6 +744,7 @@ extern const FFCodec ff_libcodec2_decoder;
 extern const FFCodec ff_libdav1d_decoder;
 extern const FFCodec ff_libdavs2_decoder;
 extern const FFCodec ff_libfdk_aac_encoder;
+extern const AVCodec ff_libmad_decoder;
 extern const FFCodec ff_libfdk_aac_decoder;
 extern const FFCodec ff_libgsm_encoder;
 extern const FFCodec ff_libgsm_decoder;
diff --git a/libavcodec/codec_id.h b/libavcodec/codec_id.h
index 8b317fa121..be70f4a71c 100644
--- a/libavcodec/codec_id.h
+++ b/libavcodec/codec_id.h
@@ -519,6 +519,7 @@ enum AVCodecID {
     AV_CODEC_ID_FASTAUDIO,
     AV_CODEC_ID_MSNSIREN,
     AV_CODEC_ID_DFPWM,
+    AV_CODEC_ID_LIBMAD,
 
     /* subtitle codecs */
     AV_CODEC_ID_FIRST_SUBTITLE = 0x17000,          ///< A dummy ID pointing at the start of subtitle codecs.
diff --git a/libavcodec/libmaddec.c b/libavcodec/libmaddec.c
new file mode 100644
index 0000000000..7082c53f4d
--- /dev/null
+++ b/libavcodec/libmaddec.c
@@ -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
+};
+
-- 
2.17.6


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