Unable to decode Fraunhofer FDK AAC -encoded files

[davidgrisham]

Hi!

I'm working on integrating uaac.h as an AAC decoder for miniaudio, but I'm having issues getting uaac to decode my AAC files in the first place. I'm using ffmpeg and libfdk_aac (Fraunhofer FDK AAC) to encode my files. I've tested with your aplay+ program and am getting the same issue.

I have two files in the following test:

• output.m4a was encoded with the default ffmpeg/libfdk_aac settings
• output-moov.m4a was encoded the same way, but I passed the -movflags +faststart options so that the moov atom would be at the start of the file (since uaac wasn't able to find it)

$ ./aplay+ files/

/home/grish/scratch/aplay-/files/output.m4a
[moov] is not found!
[trak] is not found!
[mdia] is not found!
[hdlr] is not found!
type:4d344120/736f756e 0 fffffff8
Error: cannot read AAC data

/home/grish/scratch/aplay-/files/output-moov.m4a
[mdat] is not found!
Error: cannot read AAC data

I've been playing around with the uaac_extract_aac function, but am not well-versed in decoding audio files. Curious if you have any ideas here, or at least a sense of how much work it would be to get this working. I have a rather large audio library that's encoded in the same way, so I'm hoping to get this working without modifying the audio files.

[davidgrisham]

I've now gotten aplay+ to output very clicky/noisy audio, and I can sort of hear the actual song underneath. I updated the uaac_extract_aac function to:

uint8_t *uaac_extract_aac(int fd, int *len, int *samplerate, int *channels)
{
	_ATOM ftyp = uaac_findMp4Atom("ftyp", 0, 0, fd);
	if (!ftyp.size) return 0; // no mp4/m4a file
	_ATOM free = uaac_findMp4Atom("free", ftyp.pos + ftyp.size, 1, fd);
	if (!free.size) return 0; // no mp4/m4a file

	_ATOM mdat = uaac_findMp4Atom("mdat", free.pos + free.size, 1, fd);
	printf("mdat.size: %d\n", mdat.size);
	*len = mdat.size;
	uint8_t *data = (uint8_t*)malloc(mdat.size);
	if (!data) return 0; // memory error!

	ssize_t bytes_read = read(fd, data, mdat.size);
	printf("bytes_read: %zd\n", bytes_read);

	_ATOM moov = uaac_findMp4Atom("moov", mdat.pos + mdat.size, 0, fd);
	_ATOM trak = uaac_findMp4Atom("trak", moov.pos + 8, 1, fd);
	_ATOM mdia = uaac_findMp4Atom("mdia", trak.pos + 8, 1, fd);
	_ATOM mdhd = uaac_findMp4Atom("mdhd", mdia.pos + 8, 1, fd);
	_ATOM hdlr = uaac_findMp4Atom("hdlr", mdhd.pos + mdhd.size, 1, fd);
	_ATOM minf = uaac_findMp4Atom("minf", hdlr.pos + hdlr.size, 1, fd);
	_ATOM stbl = uaac_findMp4Atom("stbl", minf.pos + 8, 1, fd);
	_ATOM stsd = uaac_findMp4Atom("stsd", stbl.pos + 8, 1, fd);
	if (!stsd.size) return 0; // something is not ok

	*channels = uaac_read16(stsd.pos + 8 + 0x20, fd);
	*samplerate = uaac_read32(stsd.pos + 8 + 0x26, fd);

	printf("channels: %d\n", *channels);
	printf("samplerate: %d\n", *samplerate);

	return data;
}

And after recompiling + running aplay+ the terminal output is:

$ ./aplay+ -d 'hw:2,0' files/

/home/grish/scratch/aplay-/files/output.m4a
mdat.size: 7727795
bytes_read: 7727795
channels: 2
samplerate: 48000
48000Hz 2ch

In case it's helpful, here's the data ffmpeg extracts from the same audio file:

$ ffprobe -i output.m4a
ffprobe version n6.0 Copyright (c) 2007-2023 the FFmpeg developers
  built with gcc 13.2.1 (GCC) 20230801
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-amf --enable-avisynth --enable-cuda-llvm --enable-lto --enable-fontconfig --enable-gmp --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --enable-libjxl --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librav1e --enable-librsvg --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpl --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-libzimg --enable-nvdec --enable-nvenc --enable-opencl --enable-opengl --enable-openssl --enable-shared --enable-vaapi --enable-version3 --enable-vulkan --enable-libfdk_aac --enable-nonfree --disable-htmlpages
  libavutil      58.  2.100 / 58.  2.100
  libavcodec     60.  3.100 / 60.  3.100
  libavformat    60.  3.100 / 60.  3.100
  libavdevice    60.  1.100 / 60.  1.100
  libavfilter     9.  3.100 /  9.  3.100
  libswscale      7.  1.100 /  7.  1.100
  libswresample   4. 10.100 /  4. 10.100
  libpostproc    57.  1.100 / 57.  1.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'output.m4a':
  Metadata:
    major_brand     : M4A
    minor_version   : 512
    compatible_brands: M4A isomiso2
    title           : Irresistible
    artist          : Deafheaven
    composer        : George Clarke/Kerry McCoy
    album           : Sunbather (10th Anniversary Remix / Remaster)
    genre           : Black Metal
    track           : 2
    disc            : 0
    date            : 2023
    compilation     : 0
    encoder         : Lavf60.3.100
    ORIGINAL YEAR   : 0000
    lyrics          :
    sort_album_artist:
    sort_artist     :
    sort_composer   :
    grouping        :
    comment         : ~ N ~
    album_artist    : Deafheaven
  Duration: 00:03:13.15, start: 0.000000, bitrate: 321 kb/s
  Stream #0:0[0x1](und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 320 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
      vendor_id       : [0][0][0][0]

[yui0]

Thank you for your useful information!