Fraunhofer FDK AAC

Current AAC encoders (most to least recommended)
1	Apple AAC M/W
2	FhG AAC (Winamp) W
3	Fraunhofer FDK AAC S/L/M/W
4	Nero AAC L/W
5	FFmpeg 3.0+ AAC encoder S/L/M/W
6	FAAC S/L/M/W
7	Libav (pre-3.0 FFmpeg) AAC encoder S/L/M/W
S Source code available; L Linux; M macOS; W Windows
List of AAC encoders

The Fraunhofer FDK AAC is a high-quality open-source AAC encoder library developed by Fraunhofer IIS. It was officially released for Android, but has been ported to other platforms.

The licensed Fraunhofer AAC codec included in Winamp (often called FhG AAC) is not the same as the FDK AAC codec. While they use the same approach, they are developed by different teams, and target different platforms. The FDK library is built around fixed-point math and originally targeted low-delay communication on mobile devices.

FDK AAC is considered a favorable alternative to the Nero AAC codec, which is no longer developed.

Software Versions

Package/Component	Version	Developer/Maintainer	License	Description
FDK Encoder	4.0.0 [1]	Fraunhofer IIS	FDK License	The FDK AAC library included in Android.
FDK SBR/PS Encoder (for HE & HEv2)	4.0.0 [2]
FDK Decoder	3.0.0 [3]
FDK SBR/PS Decoder	2.2.6 [4]
fdk-aac	2.0.0 (2018-11-22) (watch) based on FDK AAC (4.0.0/3.0.0) shared library version 2.0.0	Martin Storsjö/Opencore AMR project	FDK License with additions under Apache 2.0 license	The FDK AAC encoder and decoder as a portable library separate from Android.
fdk-aac (debian)	0.1.6-1 [5] libfdk-aac0 (shared library version 1.1.0)			The Debian source package for fdk-aac. Includes libfdk-aac* and the aac-enc encoding front end.
fdkaac	1.0.0 (using libfdk-aac 0.1.6)	nu774	zlib	An advanced front-end to the FDK AAC encoder using libfdk-aac.
FFmpeg/Libav support	Libav: encode, decode FFmpeg: encode, decode	Martin Storsjö	ISC license	A wrapper for libfdk-aac that adds support to FFmpeg and Libav/avconv. It is included in both projects, and is the recommended AAC encoder for FFmpeg.

FDK License

The license included by Fraunhofer in the FDK source code specifically allows distribution in source or binary forms, but does not license patented technologies described by the source code. It goes on to say "you may use this FDK AAC Codec software or modifications thereto only for purposes that are authorized by appropriate patent licenses". As this governs use, it should not have anything to do with distribution. The Free Software Foundation (FSF) considers it fishy to have an invitation to purchase patent licenses in the text, but concedes that "any program is potentially threatened by patents". Considering the patent terms, all AAC software are indeed equally affected.

This license puts a limitation on charging for software that includes the library, leading Debian to consider it non-free. Debian does not comment on the patent situation.

The position of FFmpeg is that although the license is GPL-incompatible (and therefore nondistributable with GPL parts), it is acceptable to distribute the library with LGPL parts. FFmpeg does not care about patents. (The AAC patent license covers both encoder and decoder, so using fdk_aac does not add patent violations to FFmpeg.)

Free Software?

Party	Classification	Note
Debian	Non-free	[6][7] (fee clause)
Fedora/Red Hat	Free but not Allowed	[8]: Fedora has since adopted a more defensive posture to patent language, making it not Allowed
FSF	Free (but warns about patents)	[9]

Afterburner

Afterburner is "a type of analysis by synthesis algorithm which increases the audio quality but also the required processing power." Fraunhofer recommends to always activate this feature.

Audio Object Types

The library supports the following MPEG-2/4 AOTs:

Object Type ID	Audio Object Type	Description
2	AAC-LC	"AAC Profile" MPEG-2 Low-complexity (LC) combined with MPEG-4 Perceptual Noise Substitution (PNS)
5	HE-AAC	AAC LC + SBR (Spectral Band Replication)
29	HE-AAC v2	AAC LC + SBR + PS (Parametric Stereo)
23	AAC-LD	"Low Delay Profile" used for real-time communication
39	AAC-ELD	Enhanced Low Delay
129	MPEG-2 AAC LC
132	MPEG-2 HE-AAC (SBR)
156	MPEG-2 HE-AAC v2 (SBR+PS)

Bitrate Modes

AACENC_BITRATEMODE	Mode	Stream Bitrate
0	Constant Bitrate (CBR)	As specified by AACENC_BITRATE
1-5	Variable Bitrate (VBR)	Calculated based on channel layout (See table below)
6	Fixed frame mode.
7	Superframe mode.
8	LD/ELD full bitreservoir for packet based transmission

The bitrate limit for each variable bitrate mode. [10] HE and HEv2 will often end up with actual bitrates far below these limits.

AACENC_BITRATEMODE (VBR Modes)	Mode	Bitrate per channel (LC)		AOTs
		Mono	Stereo a
1	VBR	32 kbps	20 kbps	LC, HE, HEv2
2	VBR	40 kbps	32 kbps	LC, HE, HEv2
3	VBR	56 kbps	48 kbps	LC, HE, HEv2
4	VBR	72 kbps	64 kbps	LC
5	VBR	112 kbps	96 kbps	LC

^a Note that a "stereo" channel is any that is bonded with another channel, as noted with a plus sign in the channel layouts table.

Example Bitrate Calculations

Profile	VBR Mode	Channel layout	Expected stream bitrate
LC	3	L+R	2 "stereo" channels at 48kbps = 96kbps
LC	3	C, L+R	1 "mono" center channel at 56 kbps and 2 "stereo" channels at 48kbps = 152kbps
LC	4	C, L+R, LS+RS, LFE	1 "mono" center channel and 1 mono LFE channel each at 72kbps, and 4 "stereo" channels (2 sets of 2) each at 64kbps = 400kbps

Bandwidth

A spectrogram showing the effect of the FDK AAC low-pass filter.

The default bandwidth (or low-pass filter cutoff) for each bitrate mode will be the minimum of the appropriate value in the tables below or half the sample rate. This can be overridden, but the maximum value is 20000 Hz. [11]

The fdk-aac parameter is AACENC_BANDWIDTH. More information can be found in the official documentation, section 3.1 Bandwidth.

HE-AAC/SBR

The HE-AAC and HE-AACv2 profiles encode audio using AAC-LC at one half the sample rate, relying on Spectral Band Replication (SBR) to attempt reconstruction of the missing higher frequencies. The end result is an apparent full bandwidth transmission (as if no low-pass filter was applied), even though the actual AAC-LC encoded audio is only storing frequencies up to 1/4 the original sample rate.

VBR Modes

AACENC_BITRATEMODE	Mono	Two or More Channels
1	13050 Hz
2	13050 Hz
3	14260 Hz
4	15500 Hz
5	Full range, no filter

CBR Mode

AOT/Sample Rates	Bitrate per channel	Mono	Two or More Channels
LC / Any	Below 12kbps	3700 Hz	5000 Hz
	12-20 kbps	5000 Hz	6400 Hz
	20-28 kbps	6900 Hz	9640 Hz
	28-40 kbps	9600 Hz	13050 Hz
	40-56 kbps	12060 Hz	14260 Hz
	56-72 kbps	13950 Hz	15500 Hz
	72-96 kpbs	14200 Hz	16120 Hz
	96kbps and above	17000 Hz
...
LD / 44100 Hz	56kbps	11000 Hz	12900 Hz
	64kbps	14400 Hz	15500 Hz
...

Sample Format

The FDK library is based on fixed-point math and only supports 16-bit integer PCM input.

Sample Rates

FDK library officially supports sample rates for input of 8000, 11025, 12000, 16000, 22050, 24000, 32000, 44100, 48000, 64000, 88200, and 96000 Hz.

See Issues/GetInvInt table limit if experiencing crashes with high sample rates and VBR.

Also see Recommended Sampling Rate and Bitrate Combinations.

Channel Layouts

Channels	Layout	Mode	Description
1	C	MODE_1	Mono
2	L+R	MODE_2	Stereo
3	C, L+R	MODE_1_2
4	C, L+R, Rear	MODE_1_2_1	fdkaac calls it "C L R Cs"
5	C, L+R, LS+RS	MODE_1_2_2
5.1	C, L+R, LS+RS, LFE	MODE_1_2_2_1
7.1	C, LC+RC, L+R, LS+RS, LFE	MODE_1_2_2_2_1 MODE_7_1_FRONT_CENTER
7.1 (Rear)	C, L+R, LS+RS, Lrear+Rrear, LFE	MODE_7_1_REAR_SURROUND

The plus sign (+) denotes "stereo" channels.

Issues

GetInvInt table limit

As of FDK version 3.4.12, not all combinations of audio object types, bitrate modes, channel layouts, and sample rates can be used together, due to a limited table of pre-computed values used by the encoder.

For example, using 96kHz stereo input with the AAC-LC audio object type and bitrate mode 5 (VBR 96-112kbps/channel) will result in catastrophic failure: [12]

./libFDK/include/fixpoint_math.h:459: FIXP_DBL GetInvInt(int): Assertion `(intValue > 0) && (intValue < 50)' failed.
Aborted (core dumped)

A recent (August 2014) patch to libfdk-aac fixes most of the previously unsupported combinations [13], and is expected to be included in the next official version of the FDK AAC library.

See Libav/avconv for a workaround.

Recommended Sampling Rate and Bitrate Combinations

This table is from the documentation included in the FDK library source code. (PDF section 2.12 or source code: [14])

The following table provides an overview of recommended encoder configuration parameters which [Fraunhofer] determined by virtue of numerous listening tests.

Audio Object Type	Bit Rate Range [bit/s]	Supported Sampling Rates [kHz]	Recommended Sampling Rate [kHz]	Number of Channels
[29] HE-AAC v2 (AAC LC + SBR + PS)	8000 - 11999	22.05, 24.00	24.00	2
	12000 - 17999	32.00	32.00	2
	18000 - 39999	32.00, 44.10, 48.00	44.10	2
	40000 - 56000	32.00, 44.10, 48.00	48.00	2
[5] HE-AAC (AAC LC + SBR)	8000 - 11999	22.05, 24.00	24.00	1
	12000 - 17999	32.00	32.00	1
	18000 - 39999	32.00, 44.10, 48.00	44.10	1
	40000 - 56000	32.00, 44.10, 48.00	48.00	1
	16000 - 27999	32.00, 44.10, 48.00	32.00	2
	28000 - 63999	32.00, 44.10, 48.00	44.10	2
	64000 - 128000	32.00, 44.10, 48.00	48.00	2
[5] HE-AAC (AAC LC + SBR)	64000 - 69999	32.00, 44.10, 48.00	32.00	5, 5.1
	70000 - 159999	32.00, 44.10, 48.00	44.10	5, 5.1
	160000 - 245999	32.00, 44.10, 48.00	48.00	5
	160000 - 265999	32.00, 44.10, 48.00	48.00	5.1
[2] AAC LC	8000 - 15999	11.025, 12.00, 16.00	12.00	1
	16000 - 23999	16.00	16.00	1
	24000 - 31999	16.00, 22.05, 24.00	24.00	1
	32000 - 55999	32.00	32.00	1
	56000 - 160000	32.00, 44.10, 48.00	44.10	1
	160001 - 288000	48.00	48.00	1
[2] AAC LC	16000 - 23999	11.025, 12.00, 16.00	12.00	2
	24000 - 31999	16.00	16.00	2
	32000 - 39999	16.00, 22.05, 24.00	22.05	2
	40000 - 95999	32.00	32.00	2
	96000 - 111999	32.00, 44.10, 48.00	32.00	2
	112000 - 320001	32.00, 44.10, 48.00	44.10	2
	320002 - 576000	48.00	48.00	2
[2] AAC LC	160000 - 239999	32.00	32.00	5, 5.1
	240000 - 279999	32.00, 44.10, 48.00	32.00	5, 5.1
	280000 - 800000	32.00, 44.10, 48.00	44.10	5, 5.1

(lib)fdk-aac

Martin Storsjö (as the opencore-amr project) maintains a source code distribution of the Fraunhofer library as fdk-aac. It is distributed in a binary form in Debian (and Debian derivatives like Ubuntu) as the package fdk-aac, which includes the libfdk-aac* and aac-enc binaries.

See Software Versions for latest release information.

Links

• Source at Github
• fdk-aac at Debian package tracker. Package includes libfdk-aac* and the aac-enc binary.

aac-enc

fdk-aac includes a very, very basic command-line interface encoding utility, called aac-enc, that can encode to AAC from WAV.

Usage

aac-enc [-r bitrate] [-t aot] [-a afterburner] [-s sbr] [-v vbr] in.wav out.aac

-r <bitrate>

Bitrate in bits per seconds (for CBR). Default is 64000.

-t <aot>

The Audio Object Type. Default is 2 (AAC-LC).

-a <0,1>

Enable Afterburner. 0=Disabled, 1=Enabled (recommended). Default is 1.

-s <-1,0,1>

Spectral Band Replication (ELD AOT only). -1=Use ELD SBR auto configurator (default,recommended), 0=Disabled, 1=Enabled. Default is -1.

-v <0-5>

Bitrate mode. Only 0-5 used. 0=CBR @ value given in -r. Default is 0.

fdkaac

fdkaac is a command-line interface encoding and metadata utility. It is maintained by nu774 and is licensed under the zlib license. It employs libfdk-aac for encoding.

See Software Versions for latest release information.

Examples

# Convert a FLAC file to m4a using fdkaac configured for AAC-LC at about 50kbps/channel (100kbps for stereo).
flac -s -d -c song.flac | fdkaac --ignorelength --profile 2 --bitrate-mode 3 -o song.m4a -

Usage

fdkaac [options] input_file

-p, --profile <n>

The Audio Object Type.

-b, --bitrate <n>

Bitrate in bits per seconds (for CBR)

-m, --bitrate-mode <n>

Bitrate mode. Only 0-5 used. 0=CBR.

-w, --bandwidth <n>

Frequency bandwidth in Hz (AAC LC only)

-a, --afterburner <n>

Enable Afterburner. 0=Disabled, 1=Enabled (recommended). Default is 1.

-L, --lowdelay-sbr <-1,0,1>

Configure SBR activity on AAC ELD

-1	Use ELD SBR auto configurator
0	Disable SBR on ELD (default)
1	Enable SBR on ELD

-s, --sbr-ratio <0,1,2>

Controls activation of downsampled SBR

0	Use lib default (default)
1	Downsampled SBR (default for ELD+SBR)
2	Dual-rate SBR (default for HE-AAC)

-f, --transport-format <n>

Transport format

0	RAW (default, muxed into M4A)
1	ADIF
2	ADTS
6	LATM MCP=1
7	LATM MCP=0
10	LOAS/LATM (LATM within LOAS)

-C, --adts-crc-check

Add CRC protection on ADTS header

-h, --header-period <n>

StreamMuxConfig/PCE repetition period in transport layer

-o <filename>

Output filename

-G, --gapless-mode <n>

Encoder delay signaling for gapless playback

0	iTunSMPB (default)
1	ISO standard (edts + sgpd)
2	Both

--include-sbr-delay

Count SBR decoder delay in encoder delay. This is not iTunes compatible, but is default behavior of FDK library.

-I, --ignorelength

Ignore length of WAV header

-S, --silent

Don't print progress messages

--moov-before-mdat

Place moov box before mdat box on m4a output

Options for raw (headerless) input:

-R, --raw

Treat input as raw (by default WAV is assumed)

--raw-channels <n>

Number of channels (default: 2)

--raw-rate <n>

Sample rate (default: 44100)

--raw-format <spec>

Sample format, default is "S16L". Spec is as follows:

1st char	S(igned), U(nsigned), or F(loat)
2nd part	bits per channel
Last char	L(ittle) or B(ig)

Last char can be omitted, in which case L is assumed. Spec is case insensitive, therefore "u16b" is same as "U16B".

Up to 32-bit integer or 64-bit floating point format is supported as input. The FDK library, however, is implemented based on fixed point math and onlysupports 16-bit integer PCM. Therefore, be wary of clipping. You might want to dither/noise shape beforehand when your input has higher resolution.

Tagging options:

--tag <fcc>

<value>: Set iTunes predefined tag with four char code. See iTunes Metadata.

--tag-from-file <fcc>:<filename>

Same as above, but value is read from file.

--long-tag <name>:<value>

Set arbitrary tag as iTunes custom metadata.

--tag-from-json <filename[?dot_notation]>

Read tags from JSON. By default, tags are assumed to be direct children of the root object(dictionary). Optionally, position of the dictionary that contains tags can be specified with dotted notation.

Option/Usage	MP4 Block Modified	Comment
--title <string>	©nam
--artist <string>	©ART
--album <string>	©alb
--genre <string>	©gen	Appears to always store the string the "user-defined" ©gen even if there is an ID3 genre id that could be used with the gnre block.
--date <string>	©day	YYYY[-MM[-DD]] format
--composer <string>	©wrt
--grouping <string>	©grp
--comment <string>	©cmt
--album-artist <string>	aART
--track <number[/total]>	trkn	Block stores both track and totaltracks in one binary value
--disk <number[/total]>	disk	Block stores both disc and totaldiscs in one binary value
--tempo <n>	tmpo	Beats per minute, stored as a 16-bit integer

Links

• Source code
• Ubuntu PPA

FFmpeg

libfdk-aac can be used with FFmpeg, but requires a custom build of FFmpeg. FFmpeg provides significant documentation for using libfdk_aac in the FFmpeg wiki.

Usage/Examples

CBR mode:

ffmpeg -i <input> -c:a libfdk_aac -b:a 128k <output>

VBR mode:

ffmpeg -i <input> -c:a libfdk_aac -vbr 3 <output>

-afterburner

Enable Afterburner. 0=Disabled, 1=Enabled (recommended). Default is 1.

-profile:a

The Audio Object Type. Value is one of LC, HE-AAC, HE-AACv2, LD, or ELD. Default is LC.

-b:a

CBR bitrate

-vbr

Values 1-5. See Bitrate mode.

--cutoff

The low-pass filter cut-off in Hz. See Bandwidth for default values. FFmpeg maximum value is 20000.

Links

• libfdk-aacenc.c in FFmpeg source tree
• libfdk-aacdec.c in FFmpeg source tree

Libav/avconv

libfdk-aac can be used with Libav's avconv, but requires a custom build of avconv with "--enable-libfdk-aac" passed to configure. See Libav AAC encoding.

Usage

CBR mode:

avconv -i <input> -c:a libfdk_aac -b:a <bitrate> -afterburner 1 <output>

VBR mode:

avconv -i <input> -c:a libfdk_aac -flags +qscale -global_quality [1-5] -afterburner 1 <output>

-afterburner

See afterburner.

-global_quality

Values 1-5. See Bitrate mode.

FLAC to M4A example with quirks

Using a FLAC example with 24-bit/96kHz 5.1 channel audio, and embedded album art to demonstrate workarounds for some quirks/bugs. The sample used is from the Diatonis Free Surround Sound Music page. The track used is titled "Rock".

avconv -i diatonis-rock.flac -vn -sample_fmt s16 -ar 48000 -c:a libfdk_aac -flags +qscale -global_quality 5 diatonis-rock.m4a

-global_quality 5

Use VBR Mode 5.

-vn

Means drop all video. The FLAC source has embedded album art that can't be handled by avconv in this case. Libav apparently doesn't know how to embed cover art in M4A. It tries to use it as an MP4 video stream. Using -c:v mjpeg, as can be done with MP3, doesn't work either. See NeroAacTag for a tool that can easily add M4A album art.

-sample_fmt s16 -ar 48000

The FLAC source's 96kHz sample rate combined with VBR mode 5 triggers the GetInvInt table limit bug in libfdk_aac 0.1.3 and earlier. These options resample the audio before sending it to the FDK encoder, to avoid the crash.

Links

• libfdk-aacenc.c in Libav source tree
• libfdk-aacdec.c in Libav source tree

Links

• Official web page
• Fraunhofer FDK AAC at Wikipedia
• Release information HydrogenAudio forums
• FDK in Android source code
• fdk-aac source (github)
• fdk-aac source code (sourceforge)