Introduction of the Ex-HEAAC Decoder

Ex-HEAAC (Extended HE AAC), the latest upgrade to the MPEG AAC codec family, is the audio codec of choice for digital radio and low bit rate streaming applications. Selected as the mandatory audio codec for DRM (Digital Radio Mondiale), Ex-HEAAC bridges the gap between speech and audio coding, and provides consistent high quality audio for all signal types, such as speech, music or mixed content.

Another important feature is the codec’s coding efficiency. The audio codec produces excellent sound for both music and speech, delivering high quality performance at bit rates starting as low as 6 kbit/s for mono and 12 kbit/s for stereo services. Thus Ex-HEAAC streaming apps and streaming radio players may switch to very low bit rate streams and offer a continuous playback even while the network is congested. Once more bandwidth becomes available on the network again, the Ex-HEAAC client can request a higher bitrate version and seamless switch over the full range of bitrates. Audio bitrate that’s being saved due to the improved coding efficiency can be used to improve video quality. Ex-HEAAC supports AAC and HEAAC v2 as well.

AAC is a popular audio coding technique recommended by MPEG committee. The codec handles audio signals sampled in the range of 8 kHz to 96 kHz. It operates on a frame of 1024 samples. The bit-rates supported are in the range of 8 kbps to 576 kbps per channel.

SBR and PS are the tools used in combination with the AAC general audio codec resulting in HEAAC v2 (also known as Enhanced AAC Plus). It provides significant increase in coding gain. In SBR, the high-band, i.e. the high frequency part of the spectrum is replicated using the low-band. In PS, channel redundancy is exploited and parameters are extracted from a down-mixed channel.The bit-rate is by far below the bit-rate required when using conventional AAC coding. This translates into better quality at lower bit-rates.

Introduction to Ex-HEAAC Decoder APIs

Files to be included are

• ixheaacd_apicmd_standards.h
• ixheaacd_type_def.h
• ixheaacd_memory_standards.h
• ixheaacd_error_standards.h
• ixheaacd_error_handler.h
• ixheaacd_aac_config.h

Decoder APIs

A single API is used to get and set configurations and execute the decode thread, based on command index passed.

• ia_xheaacd_dec_api

Flowchart of calling sequence

Building the Ex-HEAAC Decoder

Building for AOSP

• Makefiles for building the Ex-HEAAC decoder library is provided in root(libxaac/) folder.
• Makefiles for building the Ex-HEAAC decoder testbench is provided in test folder.
• Build the library followed by the application using the below commands: Go to root directory

$ mm

Using CMake

Users can also use cmake to build for x86, x86_64, and Windows (MSVS project) platforms.

Building for native platforms

Run the following commands to build the Ex-HEAAC Decoder for native platform:

Go to the root directory(libxaac/) of the Ex-HEAAC Decoder.
Create a new folder in the project root directory and move to the newly created folder.

$ cd <path to libxaac>
$ mkdir bin
$ cd bin
$ cmake ..
$ cmake --build .

Creating MSVS project files

To create MSVS project files for the Ex-HEAAC decoder from cmake, run the following commands:

Go to the root directory(libxaac/) of the Ex-HEAAC Decoder.
Create a new folder in the project root directory and move to the newly created folder.

$ cd <path to libxaac>
$ mkdir bin
$ cd bin
$ cmake -G "Visual Studio 15 2017" ..

The above command creates MSVS 2017 project files. If the version is different, modify the generator name accordingly. The Ex-HEAAC decoder can be built using these project files.

Running the Ex-HEAAC Decoder

The Ex-HEAAC Decoder can be run by providing command-line parameters(CLI options) directly or by providing a parameter file as a command line argument.

Command line usage :

<exceutable> -ifile:<input_file> -imeta:<meta_data_file> -ofile:<out_file> [options]

[options] can be,
[-pcmsz:<pcmwordsize>]
[-dmix:<down_mix>]
[-tostereo:<interleave_to_stereo>]
[-dsample:<down_sample_sbr>]
[-fs:<RAW_sample_rate>]
[-nosync:<disable_sync>]
[-sbrup:<auto_sbr_upsample>]
[-maxchannel:<maximum_num_channels>]
[-mp4:<mp4_flag>]

where,
  <inputfile>        	 is the input AAC file name.
  <meta_data_file> 	 	 is a text file which contains metadata for USAC files with an MP4 container.
  <outputfile>       	 is the output file name.
  <pcmwordsize>      	 is the bits per sample info. Only 16 is valid
  <down_mix> 		 	 is to enable/disable always mono output. Default 0.
  <interleave_to_stereo> is to enable/disable always interleaved to stereo output. Default 1.
  <down_sample_sbr> 	 is to enable/disable down-sampled SBR output. Default auto identification from header.
  <RAW_sample_rate> 	 is to indicate the core AAC sample rate for a RAW stream. If this is specified no other file format headers are searched for.
  <disable_sync> 		 is to disable the ADTS/ADIF sync search i.e when enabled the decoder expects the header to be at the start of input buffer. Default 0.
  <auto_sbr_upsample> 	 is to enable(1) or disable(0) auto SBR upsample in case of stream changing from SBR present to SBR not present. Default 1.
  <maximum_num_channels> is the number of maxiumum channels the input may have. Default is 6 (5.1).
  <mp4_flag> 			 is a flag that should be set to 1 when passing raw stream along with meta data text file.

Sample CLI:

<xaac_dec_exe> -ifile:in_file.aac -ofile:out_file.wav -pcmsz:16 					 

Validating the Ex-HEAAC Decoder

Conformance testing for AAC/HEAAC v1/HEAAC v2 mainly involves comparing decoder under test output with the ISO and 3GPP reference decoded output.

Testing for USAC is done using encoded streams generated using ISO USAC reference encoder. The output generated by ITTIAM USAC decoder is compared against the output generated by ISO USAC decoder for 16-bit conformance on the respective(ARMv7, ARMv8, X86_32, X86_64) platforms.