exhale

exhale, which is an acronym for "Ecodis eXtended High-efficiency And Low-complexity Encoder", is a lightweight library and application to encode uncompressed WAVE-format audio files into MPEG-4-format files complying with the ISO/IEC 23003-3 (MPEG-D) Unified Speech and Audio Coding (USAC, also known as Extended High-Efficiency AAC) standard.

In addition, exhale writes program peak-level and loudness data into the generated MPEG-4 files according to the ISO/IEC 23003-4, Dynamic Range Control (DRC) specification for use by decoders providing DRC.

exhale currently makes use of all frequency-domain (FD) coding tools in the scalefactor based MDCT processing path. Its objective is high quality mono, stereo, and multichannel coding at medium and high bit rates, so the lower-rate USAC coding tools (ACELP, TCX, Enhanced SBR and MPEG Surround with Unified Stereo coding) won't be integrated.

Important: Due to the missing lower-rate coding tools, the audio quality at the lowest of exhale's bit-rate modes (18 kbit/s mono, 36 kbit/s stereo) doesn't reflect the full capabilities of the Extended HE-AAC standard. Therefore, use the lowest bit-rate modes only when required. Also, please don't attempt to modify exhale's source code or to configure the command-line encoder to produce lower bit-rates. Use only existing presets and input sampling rates of 32...48 kHz.

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Copyright

(c) 2024 Christian R. Helmrich, project ecodis. All rights reserved.

License

exhale is being made available under an open-source license which is based on the 3-clause BSD license but modified to address particular aspects dictated by the nature and the output of this application.

The license text and release notes for the current version 1.2.1 can be found in the include subdirectory of the exhale distribution.

Compilation

This section describes how to compile the exhale source code into an executable application under Linux and Microsoft Windows. The binary application files will show up in a newly created bin subdirectory of the exhale distribution directory and/or a subdirectory thereof.

Note that, for advanced use cases, cmake files are provided as well. See https://gitlab.com/ecodis/exhale/-/merge_requests/2 for details.

Linux and MacOS (GNU Compiler Collection, gcc):

In a terminal, change to the exhale distribution directory and enter

make release

to build a release-mode executable with the default (usually 64-bit) configuration. A 32-bit debug-mode executable can be built by typing

make BUILD32=1 debug

Microsoft Windows (Visual Studio 2012 and later):

Doubleclick the exhale_vs2012.sln file to open the project in Visual Studio. Once it's loaded, rightclick on exhaleApp in the "Solution Explorer" window on the right-hand side, then select Set as StartUp Project. Now simply press F7 to build the solution in debug mode.

To change the debugging command, rightclick again on exhaleApp and select Properties. In the newly opened window click on Debugging under "Configuration Properties" on the left-hand side. Then you can edit the "Command Arguments" entry on the right-hand side as needed.

For fastest encoding speed, please select Release and x64 before building the solution. This will create a release-mode 64-bit binary. If you would like to build a dynamically linked library (DLL) of the exhale source instead of an application binary, select Release DLL instead of Release, rightclick on exhaleLib, and select Build.

Usage

This section describes how to run the exhale application either from the command-line or using a third-party software providing WAVE data to exhale's standard input pipe (stdin), such as foobar2000. See the Wiki at https://gitlab.com/ecodis/exhale/-/wikis/home for more info.

Standalone (command-line):

In a terminal, change to exhale's bin subdirectory and then enter

./exhale (on Linux and MacOS) or exhale.exe (on Windows)

to print out usage information. As an example, the following command

exhale.exe 5 C:\Music\Input.wav C:\Music\Output.m4a

converts file Input.wav to file Output.m4a at roughly 128 kbit/s (if the input signal is 2-channel stereo) and in Extended HE-AAC format.

There is also an expert mode providing two additional arguments:

exhale.exe b s 42 C:\Music\Input.wav C:\Music\Output.m4a

e.g. encodes Input.wav to Output.m4a at roughly 48 kbit/s stereo and with SBR enabled, seamless operation (s forces media time 0 in the edit list), and an independent frame interval of 42 (range 10...99).

Third-party stdin (foobar2000):

After downloading from www.foobar2000.org and starting the software, load the desired input audio files into the playlist. Mark all files to be converted, rightclick on one of them, and select Convert -> .... In the newly opened window click on Output format. Once the window content changed, double-click on entry AAC (exhale) and set up the conversion. If that entry does not exist, click on Add New, select Custom under "Encoder" and enter the following information:

• Encoder file: exhale.exe (including path to the executable)
• Extension: m4a
• Parameters: # %d (where # is the bit-rate mode, i.e., 0...9 when SBR is disabled, or a...g when SBR is enabled)
• Format is: lossy
• Highest BPS mode supported: 24 (or 32, doesn't matter much)
• Encoder name: Extended HE-AAC (exhale)
• Bitrate (kbps): (depends on bit-rate mode, see Usage above)
• Settings: CVBR mode # (where # equals that in Parameters)

Then click on OK and on Back and, in the first "Converter Setup" window, on Other and ensure the "Transfer..." box for the class of input metadata that you wish to copy to the output files is checked. Now set the destination settings as desired and click on Convert.

Development

If you are interested in contributing to exhale, please email one of the developers. Merge requests with fixes and/or speedups are highly appreciated.