Virtual audio device, real-time encoder and SPDIF forwarder for Mac OS X
C C++ Objective-C++ Objective-C
Latest commit 74f27a5 Nov 5, 2016 @q-p Update README for building cut-down FFmpeg
macOS Sierra’s otool still prints the library header when using -X.

Newer versions of FFmpeg always link against CoreServices and
VideoDecodeAcceleration framework (all of which we don't want / need).
I haven’t found a way to avoid this other than editing `configure`.

Sound Pusher — real-time encoded audio output for OS X

Provides a virtual 5.1-channel audio output device whose contents are real-time encoded to AC3 and sent as digital SPDIF stream to a real audio device output.

WARNING: Please turn down the speaker / headphone volume before running this application, as any component misinterpreting a compressed audio stream as "normal" audio signals may result in unexpectedly loud noise.


The application has been developed on OS X 10.11 "El Capitan", but should work on OS X 10.10 "Yosemite" as well. It requires a compatible audio device capable of outputting compressed AC3 audio formats (format IDs ac-3, cac3, IAC3, iac3). All testing was done with cac3 (aka kAudioFormat60958AC3).


There are two component required to make this work:

  1. LoopbackAudio — A user-space CoreAudio driver (aka AudioServerPlugin) that provides a 5.1 output stream whose contents are mirrored ("looped back") to its own input stream. This allows applications to process the audio output of the system (if it is sent to the LoopbackAudio device). LoopbackAudio.driver must be installed (copied) into the /Library/Audio/Plug-Ins/HAL directory.
  2. SoundPusher — An application that continuously reads audio from the LoopbackAudio device, encodes in into a compressed format, and then sends that compressed stream to a (hopefully) real sound device's digital output stream. This is controlled through its menu bar extra.

Once the driver is installed and SoundPusher running, you should be able to select supported digital output devices which will forward any sound output sent to the Loopback Audio device to the designated digital output. SoundPusher will set the default output device to Loopback Audio if it was previously set to the device used for digital output.

Contact & Support

Please report any issues on GitHub.


FFmpeg — libavcodec, libavformat

Here's a script that builds a cut-down version of FFmpeg that includes all that's required for SoundPusher:

./configure --prefix=$FFMPEG_HOME --cc=clang --disable-static --enable-shared --disable-all --disable-doc --disable-everything --disable-pthreads --disable-iconv --disable-securetransport --disable-audiotoolbox --disable-videotoolbox --disable-vda --enable-avutil --enable-swresample --enable-avcodec --enable-avformat --enable-encoder=ac3 --enable-muxer=spdif
make -j8
make install
# update shared library install names
AVUTIL_ID=`otool -D libavutil.dylib | tail -1`
AVCODEC_ID=`otool -D libavcodec.dylib | tail -1`
AVFORMAT_ID=`otool -D libavformat.dylib | tail -1`
SWRESAMPLE_ID=`otool -D libswresample.dylib | tail -1`
for lib in libavutil libavcodec libavformat libswresample
    install_name_tool -id @rpath/$lib.dylib -change $AVUTIL_ID @rpath/libavutil.dylib -change $AVCODEC_ID @rpath/libavcodec.dylib -change $AVFORMAT_ID @rpath/libavformat.dylib -change $SWRESAMPLE_ID @rpath/libswresample.dylib $lib.dylib


This software is built on the experience of others:

  • LoopbackAudio is based on the NullAudio user-space driver provided by Apple as example code.
  • TPCircularBuffer by Michael Tyson is used to move audio and packet data around.
  • FFmpeg is used to encode to compressed formats and then mux those packets into an SPDIF compliant bit-stream.