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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 is being developed on OS X 11 "Big Sur", but should work back to OS X 10.14 "Mojave" 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 components required to make this work:

  1. SoundPusherAudio — A user-space (sand-boxed) CoreAudio driver (aka AudioServerPlugin) that provides a 5.1 channel 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 SoundPusher Audio device). SoundPusherAudio.driver must be installed (copied) into the /Library/Audio/Plug-Ins/HAL directory. This driver was previously called Loopback Audio and you may have to remove the old version (called LoopbackAudio.driver) manually. Instead of this, you can also use any other suitable (6 channel, 48kHz) input device, e.g. Rogue Amoeba's Loopback.
  2. SoundPusher — An application that continuously reads audio from SoundPusher Audio (or any other suitable) device, encodes it 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 the desired combination of supported input and digital output devices (which will forward any sound output from the input device to the designated digital output). SoundPusher will set the default output device to the selected input device (e.g. SoundPusher Audio) if it was previously set to the device used for digital output.

You can override the default IOCycle timing adjustment (which attempts to postpone the encoding to as late as possible to just before the hardware needs the next packet) in case you notice sound drop-outs or overloads. The default is a safety-factor of 8. This is done via defaults write de.maven.SoundPusher IOCycleSafetyFactor -float <value>. Any value less than 1 disables IOCycle adjustment altogether, which means that CoreAudio starts preparing the next packet roughly when the current packet is sent to the hardware (which is usually a bit of a waste as current hardware can easily encode 100x faster than real-time).

Contact & Support

Please report any issues on GitHub.


FFmpeg — libavcodec, libavformat, libswresample

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

# when compiling for x86_64 on Apple Silicon (aarch64) you probably want to install yasm and add the following:
# --extra-cflags="-target x86_64-apple-macos10.14" --extra-ldflags="-target x86_64-apple-macos10.14" --arch=x86 --x86asmexe=<PATH_TO_YASM>/bin/yasm
./configure --prefix=$FFMPEG_HOME --cc=clang --extra-cflags="-fno-stack-check" --disable-static --enable-shared --disable-all --disable-autodetect --disable-programs --disable-doc --disable-everything --disable-pthreads --disable-network --disable-dct --disable-dwt --disable-lsp --disable-lzo --disable-rdft --disable-faan --disable-pixelutils --enable-avutil --enable-avcodec --enable-avformat --enable-swresample --enable-encoder=ac3 --enable-muxer=spdif
make -j8
make install
# update shared library install names
AVUTIL_ID=`otool -D libavutil.dylib | tail -1`
AVCODEC_ID=`otool -D libavcodec.dylib | tail -1`
AVFORMAT_ID=`otool -D libavformat.dylib | tail -1`
SWRESAMPLE_ID=`otool -D libswresample.dylib | tail -1`
for lib in libavutil libavcodec libavformat libswresample
    install_name_tool -id @rpath/$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

If you're building a universal binary (for Intel (x86_64) and Apple Silicon (aarch64)) you probably want to compile FFmpeg twice and then lipo -create -output <outLib> <inLibA> <inLibB> all the resulting libs together. Note that you should add FFMPEG_HOME in Xcode's Preferences -> Locations -> Custom Paths and point it towards the install directory of FFmpeg so that the Xcode project can find it.


This software is built on the experience of others:

  • SoundPusherAudio (formerly known as 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.