cl mixed


Bindings to libmixed, a sound mixing and processing library.

About cl-mixed

This is a bindings library to libmixed, an audio mixing and processing library.

How To

Precompiled versions of the underlying library are included in this for most common system combinations. If you want to build it manually however, refer to the libmixed repository.

Examples on how to use cl-mixed can be found in the examples directory. This also includes an example on how to create a custom audio processing element in pure lisp.

cl-mixed also ships a variety of extension systems providing segments that can be used for playing back audio on various sound systems, or reading audio from various formats.

Fully implemented extensions:

  • Alsa

  • CoreAudio

  • Flac

  • Mpg123

  • Ogg/vorbis

  • Openmpt

  • Oss

  • Out123

  • PulseAudio

  • Wasapi

  • Wave

Basic Concepts

This library is a wrapper around libmixed. As such, most of the functionality is inherent to the underlying library. However, for convenience, we'll explain the basic concepts here.


Since we're dealing with a foreign library, we need an interface to manage the shared memory. This is done via c-object, a base class for everything in cl-mixed.

When an instance is created, allocate-handle is called to manage the allocation and initialisation of the foreign data. The resulting pointer is retained in the handle slot, and registered in a reverse table that allows retrieving the Lisp object from the handle pointer.

Once the object is no longer needed, free must be called on it to clean up the foreign memory. It is safe to call free repeatedly. Once freed, the object should not be used for anything else anymore.

For dynamic-extent use of c-objects, with-objects may be used, which ensures the cleanup.


A container for raw audio samples. Each buffer represents one "channel" of audio signals, encoded as 32-bit floating point numbers in the range [-1,+1]. A buffer has a maximum number of samples it can store, and keeps track of read and write heads.

Internally, buffers are implemented as lock-free bipartite buffers, meaning that whenever you request a write or read, you will always get a consecutive region of memory, and you can write and read from it in parallel. Note however, that only one simultaneous reader and writer are supported.

To start a transaction use request-read or request-write, which will give you a start and end index to use. Once your transaction is completed, use finish-read or finish-write and pass the number of samples that were actually consumed. If you want to abort a transaction, you must still call the finish function, but should simply pass 0 for the number of processed elements.

You can also query the available space with available-read and available-write, and perform transaction more conveniently with with-buffer-tx, or a sample-wise transfer from one buffer to another with with-buffer-transfer.

Buffers can be resized simply using (setf size), and cleared using clear.


Note that there's two types of buffers in libmixed, the regular buffer as explained above that carries samples, and packs, which carry encoded audio frames in a byte buffer. Both use the same interface to read/write, but beware that when dealing with a pack you must appropriately encode and decode the samples from the packed byte frames.

In addition to the bip-buffer content, a pack also holds data about the representation of the audio data, such as the sample encoding, channels, framesize, and samplerate. You can conveniently transfer pack data from/to buffers using transfer.

Also see the unpacker and packer segments, which efficiently handle the encoding and decoding operations, including resampling.


A segment is a representation of some audio processing unit. A segment may take samples from a number of input buffers, and may put samples into a number of output buffers. Segments represent the base part of libmixed and have a standardised interface to deal with their buffers, parameters, and metadata.

You can retrieve information about the available fields, outputs, inputs, etc. using info. Connected inputs and outputs can also be fetched, as long as they were connected using the appropriate Lisp functions ((setf input), (setf output)). Note: libmixed does not offer a way for us to get notified when foreign code changes fields, and so cl-mixed will miss buffer connections when they happen outside of lisp-land.

Fields in general can be accessed with input-field, output-field, and field. Please consult the respective field descriptions in the info to determine when it is safe to change these fields, and which values are permitted.

Once a segment has been properly connected, it can be started to ready it for use. After that, mix can be called repeatedly to process audio samples. Once the segment is no longer used, or when specific parameters need to be reconfigured, end should be called to park the segment.


Libmixed offers a full reflection API, including the listing of supported segments, which may also be contributed by outside libraries not directly known to cl-mixed. You can list all available segment types using list-segments and construct an instance using make-generic-segment.


A mixer is a special kind of segment that takes an arbitrary number of input buffers and mixes them together in particular ways to produce a fixed set of output buffers. libmixed offers three types of mixers out of the box: basic-mixer, plane-mixer, and space-mixer.


A source is a special kind of segment that does not have any output or input buffers, only a connected pack that it writes raw audio frames into. Sources also offer special methods to handle the audio input stream, such as byte-position, frame-position, frame-count, channel-order, and seek.

All audio sources that produce samples from an audio file or similar will be a subclass of source.


Analogous to sources, a drain does not have any input or output buffers, only a pack that it reads raw audio frames from. Drains are used to output audio data to a file or playback device. They similarly support the channel-order hint.

A special subclass of drain, device-drain further allows discovery of available playback devices using list-devices and the selection of a specific device either through the :device initarg, or using the device accessor. The format of the device argument is typically a string, but is dependent on the type of drain used.


Thanks to libmixed's standardised segment structure, we can also create segments from Lisp that integrate with any other part that consumes the libmixed API. To do so, create a subclass of virtual and implement methods as needed on start, mix, end, etc.

This is especially handy for experimentation with audio data, as we can quickly update and change processing functions. All of cl-mixed's extensions are implemented in part through a virtual segment.

See Also

  • libmixed The underlying C library implementing the high-performance processing functionality and base API.

  • Harmony A capable sound server implemented on top of cl-mixed, which offers a more convenient interface to manage playback and construct mixing pipelines.

System Information

Nicolas Hafner

Definition Index