aucat(1) audio files manipulation tool


aucat [-dn ] [-b size ] [-c min : max ] [-e enc ] [-f device ] [-h fmt ] [-i file ] [-j flag ] [-o file ] [-q port ] [-r rate ] [-v volume ]


The aucat utility can play, record, mix, and process audio files on the fly. During playback, aucat reads audio data concurrently from all played files, mixes it and plays the result on the device. Similarly, it stores audio data recorded from the device into corresponding files. An off-line mode could be used to process audio files without involving audio hardware. Processing includes:

  • Change the sound encoding.
  • Route the sound from one channel to another.
  • Control the per-file playback volume.

Finally, aucat can accept MIDI messages usable for:

  • Volume control.
  • Start, stop and relocate playback and recording.

The options are as follows:

-b size
The buffer size of the audio device in frames. Default is 7680.
-c min : max
The range of audio file channel numbers. The default is 0:1 i.e. stereo.
Increase log verbosity.
-e enc
Encoding of the audio file. The default is s16 Encoding names use the following scheme: signedness Po s or u Pc followed by the precision in bits, the byte-order Po le or be Pc , the number of bytes per sample, and the alignment Po msb or lsb Pc . Only the signedness and the precision are mandatory. Examples: u8 , s16le , s24le3 , s24le4lsb
-f device
Use this sndio(7) audio device. Device mode and parameters are determined from audio files. Default is default
-h fmt
Audio file type. The following file types are supported:

Headerless file.
Microsoft WAV file format.
Apple's audio interchange file format.
Sun/NeXT audio file format.
Try to guess, depending on the file name. This is the default.

-i file
Play this audio file. If the option argument is `-' then standard input will be used.
-j flag
Control whether source channels are joined or expanded if they don't match the destination number of channels. If the flag is off then each source channel is routed to a single destination channel, possibly discarding channels. If the flag is on then a single source may be sent to multiple destinations and multiple sources may be mixed into a single destination. For instance, this feature could be used to convert a stereo file into a mono file mixing left and right channels together. The default is off
Off-line mode. Read input files and store the result in the output files, processing them on the fly. This mode is useful to mix, demultiplex, resample or re-encode audio files off-line. It requires at least one input (-i ) and one output (-o )
-o file
Record into this audio file. If the option argument is `-' then standard output will be used.
-q port
Control audio device properties through this MIDI port. This includes per-stream volumes and the ability to synchronously start, stop and relocate audio files.
-r rate
Sample rate in Hertz of the audio file. The default is 48000
-v volume
Software volume attenuation of the file to play. The value must be between 1 and 127, corresponding to -42dB and -0dB attenuation in 1/3dB steps. The default is 127, i.e. no attenuation.

On the command line, per-file parameters (-cehjrv ) must precede the file definition (-io )

If aucat is sent SIGHUP SIGINT or SIGTERM it terminates recording to files.


aucat can be controlled through MIDI (-q ) as follows: a MIDI channel is assigned to each stream, and the volume is changed using the standard volume controller (number 7).

The master volume can be changed using the standard master volume system exclusive message.

All audio files are controlled by the following MMC messages:

All files are relocated to the requested time position. If it is beyond the end of a file, the file is temporarly disabled until a valid position is requested.
Playback and/or recording is started.
Playback and/or recording is stopped and all files are rewound back to the starting position.

MIDI control is intended to be used together with sndiod(8). For instance, the following command will create two devices: the default snd/0 and a MMC-controlled one snd/0.mmc

$ sndiod -r 48000 -z 480 -s default -t slave -s mmc

Programs using snd/0 behave normally, while programs using snd/0.mmc wait for the MMC start signal and start synchronously. Then, the following command will play a file on the snd/0.mmc audio device, giving full control to MIDI software or hardware connected to the midithru/0 MIDI port:

$ aucat -f snd/0.mmc -q midithru/0 -i file.wav

At this stage, aucat will start, stop and relocate automatically following all user actions in the MIDI sequencer, assuming it's configured to transmit MMC on midithru/0 Furthermore, the MIDI sequencer could be configured to use the snd/0 port as MTC clock source, assured to be synchronous to playback of file.wav


Mix and play two files while recording a third file:
$ aucat -i file1.wav -i file2.wav -o file3.wav

Record channels 2 and 3 into one stereo file and channels 6 and 7 into another stereo file using a 44.1kHz sampling rate for both:

$ aucat -r 44100 -c 2:3 -o file1.wav -c 6:7 -o file2.wav

Split a stereo file into two mono files:

$ aucat -n -i stereo.wav -c 0:0 -o left.wav \
        -c 1:1 -o right.wav


Resampling is low quality.

There are limitations inherent to the wav aiff and au file formats: not all encodings are supported, file sizes are limited to 2GB, and the files must support the lseek(2) operation (e.g. pipes do not support it).