audio files manipulation
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
mode could be used to process audio files without involving audio hardware.
- Change the sound encoding.
- Route the sound from one channel to another.
- Control the per-file playback volume.
aucat can accept MIDI messages
- Volume control.
- Start, stop and relocate playback and recording.
The options are as follows:
- The buffer size of the audio device in frames. Default is 7680.
- The range of audio file channel numbers. The default is
0:1, i.e. stereo.
- Increase log verbosity.
- Encoding of the audio file. The default is s16. Encoding names use the following scheme: signedness (s or u) followed by the precision in bits, the byte-order (le or be), the number of bytes per sample, and the alignment (msb or lsb). Only the signedness and the precision are mandatory. Examples: u8, s16le, s24le3, s24le4lsb.
- Use this sndio(7) audio device. Device mode and parameters are determined from audio files. Default is default.
- Go to the given time position and start playback or recording there. This option is equivalent to an incoming MMC relocate message with the same position. The position is expressed as the number of samples (at device sample rate).
- 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.
- Play this audio file. If the option argument is ‘-’ then standard input will be used.
- 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-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 (
- Record into this audio file. If the option argument is ‘-’ then standard output will be used.
- Time offset where the beginning of the file belongs. The first sample of the file will be played or recorded when the device reaches the given postion. The position is expressed as the number of samples (at device sample rate).
- Control audio device properties through this MIDI port. This includes per-stream volumes and the ability to synchronously start, stop and relocate audio files.
- Sample rate in Hertz of the audio file. The default is
- 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
aucat is sent
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
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
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
audioctl(1), cdio(1), mixerctl(1), audio(4), sndio(7), sndiod(8)
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).