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NAME

aucat — audio/MIDI stream manipulation tool

SYNOPSIS

DESCRIPTION

The aucat utility can play, record, mix, and convert audio files. During playback, aucat reads audio data concurrently from all played files, mixes it and sends the result to the device. Similarly, during recording it duplicates audio data recorded from the device and stores it into corresponding files. It can process audio data on the fly:

• Change the sound encoding.
• Route the sound from one channel to another, join stereo or split mono.
• Control the per-file playback volume.
• Monitor the sound being played, allowing the playback mix to be recorded.

Finally, aucat can accept MIDI messages usable for:

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

The options are as follows:

-C min:max, -c min:max

The range of stream channel numbers for recording and playback directions, respectively. The default is 0:1, i.e. stereo.

-d

Increase log verbosity.

-e enc

Encoding of the playback or recording stream (see below). The default is signed, 16-bit, native byte order.

-f device

Use this sndio(7) audio device. Preceding per-device options apply to this device. Streams (-io) and control MIDI ports (-q) that are applied after will be attached to this device. Device mode and parameters are determined from streams attached to it.

-h fmt

File format of the playback or record stream (see below). The default is auto.

-i file

Add this file to the list of streams to play. If the option argument is ‘-’ then standard input will be used.

-j flag

Control whether stream channels are joined or expanded if the stream number of channels is not equal to the device number of channels. If the flag is off then stream channels are routed to the corresponding device channel, possibly discarding channels not present in the device. If the flag is on, then a single stream channel may be sent on multiple device channels, or multiple stream channels may be sent to a single device channel. For instance, this feature could be used to request mono streams to be sent on multiple outputs or to record a stereo input into a mono stream. The default is on.

-M

Create a MIDI thru box (i.e. MIDI-only pseudo device). It merges any number of MIDI inputs and broadcasts the result to any number of MIDI outputs, similarly to a hardware MIDI thru box. Only MIDI ports (-q) and MIDI files can be attached to it.

-n

Create a loopback pseudo audio device. Send input streams to the output, processing them on the fly. This pseudo-device is useful to mix, demultiplex, resample or re-encode audio files offline. It requires at least one input (-i) and one output (-o).

-o file

Add this file to the list of recording streams. If the option argument is ‘-’ then standard output will be used.

-q port

Allow audio device properties to be controlled through this MIDI port. This includes per-stream volumes and the ability to synchronously start, stop and relocate streams created in MIDI Machine Control (MMC) slave mode (-t).

-r rate

Sample rate in Hertz of the stream. The default is 48000.

-t mode

Select the way streams are controlled by MIDI Machine Control (MMC) messages. If the mode is off (the default), then streams are not affected by MMC messages. If the mode is slave, then streams are started synchronously by MMC start messages.

-v volume

Software volume attenuation of the playback stream. 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.

-w flag

Control aucat behaviour when the maximum volume of the hardware is reached and a new stream is connected. This happens only when stream volumes are not properly set using the -v option. If the flag is on, then the master volume (corresponding to the mix of all playback streams) is automatically adjusted to avoid clipping. Using off makes sense when all streams are recorded or produced with properly lowered volumes. The default is on.

-x policy

Action when the output stream cannot accept recorded data fast enough or the input stream cannot provide data to play fast enough. If the policy is ignore (the default) then samples that cannot be written are discarded and samples that cannot be read are replaced by silence. If the policy is sync then recorded samples are discarded, but the same amount of silence will be written once the stream is unblocked, in order to reach the right position in time. Similarly silence is played, but the same amount of samples will be discarded once the stream is unblocked. If the policy is error then the stream is closed permanently.

If a stream is created with the -t option, the ignore action is disabled for any stream connected to it to ensure proper synchronization.

On the command line, per-device parameters must precede the device definition (-fMn), and per-stream parameters (-Ccehjmrtvx) must precede the stream definition (-io). MIDI ports (-q) and stream definitions (-io) must follow the definition of the device (-fMn) to which they are attached.

If no audio devices (-fMn) are specified, settings are applied as if the default device is specified.

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

File formats are specified using the -h option. The following file formats are supported:

Encodings are specified using the -e option. The following encodings are supported:

MIDI CONTROL

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). Similarly, when the audio client changes its volume, the same MIDI controller message is sent out; it can be used for instance for monitoring or as feedback for motorized faders.

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

Streams created with the -t option are controlled by the following MMC messages:

MIDI control is intended to be used together with sndiod(1). 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 400 -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 -t slave -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.

EXAMPLES

Mix and play two stereo streams, the first at 48kHz and the second at 44.1kHz:

$ aucat -r 48000 -i file1.raw -r 44100 -i file2.raw

Record channels 2 and 3 into one stereo file and channels 6 and 7 into another stereo file using a 96kHz sampling rate for both:

$ aucat -j off -r 96000 -C 2:3 -o file1.raw -C 6:7 -o file2.raw

Split a stereo file into two mono files:

$ aucat -n -j off -i stereo.wav -C 0:0 -o left.wav -C 1:1 \
	-o right.wav

SEE ALSO

audioctl(1), cdio(1), mixerctl(1), sndiod(1), audio(4), sndio(7)

BUGS

Resampling is low quality; down-sampling especially should be avoided when recording.

Processing is done using 16-bit arithmetic, thus samples with more than 16 bits are rounded. 16 bits (i.e. 97dB dynamic) are largely enough for most applications though.