---
name: "virtual_oss"
section: "8"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/virtual_oss"
---

# virtual\_oss(8)

daemon to multiplex and demultiplex an OSS device

## NAME

**virtual\_oss** – daemon to multiplex and demultiplex an OSS device

## SYNOPSIS

**virtual\_oss** \[**-h**\]

## DESCRIPTION

**virtual\_oss** is an audio mixing application that multiplexes and demultiplexes a single OSS device into multiple customizable OSS compatible devices using character devices from userspace. These devices can be used to record played back audio and mix the individual channels in multiple ways.

**virtual\_oss** requires the [cuse(3)](/cuse/3.md) kernel module. To load the driver as a module at boot time, place the following line in [loader.conf(5)](/loader.conf/5.md):

       cuse\_load="YES"

All channel numbers start at zero. Left channel is zero and right channel is one.

The following options are available:

- **-B**

  Run program in background.

- **-S**

  Enable automatic DSP rate resampling.

- **-Q** *quality*

  Set resampling quality: 0=best, 1=medium and 2=fastest (default).

- **-b** *bits*

  Set sample depth to *bits* for the subsequent commands. Valid values are 8, 16, 24 and 32.

- **-r** *rate*

  Set default sample-rate for the subsequent commands.

- **-s** *value*

  Set default buffer size to *value*. If the argument is suffixed by "ms" it is interpreted as milliseconds. Else the argument gives number of samples. The buffer size specified is per channel. If there are multiple channels, the total buffer size will be larger.

- **-i** *priority*

  Set real-time priority to *priority*. Refer to [rtprio(1)](/rtprio/1.md) for more information.

- **-a** *log2\_amp*

  Set the default DSP output and input device amplification to *log2\_amp*. The specified amplification is logarithmic. Valid values range from -63 to 63 inclusivly. The device input amplification gets set to minus *log2\_amp* and the device output amplification gets set to *log2\_amp*.

- **-a** *i,log2\_amp*

  Set the default DSP input device amplification to *log2\_amp*. The specified amplification is logarithmic. Valid values range from -63 to 63 inclusivly.

- **-a** *o,log2\_amp*

  Set default DSP output device amplification to *log2\_amp*. The specified amplification is logarithmic. Valid values range from -63 to 63 inclusivly.

- **-p** *polarity*

  Set default polarity of DSP device. A value of zero means normal polarity. A value of one means negative polarity.

- **-e** *rx\_mute,tx\_mute*

  Set default mute state of DSP device. A value of zero means unmuted. A value of one means muted.

- **-m** *rx\_ch,tx\_ch,....*

  Set default channel mapping of DSP device, as a comma separated list of integers. The first integer selects the receive channel, the second value selects the transmit channel and then it repeats. A value of zero indicates the first receive or transmit channel.

- **-C** *num*

  Set the maximum number of mix channels to *num*.

- **-c** *num*

  Set mix channels for the subsequent commands.

- **-M** *type,src\_ch,dst\_ch,pol,mute,log2\_gain*

  Add a monitoring filter. The filter consists of a list of comma separated arguments. The first argument indicates the type of monitoring filter:

  - i

    Feedback one mix input channel into another mix output channel, for remote feedback.

  - o

    Add one mix output channel into another mix output channel, for creating a mix of multiple output channels.

  - x

    Feedback one mix output channel into another mix input channel, for local feedback.

    The second argument gives the source mix channel. The third argument gives the destination mix channel. The fourth argument gives the polarity, default is zero. The fifth argument gives the mute state, default is one or muted. The sixth argument gives the amplitude, default is zero or no gain.

- **-t** *devname*

  Set control device name.

- **-P** *devname*

  Set playback DSP device only. Specifying */dev/null* is magic and means no playback device. Specifying a [sndio(7)](/sndio/7.md) device descriptor prefixed by */dev/sndio/* is also magic, and will use a sndio backend rather than an OSS device. Specifying path prefixed by */dev/bluetooth/* is also magic, and will use Bluetooth audio backend rather than an OSS device. Suffix after */dev/bluetooth/* is Bluetooth device address or alias as configured in [bluetooth.hosts(5)](/bluetooth.hosts/5.md).

- **-O** *devname*

  Set playback DSP device only which acts as a master device. This option is used in conjunction with **-R** */dev/null*.

- **-R** *devname*

  Set recording DSP device only. Specifying */dev/null* is magic and means no recording device. Specifying path prefixed by */dev/bluetooth/* is also magic, and will use Bluetooth audio backend rather than an OSS device. Suffix after */dev/bluetooth/* is Bluetooth device address or alias as configured in [bluetooth.hosts(5)](/bluetooth.hosts/5.md).

- **-f** *devname*

  Set both playback and recording DSP device. Specifying path prefixed by */dev/bluetooth/* is magic, and will use Bluetooth audio backend rather than an OSS device. Suffix after */dev/bluetooth/* is Bluetooth device address or alias as configured in [bluetooth.hosts(5)](/bluetooth.hosts/5.md).

- **-w** *name*

  Create a WAV file format compatible companion device by given name. This option should be specified before the -d and -l options.

- **-d** *name*

  Create an OSS device by given name.

- **-l** *name*

  Create a loopback OSS device by given name.

- **-L** *name*

  Create a loopback OSS device which acts as a master device. This option is used in conjunction with **-f** */dev/null*.

- **-F** *size*

  Set receive filter size in number of samples or \<milliseconds\>ms for the next device to be created.

- **-G** *size*

  Set transmit filter size in number of samples or \<milliseconds\>ms for the next device to be created.

- **-D** *file*

  Write process ID of virtual\_oss to file.

- **-g** *knee,attack,decay*

  Enable device compressor in receive direction. See description of -x option.

- **-x** *knee,attack,decay*

  Enable output compressor and set knee, attack and decay. Knee is in the range 0..255, while attack and decay are between 0 and 62. Samples having an absolute value lower than the knee are transmitted unchanged. Sample values over the knee are lowered "a little bit". You can think about attack and decay as a measure of how fast or slow the gain of the compressor will work. It is advised that attack is low, so it reacts fast once too high sample values appear. It is also advised that the decay value is higher than the attack value so that the gain reduction is gradually removed. The reasoning behind this is that the compressor should react almost immediately when high volume signals arrive to protect the hardware, but it slowly changes gain when there are no loud signals to avoid distorting the signal. The default values are 85,3,20 .

- **-E** *enable\_recording*

  If the value passed is non-zero, recording is enabled. Else recording is disabled. This can be used to synchronize multiple recording streams.

- **-h**

  Show usage and all available options.

## EXAMPLES

Split a 2-channel OSS compatible sound device into multiple subdevices:

```
virtual_oss \
	-S \
	-c 2 -r 48000 -b 16 -s 4ms -f /dev/dspX \
	-a 0 -b 16 -c 2 -m 0,0,1,1 -d vdsp.zyn \
	-a 0 -b 16 -c 2 -m 0,0,1,1 -d vdsp.fld \
	-a 0 -b 16 -c 2 -m 0,0,1,1 -d dsp \
	-a 0 -b 16 -c 2 -m 0,0,1,1 -w vdsp.jack.wav -d vdsp.jack \
	-a 0 -b 16 -c 2 -m 0,0,1,1 -w vdsp.rec.wav -l vdsp.rec \
	-M i,0,0,0,1,0 \
	-M i,0,0,0,1,0 \
	-M i,0,0,0,1,0 \
	-M i,0,0,0,1,0 \
	-t vdsp.ctl
```

Split an 8-channel 24-bit OSS compatible sound device into multiple subdevices:

```
sysctl dev.pcm.X.rec.vchanformat=s24le:7.1
sysctl dev.pcm.X.rec.vchanrate=48000
sysctl dev.pcm.X.play.vchanformat=s24le:7.1
sysctl dev.pcm.X.play.vchanrate=48000
sysctl dev.pcm.X.bitperfect=1
mixer vol.volume=1 pcm.volume=1
virtual_oss \
	-S \
	-i 8 \
	-x 85,3,20 \
	-C 16 -c 8 -r 48000 -b 32 -s 4ms -f /dev/dspX \
	-a 12 -b 16 -c 2 -m 0,4,1,5 -d dsp \
	-a 12 -b 16 -c 2 -m 8,8,9,9 -d vdsp \
	-a 13 -b 16 -c 2 -m 10,10,11,11 -d vdsp.fld \
	-a 0 -b 32 -c 4 -m 4,2,5,3,6,4,7,5 -d vdsp.jack \
	-a -3 -b 32 -c 2 -m 14,14,15,15 -d vdsp.zyn \
	-e 0,1 \
	-a 0 -b 32 -c 8 -m 0,8,1,9,2,8,3,9,4,8,5,9,6,8,7,9 -w vdsp.rec.mic.wav -d vdsp.rec.mic \
	-a 0 -b 32 -c 2 -m 0,8,1,9 -w vdsp.rec.master.wav -d vdsp.master.mic \
	-a 0 -b 32 -c 2 -m 10,10,11,11 -w vdsp.rec.fld.wav -l vdsp.rec.fld \
	-a 0 -b 32 -c 2 -m 12,12,13,13 -w vdsp.rec.jack.wav -l vdsp.rec.jack \
	-a 0 -b 32 -c 2 -m 14,14,15,15 -w vdsp.rec.zyn.wav -l vdsp.rec.zyn \
	-M o,8,0,0,0,0 \
	-M o,9,1,0,0,0 \
	-M o,10,0,0,0,0 \
	-M o,11,1,0,0,0 \
	-M o,12,0,0,0,0 \
	-M o,13,1,0,0,0 \
	-M o,14,0,0,0,0 \
	-M o,15,1,0,0,0 \
	-M i,14,14,0,1,0 \
	-M i,15,15,0,1,0 \
	-M x,8,0,0,1,0 \
	-M x,8,1,0,1,0 \
	-t vdsp.ctl
```

Create a secondary audio device sending its output audio into both input and output channels of the main DSP device.

```
virtual_oss \
	-C 4 -c 2 \
	-r 48000 \
	-b 24 \
	-s 4ms \
	-f /dev/dsp3 \
	-c 2 \
	-d dsp \
	-m 2,2,3,3 \
	-d dsp.speech \
	-M o,2,0,0,0,0 \
	-M o,3,1,0,0,0 \
	-M x,2,0,0,0,0 \
	-M x,3,1,0,0,0
```

Connect to a bluetooth audio headset, playback only:

```
virtual_oss \
	-C 2 -c 2 -r 48000 -b 16 -s 4ms \
	-R /dev/null -P /dev/bluetooth/xx:xx:xx:xx:xx:xx -d dsp
```

Connect to a bluetooth audio headset, playback and recording:

```
virtual_oss \
	-C 2 -c 2 -r 48000 -b 16 -s 4ms \
	-f /dev/bluetooth/xx:xx:xx:xx:xx:xx -d dsp
```

Create recording device which outputs a WAV-formatted file:

```
virtual_oss \
	-C 2 -c 2 -r 48000 -b 16 -s 4ms \
	-f /dev/dspX -w dsp.wav -d dsp
```

Create a device named dsp.virtual which mix the samples written by all clients and outputs the result for further processing into dsp.virtual\_out:

```
virtual_oss \
	-S -Q 0 -b 16 -c 2 -r 96000 -s 100ms -i 20 \
	-f /dev/null -d dsp.virtual -L dsp.virtual_out
```

Create a playback-only audio device which sends its output to a remote [sndio(7)](/sndio/7.md) server:

```
virtual_oss \
	-b 16 -c 2 -r 44100 -s 50ms \
	-R /dev/null -O /dev/sndio/snd@remotehost/0 -d dsp
```

Create a full-duplex audio device exchanging audio using the default [sndio(7)](/sndio/7.md) server:

```
virtual_oss -S -b 16 -C 2 -c 2 -r 48000 -s 4ms \
	-f /dev/sndio/default -d dsp
```

## NOTES

Users have to be part of the audio group to access any device node created by **virtual\_oss**.

## SEE ALSO

[cuse(3)](/cuse/3.md), [sound(4)](/sound/4.md), [loader.conf(5)](/loader.conf/5.md), [sndio(7)](/sndio/7.md), [mixer(8)](/mixer/8.md), [sysctl(8)](/sysctl/8.md), [virtual\_bt\_speaker(8)](/virtual_bt_speaker/8.md), virtual\_equalizer(8), [virtual\_oss\_cmd(8)](/virtual_oss_cmd/8.md)

## AUTHORS

**virtual\_oss** was written by Hans Petter Selasky hselasky@freebsd.org.