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.\" Copyright (c) 2017-2022 Hans Petter Selasky <hselasky@freebsd.org>
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.Dd April 17, 2026
.Dt VIRTUAL_OSS 8
.Os
.Sh NAME
.Nm virtual_oss
.Nd daemon to multiplex and demultiplex an OSS device
.Sh SYNOPSIS
.Nm
.Op Fl h
.Sh DESCRIPTION
.Nm
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.
.Pp
.Nm
requires the
.Xr cuse 3
kernel module.
To load the driver as a module at boot time, place the following line in
.Xr loader.conf 5 :
.Pp
       cuse_load="YES"
.Pp
All channel numbers start at zero.
Left channel is zero and right channel is one.
.Pp
The following options are available:
.Bl -tag -width indent
.It Fl B
Run program in background.
.It Fl S
Enable automatic DSP rate resampling.
.It Fl Q Ar quality
Set resampling quality: 0=best, 1=medium and 2=fastest (default).
.It Fl b Ar bits
Set sample depth to
.Fa bits
for the subsequent commands.
Valid values are 8, 16, 24 and 32.
.It Fl r Ar rate
Set default sample-rate for the subsequent commands.
.It Fl s Ar value
Set default buffer size to
.Fa 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.
.It Fl i Ar priority
Set real-time priority to
.Fa priority .
Refer to
.Xr rtprio 1
for more information.
.It Fl a Ar log2_amp
Set the default DSP output and input device amplification to
.Fa log2_amp .
The specified amplification is logarithmic.
Valid values range from -63 to 63 inclusivly.
The device input amplification gets set to minus
.Fa log2_amp
and the device output amplification gets set to
.Fa log2_amp .
.It Fl a Ar i,log2_amp
Set the default DSP input device amplification to
.Fa log2_amp .
The specified amplification is logarithmic.
Valid values range from -63 to 63 inclusivly.
.It Fl a Ar o,log2_amp
Set default DSP output device amplification to
.Fa log2_amp .
The specified amplification is logarithmic.
Valid values range from -63 to 63 inclusivly.
.It Fl p Ar polarity
Set default polarity of DSP device.
A value of zero means normal polarity.
A value of one means negative polarity.
.It Fl e Ar rx_mute,tx_mute
Set default mute state of DSP device.
A value of zero means unmuted.
A value of one means muted.
.It Fl m Ar 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.
.It Fl C Ar num
Set the maximum number of mix channels to
.Fa num .
.It Fl c Ar num
Set mix channels for the subsequent commands.
.It Fl M Ar 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:
.Bl -tag -width indent
.It i
Feedback one mix input channel into another mix output channel, for remote
feedback.
.It o
Add one mix output channel into another mix output channel, for creating a mix
of multiple output channels.
.It x
Feedback one mix output channel into another mix input channel, for local
feedback.
.El
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.
.It Fl t Ar devname
Set control device name.
.It Fl P Ar devname
Set playback DSP device only.
Specifying /dev/null is magic and means no playback device.
Specifying a
.Xr sndio 7
device descriptor prefixed by "/dev/sndio/" is also magic, and will use a sndio
backend rather than an OSS device.
.It Fl O Ar devname
Set playback DSP device only which acts as a master device.
This option is used in conjunction with -R /dev/null .
.It Fl R Ar devname
Set recording DSP device only.
Specifying /dev/null is magic and means no recording device.
.It Fl f Ar devname
Set both playback and recording DSP device
.It Fl w Ar name
Create a WAV file format compatible companion device by given name.
This option should be specified before the -d and -l options.
.It Fl d Ar name
Create an OSS device by given name.
.It Fl l Ar name
Create a loopback OSS device by given name.
.It Fl L Ar name
Create a loopback OSS device which acts as a master device.
This option is used in conjunction with -f /dev/null .
.It Fl F Ar size
Set receive filter size in number of samples or <milliseconds>ms for the next
device to be created.
.It Fl G Ar size
Set transmit filter size in number of samples or <milliseconds>ms for the next
device to be created.
.It Fl D Ar file
Write process ID of virtual_oss to file.
.It Fl g Ar knee,attack,decay
Enable device compressor in receive direction.
See description of -x option.
.It Fl x Ar 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 .
.It Fl E Ar 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.
.It Fl h
Show usage and all available options.
.El
.Sh EXAMPLES
Split a 2-channel OSS compatible sound device into multiple subdevices:
.Bd -literal -offset indent
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
.Ed
.Pp
Split an 8-channel 24-bit OSS compatible sound device into multiple subdevices:
.Bd -literal -offset indent
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

.Ed
.Pp
Create a secondary audio device sending its output audio into both
input and output channels of the main DSP device.
.Bd -literal -offset indent
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
.Ed
.Pp
Connect to a bluetooth audio headset, playback only:
.Bd -literal -offset indent
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
.Ed
.Pp
Connect to a bluetooth audio headset, playback and recording:
.Bd -literal -offset indent
virtual_oss \\
	-C 2 -c 2 -r 48000 -b 16 -s 4ms \\
	-f /dev/bluetooth/xx:xx:xx:xx:xx:xx -d dsp
.Ed
.Pp
Create recording device which outputs a WAV-formatted file:
.Bd -literal -offset indent
virtual_oss \\
	-C 2 -c 2 -r 48000 -b 16 -s 4ms \\
	-f /dev/dspX -w dsp.wav -d dsp
.Ed
.Pp
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:
.Bd -literal -offset indent
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
.Ed
.Pp
Create a playback-only audio device which sends its output to a remote
.Xr sndio 7
server:
.Bd -literal -offset indent
virtual_oss \\
	-b 16 -c 2 -r 44100 -s 50ms \\
	-R /dev/null -O /dev/sndio/snd@remotehost/0 -d dsp
.Ed
.Pp
Create a full-duplex audio device exchanging audio using the default
.Xr sndio 7
server:
.Bd -literal -offset indent
virtual_oss -S -b 16 -C 2 -c 2 -r 48000 -s 4ms \\
	-f /dev/sndio/default -d dsp
.Ed
.Sh NOTES
All character devices are created using the 0666 mode which gives
everyone in the system access.
.Sh SEE ALSO
.Xr cuse 3 ,
.Xr sound 4 ,
.Xr loader.conf 5 ,
.Xr sndio 7 ,
.Xr mixer 8 ,
.Xr sysctl 8 ,
.Xr virtual_bt_speaker 8 ,
.Xr virtual_equalizer 8 ,
.Xr virtual_oss_cmd 8
.Sh AUTHORS
.Nm
was written by
.An Hans Petter Selasky hselasky@freebsd.org .