NAME

       oss_audioloop - Loopback audio driver.

DESCRIPTION

       The  loopback  audio  driver  makes  it  possible  to create special purpose virtual audio
       devices based on user land server processes.

       Loopback devices are driven by a timer interrupt and no real audio hardware is required.

   INTRODUCTION
       Audio loopback devices are like named pipes or  pseudo  terminals.  They  are  grouped  in
       client and server device pairs. The server side device must be open before the client side
       device can be opened.

       Loopback devices are typically used  to  implement  server  based  special  purpose  audio
       devices.  This kind of server can for example transfer the audio data played by the client
       application to some remote system using some VoIP protocol. However the server application
       doesn't  need to be any dedicated server. Practically any audio application can be used as
       the server.

   SERVER SIDE DEVICE
       The server side applications sets up the native sampling rate and sample format (number of
       bits  and  channels).  The server side device can be opened for input (O_RDONLY) pr output
       (O_WRONLY). Opening for simultaneous input and output (O_RDWR) is not permitted.

       The server application will automatically be paused at the moment  it  tries  to  read  or
       write  audio  data  for  the  first  time.  It will be kept in sleep until the client side
       application starts writing or reading data. This sleep period may last forever and in some
       cases the first write/read call never returns. For this reason it's not recommended to use
       GUI based audio applications as the server. Note that this wait will  occur  even  in  the
       non-blocking (O_NONBLOCK) mode (this is intentional feature and not a bug).

   CLIENT SIDE DEVICE
       The  client  side  device  is  typically used by any ordinary audio application.  There is
       nothing special in loopback devices.

       Since the loop is unidirectional the client side will be forced to be write  only  if  the
       server side device is open for recording and vice versa.

       The  loop  will use the sample rate and sample format (number of bits and channels) set by
       the server side application. If the client uses different settings then OSS  will  perform
       the required sampling rate and format conversions automatically.

COMPATIBILITY ISSUES

       Audio loopback devices differ from "normal" audio devices because an application is needed
       at the both ends of the loop. The loop device will return a  "Connection  reset  by  peer"
       error  (ECONNRESET)  error.  Applications  designed  to  be  used as loopback based server
       applications can/should use this error (returned by read or  write)  as  an  end-of-stream
       indication.

OPTIONS

       o audioloop_instances:
              Specifies how many loopback client/server audio device pairs to be created.

              Values: 1-16
                     Default: 1

KNOWN PROBLEMS

       •  There  is  no  mixer  (volume  control)  related  with loopback audio devices. This may
          prevent poorly designed audio applications (that expect/require a mixer) from  working.
          There is no workaround available.

       •  The  server  side  application will wait until the client side application starts using
          it. This wait may last forever which in turn may cause unrecoverable (network) problems
          with some applications.

       •  Loopback devices may return "Connection reset by peer" error when the reote side of the
          loop disconnects the device. Some recording applications may fail to save the  recorded
          data  properly  because of this. Use some other application (such as ossrecord) if this
          happens.

FILES

       /etc/oss4/conf/oss_audioloop.conf Device configuration file

AUTHOR

       4Front Technologies

                                         03 December 2013                        oss_audioloop(7)