Provided by: oss4-base_4.2-build2005-2ubuntu1_i386 bug


       oss_audioloop - Loopback audio driver.


       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.

       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.

       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).

       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

       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.


       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.


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

              Values: 1-16
                     Default: 1


       ·  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

       ·  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

       ·  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


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


       4Front Technologies

                               05 February 2012               oss_audioloop(7)