           isdn_audio - audio extension of ttyI ISDN character devices

## DESCRIPTION

           This document describes the audio  related  extensions  of
           the modem emulation, provided by /dev/ttyIx.

           The  ISDN  tty devices are equipped with a modem emulation
           implementing a set of traditional and some special AT com
           mands.  This  provides  easy  use together with almost all
           communication software that  uses  modem  commands:  mini
           com(1),  seyon(1), XCept(1), uucico(8), mgetty(8), dip(8),
           pppd(8) and more. When enabled  during  kernel  configura
           tion, the emulator is capable of a reduced set of commands
           to support audio. To use audio features, an ISDN card with
           a  audio-capable low-level driver is needed. Currently the
           only audio-capable drivers are the teles and HiSax driver.

           The  port  speed  of  the ISDN tty devices is always 64000
           bps.

## AUDIO RELATED AT COMMAND SET

           This description does not include the standard AT  command
           set.  For  a  description  of the standard AT command set,
           provided by the Linux ISDN subsystem,  see  ttyI(4).   All
           commands  starting  with  AT+ except AT+FCLASS are enabled
           only, if  audio  mode  is  active.   The  following  audio
           related AT commands are supported by ISDN tty devices:

           AT+FCLASS=0
                  Disable  audio  mode  (default).  This command sets
                  register 18 to 4.

           AT+FCLASS=8
                  Enable audio mode (default). This command sets reg
                  ister 18 to 5.

           AT+FCLASS=?
                  Show possible modes.

           AT+FCLASS?
                  Show current mode.

           AT+VNH=x
                  No function, provided for compatibility only.

           AT+VNH=?
                  Always showing "1".

           AT+VNH?
                  Always showing "1".

           AT+VLS=x
                  Select line x, where x is one of the following:
                  0 = No device.
                  2 = Phone line.

           AT+VLS=?
                  Always showing "0,2".

           AT+VLS?
                  Show current line.

           AT+VRX Start  audio recording. See below for more informa
                  tion.

           AT+VSD=x,y
                  No function, provided for compatibility only.  Pos
                  sible parameters:
                  x = 0 ... 31
                  y = 0 ... 255

           AT+VSD=?
                  Show possible parameters.

           AT+VSD?
                  Show current parameters.

           AT+VSM=x
                  Select audio data format to x, where x is:
                  2 = ADPCM-2
                  3 = ADPCM-3
                  4 = ADPCM-4
                  5 = aLAW
                  6 = uLAW

           AT+VSM=?
                  Show possible audio data formats.

           AT+VSM?
                  Show current audio data format.

           AT+VTX Start  audio  playback. See below for more informa
                  tion.

## GENERAL BEHAVIOUR

           On incoming calls, if the application responds to  a  RING
           by sending ATA, depending on the calling service, the emu
           lator responds with either CONNECT  (data  call)  or  VCON
           (voice call).

           When  starting  audio  playback or recording, the emulator
           responds with CONNECT.   After  that,  the  data  transfer
           in  a  single command. In this case, no CONNECT message is
           sent to the application and the protocol  described  below
           is started immediately.

           When  stopping  audio  playback  or recording, the message
           returned by the emulator depends on how the data  transfer
           is  stopped.  If  the data transfer is stopped by a remote
           hangup, NO CARRIER is sent to the application, while stop
           ping  data transfer with one of the DLE sequences, VCON is
           sent. In any case, the message is preceeded by the  appro
           priate  DLE  sequence  for  end of audio data and/or abort
           respectively.

           On outgoing voice calls, the emulator responds  with  VCON
           upon connection setup.

## AUDIO DATA FORMAT

           When recording or playing back audio data, a kind of modi
           fied bisync protocol is used. When recording  audio  data,
           the  modem  emulator is considered to be the sender, while
           the application is the receiver.  When playing back,  this
           relationship  is  reversed:  The  modem  emulator  acts as
           receiver, while the application is the sender.   With  the
           bisync  protocol  the  character  DLE (0x10) has a special
           meaning and is used  like  an  escape  character.   It  is
           always  followed  by a second character which has the fol
           lowing meanings defined:

           ETX (0x03)
                  End of audio data. This code has to be sent by  the
                  sender, indicating end of audio data.

           DC4 (0x14)
                  Abort  sending.  This  code  has  to be sent by the
                  receiver, when it wishes to abort  receiving  data.
                  The  sender  has  to reply by stop sending data and
                  finally indicating end of audio data.

           DLE (0x10)
                  a single byte with value 0x10 in the data stream.

           0      Touch-tone "0" received. Valid  only  in  direction
                  from emulator to application.

           1      Touch-tone  "1"  received.  Valid only in direction
                  from emulator to application.

           2      Touch-tone "2" received. Valid  only  in  direction
                  from emulator to application.

           3      Touch-tone  "3"  received.  Valid only in direction
                  from emulator to application.
                  from emulator to application.

           5      Touch-tone  "5"  received.  Valid only in direction
                  from emulator to application.

           6      Touch-tone "6" received. Valid  only  in  direction
                  from emulator to application.

           7      Touch-tone  "7"  received.  Valid only in direction
                  from emulator to application.

           8      Touch-tone "8" received. Valid  only  in  direction
                  from emulator to application.

           9      Touch-tone  "9"  received.  Valid only in direction
                  from emulator to application.

           #      Touch-tone "#" received. Valid  only  in  direction
                  from emulator to application.

           *      Touch-tone  "*"  received.  Valid only in direction
                  from emulator to application.

           A      Touch-tone "A" received. Valid  only  in  direction
                  from emulator to application.

           B      Touch-tone  "B"  received.  Valid only in direction
                  from emulator to application.

           C      Touch-tone "C" received. Valid  only  in  direction
                  from emulator to application.

           D      Touch-tone  "D"  received.  Valid only in direction
                  from emulator to application.

## AUTHOR

           Fritz Elfert <fritz@wuemaus.franken.de>

## SEE ALSO

           ttyI(4).