NAME
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 commands. This provides easy use
together with almost all communication software that uses modem
commands: minicom(1), seyon(1), XCept(1), uucico(8), mgetty(8), dip(8),
pppd(8) and more. When enabled during kernel configuration, 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. This command sets register 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+VIP Reset all audio parameters.
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 information.
AT+VSD=x,y
Set silence-detection parameters.
Possible parameters:
x = 0 ... 31 sensitivity threshold level.
(default 0 , deactivated)
y = 0 ... 255 range of interval in units of 0.1 second. (default
70)
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 information.
GENERAL BEHAVIOUR
On incoming calls, if the application responds to a RING by sending
ATA, depending on the calling service, the emulator 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 protocol described below is
used. It is possible to initiate full duplex data transfer by issuing
AT+VRX+VTX 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 stopping data transfer with one of the DLE
sequences, VCON is sent. In any case, the message is preceeded by the
appropriate 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 modified 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 following 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.
4 Touch-tone "4" received. Valid only in direction 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@isdn4linux.de>
SEE ALSO
ttyI(4).