diff --git a/Documentation/isdn/README.audio b/Documentation/isdn/README.audio
new file mode 100644
index 0000000..8ebca19
--- /dev/null
+++ b/Documentation/isdn/README.audio
@@ -0,0 +1,138 @@
+$Id: README.audio,v 1.8 1999/07/11 17:17:29 armin Exp $
+
+ISDN subsystem for Linux.
+  Description of audio mode.
+
+When enabled during kernel configuration, the tty emulator of the ISDN
+subsystem is capable of a reduced set of commands to support audio.
+This document describes the commands supported and the format of
+audio data.
+
+Commands for enabling/disabling audio mode:
+
+        AT+FCLASS=8      Enable audio mode.
+                         This affects the following registers:
+                           S18: Bits 0 and 2 are set.
+                           S16: Set to 48 and any further change to
+                                larger values is blocked.
+        AT+FCLASS=0      Disable audio mode.
+                         Register 18 is set to 4.
+        AT+FCLASS=?      Show possible modes.
+        AT+FCLASS?       Report current mode (0 or 8).
+
+Commands supported in audio mode:
+
+All audio mode commands have one of the following forms:
+
+        AT+Vxx?          Show current setting.
+        AT+Vxx=?         Show possible settings.
+        AT+Vxx=v         Set simple parameter.
+        AT+Vxx=v,v ...   Set complex parameter.
+
+where xx is a two-character code and v are alphanumerical parameters.
+The following commands are supported:
+
+        AT+VNH=x         Auto hangup setting. NO EFFECT, supported
+                         for compatibility only.
+        AT+VNH?          Always reporting "1"
+        AT+VNH=?         Always reporting "1"
+
+        AT+VIP           Reset all audio parameters.
+
+        AT+VLS=x         Line select. x is one of the following:
+                           0 = No device.
+                           2 = Phone line.
+        AT+VLS=?         Always reporting "0,2"
+        AT+VLS?          Show current line.
+
+        AT+VRX           Start recording. Emulator responds with
+                         CONNECT and starts sending audio data to
+                         the application. See below for data format
+
+        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=?         Report possible parameters.
+        AT+VSD?          Show current parameters.
+
+        AT+VDD=x,y       Set DTMF-detection parameters.
+                         Only possible if online and during this connection.
+                         Possible parameters:
+                           x = 0 ... 15  sensitivity threshold level.
+                                         (default 0 , I4L soft-decode)
+                                         (1-15 soft-decode off, hardware on)
+                           y = 0 ... 255 tone duration in units of 5ms.
+                                         Not for I4L soft decode (default 8, 40ms)
+        AT+VDD=?         Report possible parameters.
+        AT+VDD?          Show current parameters.
+
+        AT+VSM=x         Select audio data format.
+                         Possible parameters:
+                           2 = ADPCM-2
+                           3 = ADPCM-3
+                           4 = ADPCM-4
+                           5 = aLAW
+                           6 = uLAW
+        AT+VSM=?         Show possible audio formats.
+
+        AT+VTX           Start audio playback. Emulator responds
+                         with CONNECT and starts sending audio data
+                         received from the application via phone line.
+General behavior and description of data formats/protocol.
+    when a connection is made:
+
+      On incoming calls, if the application responds to a RING
+      with ATA, depending on the calling service, the emulator
+      responds with either CONNECT (data call) or VCON (voice call).
+      
+      On outgoing voice calls, the emulator responds with VCON
+      upon connection setup.
+
+  Audio recording.
+
+    When receiving audio data, a kind of bisync protocol is used.
+    Upon AT+VRX command, the emulator responds with CONNECT, and
+    starts sending audio data to the application. There are several
+    escape sequences defined, all using DLE (0x10) as Escape char:
+
+    <DLE><ETX>              End of audio data. (i.e. caused by a
+                            hangup of the remote side) Emulator stops
+                            recording, responding with VCON.
+    <DLE><DC4>		    Abort recording, (send by appl.) Emulator
+			    stops recording, sends DLE,ETX.
+    <DLE><DLE>              Escape sequence for DLE in data stream.
+    <DLE>0                  Touchtone "0" received.
+         ...
+    <DLE>9                  Touchtone "9" received.
+    <DLE>#                  Touchtone "#" received.
+    <DLE>*                  Touchtone "*" received.
+    <DLE>A                  Touchtone "A" received.
+    <DLE>B                  Touchtone "B" received.
+    <DLE>C                  Touchtone "C" received.
+    <DLE>D                  Touchtone "D" received.
+
+    <DLE>q                  quiet. Silence detected after non-silence.
+    <DLE>s                  silence. Silence detected from the
+                            start of recording.
+
+    Currently unsupported DLE sequences:
+
+    <DLE>c                  FAX calling tone received.
+    <DLE>b                  busy tone received.
+
+  Audio playback.
+
+    When sending audio data, upon AT+VTX command, emulator responds with
+    CONNECT, and starts transferring data from application to the phone line.
+    The same DLE sequences apply to this mode.
+
+  Full-Duplex-Audio:
+
+    When _both_ commands for recording and playback are given in _one_
+    AT-command-line (i.e.: "AT+VTX+VRX"), full-duplex-mode is selected.
+	In this mode, the only way to stop recording is sending <DLE><DC4>
+    and the only way to stop playback is to send <DLE><ETX>.
+