The modem may be configured in response to AT voice commands to provide enhanced Adaptive Differential Pulse Code Modulation (ADPCM) coding and decoding for the compression and decompression of digitised voice. ADPCM compression supports the efficient storage of voice messages while optional coder silence deletion and decoder silence interpolation significantly increase compression rates.

The ADPCM voice mode supports three sub-modes once a voice connection is established (see #CLS command): on-line voice command mode, voice receive mode and voice transmit mode.

If the modem is the answerer, it enters on-line voice command mode immediately after going off hook, and can report instances of DTMF tones and calling tones to the DTE. If the modem is the originator, it enters on-line voice command mode based on detection of the ring back cadence going away, upon expiration of the ring back never came timer, or upon detection of answer tone; the modem can report DTMF tones, answer tones, busy tones and dial tone to the DTE. Note that DTMF tone reporting is supported in this mode if DTMF reporting is enabled via the #VTD command.

In voice receive mode, voice samples from the modem analogue-to-digital converter (ADC) are sent to the ADPCM codec for compression, and can then be read by the host. AT commands control the codec-bits-per-sample rate and select (optional) silence deletion including adjustment of the silence detection period.

In this mode, the modem detects and reports DTMF, dial tone, busy tone cadence, and inactivity (periods of silence) as enabled by the #VTD and #VSS commands, respectively. The modem can exit the voice receive mode only via a DTE key Abort or via Deadman timer expiration (S30).

When configured for voice mode (#CLS 8), the modem enters on-line voice command mode immediately after going off hook. When the #CLS=8 command is entered, the modem can be programmed to look for 1100 Hz and 1300 Hz calling tones (see #VTD), thus eliminating the need to do so as part of A command processing. After the VCON message is issued, the modem re-enters on-line voice command mode while sending any incoming DTMF or calling tone indications to the DTE.

After answering in voice mode (#CLS 8), the DTE, as part of its call discrimination processing, can decide to change the #CLS setting to attempt receiving a fax in Class1, or make a data connection. In such a case, the DTE commands the modem to proceed with the data or fax handshake via the A command even though the modem is already off hook.

The modem attempts to determine if the remote picked up the telephone line and once determined, the VCON message is sent to the DTE. This determination is initially made based upon the ring back detection and disappearance (see #VRA and #VRN commands). DTMF tone and status reporting to the DTE is enabled.

The H command forces #CLS=0, but does not destroy any of the voice parameter settings such as #VBS, #VSP, etc. If the DTE needs to issue an H command and then pursue another voice call, it must issue a subsequent #CLS=8 command. There is no need to amend the voice parameter settings again unless a change in the settings is required.

The #BDR setting is forced back to 0, re-enabling auto baud.

If the #VLS setting selects a device which does not include the telephone line (such as a local handset or microphone), the H command de-selects this device and re-selects the default setting (#VLS=0).

When in on-line voice command mode and #BDR is not zero (no auto baud selected), the modem supports a full duplex DTE interface. The DTE can enter commands at any time, even when the modem is sending a shielded code indicating DTMF detection to the DTE. When #BDR is not set as zero, the modem employs the S30 deadman timer (See S Registers chapter). The deadman timer starts at the point where #BDR is non zero. If this period expires (after 60 seconds) with no activity on the DTE interface, the modem reverts to #BDR=0 and #CLS=0.

#BDR? returns the current setting of the #BDR command as an ASCII decimal value in result code format.

#BDR=? returns a message indicating the speeds that are supported by the modem.

#BDR=0 enables auto baud detection on the DTE interface.

#BDR=n where n has the range 1 to 48. This command sends an OK message at current speed, then switches to the new speed defined by n*2400 bps until another #BDR=n command is received. Auto baud is disabled, and the character format is maintained at the most recently detected format.

#CLS? returns the current setting (0, 1, 2, or 8) of the #CLS=0 command as an ASCII decimal value in result code format.

#CLS=? returns the message, '0, 1, 2, 8'.

#CLS=0 data. This is similar to setting +FCLASS=0, and instructs the modem to act like a data modem on subsequent answer or originate operations.

When a disconnect or activity timeout in the non-auto baud mode is detected, the modem automatically sets the #CLS setting to 0 and hangs up. This ensures that the modem is always in a known state.

#CLS=1 class 1 fax. This is similar to setting +FCLASS=1, and instructs the modem to be a Class 1 fax modem. Once set, either the +FAA or +FAE command can be used to force subsequent answers to be Class 1 adaptive.

#CLS=2 class 2 fax. This is similar to setting +FCLASS=2, and instructs the modem to be a Class 2 fax modem. Once set, the +FAA command can be used to force subsequent answers to be Class 2 adaptive.

#CLS=8 Voice Mode. This is the main setting the DTE uses to effect directed or adaptive answer or originate sequences involving voice modes. All telephone calls initialised by #CLS=8 result (after answer or successful call progress) in the modem in entering on-line voice command mode.

#VBS? returns the current setting of the #VBS command as an ASCII decimal value in result code format.

#VBS=? returns the message '2, 4', which are the ADPCM compression bits/ sample values available. Selected models also return '8, 16', which is the PCM bits per sample.

#VBS=2 selects 2 bits per sample ADPCM.

#VBS=4 selects 4 bits per sample ADPCM.

#VBS=8 selects 8 bits per sample PCM (selected models only).

#VBS=16 selects 16 bits per sample PCM (selected models only).

#VBT? returns the current setting of the #VBT command as an ASCII decimal value in result code format.

#VBT=? returns the message '0-40'.

#VBT=0 disables the tone generation capability.

#VBT=n where n has the range 1 to 40. This command sets the tone duration time (10 = 1 second).

'ROCKWELL;ADPCM;16'.

#VLS? returns the current setting of the #VLS command as an ASCII decimal value in result code format.

#VLS=? requests a report of the device types available for selection. The response is a series of numbers separated by commas, and each number indicates a device position number. Currently, device position numbers correspond to the device type numbers as shown, but this correlation is not required (i.e., 0's could be placed in the first few device position number locations to correspond to multiple telephone line connections).

#VLS=0 is the default option on the modem. This command instructs the modem that when entering any of the three voice operating sub-modes (on-line command, transmit or receive), to route the telephone line interface through the modem. The OK response is sent to the DTE, and any previous connection is lost.

#VLS=1 instructs the modem to route the handset through the modem. This setting can be chosen before recording a greeting message.

#VLS=2 instructs the modem to route the speaker through the modem. This setting can be chosen before playing back any message. Upon receipt of this command the modem immediately switches to on-line voice command mode, and generates the VCON response. Since the speaker is an output only device, very little occurs until the DTE sends the #VTX command.

#VLS=3 instructs the modem to route the auxiliary input device (microphone) through the modem. This setting can be chosen before recording a greeting message.

#VLS=4 is the same as #VLS=0, except that the modem enables the internal speaker and telephone line handset.

#VRA? returns the current setting of the #VRA command as an ASCII decimal value in result code format.

#VRA=? returns the message '0-255'.

#VRA=0 switches off the "Ring back Goes Away" timer. After one ring back, the dialling modem sends VCON and enters on-line voice command mode immediately.

#VRA=n where n has the range 0 to 255. Defines the period without ring back (after at least one ring back has been detected) in 100 ms units.

#VRN? returns the current setting of the #VRN command as an ASCII decimal value in result code format.

#VRN=? returns the message '0-255'.

#VRN=0 turns off the 'ring back never came timer'. After dialling, the modem sends VCON and enters on-line voice command mode immediately.

#VRN=n where n has the range 0 to 255 (100 ms units), and defines the period without ring back after dialling.

#VSD? returns the current setting of the #VSD command as an ASCII decimal value in result code format.

#VSD=? returns the message '0, 1'.

#VSD=0 turns off the silence deletion.

#VSD=n turns on the silence deletion if the silence sensitivity setting.

#VSD is non zero. The aggressiveness of the silence deletion is controlled by #VSS.

#VSP? returns the current setting of the #VSP command as an ASCII decimal value in result code format.

#VSP=? returns the message '0-255'.

#VSP=0 switches off the silence period detection timer.

#VSP=n where n has the range 0 to 255. This command defines the period without received data in 100 ms units.

7200 Hz is available for ADPCM or PCM.

11025 Hz is available for PCM only.

#VSS? returns the current setting of the #VSS command as an ASCII decimal value in result code format.

#VSS=? returns the message '0-3'.

#VSS=0 disables silence detection by the modem when in voice receive mode.

#VSS=1 is the least sensitive setting. This command configures the system to a state which is least likely to detect and compress periods of silence, but is still able to if the line is really quiet.

#VSS=2 is the mid range setting. This command configures the system to a state which is likely to be the best overall compromise on normal telephone lines.

#VSS=3 is the most sensitive setting. This command configures the system to a state which is most likely to detect and compress periods of silence.

#VTD? returns the current setting of the #VTD command as an ASCII decimal value in result code format.

#VTD=? returns the tone reporting capabilities of the modem.

#VTD=i,j,k where i,j,k corresponds (in ASCII hexadecimal) to the desired capabilities (see the table below) for the voice transmit, voice receive and on-line voice command modes, respectively.

#VTM = 0 disables timing marks.

#VTM = 10 enables timing marks every 1 second.

Dual or single tones are represented by a sub-string enclosed in square brackets ([ ]) within the parameter. Each such sub-string comprises three sub-elements corresponding to two frequencies (0 Hz or 200-3000 Hz) and a duration (ASCII decimal in units of 100 ms).

Varying DTMF digits are represented by a sub-string enclosed in curly brackets ({ }) within the parameter. Each such sub-string consists of two sub-elements corresponding to a DTMF digit (0-9, A-D, *, #) and alternate durations in units of 100 ms.