Voice/Audio Operation

		Voice/Audio Operation

You should use the voice software supplied with your modem to send and receive verbal messages or record telephone conversations. Consult the documentation associated with that software for more information.

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.

Online voice command mode

On-line voice command mode is the default voice sub-mode entered when the #CLS=8 command is issued, and may also be entered from voice receive mode and voice transmit mode. Entry into on-line voice command mode is indicated in the DTE via the VCON message, after which AT commands can be entered without aborting the telephone line connection.

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.

Voice receive mode

Voice receive mode is entered when the DTE issues the #VRX command because it wants to receive voice data. This typically occurs when either recording a greeting message or when recording voice messages from a remote station.

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).

Voice transmit mode

Voice transmit mode is entered when the DTE issues the #VTX command because it wants to transmit voice data. In this mode, the modem continues to detect and report DTMF and calling tones if enabled by the #VTD command. This mode is typically used when playing back greeting messages or previously received/recorded messages. In this mode, voice decompression is provided by the codec, and decompressed data is reconstituted into analogue voice by the DAC at the original voice compression quantisation sample-per-bits rate. Optional silence interpolation is enabled if silence deletion was selected for voice compression.

Tone and status monitoring

The modem can detect specific tones and other status information, and report these to the DTE while in any of the three voice sub-modes. The modem simultaneously looks for 1300 and 1100 Hz calling tones when answering, and for CCITT (ITU-T) and Bell answer tones when originating. The modem can also detect dial or busy tones in any of the three voice sub-modes. All detected tones, as well as certain other status's addressed in the following table such as "silence" and "teleset-off-hook" (i.e. handset off hook) are reported as shielded codes. When in on-line voice command mode or voice transmit mode, the codes in the following table are sent to the DTE immediately on verification by the modem of the associated tone, status or cadence.

**Shielded codes sent to the DTE**
<DLE>0 - <DLE>9 <DLE>* - <DLE># <DLE>A - <DLE>D	DTMF digits 0 to 9, *, # and A to D detected by the modem.
<DLE>a	Answer tone detected. V.25 or T.30.
<DLE>b	The BUSY signal is detected.
<DLE>c	The T.30 calling tone is detected.
<DLE>d	Dial tone is detected.
<DLE>e	The V.25 calling tone is detected.
<DLE>f	Bell answer tone is detected.
<DLE>h	Local handset has been hung up.
<DLE>o	An overrun has occurred in voice receive mode.
<DLE>q	Silence has been detected in voice receive mode and there has been valid voice previously detected.
<DLE>s	Silence has been detected in voice receive mode and there has not been valid voice previously detected.
<DLE>t	Local handset has been picked up.
<DLE>u	An under-run has occurred in voice transmit mode.
<DLE><ETX>	Acknowledgement to a key press abort in voice receive mode.

Shielded codes sent from the DTE
<DLE><ETX> Sent to indicate the end of voice transmission.
<DLE><CAN> Sent to cancel current voice transmission and flush any data in the transmit buffer.
<DLE>p Sent to pause the current voice transmission.
<DLE>r Sent to resume a paused voice transmission.

**Shielded codes sent from the DTE**
<DLE><ETX>	Sent to indicate the end of voice transmission.
<DLE><CAN>	Sent to cancel current voice transmission and flush any data in the transmit buffer.
<DLE>p	Sent to pause the current voice transmission.
<DLE>r	Sent to resume a paused voice transmission.

In this mode, the 2-character code is not buffered, nor does the DTE have the ability to stop the code with flow control. If the DTE has started (but not completed) sending any AT command, the tone monitoring function is disabled until the command has been received or processed.