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AT Command Reference Manual
7-4 1048
7.2.3 Voice/Audio Data Transfer
A significant area of concern when handling the transfer of voice/audio data is the data transfer rate on the modem/DTE
interface. Data transfer rates can be expressed as the number of interrupts which must be serviced per time period to keep
up. This is a function of the sampling rate and compression method (if any) used by the modem, and the DTE interface
speed required to handle the data flow on the telephone line side.
Tables 7-1a and 7-1b demonstrate the relationship between the sampling rate, interrupt rate, and DTE speed necessary for
the modem to support various compression ratios. The built-in 16C550A UART-compatible interface in the MCU hardware is
required to support Voice/Audio Mode operation in the Microsoft Windows environment.
Table 7-1. DTE Speeds
a. 7.2 kHz Sample Rate
Modem Samples per 8-bit Byte Required Asynchronous DTE Speed
(8N1 Format)
Implied DTE Speed Supported
1 (8-bits) 72,000 bps 115.2 kbps
2 (4-bits) 36,000 bps 38.4 kbps
4 (2-bits) 18,000 bps 19.2 kbps
b. 11.025 kHz Sample Rate
Modem Samples per 8-bit Byte Required Asynchronous DTE Speed
(8N1 Format)
Implied DTE Speed Supported
1 (8-bits) 110,250 bps 115.2 kbps
7.2.4 Tone and Status Monitoring Shielded <DLE> Statuses
The modem can detect specific tones and other status information, and report these to the DTE while in any of the three
voice submodes. The modem simultaneously looks for 1300 and 1100 Hz calling tones when answering, and for CCITT and
Bell answer tones when originating. The modem can also detect dial or busy tones in any of the three voice submodes. All
detected tones, as well as certain other statuses addressed in Table 7-2 such as silence and “telset off-hook” (i.e., handset
off-hook) are reported as shielded codes.
When in Online Voice Command Mode or Voice Transmit Mode, the codes in Table 7-2 are sent to the DTE immediately
upon verification by the modem of the associated tone, status, or cadence. 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 and processed.
The modem can discriminate between single and multiple DTMF tones received. If calling tone, dial tone, busy tone, or
answer tone is detected, this detection is reported repeatedly (at reasonable intervals) if the DTE takes no action, and the
tone continues to be detected.