ST75C540 datasheet(75/84 Pages) STMICROELECTRONICS

75 / 84 page

XIII.4 - Receive (CODER)

This function can be entered either by :

- The CONF CODER Command. This corresponds

to the “Normal Answering Machine” function.

- The MODC Command with MODC_COD = 1, in

the HANDSET Mode. This corresponds, in the

HANDSET mode to the “Conversation Record-

ing” function. This reduced sub-mode does not

allow ADPCM format and does not perform VAD

(Voice Activity Detector).

Once this function is selected, the ST75C530/540

starts to code the voice signal, writes one frame of

compressed voice into the VOCDATA bytes (if the

low bit rate mode is selected, computes always the

Corrector bytes and writes them in the VOCCORR

bytes) then writes the VOCSTAbyte and generates

the IT1 interrupt.

The IT1 interrupt rate is as follows :

Mode

Interrupt

Time

(ms)

Number of Voice

Samples

in the Buffer

(8kHz sampling)

ADPCM 32Kpbs

4.5

ADPCM 24Kpbs

ADPCM 16Kpbs

Low Bit Rate (with and

without error correction)

240

Note that the VOCCORR are always computed,

whatever the value of CONF_ERCOR.

The format of the VOCSTA byte is as follows :

VOCSTAT

Format

Field

Pos. Value

Definition

Low Bit

Rate

VOC

_VAD

VAD Unvoiced Signal.

VAD Voice Signal.

VOC

_NUM

4..0

10100 (20 decimal) Number

of VOCDATA Bytes

ADPCM

VOC

_VAD

VAD Unvoiced Signal.

VAD Voice Signal.

VOC

_NUM

4..0

10010 (18 decimal) Number

of VOCDATA Bytes

Note that in “Conversation recording” the VOCSTA

byte is always $14.

The user must read the VOCDATA (and optionally

the VOCCORR) bytes and clear the VOCSTA byte

(writing $00).

If the user does not clear the VOCSTA byte within

the interrupt time, the ST75C530/540 will signal

this error by rising the ERR_VOCO in the SYSERR

byte and rising the IT0 Interrupt. In this case the

current frame is lost.

If the CONF_SUPSIL bit is 1 in the CONF CODER

command, the interrupts IT1 appears only when

the VAD has detected a voiced signal.

XIII - VOCODER DATA EXCHANGE (continued)

ST75C530 - ST75C540

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