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ST75C530

SUPER INTEGRATED DEVICES WITH DSP, AFE & MEMORIES FORTELEPHONY, MODEM, FAX OVERINTERNET & POTS LINES

STMicroelectronics 

ST75C530 - ST75C540

V - FUNCTIONAL DESCRIPTION (continued)

Figure 1

RING

T1

T2

T3

STA_RING

1/Fmax prog. < T1 < 1/Fmin prog.

T2 < 1/Fmax prog.

T3 ≈ 1/Fmin prog.

V.3.10 - VOCODER Description

The Vocoder mode allows the implementation of

an answering machine function. In the CODER

mode the received samples from one of the two

analog inputs, Line or Audio, are compressed by

the ST75C530/540 and written into the dual port

RAM Vocoder Buffer (VOCxxx). At the same time

the ST75C530/540 is looking for an incoming

DTMF tone and 4 different programmable tones.

In the DECODER mode the compressed samples

are read from the dual port RAM, decompressed

and transmitted to one of the two analog output,

Line or Micx. The ST75C530/540 synthesises an

estimation of its echo and subtracts it from the

received signal. At the same time the

ST75C530/540 is looking for an incoming DTMF

tone and 4 different tones.

Two algorithms of voice coding are implemented :

- Low bit rate speech coder (4800bps or 5300bps

with forward error correction).

- ADPCM (ST proprietary algorithm) at 32, 24 and

16Kbps.

If the low bit rate coder algorithm is selected the

ST75C530/540 has the capability to slow down or

speed up the DECODER flow up to ±50%. This

Figure 2

function allows a quick message listening if speed

up is used, or at the opposite if slow down is used,

an enhancement of the voice intelligibility.

V.3.11 - Voice Activity Detector (VAD)

In CODER Mode, for both of the Voice Coding

algorithms, a Voice Activity Detector is imple-

mented while coding by the ST75C530/540. The

STA_109 bit and STA_109F bit reflect the state of

the VAD. After the CONF command the VAD is on

(assume voice). The default time-out to detect si-

lence is 2 secondsand the set-up time to detect the

voice is 15ms. This VAD information is also copied

into the Receive Buffer Status Word MSB (VOC-

STA bit7). This detector is fully programmable in

level sensitivity (down to -60dBm), hysteresis, and

various criteria.

An optional silence suppressor is implemented in

the Coder section to suppress long silence in the

incoming message. When enabled (CONF_SUP-

SIL equal 1) if a long silence is detected (STA_109

equal 0) the ST75C530/540stops generating Buff-

er Interrupts. After that if a voice is again detected

the ST75C530/540will resume the Buffer Interrupt

mechanism.

Rx Signal

2s

STA_109

(or VOCSTA bit 7)

Interrupt (IT1)

14/84

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