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| 型號： | ST75C540FP-A |
| 廠商： | 意法半導體 |
| 英文描述： | SUPER INTEGRATED DEVICESWITH DSP, AFE & MEMORIES FORTELEPHONY,MODEM, FAXOVERINTERNET& POTSLINES |
| 中文描述： | 超級綜合DEVICESWITH數(shù)字信號處理器，模擬前端 |
| 文件頁數(shù)： | 14/84頁 |
| 文件大小： | 599K |
| 代理商： | ST75C540FP-A |
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V.3.10- VOCODERDescription
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 ST75C530/540and written into the dual port
RAM Vocoder Buffer (VOCxxx). At the same time
the ST75C530/540 is looking for an incoming
DTMFtone and 4 differentprogrammabletones.
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 differenttones.
Two algorithms of voicecoding are implemented :
- Low bit rate speech coder (4800bpsor 5300bps
with forward error correction).
- ADPCM (STproprietaryalgorithm) at 32, 24 and
16Kbps.
If the low bit rate coder algorithm is selected the
ST75C530/540has the capabilityto slow down or
speed up the DECODER flow up to
±
50%. This
Figure 2
functionallows a quickmessagelistening if speed
up is used, or at the oppositeif slow down is used,
an enhancementof the voice intelligibility.
V.3.11- VoiceActivity 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_109bit and STA_109Fbit reflect the state of
the VAD. After the CONFcommandthe VADis on
(assume voice). The default time-out to detect si-
lenceis2 secondsandtheset-uptimeto detectthe
voiceis 15ms. This VADinformationis alsocopied
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-
SILequal1) if along silenceis detected(STA_109
equal0) the ST75C530/540stopsgeneratingBuff-
er Interrupts.After that if a voice is again detected
theST75C530/540will resumethe BufferInterrupt
mechanism.
RING
STA_RING
T1
1/Fmax prog. < T1< 1/Fminprog.
T2 < 1/Fmaxprog.
T3
1/Fmin prog.
≈
T2
T3
7
Figure 1
V - FUNCTIONALDESCRIPTION
(continued)
Rx Signal
STA_109
(or VOCSTA bit 7)
2s
Interrupt (IT1)
7
ST75C530 - ST75C540
14/84
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