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ST75C540 数据表(PDF) 14 Page - STMicroelectronics |
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ST75C540 数据表(HTML) 14 Page - STMicroelectronics |
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14 / 84 page  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 r e ce i v ed s i gnal . A t t he s a me t i m e th e 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 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 ST75C530/540 stops generating Buff- er Interrupts. After that if a voice is again detected the ST75C530/540 will resume the Buffer Interrupt mechanism. RING STA_RING T1 1/Fmax prog. < T1 < 1/Fmin prog. T2 < 1/Fmax prog. T3 1/Fmin prog. ≈ T2 T3 Figure 1 V - FUNCTIONAL DESCRIPTION (continued) Rx Signal STA_109 (or VOCSTA bit 7) 2s Interrupt (IT1) Figure 2 ST75C530 - ST75C540 14/84 |
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