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TEA7540FP датащи(PDF) 3 Page - STMicroelectronics |
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TEA7540FP датащи(HTML) 3 Page - STMicroelectronics |
3 / 12 page FUNCTIONAL DESCRIPTION SWITCHED ATTENUATORS Fig.A represents a block diagram of a handsfree subset with attenuators in signal mode. To pre- vent the system from howling, the total loop gain, including acoustic feedback through the housing and sidetone coupling, must be less than 0dB. For this purpose, two switched attenuators are in- serted in each mode (emission and reception). The attenuation is shifted from one mode to the other, resulting from the speech level comparison between each way. To prevent the circuit to switch continuously in one way, the operation of the IC must be fully symetrical in both ways. This involves signal com- parison, attenuation value. GAIN COMPRESSOR In TEA7540, two signal compressors are inserted in each mode before the signal comparison, so the signal coming from each end has the same level (100mV peak), the losses in each way (for instance losses resulting from the line length in receiving mode) are compensated and the signal comparison is fully symetrical. The time constant of each signal compressor decreases 80 times more quickly than it increases to prevent from noise increasing between words. The compress- ing depth is 38dB. BACKGROUND NOISE DISCRIMINATION An additional feature provided in TEA7540 is background noise level discrimination in each way. The IC stores the background sound level with a long time constant (3 to 5 seconds depend- ing on an external RC) and compares it with the incoming signal in order to distinguish a useful signal (speech) from the background noise. This background noise memorization is also used to compensate the noise in each mode before signal comparison: the noise level in each mode is sub- N ° Name Function 1 GND Ground 2 CS Chip Select 3 OUTE Transmit Attenuator Output 4 ATTE Transmit Attenuator Input 5 HYST1 Transmit Channel Hysteresis 6 HYST2 Receive Channel Hysteresis 7 TIM RC Timer 8 CCE Time Constant of the Transmit Signal Compressor 9 CE3 Transmit Background Noise Memorization Output 10 CE2 Transmit Peak Detector Output 11 CE1 Transmit Rectifier Input 12 MICOUT Transmit Signal Compressor Output 13 INE Transmit Signal Compressor Input 14 Vref V+/2 - Reference Voltage 15 INR Receive Signal Compressor Input 16 RECOUT Receive Signal Compressor Output 17 CR1 Receive Rectifier Input 18 CR2 Receive Peak Detector Output 19 CR3 Receive Background Noise Memorization Output 20 CCR Time Constant of the Receive Signal Compressor 21 AMP IN Handset Preamplifier Input 22 AMP SUP Handset Preamplifier Power Supply 23 VOL Volume Control 24 ATR Attenuation Value 25 ATTR Receive Attenuator Input 26 OUTR Receive Attenuator Output 27 Iref Reference Current Source 28 V+ PIN FUNCTION TEA7540 3/12 |
Аналогичный номер детали - TEA7540FP |
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Аналогичное описание - TEA7540FP |
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