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SL34118 Datasheet(PDF) 4 Page - System Logic Semiconductor |
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SL34118 Datasheet(HTML) 4 Page - System Logic Semiconductor |
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4 / 12 page  SL34118 System Logic Semiconductor SLS the other (RLI2) is at the output of the speaker amplifier. On the transmit side, one level detector (TLI2) is at the output of the microphone amplifier, while the other (TLI1) is at the hybrid output. Outputs RLO1 and TLO1 feed a comparator, the output of which goes to the Attenuator Control Block. Likewise, outputs RLO2 and TLO2 feed a second comparator which also goes to the Attenuator Control Block. The truth table for the effects of the level detectors on the Control Block is given in the section describing the Control Block. BACKGROUND NOISE MONITORS The purpose of the background noise monitors is to distinguish speech (which consists of bursts) from background noise (a relatively constant signal level). There are two background noise monitors - one for the receive path and one for the transmit path. Refering to Figure 2, the receive background noise monitor is operated on by the RLI1-RLO1 level detector, while the transmit background noise monitor is operated on by the TLI2-TLO2 level detector. They monitor the background noise by storing a dc voltage representative of the respective noise levels in capacitors at CPR and CPT. The voltages at these pins have slow rise times (determined by the external RC), but fast decay times. If the signal at RLI1 (or TLI2) changes slowly, the voltage at CPR (or CPT) will remain more positive than the voltage at the non- inverting input of the monitor’s output comparator. When speech is present, the voltage on the noninverting input of the comparator will rise quicker than the voltage at the inverting input (due to the burst characteristic of speech), causing its output to change. This output is sensed by the Attenuator Control Block. The 36 mV offset at the comparator’s input keeps the comparator from changing state unless the speech level exceeds the background noise by ≈4.0 dB. The time constant of the external RC ( ≈4.7 seconds) determines the response time to background noise variations VOLUME CONTROL The volume control input at VLC (Pin 13) is sensed as a voltage with respect to VB. The volume control affects the attenuators only in the receive mode. It has no effect in the idle or transmit modes. When in the receive mode, the gain of the receive attenuator will be +6.0 dB, and the gain of the transmit attenuator will be -46 dB only when VLC is equal to VB. As VLC is reduced below VB, the gain of the receive attenuator is reduced, and the gain of the transmit attenuator is increased such that their sum remains constant. Changing the voltage at VLC changes the voltage at CT (see the Attenuator Control Block section), which in turn controls the attenuators. The volume control setting does not affect the maximum attenuator input signal at which notice able distortion occurs. The bias current at VLC is typically 60 nA out of the pin, and does not vary significantly with the VLC voltage or with VCC. DIAL TONE DETECTOR The dial tone detector is a comparator with one side connected to the receive input (RXI) and the other input connected to VB with a 15 mV offset (see Figure 3). If the circuit is in the receive mode, and the incoming signal is greater than 15 mV (10 mVrms), the comparator’s output will change, disabling the receive idle mode. Tthe receive attenuator will then be at a setting determined solely by the volume control. The purpose of this circuit is to prevent the dial tone (which would be considered as continuous noise) from fading away as the circuit would have the tendency to swich to the idle mode. By disabling the receive idle mode, the dial tone remains at the normally expected full level. Figure 3. Dial Tone Detector AGS The AGS circuit affects the circuit only in the receive mode, and only when the supply voltage (VCC) is less than 3.5 volts. As VCC falls below 3.5 volts, the gain of the receive attenuator is reduced. The transmit path attenuation changes such that the sum of the transmit and receive gains remains constant. The purpose of this feature is to reduce the power (and current) used by the speaker when a line- powered speakerphone is connected to a long line, where the available power is limited. By reducing the speaker power, the voltage sag at VCC is controlled, preventing possible erratic operation. |
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