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TEA1095T Scheda tecnica(PDF) 11 Page - NXP Semiconductors |
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TEA1095T Scheda tecnica(HTML) 11 Page - NXP Semiconductors |
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11 / 28 page  1997 Nov 25 11 Philips Semiconductors Product specification Voice switched speakerphone IC TEA1095 DECISION LOGIC: PINS IDT AND SWT The TEA1095 selects its mode of operation (transmit, receive or idle mode) by comparing the signal and the noise envelopes of both channels. This is executed by the decision logic. The resulting voltage on pin SWT is the input for the voice-switch. To facilitate the distinction between signal and noise, the signal is considered as speech when its envelope is more than 4.3 dB above the noise envelope. At room temperature, this is equal to a voltage difference VENV−NOI = 13 mV. This so called speech/noise threshold is implemented in both channels. The signal on TXIN contains both speech and the signal coming from the loudspeaker (acoustic coupling). When receiving, the contribution from the loudspeaker overrules the speech. As a result, the signal envelope on TENV is formed mainly by the loudspeaker signal. To correct this, an attenuator is connected between TENV and the TENV/RENV comparator. Its attenuation equals that applied to the transmit amplifier. When a dial tone is present on the line, without monitoring, the tone would be recognized as noise because it is a signal with a constant amplitude. This would cause the TEA1095 to go into the idle mode and the user of the set would hear the dial tone fade away. To prevent this, a dial tone detector is incorporated which, in standard application, does not consider the input signals at RXIN as noise when they have a level greater than 42 mV (RMS). This level is proportional to RRSEN. As can be seen from Fig.8, the output of the decision logic is a current source. The logic table gives the relationship between the inputs and the value of the current source. It can charge or discharge the capacitor CSWT with a current of 10 µA (switch-over). If the current is zero, the voltage on SWT becomes equal to the voltage on IDT via the high ohmic resistor RIDT (idling). The resulting voltage difference between SWT and IDT determines the mode of the TEA1095 and can vary between −400 mV and +400 mV. Table 1 Modes of TEA1095 VSWT − VIDT (mV) MODE < −180 transmit mode 0 idle mode >180 receive mode The switch-over timing can be set with CSWT, the idle mode timing with CSWT and RIDT. In the basic application given in Fig.12, CSWT is chosen at 220 nF and RIDT at 2.2 MΩ. This enables a switch-over time from transmit to receive mode or vice-versa of approximately 13 ms (580 mV swing on SWT). The switch-over time from idle mode to transmit mode or receive mode is approximately 4 ms (180 mV swing on SWT). The switch-over time from receive mode or transmit mode to idle mode is equal to 4 × RIDT CSWT and is approximately 2 s (idle mode time). The inputs MUTETX and MUTERX overrule the decision logic. When MUTETX goes HIGH, the capacitor CSWT is charged with 10 µA resulting in the receive mode. When the voltage on pin MUTERX goes HIGH, the capacitor CSWT is discharged with 10 µA resulting in the transmit mode. VOICE-SWITCH: PINS STAB AND SWR A diagram of the voice-switch is illustrated in Fig.9. With the voltage on SWT, the TEA1095 voice-switch regulates the gains of the transmit and the receive channel such that the sum of both is kept constant. In the transmit mode, the gain of the transmit amplifier is at its maximum and the gain of the receive amplifier is at its minimum. In the receive mode, the opposite applies. In the idle mode, both transmit and receive amplifier gains are halfway. The difference between maximum and minimum is the so called switching range. This range is determined by the ratio of RSWR and RSTAB and is adjustable between 0 and 52 dB. RSTAB should be equal to 3.65 kΩ and sets an internally used reference current. In the basic application diagram given in Fig.12, RSWR is equal to 365 k Ω which results in a switching range of 40 dB. The switch-over behaviour is illustrated in Fig.10. In the receive mode, the gain of the receive amplifier can be reduced using the volume control. Since the voice-switch keeps the sum of the gains constant, the gain of the transmit amplifier is increased at the same time (see dashed curves in Fig.10). In the transmit mode however, the volume control has no influence on the gain of the transmit amplifier or the gain of the receive amplifier. Consequently, the switching range is reduced when the volume is reduced. At maximum reduction of volume, the switching range becomes 0 dB. |
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