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TEA1085A датащи(PDF) 11 Page - NXP Semiconductors |
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TEA1085A датащи(HTML) 11 Page - NXP Semiconductors |
11 / 32 page March 1992 11 Philips Semiconductors Preliminary specification Listening-in circuit for line-powered telephone sets TEA1085; TEA1085A Larsen limiter detector (DTI and DCA) pins 15 and 16 The QLA output signal is AC coupled to the detector input DTI. DTI is biased by potential divider R30 and R31. The voltage applied to DTI of the Larsen level limiter is converted into a current for further processing in this circuit. Current adjustment is achieved using the network connected between DCA and VBB (see Fig.8). The equation for DC current is: The equation for AC current is: In the basic application: R30 = 100 k Ω, R31 = 220 kΩ, R33 = 500 Ω, R32 = 100 kΩ and C25 = 330 nF This results in IDCA = 11 µA and the equation: High-pass filter A third order high-pass filter is created between the microphone input voltage and the current flowing into DCA. The cut-off frequencies (see Fig.9) of the three sections are: Where: R25 = R26 and C22 = C23 The filter reduces the sensitivity of the system to own speech. Normal speech is in the frequency range 300 Hz to 3400 Hz, however, the Larsen signal normally occurs at a frequency > 3 kHz. With the component values as used in the basic application (see Fig.16); f1 = 500 Hz, f2 = 1 kHz and f3 = 3 kHz I DCA R30 R30 R31 + ----------------------------- V BB 1 R32 R33 + ----------------------------- × × = i DCA V DTI R33 ------------ forf> 1 2 --- π R33 C25 = i DCA V DTI ------------ 2 (mA/V) = f1 1 2 πR eg C24 ----------------------------- where R eq R30 R31 × R30 R31 + ----------------------------- == f2 1 2 πR33C24 ------------------------------ = f3 1 2 πR26C23 ------------------------------ 1/(2 πR25C22 ) == Where: Larsen limiter capacitor (LLC) pin 12 A 1 µF capacitor (C26) is connected externally between VSS and LLC to determine the attack and release timing of the Larsen level limiter in the listen-in and Larsen mode. The timing is also dependent on the value of the resistor connected between SIC and VSS. Larsen level limiter threshold (THL1 and THL2) pins 13 and 14 When the signal at DTI exceeds the first threshold level the capacitor connected to LLC will start to discharge. The first threshold level is determined by the value of the resistor, R35, connected to THL1 and VSS. The amount of discharge of C26 depends on how much the level of the signal at DTI exceeds the first threshold level (for normal speech the discharge is small). The Larsen effect is generally defined as a signal level of ≥ 100 mV(RMS), on line, for a period of more than 100 ms. The Larsen signal must be reduced to a low level within 200 ms. For Larsen signal levels (f > f3 in Fig.9) of ≥ 100 mV(RMS) at DTI and, with the component values of Fig.16, the system will switch from the listen-in mode to the Larsen mode in a time period of 100 ms to 200 ms; consequently, the initial Larsen effect will last only for a short period of time. Fig.9 Third-order high-pass filter. handbook, halfpage MGR038 20 log (dB) g go 20 log f f3 f2 f1 0 12 dB per octave 6 dB per octave 18 dB per octave speech Larsen g i DCA V m ----------- = g o A pre R33 ----------- = |
Аналогичный номер детали - TEA1085A |
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Аналогичное описание - TEA1085A |
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