Product Info

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VTech Communications LTD.
R&D Department
The document is VTech’s proprietary information, Please do not disclose to third party without VTech’s consent
Ref. No.: 20191009 Version: 01 Date: 09 Oct, 2019 Page 17 of 33
7.2.3.3 DAC digital FIR filte
The DAC contains an integrated digital FIR filter with the following modes:
• A default long FIR filter for best performance at ≥ 44.1 kHz.
• A short FIR to reduce latency.
• A narrow FIR (a sharp roll-off at Nyquist) for G.722 compliance. Best for 8 kHz/16 kHz.
7.2.4 Microphone bias generator
VTBM-A01 BT Module contains an independent low-noise microphone bias generator. The microphone bias
generator is recommended for biasing electret condensor microphones.
Figure 7-4 shows a typical biasing circuit for electret condenser microphones.
Figure 7-4 Microphone biasing
The microphone bias generator provides a selectable output voltage of 1.8 V or 2.6 V nominal and derives its power
from VBAT or VOUT_3V3. No output capacitor is required.
The bias resistor R1 should match the microphone load impedance, and typically is 2.2 kΩ. C1 and C2 are typically
100/150 nF to give a bass roll-off to limit wind noise on the microphone.
The mic bias generator has a maximum drop out of 300 mV, if VBAT drops below (selected output voltage – drop out
voltage), the output voltage will fall below specification. The generator will continue to operate but noise performance
will be impaired.
7.2.5 Line input
Figure 7-5 and Figure 7-6 show 2 circuits for line input operation and show connections for either differential
or single-ended inputs.