Speech Processing; Microphone Circuit; Single-Ended Microphone Input; Figure 18: Single Ended Microphone Input - Siemens TC65 Hardware Interface Description

TC65 Hardware Interface Description
Strictly confidential / Draft
3.13.1

Speech Processing

The speech samples from the ADC or DAI are handled by the DSP of the baseband
controller to calculate e.g. amplifications, sidetone, echo cancellation or noise suppression
depending on the configuration of the active audio mode. These processed samples are
passed to the speech encoder. Received samples from the speech decoder are passed to
the DAC or DAI after post processing (frequency response correction, adding sidetone etc.).
Full rate, half rate, enhanced full rate, adaptive multi rate (AMR), speech and channel
encoding including voice activity detection (VAD) and discontinuous transmission (DTX) and
digital GMSK modulation are also performed on the GSM baseband processor.
3.13.2

Microphone Circuit

TC65 has two identical analog microphone inputs. There is no on-board microphone supply
circuit, except for the internal voltage supply VMIC and the dedicated audio ground line
AGND. Both lines are well suited to feed a balanced audio application or a single-ended
audio application.
The AGND line on the TC65 board is especially provided to achieve best grounding
conditions for your audio application. As there is less current flowing than through other GND
lines of the module or the application, this solution will avoid hum and buzz problems.
3.13.2.1

Single-ended Microphone Input

Figure 18 as well as Figure 38 show an example of how to integrate a single-ended
microphone input.
R R
A
C
V
Bias
R

Figure 18: Single ended microphone input

R has to be chosen so that the DC voltage across the microphone falls into the bias voltage
A
range of 1.0V to 1.6V and the microphone feeding current meets its specification.
The MICNx input is automatically self biased to the MICPx DC level. It is AC coupled via C
to a resistive divider which is used to optimize supply noise cancellation by the differential
microphone amplifier in the module.
TC65_HD_V00.521
VMIC
R
VMIC
A
MICPx
F
MICNx
C
K
B
AGND
Page 55 of 99
R = typ. 2k
A
R = typ. 5k
B
R
VMIC
C = typ. 100nF
k
C = typ. 22µF
F
V = typ. 2.5V
MIC
GSM module
V = 1.0V ... 1.6V, typ. 1.5V
bias
s
= typ. 470Ohm
24.05.2005
K