Specifications

XT65/XT75 Hardware Interface Description
3.14 Audio Interfaces
s
XT65_XT75_HD_v01.001 Page 60 of 133 2007-1-8
Confidential / Released
3.14.1 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.
3.14.2 Microphone Circuit
XT65/XT75 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 XT65/XT75 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.
While XT65/XT75 is in Power-down mode, the input voltage at any MIC pin must not exceed ±0.3V relative to
AGND (see also Section 7.1). In any other operating state the voltage applied to any MIC pin must be in the range
of +2.7V to -0.3V, otherwise undervoltage shutdown may be caused.
If VMIC is used to generate the MICP-pin bias voltage as shown in the following examples consider that VMIC
is switched off (0V) outside a call. Audio signals applied to MICP in this case must not fall below -0.3V.
If higher input levels are used especially in the line input configuration the signal level must be limited to 600mV
pp
outside a call, or AT^SNFM=,1 should be used to switch on VMIC permanently.
3.14.2.1 Single-ended Microphone Input
Figure 23 as well as Figure 54 show an example of how to integrate a single-ended microphone input.
R
A
= typ. 2k
R
B
= typ. 5k
R
VMIC
= typ. 470Ohm
C
k
= typ. 100nF
C
F
= typ. 22µF
V
MIC
= typ. 2.5V
V
bias
= 1.0V … 1.6V, typ. 1.5V
Figure 23: Single ended microphone input