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AC75 Hardware Interface Description
Strictly confidential / Preliminary
3.15.1
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.
3.15.2
Microphone Circuit
AC75 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 AC75 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 AC75 is in Power-down mode, the input voltage at any MIC pin must not exceed ±0.3V
relative to AGND (see also section 5.1). 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
permanently.
AC75_HD_V00.202
outside a call, or AT^SNFM=,1 should be used to switch on VMIC
Page 64 of 120
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27.04.2006
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