MC55i Terminal Hardware Interface Description
3.7 Audio Interface
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MC55i_Terminal_HD_v01.201a
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2010-04-16
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3.7.2
Speech processing
The voiceband filter includes a digital interpolation low-pass filter for received voiceband sig-
nals with digital noise shaping and a digital decimation low-pass filter for voiceband signals to
be transmitted.
After voiceband (interpolation) filtering the resulting 4Mbit/s data stream is digital-to-analog
converted and amplified by a programmable gain stage in the voiceband processing part. The
output signal can directly be connected to the earpiece of the GSM cellular device or to an ex-
ternal handset earpiece (via I/O connector). In the opposite direction the input signal from the
microphone is first amplified by a programmable amplifier. After analog-to-digital conversion a
4Mbit/s data stream is generated and voiceband (decimation) filtering is performed.
The resulting speech samples from the voiceband filters are handled by the DSP of the base-
band controller to calculate e.g. amplifications, sidetone, echo cancellation or noise suppres-
sion.
Full rate, half rate and enhanced full rate, 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.
Note:
With regard to acoustic shock, the cellular application must be designed to avoid sending
false AT commands that might increase the amplification, e.g. for a high sensitive earpiece.
