CMX981 Consumer Microcircuits Limited, CMX981 Datasheet - Page 15

• Manufacturer Part Number: CMX981
• Description: Advanceddigital Radio Baseband Processor (tetra Etc.)
• Manufacturer: Consumer Microcircuits Limited

Advanced Digital Radio Baseband Processor
5.5.3
5.5.4
5.5.5
5.5.6
5.5.7
 2002 CML Microsystems Plc
MIC 1
MIC 2
Microphone Amplification
The microphone input signal is passed through a variable gain amplifier. The gain of this amplifier
is selectable between 0 and 22.5dB in 1.5dB steps via the CodecGain1 register.
Encoder Filtering
The analogue input signal is passed through an ADC and then a digital bandpass filter conforming
to the G.712 standard. The highpass function of this filter is selectable. The filtered signal is then
decimated to 8kHz, and can be read via the serial interface. See figure 4.
Tone Generator
The tone generator can be enabled by setting bit 1 of the CodecSetup2 register. The frequency of
the tone is controlled by the CodecToneFreq register, and is variable from 0 to 4kHz in
approximately 0.5Hz increments.
Decoder Filtering
The data written via the serial interface is interpolated to 32kHz, and passed through the same
filter as described in section 5.5.4. The sidetone and ring tone are added to the decoder signal.
The decoder signal can be disabled at this point.
Output Amplification
The decoded signal is converted to an analogue signal using a sigma delta converter. The output
of this converter is passed through a variable gain amplifier. The gain of this amplifier is selectable
between 0 and -30dB in -2dB steps via the CodecGain1 register.
Earpiece Amp
0 or 6dB
Power Amp
0 or 6dB
Input Amp
0 or 20dB
(With mute)
20dB
Volume Amp
0 to -30dB
-2dB Steps
Mic Amp
0 to 22.5dB
1.5dB Steps
Figure 3 Voice Codec Block Diagram
DAC
ADC
15
Bandpass
Bandpass
SideTone Amp
-12.5dB to -27.5dB
-1dB Steps
Filter
Filter
Tone Amp
0 to -30dB
-2dB Steps
Interpolation
Decimation
Generator
Filter
Filter
Tone
Register BUS
CMX981
D/981/1