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10 Circuit Description
The SpeechMike II consists of 5 main function groups, which are:
• Analogue processing
• Digital processing
• Microprocessor
• I/O expander
• Optical trackball sensor
• Buttons and LED's
10.1 Analogue processing
IC 301
The sound is taken up by the microphone of the SpeechMike and taken up by
IC301. The RC filter on the + Input (Pin 3) of the IC 301 is designed as filter with a LP cut
frequency of 200Hz and an HP cut frequency at 7 kHz. There is a fix amplification of the
incoming signal of about 32dB based on the ratio of R305 and R304. 560k/18K +1
The output of IC301 (pin 1) is connected via a LP filter consisting of C346 // C347 and C331
to Pin 27 of the Signal processor IC 302. The 2
mid. Voltage of 1,5 V. On + Input (Pin 5) the 2 100 k Resistors R306 and R307 divide the 3V
supply Voltage to 1,5V which is available on the Output (Pin 7) and supplied via R303 to the
+ Input (Pin 3) of IC 301.
10.2 Digital processing
IC302
IC302 is a programmable analogue /digital converter which converts the analogue audio
signal from IC301 into a digital signal.
The analogue signal is supplied via Pin 27 to IC 302 processed via a 20Db amplifier, which is
switch off because of high noise level followed by a ½ db programmable amplifier, which is
used by the software (slider) to adjust input level of the signal. The maximum sensitivity is –
55dBV (+1,8mv RMS) and the lowest level is –19dBV (+106mv RMS). The next step is than
the analogue/digital conversion
On pin 28 the MPI signal is used via TS303 and TS302 to switch on/off the microphone
amplifier. This is done via Pin 28 from IC 302. In case Pin 28 of IC302 is "H" Collector of TS
303 becomes "L" and TS 302 is switching on the 3V. This is to fulfil the USB low power
specification < 500 µA
Connections to the µ Controller
Pin 5 receives the Power down information.
Pin 6 is the input for the Chip Select (L) information
Pin 7 in the input for the Control Clock
Pin 8 the input for the Control Data
Pin 9 is Serial Data in
Pin 10 Serial Data out
Pin 11 Frame clock and
Pin 12 Bit Clock
Pin 15 Master Clock input
nd
part of IC 301 is used to supply the required
- 24 / 26-
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