TM1215 Iss. 6.0 RPR750
4.19MHz, with a sub-system clock XL2 operating on either 32.768kHz or 38.4kHz,
depending upon the Baud rate selected for the pager (512 or 1200).
The radio circuits are activated by IC1/IC12 (RXEN-SK1/13) for a brief period every
second, so that a check may be made for the presence of a preamble signal. If preamble
is detected, then the radio is held on and the demodulated data signal is downloaded
(DATAP) from the Radio board via SK1/11, for decoding by IC1/IC12.
At the same time IC1/IC12 reads the pager address and the programmed options data
held in the EEPROM IC2 and if the decoded data is recognised as valid, then IC1/IC12
generates a call alert sequence which includes the options of flashing the Alert LED (HS),
an audible tone alert (BEEP), message text display and speech and/or vibrate (VIBRATE).
Communication between IC1/IC12 and the Display Module is via a high-speed serial data
bus, along a 12 way flexible connecting strip (signals SI, SCL, A0 & SO). The LCM
contains the LC display, a driver IC, control switches (OCR & MUTE), the red Alert LED
and LED backlighting facilities.
IC1/IC12 controls the radio circuits via SK1 pins 13 (RXEN), which enables the power
ON/OFF function and pin 12 (F_CHG), which controls the FSK reference voltage for the
data comparator (see preceding Radio PCB description).
Battery status data is also fed from IC401 via SK1 pin 10 (LOWB), which is processed by
IC1/IC12 to sound a “low battery” alert. In the event of the main battery being removed,
IC1/IC12 will also cause the switchover to the back-up battery, to maintain the internal
clock and messages.
3.2.4.2 Audio
The Audio section consists of speech storage chip IC3, a pre-amplifier IC9, digital
potentiometers IC7 and IC11 (MKII only) and power amplifier IC8.
When a speech call is received, the audio from the radio is fed from SK1 pin 9 (RX
AUDIO) into the pre-amplifier IC9 (pin 3). IC9 and IC8 are normally held quiet until a
speech call is detected by IC1/IC12, which then activates IC9 via (EN2) and IC8 via (EN1)
control lines. When the pager is in battery economy mode, or receiving calls, both the
control lines are cleared, putting the audio section into standby mode.
The output from IC9 (pin 4) is fed jointly to the input of stored-speech chip IC3 (SPEECH
IN) and also directly to input of IC8 (pin 1). In company with this, the output from IC3
(SPEECH OUT) is also fed to the input of IC8 (pin 1).
With this configuration, when a speech call is first received, both (EN1) and (EN2) control
lines are activated by IC1/IC12 and the audio is fed to both the loudspeaker and the
speech storage chip IC3 simultaneously. When a stored speech call is selected to be
replayed from IC3, only line (EN1) is enabled.
The functional gain of the power amplifier IC8 is controlled by the agc circuit combination
of TR7 and IC7, the latter being a digital potentiometer. The gain value set in IC7 is
determined from IC1/IC12, by toggling the control lines (CS1) and (U/D), enabling the user
to set the speech audio signal level entering IC8.
The alert tones are fed directly from IC1/IC12 (BEEP) output to digital potentiometer IC11
and then to the final power amplifier IC8.
IC8 provides a balanced drive to the loudspeaker. The overall gain of this final device is
held by R49, R50, R24 and R65.
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