SIGMA BENEFON TDP-52-SN3 - Manual (Page 44)

27.1.1999
4A0701GB.__1 10
Product: TDP-52-SN3 (Sigma Gold) / OA0700 Audio
Audio
5.1.5 Function Description
OA0700 Audio
The audio module is comprised of the following functions:
1. TX-audio signal handling
2. RX-audio signal handling
3. Fii signal handling
4. FFSK modem
5. DTMF generator/receiver
6. Signal level detectors
7. Compander
8. Expander
9. Buzzer
The audio functions are mainly located in a single circuit AK2339. This chip
from AKM is controlled by a serialbus. It is possible to shut down parts of the
circuit, one block at a time to minimise power consumption.
5.1.6 TX-audio
The input from the microphone is fed to the audio circuit I405 pin 61, which is
an operational amplifier (AMP1) inverting input. The operational amplifier gain
is set by resistors R413 and R416. The amplifier is connected as a low pass
filter. After the amplifier is the microphone switch and then summing junction of
MIC input signal, EXTMIC input signal and transmit DTMF signal. VR1 is a
programmable amplifier, which sets the microphone signal (sensitivity) to the
correct level. After VR1 comes band-pass filter for transmitting the voice signal.
After the TXBPF is the summing junction (AMP6) of the tx signal and transmit
answer signal (ANAOUT). TXDET is the transmit voice signal detection circuit
which works as a full wave rectifier. Next comes ATT1 which is an attenuate
circuit to set the transmit signal level in the HF-mode. COMP is the compressor
circuit. Compress the transmitting signal amplitude with square root law. It can
be hypassed. The linearity is adjustable by the control register CVR. Next is
VR2, normal deviation gain control circuit to set the signal level. The P/E &
LIMIT pre-emphasis circuit and limiting circuit, emphasize the higher frequency
component of the signal in order to improve the signal-to-noise ratio of
modulated signal. This block includes a limiting circuit for signal amplitude in
order to confine the maximum deviation of the transmit modulated signal.
TXLPF is the low pass filter to reject the higher frequency component on the