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Audio
5.1.4 Function Description
OA1900 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.
5.1.5 TX-audio
The input from the microphone is fed to the audio circuit I401 pin 61, which is
an operational amplifier (AMP1) inverting input. The operational amplifier gain
is set by resistors R403 and R404. 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.
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.
Before TXLPF is the FFSKTX switch. TXLPF is the low pass filter to reject the
higher frequency component on the transmit signal. VR3 is a maximum
4A1900GB.fm
Product: TDP-60-HN /
1.3.1999
OA1900 Audio
9
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