Tait TM8000 Series, Service Manual

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TM8100 Mobile Radio Service Manual General Description 57
May 2004 © Tait Electronics Limited
Output to Speakers
The output of the CODEC is fed to an audio power amplifier and to the
control head via a buffer amplifier. The output configuration of the
audio power amplifier is balanced and drives an internal speaker and,
optionally, an external speaker. The speaker loads are connected in
parallel rather than being switched. The power delivered to each speaker
is limited by its impedance. The internal speaker has 16
Ω
impedance
whereas the external speaker can be as low as 4
Ω
.
2.5 Operation in Transmit Mode
Transmit Path
This subsection describes the functioning of the radio in transmit mode.
Detailed descriptions of the circuitry involved are given in
Section 3
(on page 63) . As shown in Figure 2.14 , the transmit path exists in the
hardware, cu
s
tom-logic and DSP domains. From a functional point of
view there are three major parts:
■
audio processing and signalling
■
frequency synthesizer
■
RF transmitter
These functional parts are described in detail below.
Audio Processing and Signalling
Microphone Input
The input to the transmitter path begins at the microphone input. There
are two microphone sources: a fist microphone connected to the control
head and an auxiliary microphone connected via the auxiliary or external
options connector. Only electret-type microphones are supported.
Support for optional dynamic fist microphones is facilitated by a
hardware amplifier and filter; this circuitry is mounted on a separate
board that needs to be installed in the control head.
Analogue Processing
of Microphone Signal
The CODEC (AD6521) performs microphone selection and
amplification. The microphone amplifier consists of an amplifier with a
fixed gain of 16 dB followed by a programmable-gain amplifier with 0 to
22 dB gain. The amplified microphone signal is converted to a digital
stream by a 16-bit ADC with integral anti-alias filtering (0.1 to 3.2 kHz).
The digital stream is transported to the DSP for further audio processing.
Automatic Level
Control
The ALC (automatic level control) follows, and is used to effectively
increase dynamic range by boosting the gain of the microphone pre-
amplifier under quiet conditions and reducing the gain under noisy
acoustic conditions. The ALC function resides in the DSP and controls
the microphone programmable-gain amplifier in the CODEC. The
ALC has a fast-attack (about 10 ms) and slow-decay (up to 2 s)
characteristic. This characteristic ensures that the peak signal level is
regulated near full scale to maximise dynamic range.