Tait TB7100, Service Manual

Page: 25 / 370

TB7100 Service Manual Functional Description 25
© Tait Electronics Limited October 2005
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.
DSP Audio
Processing
The output of the automatic level control provides the input to the DSP
audio-processing chain at a sample rate of 8 kHz. Optional processing such
as encryption or companding is done first if applicable. Pre-emphasis, if
required, is then applied. The pre-emphasised signal is hard limited to
prevent over deviation, and filtered to remove high-frequency components.
The sample rate is then interpolated up to 48 kHz and scaled to be suitable
for the frequency synthesizer.
Data and Signalling
Encoders
The data and signalling encoders inject their signals into various points
within the audio-processing chain. The injection point depends on the
bandwidth of the encoders and whether pre-emphasis is required.
3.2.2 Frequency Synthesizer
Main Parts of
Synthesizer
The frequency synthesizer consists of two main parts:
■
FCL (frequency control loop)
■
RF PLL (phase-locked loop)
The FCL and RF PLL are described briefly below. Note that patents are
pending for several aspects of the synthesizer design.
Frequency Control
Loop
The FCL consists of the following:
■
TCXO
■
mixer
■
loop filter
■
VCXO (voltage-controlled crystal oscillator)
■
frequency control block
The FCL provides the reference frequency for the RF PLL. It generates a
high-stability reference frequency that can be both modulated and offset in
fine resolution steps.