Tait TB7100, Service Manual

Page: 23 / 370

TB7100 Service Manual Functional Description 23
© Tait Electronics Limited October 2005
3.1.3 Audio Processing and Signalling
Audio Processing
Raw demodulated data from the receiver is processed within the DSP. The
sample rate at this point is 48kHz with signal bandwidth limited only by the
IF filtering. Scaling (dependent on the bandwidth of the RF channel) is then
applied to normalise the signal level for the remaining audio processing.
The sample rate is decimated to 8kHz and bandpass audio filtering (0.3 to
3 kHz) is applied. The base station takes the audio from the receiver mode
at Tap R4, this point has no de-emphasis.
Data and Signalling
Decoders
The data and signalling decoders obtain their signals from various points
within the audio processing chain. The point used depends on the
bandwidth of the decoders and whether de-emphasis is required. Several
decoders may be active simultaneously.
Side Tones
Side tones are summed in at the end of the audio-processing chain.
These are tones that provide some form of alert or give the user confidence
an action has been performed. The confidence tones may be generated in
the receiver. The side-tone level is a fixed proportion (in the order of
– 10dB) relative to full scale in the receive path.
CODEC
The combined audio and side-tone signal is converted to analog form by a
16-bit DAC with integral anti-alias filtering. This is followed by a
programmable-gain amplifier with a range of 45 dB in 1.5dB steps.
The amplifier performs muting. The DAC is part of the same CODEC
device (AD6521).
Output to Speakers
The output of the CODEC is fed to an audio power amplifier and to the
UI board via a buffer amplifier. The output configuration of the audio
power amplifier is balanced and drives an internal speaker. The power
delivered to the speaker is limited by its impedance. The speaker has 16
Ω
impedance.