40 Description TP9100 Service Manual
© Tait Electronics Limited May 2005
Rx Vocoder
(Digital Mode)
The 88 bits from the FEC are decoded to generate the fundamental
frequency of the frame, the voiced/unvoiced decisions for each frequency
band, and the spectral amplitudes. 20ms of speech is synthesized from this
information, and is interpolated between the previous frame and the next
frame to minimize any artefacts due to the transition from one frame to the
next.
Arbitrator
(Digital Mode)
Transmission over the air-interface from a radio terminal is governed by
channel access procedures. The radio must monitor the status symbols on
the inbound channel, and wait for the status to indicate that the channel is
free to use (idle) before transitioning to transmit mode. The channel access
procedures are supervised by various timers. Normally, these procedures
must be applied before transitioning from receive mode to transmit mode.
However, they may be overridden under emergency conditions.
CODEC The combined audio and side-tone signal is converted to analogue form by
a 16-bit DAC with integral anti-alias filtering. This is followed by a
programmable-gain amplifier with 45dB range in 1.5dB steps, that performs
volume control and muting. The DAC and the volume control are part of
the same CODEC device (AD6521).
Output to Speakers The output of the CODEC is fed to an audio power 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 but the internal speaker can be switched under
software control. The power delivered to each speaker is limited by its
impedance. The internal speaker has 16Ω impedance whereas the external
speaker can be higher than this.
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