TM9100 Service Manual Description 53
© Tait Electronics Limited August 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.
Arbitration
(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 analog 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
primary volume control and muting. The DAC and primary 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 via a
secondary volume control and to the control head via a buffer amplifier.
The output configuration of the audio power amplifier is balanced and
drives an internal speaker in non-remote control-head configuration 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 16Ω impedance whereas the
external speaker can be as low as 4Ω.
Volume Control
Configurations
There are two volume controls in the radio but only one is active at any time
when audio is being output to the speaker(s). The inactive volume control
is set to maximum. For non-remote control-head configuration, the
primary volume control is active. For remote control-head configuration,
the secondary volume control is active. This enables fixed level audio feed
to the remote control head, and independent volume control of the external
speaker and the speaker of the remote control head.
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