Tait TP9100 Service Manual

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Contents

40 Description TP9100 Service Manual© Tait Electronics Limited May 2005Rx Vocoder(Digital Mode) The 88 bits from the FEC are decoded to generate the fundamentalfrequency of the frame, the voiced/unvoiced decisions for each frequencyband, and the spectral amplitudes. 20ms of speech is synthesized from thisinformation, and is interpolated between the previous frame and the nextframe to minimize any artefacts due to the transition from one frame to thenext.Arbitrator(Digital Mode) Transmission over the air-interface from a radio terminal is governed bychannel access procedures. The radio must monitor the status symbols onthe inbound channel, and wait for the status to indicate that the channel isfree to use (idle) before transitioning to transmit mode. The channel accessprocedures are supervised by various timers. Normally, these proceduresmust 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 bya 16-bit DAC with integral anti-alias filtering. This is followed by aprogrammable-gain amplifier with 45dB range in 1.5dB steps, that performsvolume control and muting. The DAC and the volume control are part ofthe same CODEC device (AD6521).Output to Speakers The output of the CODEC is fed to an audio power amplifier. The outputconfiguration of the audio power amplifier is balanced and drives an internalspeaker and, optionally, an external speaker. The speaker loads areconnected in parallel but the internal speaker can be switched undersoftware control. The power delivered to each speaker is limited by itsimpedance. The internal speaker has 16% impedance whereas the externalspeaker can be higher than this.