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10 ACCESSORIES
10.1 Handsfree Unit - Circuit Description
10.1.1 General Description
The handsfree unit consists of audio processing, power supply and external power for the charging circuitry within
the handheld unit.
A digital signal processor (DSP) is used to remove the local echo or feedback created between the loudspeaker
and microphone.
The analogue speech signal is converted into digital form using CODECs, which perform analogue to digital and
digital to analogue conversion on the received and transmitted audio.
The handheld unit has an internal charging circuit. The handsfree unit uses this facility to charge the attached
battery.
The connector at the base of the handheld unit contains a mechanical RF switch. When the connector is in use the
RF signal is disconnected from the internal antenna and sent out through the handsfree unit RF cable.
10.1.2 Detailed Description
TX Audio
The external microphone connects to P301. The voice signal from the microphone is amplified by U311 and
converted into a digital form by U305. U305 is a CODEC containing an analogue to digital converter for the
transmitted audio, and a digital to analogue converter for the received audio.
The digitized speech is processed by U306 (DSP echo canceller). The output from pin 13 of the DSP is re-
converted into analogue form by U304 (CODEC). Further amplification is provided by U301 and U315. The audio
signal is then transmitted via P201 to the handheld unit for further processing.
RX Audio
The voice signal from the handheld unit is amplified by the handsfree unit. P201 connects the audio signal from the
handheld to the handsfree unit via pin 4. U301 provides initial amplification of the received audio. R324 is a variable
resistor controlled by the thumbwheel on the side of the handsfree case, providing volume level control.
The voice signal is converted into digital by U304 and processed by the DSP (U306). U305 re-converts the speech
into analogue form. Further amplification is provided by U308, the output of which passes to the internal speaker
via J300 and P300. If an external speaker is connected to J300 then the path to the internal speaker is broken and
the audio is transmitted to the external source via J300.
Power Supply
Supply for the handsfree unit is provided via P203. Reverse voltage protection is provided by D201. An active low
signal from the handheld unit (nLogic Pwr) switches Q202 and Q203 ON. Activation of Q203 switches the base of
Q204 low, enabling the positive supply through Q204 to the voltage regulators. D202 provides over-voltage
protection. Should the supply exceed 16V, D202 will break down switching Q201 ON and sending the base of
Q203 low, switching Q204 OFF.
The output from Q204 takes two paths. Through Q309 the positive 13V supply is regulated down to 5V by U302
and U303. U302 supplies the DSP, CODECs and reset control. U303 supplies the audio amplifiers.
The 13V supply is also regulated down to 6.7V for the external power supply for the handheld unit. U101 is a
switching regulator that provides this function.
Level Detector
A sample of the output audio is taken from J300 pin 2. U309 detects the audio signal level on pin 1. If the detected
speaker audio level is high, then pin 6 will go low. A low signal on pin 6 switches Q305 ON, switching positive
supply onto Q306. Q306 switches R305 into the TX audio path, reducing the audio level.
MCUK971001G8
Section 10
Issue 1
Technical Guide
10 - 1
Revision 0
ACCESSORIES
