Radio reception

The following control units are involved in the radio function:

• Aerial tuner with aerial amplifier and diversity unit
  
• Audio system controller
  
• Audio amplifier (HiFi or stereo option) or amplifier (Top Hifi)
  
• Control Display
  

Brief description of components

Control Display (CD) and Controller

Control Display (CD): apart from a few exceptions, the Control Display incorporates the controls and displays for all vehicle functions.

Controller:the control display is operated by means of the controller located on the front centre armrest.

MOST bus

The MOST (Media Oriented Systems Transport) bus links the audio, video and navigation systems. Information relating to function requests is transmitted to the relevant control units by means of MOST bus messages.

Audio system controller

The audio system controller (ASK) is a control unit in the MOST network. The ASK is responsible for managing all audio signals.

Rear window antenna

The rear-window aerial is subdivided into a number of reception areas in order to improve reception. Division into left and right areas reflects the arrangement of the amplifiers.

Main functions

Rear window antenna

In contrast to a home radio, a car radio is constantly subjected to changing reception conditions. In order to improve reception, therefore, the rear-window aerial is subdivided into the following functional areas:

• one aerial for the AM (amplitude modulation) band for receiving medium-wave transmissions; the AM aerial is located in the top left, unheated area of the rear window;
  
• one aerial for the FBD (remote control)
  

• four aerials for the FM (frequency modulation) band for receiving UHF, FM1, FM2, FM3, FM4, FM5 (Japan only) transmissions
  

The HF signals received and filtered are amplified and converted by the aerial tuner and fed onto the MOST bus as LF signals. The audio system controller prepares that data for audio output.

Aerial tuner

The aerial tuner is a control unit for the radio and TV reception functions.

The aerial tuner has a separate power supply.

• In the AM band, the aerial tuner converts the HF signals from the AM aerial amplified by the diversity unit and feeds the LF signals onto the MOST bus.
  
• In the FM band, the aerial diversity unit connects the aerial with the best FM reception to the tuner. The tuner amplifies the HF signals and converts them. They are passed to the ASK as LF signals.
  

The following are connected to the aerial diversity unit:

• one aerial for the AM (amplitude modulation) band; this is used for amplifier function only;
  
• two aerial circuits (right and left) for the FM band with two aerials each; signal processing for the two left aerials and amplification is performed by the aerial diversity unit; the right aerial signals are combined into one by the aerial amplifier and passed to the aerial diversity unit as a single signal.
  

The aerial diversity unit selects the best aerial circuit by the following process:

• The aerial tuner checks the strength of the signal being received on each aerial group by continually switching back and forth between the left and right aerial group on the aerial diversity unit.
  
• The aerial diversity unit patches through the aerial with the best reception quality on the basis of the signal received from the aerial tuner.
  That aerial group is used for the next check cycle.
  
• If there is no feedback on the reception signal strength on the aerial diversity unit, e.g. due to a break in the ribbon cable between the aerial tuner and the aerial diversity unit, the aerial diversity unit automatically switches to the left aerial group.
  If either the CONTROL or DIAG signal is lost, no aerial group can be selectively activated through the diagnosis system.
  

Audio System Controller (ASK)

The audio signals include the gong functions (PDC, CC, double gong, hour gong and continuous gong), navigation system instructions and telephone. The ASK mutes certain speaker signals in order to give other signals precedence. (Example: radio ”Mute” function when a telephone call is accepted.)

Control Display control unit functions

The Control Display coordinates the function requests from the system and assigns them to the relevant functions.

The Control Display is the only control unit which knows which function is currently active on the system. If the volume is to be increased (function request), the Control Display sends a messages to that effect to the ASK/amplifier. That message also contains the information indicating to which audio source the volume adjustment applies. Infotainment audio output is always a certain amount louder than the entertainment sources.

Service functions

Checking FM reception

FM reception is best checked by driving around a circular route. The aerial diversity unit connects the aerial with the best reception to the aerial tuner.

Checking the aerial diversity unit

Connect the vehicle to the diagnostic system and select the control unit functions.

• Select a local FM station with good reception.
  
• Select control unit functions and read off the reception characteristics.
  
• Select the left aerial path and read off the reception characteristics.
  
• Select the right aerial path and read off the reception characteristics.
  
• For the remainder of the fault finding process follow the circuit diagram
  

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