Tuner Supply

Radio Reception

The following control modules are involved in the radio function:
- Antenna tuner with antenna 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 center 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 modules by means of MOST bus messages.

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

Rear window antenna
The rear window antenna 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 antenna is subdivided into the following functional areas:
- one antenna for the AM (amplitude modulation) band for receiving medium-wave transmissions; the AM antenna is located in the top left, unheated area of the rear window;
- one antenna for the FBD (remote control)
- four antennas 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 antenna tuner and fed onto the MOST bus as LF signals. The audio system controller prepares that data for audio output.

Antenna tuner
The antenna tuner is a control module for the radio and TV reception functions.

The antenna tuner has a separate power supply.
- In the AM band, the antenna tuner converts the HF signals from the AM antenna amplified by the diversity unit and feeds the LF signals onto the MOST bus.
- In the FM band, the antenna diversity unit connects the antenna 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 antenna diversity unit:
- one antenna for the AM (amplitude modulation) band; this is used for amplifier function only;
- two antenna circuits (right and left) for the FM band with two antennas each; signal processing for the two left antennas and amplification is performed by the antenna diversity unit; the right antenna signals are combined into one by the antenna amplifier and passed to the antenna diversity unit as a single signal.

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

Checking the antenna diversity unit
Connect the vehicle to the diagnostic system and select the control module functions.
- Select a local FM station with good reception.
- Select control module functions and read off the reception characteristics.
- Select the left antenna path and read off the reception characteristics.
- Select the right antenna path and read off the reception characteristics.
- For the remainder of the fault finding process follow the circuit diagram.