Radio/Audio System Description and Operation

Radio/Audio System Description and Operation

Contents

* Regular Production Options (RPO)
* Features
* Circuit Description
* Component Description

RPO Options

The entertainment system on this vehicle is configured with either a base or uplevel audio system. Both the base and uplevel audio systems contain a radio, antenna, and speakers.

The following shows the entertainment regular production options (RPOs) that are available for this vehicle:

* (U1C) AM/FM Stereo CD
* (US8) AM/FM Stereo, CD, MP3, RDS, EQ
* (US9) AM/FM Stereo, 6-Disc CD, MP3, RDS, EQ
* (UX7) Base Four Speaker
* (U79) Uplevel Four Speaker
* (UZ6) Premium Audio
* (U2K) Digital Radio

Circuit Operation

Radio Power
- The main radio power is supplied by the 15A RADIO (BATT1) fuse in the body control module (BCM). Radio switch-on power, 12 volts in ACC, RUN or RAP, is supplied by the 10A RADIO (ACCY.) in the BCM.

Radio Grounds
- The main radio ground provides a ground for the radio circuits. An additional braided ground strap provides a shielding radio case ground to reduce electromagnetic interference (EMI) noise. Both grounds are connected together inside the radio.

Radio Speaker Outputs
- At zero volume, the plus (+) and minus (-) speaker outputs are both approximately 5-6 volts, measured to vehicle ground. If a plus or minus for any speaker output is shorted to ground or voltage, the radio circuitry will turn OFF all 4 speaker outputs for component protection. Above zero volume, the plus and minus change to create a voltage difference between each other, to drive the voice coil of the speaker.

Radio Park Lamp Input
- The radio park lamp input allows the radio to sense when the vehicle parking lamps are ON. When the parking lamps are ON, this circuit goes to battery voltage, and the radio display back-lighting switches from full bright to the brightness level determined by the instrument panel (I/P) illumination input.

Radio I/P Illumination Input
- A pulse width modulated (PWM) voltage for I/P illumination is provided to the radio I/P illumination input. The radio uses this input to directly illuminate the radio buttons and adjust the radio display back-lighting when the park lamp input is on.

Amplifier Power
- The main amplifier power is provided by the 20A RADIO fuse in the underhood fuse block (UHFB).

Amplifier Radio Speaker Inputs (UZ6 only)
- The radio speaker outputs, at a reduced output level for amplified systems, are the amplifier inputs. The amplifier boosts these inputs and outputs them to the vehicle speakers. Speaker plus and minus circuits from the radio change to create a voltage difference between each other. If one speaker plus or minus is open between the radio and the amplifier, the input to the amplifier is approximately half because only one of the circuits is changing. The speaker for that channel then operates at approximately only half the normal volume.

Amplifier Speaker Outputs (UZ6 only)
- At zero volume, the plus (+) and minus (-) speaker outputs are both approximately 5-6 volts, measured to vehicle ground. If a plus or minus for any speaker output is shorted to ground or voltage, the amplifier circuitry will turn OFF either the front outputs or rear outputs for component protection. Above zero volume, the plus and minus change to create a voltage difference between each other, to drive the voice coil of the speaker.

Amplifier Radio-On (UZ6 only)
- When the radio is ON, this circuit is pulled to 12 volts by the radio. The amplifier switches ON when this circuit is 12 volts and switches OFF when this signal is 0 volts.

Remote Radio Audio Signal Inputs (UE1 only)
- Audio output from the OnStar(R) communications module connects to the remote audio signal inputs of the radio. When the cellular telephone mute signal goes to 0 volts, the radio overrides any other audio signal and uses these inputs as the source for output to the speakers.

Cellular Telephone Mute (UE1 only)
- The OnStar(R) communications module uses the cellular telephone mute signal circuit to override the radio for OnStar(R) communication. When cellular telephone mute is not active, this circuit is held at 2 volts by the radio. When the cellular telephone mute signal is pulled to ground, the radio overrides any other audio signal and uses the remote audio signals as the source for output to the speakers. If the radio was OFF when this circuit is pulled low, the radio will turn ON. Additionally, the radio fades the speakers to full front, adjusts the volume to an initial audible level, and sets an AutoTone designed for optimal use with OnStar(R). When the mute signal is no longer pulled to ground, the radio returns to the mode it was in previously.

Component Description

Antenna System

The antenna system receives broadcast AM or FM stereo signals from free space and sends the signals to the radio receiver for processing via a coaxial antenna cable. Good antenna grounding is important for good radio reception.

The antenna base and mast should be installed to the torque specifications provided in the Digital Radio Antenna Replacement procedures.

The antenna mast is a single 1/4 wave design located at the right front fender.

Ground Strap

The braided ground strap, which is connected between the radio case and the instrument panel fuse block (IPFB), is provided to improve reception and deter noise from entering the audio system.

Radio

The operator interfaces with the radio system through the radio display and controls. Through these controls, the operator is able to control system power, volume, fade, balance, bass, and treble equalizations. Control on the integrated CD, MP3, or XM Satellite Radio System is also available when equipped with these options. A vacuum florescent display (VFD) provides system feedback to the operator.

The radio processes the AM and FM signals from the antenna system or the information from the CD media, amplifies that information, and sends the output to the speaker system.

The radio is located in the instrument panel (I/P) center stack area and is fastened to the I/P by 2 fasteners. Guide pins are provided to aid in aligning the radio. Electrical connections to the radio are a 24-way connector from the I/P harness, antenna lead connector, and braided ground strap. An additional 12-way harness connector is present on OnStar(R) equipped vehicles. Additional service length is provided in the radio harnesses to allow connection prior to radio installation.

Clock time is displayed continuously on the UM7 and U1C radios when the ignition is OFF. Pressing the RCL provides momentary backlighting. Time is not displayed on the US8 and US9 radios when the ignition is OFF. Pressing RCL will temporarily display the time.

Radio amplifier outputs to the speakers are protected from damage should speaker leads become shorted to ground or shorted to vehicle power. The radio will sense these conditions and shut down the amplifier outputs in a non-destructive manner. After the short condition is removed, the radio will return to normal operation.

Speakers

The speaker system consists of 4 speakers, mounted in the doors and rear package shelf. The optional speaker system, RPO UZ6, includes an additional speaker mounted in the front driver and passenger door mirror trim panel.

Auxiliary Jack Socket

The AM/FM Stereo CD radio shall provide a 3.5 mm Auxiliary stereo jack that will allow playback of audio signals from remote devices (e.g., portable tape player, IPOD, MP3 playback device, etc.).

OnStar(R)

OnStar(R) equipped, RPO UE1, vehicles use the radio amplifier and speaker system for voice communication from the OnStar(R) operator to the vehicle. Voice communication from the vehicle to the OnStar(R) operator is through the OnStar(R) microphone and module, which is not a part of the radio system.

When OnStar(R) begins operation, the radio volume is set to a preset level, autotone preset for OnStar(R) becomes active and the fade control is adjusted to the full front speakers. The radio volume control can then be used to adjust the volume to a desired level.

The radio system and OnStar(R) system are connected through the 12-way connector at the back of the radio. Left and right channel OnStar(R) audio, mute control of radio functions by OnStar(R), and the audio signal ground are the circuits provided in the 12-way connector.

Disconnecting the 12-way connector from the radio will isolate the radio system from the OnStar(R) system. Voice communication from the vehicle to the OnStar(R) operator could be possible under this condition. However, the vehicle occupant will not be able to hear the OnStar(R) operator.

For more information regarding radio operation problems, refer to CELL LINK and Diagnostic System Check - Vehicle. For further information regarding OnStar(R) operation, refer to the OnStar(R) section of this Service Data.

Amplifier

Vehicles equipped with premium audio (RPO UZ6) include an amplifier. The amplifier has low level inputs from the radio for the 4 speaker channels and amplifies the sound. The amplifier receives a radio on signal from the radio. This signal is delayed momentarily at initial key on to prevent any speaker pops during startup.

The purpose of an amplifier is to increase the power of a voltage or current signal. The output signal of an amplifier may consist of the same frequencies as the input signal or it may consist of only a portion of the frequencies of the input signal, as in the case of a subwoofer or a mid-range amplifier.

AM/FM Reception

Radio Signal

The radio signal is sent from a broadcast station and is then received by an antenna. The strength of the signal received depends on the following:

* The power output, or wattage, of the broadcasting station
* The location of the vehicle, or receiver, relative to the broadcast tower.
* Obstacles between the tower and the receiver
* Atmospheric conditions
* Which band, AM or FM, the station is broadcasting
* Type of antenna and the ground plane

AM Reception

The AM band has a lower frequency range than the FM band. These longer wavelengths:

* Bend around obstacles
* Follow the curvature of the earth
* May reflect, or skip, off of the ionosphere

The AM frequencies have longer range due to the ground wave. The ground wave follows the curvature of the earth and is affected by its conductivity. Greater conductivity equates to less signal loss, thus transmission over water is better than over land. The AM band has a range of 80-320 km (50-200 mi).

FM Reception

The shorter wavelengths of the higher frequency FM band:

* Reflect off obstacles
* Are absorbed by the ground
* Penetrate the ionosphere

Broadcasts in the FM band are limited to "line of sight" reception which is typically 40 km (25 mi). Even when out of a direct line of sight, the signal may be reflected into areas that would be in a "shadow" otherwise. Factors which affect the line of sight include:

* Height of the broadcast antenna
* Height of the receiving antenna
* Terrain and buildings in the broadcast path

XM Reception

XM satellite radio provides digital radio reception. The XM signal is broadcast from two satellites and, where necessary, terrestrial repeaters. The high power satellites allow the antenna to receive the XM signal even when foliage and other partial obstructions block the antennas view of the satellite. Terrestrial repeaters are used in dense urban areas. These repeaters will receive the satellite signal and re-broadcast them at much higher power levels in order to ensure reception in areas with densely packed tall buildings.

XM Satellite Radio

XM is a national satellite radio service that offers up to 100 coast to coast channels including music, news, sports, talk and children's programming. XM provides digital quality audio and text information, including song title and artist name. A service fee is required in order to receive the XM service. For more information, contact XM at www.xmradio.com or call 1-800-852-9696.

Digital Radio Receiver

The radio controls communicate with the digital radio receiver via the class 2 communication circuit. The digital radio receiver sends remote radio audio signals to the radio.