Evaluation of In-Car Communication Systems - Digital Signal Processing for In-Vehicle Systems and Safety - page 71

Passenger
compartment

Loud-
speakers

Micro-
phones

Front
passengers

Rear
passengers

Amplification, AD conversion,
sample rate conversion, signal distribution

Microphone
signals

In-car
communication

Loud-
speaker
signals

Music/audio sig-
nals and prompts

Hands-
free phone

Information via
CAN bus, etc.

Remaining audio
systems (music,
voice dialog, etc.)

Phone input

Phone output

Sample rate conversion, signal mixing,
DA conversion, amplification

Fig. 5.4 Structure of an ICC system embedded in the sound system of a vehicle

Figure 5.4 sketches the structure of an ICC system with several microphones and
loudspeakers for both front and rear passengers. The ICC systems operate in a
closed electroacoustic loop since the microphones pick up at least a portion of the
loudspeaker signals. If this portion is not sufficiently small, sustained oscillations
appear which can be heard as howling or whistling. The howling margin depends on
the output gain of the ICC system, on the gains of the analog microphone and
loudspeaker amplifiers, as well as on the acoustic properties of the passenger
compartment. For this reason, all gains within an ICC system need to be adjusted
carefully.
Since ICC systems are usually incorporated into the audio system of a vehicle, not
only several restrictions but also additional possibilities arise. Cars that are equipped