30.8.
In-car Entertainment and Communication
Nowadays in-car entertainment (ICE) systems are fitted to many of the standard production cars. Facilities such as compact disc players and multiple compact disc chargers, together with automatic station search and retune, are becoming ever more popular. Hands free car telephones, so that both hands can be used to control the car, are now in use. ICE and communication systems have been developed very fast. The following is a description t)f some of the systems available and in use today.
30.8.1.
In-car Entertainment (ICE).
Most ICE systems now include six speakers; two larger speakers in the parcel shelf to produce good low frequency reproduction, two front door speakers for mid range and two front door tweeters for high frequency notes. Controls on the set include volume, treble, bass, balance and fade. Cassette tape options include Dolby, filters to reduce hiss and other tape selections such as chrome or metal. A digital display provides a visuals output of operating condition. This is also linked into the vehicle lighting to prevent glare at night. Track selection and programming for one or several compact discs is available.
The ICE systems are also coded to prevent theft. The code is activated when the main supply is disconnected. This prevents the operation of the set until the correct code has been re-entered. Some systems now use a plug in electric key, which makes the set worthless when it is removed from the car.
One of the most promising developments at present is radio data system (RDS). This is an extra inaudible digital signal, which is sent with FM broadcasts in a similar way that teletext is transmitted with TV signals. RDS provides all the information a receiver requires to act intelligently. The possibilities available now with RDS are:
(i) The station name can be displayed in place of the frequency.
(ii) Automatic tuning is incorporated to provide the best available signal for the chosen radio station.
(Hi) Traffic information broadcasts can be identified and a setting is made so that whatever the driver is listening to at the time can be interrupted. RDS, normally contains six main features as follows. Programme Identification. This allows the retune facility to follow the correct broadcasts.
Alternative Frequencies. This allows the receiver to try other signals for returning as required.
Programme Service Name. This displays the name of the station on the radios set.
Traffic Information. This provides two codes to work in conjunction route finding equipment.
Traffic Programme. This allows the set to indicate that the station broadcasts traffic information.
Traffic Announcement. This allows the receiver to adjust the volume, switch over from the cassette during a traffic announcement, lift an audio mute or of course if the driver wishes so, to do nothing.
There are two main types of radio signal transmitted. These are amplitude modulation (AM) and frequency modulation (FM). Figure 30.56 projects the difference between AM and FM signals. Electromagnetic waves used to produce sound waves in the audible range, such as speech and music, have a frequency too low for efficient transmission through the air for significant distances. Through the process of modulation, this low-frequency audio information can be impressed on a carrier wave having a much higher frequency and can propagate through space for greater distances. The transmitter at a radio station generates a carrier wave with constnat characteristics, such as amplitude and frequency. The signal containing the desired information is then used to modulate the carrier. This new carrier wave, called the modulated wave, contains the information of the signal.
With amplitude modulation, the amplitude to the carrier wave
is varied so that it contains the information of the signal. When the modulated wave reaches a radio receiver tuned to the proper frequency, it is demodulated, which is the opposite of modulation. The set can then reproduce the desired sound via an amplifier and the loudspeakers.
Amplitude modulation (AM) radio is still popular for radio broadcasting, but is has a number of disadvantages. The quality of reproduction is relatively poor due to inherent limitations in the technique and interference from other stations and other electrical signals, such as those produced by lighting or by electronic devices used in the car. Some of these drawbacks can be/ overcome by using frequency modulations. The quality of AM reception is limited by the narrow bandwidth of the signal. During the winter months reception of AM signals deteriorates due to changes in the atmosphere.
In frequency modulation (FM) the frequency of a wave is varied in response to a modulating wave. There wave that has a varying frequency is called the carrier and the modulating wave is the signals. Frequency modulation requires a higher-frequency carrier wave and a more complex method for transmitting information than amplitude modulation. An important advantage with FM is that the signal has constant amplitude. Therefore it is much less susceptible to interference from both natural and artificial sources. FM radio is generally a far better source of high fidelity music.
FM does however present problems with mobile reception. As most vehicles use a rod aerial, which is omni-directional, the aerial receives signals from all directions. Because of this, reflections from buildings, hills and other vehicles can reach the set all at the same time. This can distort the signal and is heard as a series of clicks or signal flutter as the signal is constantly enhanced or reduced. The best FM reception is considered to be within line of sight of the transmitter. In general the coverage of FM transmitters is quite high and especially with the RDS mobile reception is quite acceptable.
The aerial is worth a mention at this stage. Out of several types in use, the most popular still is the rod aerial, which is often telescopic. The advantage of a rod aerial is that is extends beyond the interference field of the vehicle. Consideration must be given to the positon of an external aerial. This has to be compromise, taking into account the following factors:
• Rod length-one metre if possible.
• Coaxial cable length- longer cable reduces the signal strength.
• Position relative to the ignition coil and leads- as far away as reasonably possible.
• Potential for vandalism- out of easy reach.
Fig. 30.56. AM and FM signals.
• Aesthetic appearance- it should fit with the style of the vehicle.
• Angle of fitting- vertical is best for AM and horizontal best for FM.
For reception in the AM bands, the aerial has a capacitance of 80 pF with a shunt resistance of about 1 MQ. The set often uses a trimmer to ensure that the aerial is matched to the set. The contact resistance between all parts of the aerial should be less than 20 mQ. This is particularly important for the earth connection.
For FM bends signalling, the length of the aerial is very important. The ideal length of a rod aerial for FM reception is one quarter of the wavelength. In the middle of the FM band (94 MHz) this is about 80 cm. Considering the magnetic and electrical field of the vehicle and the effect of the coaxial cable, the most practical length of the aerial is about 1 m.
Some smaller aerials are available but while these may be more practical the signal strength is considerably reduced. Aerials embedded into the vehicle windows or using the heated rear window element are good from the damage prevention point of view and because they are insensitive to moisture, but produce a weak signal often requiring an aerial amplifier. However this also amplifies interference. Some top range vehicle use a rod aerial and a screen aerial, the set being able to detect and use the strongest signal. This reduces the affect of reflected signals and causes less flutter.
Figure 30.57 presents a circuit typical of a modern ICE system. An electrical aerial is included in the circuit and also a connection to a multiple compact disc unit via a type of a data bus.
30.8.2.
Mobile Communications.
The vehicle communication equipment for normal business and personal use is the simple pocket sized mobile phone. For mobile communications, CB radios and short range two way systems are nowadays used by taxi firms and service industries. This is expected to continue for some time till the cellular network becomes cheaper and more convenient to use.
30.8.3.
Interference Suppression
Interference suppression is the reduction of unwanted noises produced from the speakers of an ICE system. To achieve this, first it is necessary to know the different types of interference produced. Figure 30.58 shows two signals; one clean and the other suffering from interference. The amount of interference can be presented as a signal-to-noises ratio. This ratio is defined as the useful field strength to the interference field strength at the receiver. This should be as high as possible but more than 100:1 for radio reception. Interference can propagate in four waves:
(i) Line borne- conducted through the wires.
(ii) Airborne radiated- through the air to the aerial. (Hi) Through capacitive coupling- by an electric field. (iv) Through inductive coupling- by magnetic linking.
Two types of vehicle interference are: (i) Short range interference, which is the effect is interference on the vehicle’s radio system.
Fig. 30.57. A typical modern ICE system circuit.
(ii) Long range interference, which is the effect of vehicle on external receivers such as domestic televisions.
This is covered under legislation where it is illegal to cause disturbance to radios or televisions when using a vehicle.
The sources of interference in a motor vehicle can be summarized simply as any circuit that is switched or interrupted suddenly. This includes the action of a switch and the commutation process in a motor, both of which produce rapidly increasing signals. The principle of suppression is to slow down this increase. This main areas of vehicle responsible interference production are ignition system, the charging system, motors and switches, and through static discharges.
The ignition system of a vehicle is the biggest source of interference, in specific the high tension side. Voltages up to 30 kV are now common and the current can peak is excess of 100 A for a fraction of a second when the plug fires. The interference caused by the ignition system is mostly above 30 MHz and the energy can speak for fractions of a second at up to 500 kW.
The charging system produces noise due to the sparking at the brushes. Electronic regulators with vibrating contacts cause trouble. Any motor or switch using relays is likely to produce some interference. The most common sources are the wiper motor and heater motor. The starter is not a problem because of its short usage time. Static electricity is built up due to the friction between the vehicle and the air, and the tyres and the road. This is easily prevented by using bonding straps to ensure all panels stay at the same potential. The arc to ground can be as much as 10 kV.
There are basically five techniques for suppressing radio interference. These include resistors, bonding, screening, capacitors and inductors.
Resistance is used exclusively in the ignition HT circuit, up to a maximum of about 20 kW per lead. This limits the peak current, which in turn limits the electromagnetic radiation. However, excessive resistance affects the spark quality. These resistors effectively damp down the interference waves.
Bonding straps as mentioned above are used simply to ensure that all parts of the vehicle are at the same electrical potential to prevent sparking due to build up of static potential.
Screening is generally used only for specialist’s applications such as emergency services and the military. It completely encloses the ignition system and other major sources of noise in a conductive screen, which is connected to the vehicle earth so that interference waves can escape. It is a very effective technique, but expensive.
Capacitors and inductors are used to act as filters. This filtering effect is achieved by using the variation of capacitive or inductive reactance to alternating signals with the increase of frequency. The changing value of these reactances can be calculated as follows:
Fig. 30.58. Two signals one clean and the other suffering from interference.
