CD changer

CD systems in motor vehicles represent a relatively new technology with which service technicians in the automotive industry must concern themselves. The CD medium began to conquer the market in the early 1980s. Some years later the systems were developed to such an extent that CD devices were also made available for mobile applications.

Fundamentals of CD technology

The music information (audio data) is stored on the CD in digital form (value 0 and 1). The data is scanned by the reflection of beams of light. The pick-up sends a strongly bundled beam of infrared light to the CD surface. This beam of light is reflected by the silver or gold vapour-coated surface and taken up and evaluated by an optical sensor. On a purely physical basis, the system detects elevations and depressions which, due to their length, change the reflection in certain patterns such that a differentiation can be made between the values 0 and 1.

The audio data are stored in concentric circles and not in a spiral as on a conventional vinyl disc. In addition to the music, the CD tracks therefore contain further information which serves the purpose of pick-up tracking. Among other things, this makes it possible to jump arbitrarily from one title to another during playback. At this point, it should be borne in mind that 30 of these information tracks are about as wide as one human hair. We are therefore dealing with microscopic dimensions.

In addition to tracking, irregularities on the CD surface must also be balanced out. For this reason, the focus of the reflected infrared beam is permanently monitored and corrected by raising and lowering the pick-up lens. This procedure is known as focusing.

If data are lost during the pick-up procedure (due to scratches, impact etc.), to a certain extent, these data can be reconstructed. For this purpose, the last value read is compared with the next readable value. From both values, the error correction facility calculates a kind of mean value. This procedure cannot be perceived by the human ear.

The CD changer in mobile applications

In a car CD changer, various conditions occur which are frequently misconstrued as faults. Many phenomena are, however, system-related and therefore cannot be avoided.

The mechanical part of the CD changer must be spring-mounted, however, it must not vibrate but rather it must take up (dampen) impact. For instance, unevenness in the road surface can be compensated for in this way.

The CDs are held in a magazine. Only the CD which is to be played is located completely within the mechanical part. The CD is transported from the magazine into the play position by means of 2 rubber rollers which pull the CD out of the magazine. Once the CD has reached the play position, it is lowered and fixed on a cone in the centre hole (clamping). Playback can now begin. The CD is transported back into the magazine in the reverse order. 3 photoelectric barriers are installed in the changer in order to determine the CD within the mechanical part. The position of the disc can be determined exactly by the sequence, in which these photoelectric barriers are interrupted.

Malfunctions caused by CDs

Due to the conditions, under which a CD changer operates in mobile applications, CDs which are outside or at the limit of specified tolerances can cause the following problems.

Many undesirable effect can be caused by CDs with edges which are inadequately deburred or too thick:

• The transport rollers cannot grip the CD since it is too thick.
  
• Plastic burrs can break away during transport through the rollers and adhere to the CD surface. The same problem can also occur in the case of a poorly deburred centre hole. Plastic burrs are pressed out by fixing on the cone (clamping), as a result they enter the mechanics and soil the CDs and magazine. These plastic chips (burrs) have 2 negative effects:
  The plastic chips can cause fine scratches on the surface of CDs if they enter the magazine.
  Under unfavourable conditions, larger pieces of plastic (5-7 mm) can cause pick-up errors (jumping) or they may make it not possible to read the CD directory (TOC). The CD cannot be played in such cases.
  
• Return transport into the magazine is impaired if several of these too thick CDs are stored in a magazine. On the one hand, a higher force is required in order to push the CD back into the magazine. On the other hand, however, the partitions which separate the individual CD compartments from each other can be displaced to such an extent that return transport of the CD to its original position is no longer possible.
  

Note

Do not use CDs which are inadequately deburred. Extremely thick CDs may be used only if there are not several such CDs in the same magazine.

Single CDs (8 cm) with single adapter, protective films and protective paint or with stabilizer rings:

• The CDs are thicker than normal (see above).
  
• During the transport procedure, singles can be pressed out of the adapter and thus block the mechanics. Stabilizer rings can get trapped in the transport rollers.
  

Note

Do not use such CDs with accessories in the vehicle.

Transparent CDs:

• Unfortunately, it frequently occurs that the protective film has been poorly vapourized on to the CDs. Such discs are more transparent, in part or completely, than normal. Although these CDs can still be played they no longer facilitate optical position monitoring by the photoelectric light barriers. In real terms, this means that the CD is no longer detected as such.
  

Some CDs are subject to production faults. This concerns the layer, on which the information is stored. It is sometimes possible to play such CDs on the home system but not in the car. This is not a fault of the CD changer but rather it is due to the fact that error correction in the vehicle is based on different requirements than those for stationary operation. A home player reacts very sensitively to impact and vibration. And nobody every seems to complain that such a player no longer functions correctly when it is knocked. The laser tracking can be optimized such as to pick up surface errors. On the other hand, a compromise must be found for mobile applications. Due to the possible vibrations and impact, the control range (in this case the ability to compensate for surface flaws) must be limited. And this is exactly what makes mobile applications possible.

It can sometimes be found that CDs are not completely flat, i.e. they wobble in the same as often occurred with vinyl discs. Since a CD is read out from the inside towards the outside, when using such discs, error correction is sometimes overtaxed when playing the last title. This can be reflected in the fact that a CD changer is very sensitive to impact during playback of the last title.

Notes on pick-up errors

In stationary vehicle:

• Check the CDs for impurities or scratches
  
• If the CDs are in perfect order and function satisfactorily in other devices, the changer should be sent back to the factory for inspection. If applicable, also enclose the CDs which cannot be played with a short description of when the fault occurs. For example: "No playback as of track 5, 1:34 minutes".
  

While driving:

• Dirty CDs can increase the sensitivity to vibration and impact. Please check accordingly.
  
• Have the transport screws been removed?
  
• Are the mounting springs in the correct position?
  
• Are the springs in the same position on both sides?
  
• There are, of course, also limits in the case of extremely hard chassis or high speeds. As soon as the mechanical part comes in contact with the housing as the result of vibration and impact, the electronic error correction can no longer function effectively or the pick-up tracking is no longer operable due to the lack of correction data.
  

Summary

Many customer complaints are attributed to other causes than the CD changer. These causes may include poor quality CDs, other, external effects or simply misguided expectations.