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CD Player Operation

This section is an EXTREMELY simplified explanation of the operation of a compact disc player.


Overview:
The image below shows the physical position of a few parts of a CD player. This is a compact disc drive for a computer. It's being used because it's very difficult to see the CD mechanism components in a slot-loading CD player that's used in a car audio CD mechanism. The mechanisms in CD, DVD and Blu-ray players are largely the same. They use different lasers but other than that, the theory of operation and the overall construction are the same.

Simplified Sequence of Operation (in dash mobile CD player):
When a CD is inserted into the loading port of a CD player, its presence is generally detected by an optical sensor when the disc blocks the beam from an led. The voltage on the sensor changes when the beam is blocked which tells the control chip (integrated circuit) to start the loading motor. The loading motor drives either 1 or 2 rubber coated rollers which pull the disc into the player. When the disc is fully pulled into the mechanism (to a point where the hole in the center of the disc is directly over the spindle table), the rollers will separate and the disc will be clamped down onto the spindle table (spindle). The spindle table is usually mounted directly on the shaft of the spindle motor (the motor which spins the disc). The disc may be clamped by spring tension or by magnetic attraction to the spindle table. When you tell the CD Player to play the cd, it moves the laser pickup to its innermost position (home) then applies power to the spindle motor and focuses the laser on the layer of the disc where the information is stored. The first information retrieved from the disc is its table of contents (TOC). The TOC has all of the information needed to play the cd, including (but certainly not limited to) number of songs, time for each song and total time. It will then play the first track on the disc. As the program on the disc is played, the laser pickup is moved outward. The speed at which the disc is moving, directly above the laser, remains constant this means that the disc spins more slowly as the laser moves outward. A more detailed description of some of the aforementioned functions will follow.

Motors:
In mobile cd players, virtually all of the motors are simple dc brush type (not brushless) motors.

Magnetic Disc Clamper:
A magnetic clamp simply holds the disc in place when the magnet (built into the upper half of the clamp) pulls down to the metallic spindle table while sandwiching the disc between them.

Parts of the Pickup:
The laser pickup is the most complex mechanical part of a CD player. This photo is the lens of the laser pickup, magnetic pole pieces, focusing and tracking coils.

Focusing:
Since compact discs are not exactly the same thickness, the lens in the pickup must be able to move up and down to be able to achieve focus on discs of any thickness (within reason). The focusing mechanism functions much like a speaker. It has a magnetized pole piece surrounded by a focusing coil. An electrical current is passed through the coil to achieve focus. There is a focus servo to assure that the lens stays in focus.

Tracking:
The laser pickup lens has to be moved very precisely to read the data from the disc without any significant error. The fine movement is handled by a coil which is wound around the same magnetic pole piece as the focus coil. The big difference between the two coils is that the force applied the focus coil is in the vertical plane while the force of the tracking coil is in the horizontal plane. The tracking coil is only able to move the lens approx .15 inches either way from its point of rest.

Movement of the Entire Pickup:
The larger movements of the laser pickup are commonly handled by a long screw drive mechanism. The pickup assembly is mounted on 1 or 2 chrome plated steel rods. The screw drive is in contact with a threaded part of the pickup. When the screw drive is driven by the sled motor, the pickup can be moved from the innermost to the outermost position. When the tracking coil approaches the limit of its travel, the sled motor moves the entire pickup assembly so that the lens is closer to the center of the tracking coil's range of motion.

The Suspension:
The weight of the laser pickup lens, focus and tracking coils and associated laser pickup components are held in place and supported by a 2 axis suspension system. The suspension holds the components very close to the position that they need to be for proper operation. the suspension system acts sort of like the springs which hold your car body above the axles. You know that you can easily raise the height of the body of a car by applying a small amount of force to the bumper. You also know that you would have little chance of raising the body at all, if it were not mounted on its springs. The suspension in the laser pickup does the same thing for the lens/coil assembly. The focus and tracking coils only need a fraction of the power that it would otherwise take to support the weight of the lens assembly and also move the lens through its range of motion. In the previous photo, the suspension was provided by the two wires on either side of the lens assembly.


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