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Phono cartridges probably didn't realize how easy they had it, back when tracking was simply a question of staying in the groove. It was as natural as a barge floating down the Mississippi-and about as subtle as a plowshare cutting through black Illinois topsoil. It was a primitive system, and in our Age of Information when spacewalks and supercomputers hardly excite interest anymore, a Stone Age technique of dragging a rock through a ditch could simply no longer be tolerated. Clearly it was time for a new medium and a new reader.
The Compact Disc has responded to the challenge with components from today's high technology. Modulated laser light carries the data, and nothing (except light) touches the medium surface. This poses an interesting engineering challenge: How do we track a spiral pit sequence if there is no groove to guide the pickup? The answer, of course, is the auto-tracking system found in all CD players.
The spiral pit track on a Compact Disc, running from a center circle nearly to the outer edge, makes 60 revolutions within the width of an LP microgroove. An off-center disc might exhibit track eccentricity of as much as 300 µm; in addition, vibration can challenge the pickup's ability to track within a ±0.1-µm tolerance. Hence, it is appropriate that a laser-beam system is used for tracking; it would probably be impossible for any mechanical system to track as well.
Many CD players use a "three spot" method for tracking, whereby the original beam is split by a diffraction grating to create a series of secondary beams of diminishing intensity. The two first-order beams are conveyed to the disc surface along with the central beam. The central beam spot covers the pit track while the two tracking beams are aligned above and below and to either side of the center beam.
When the beam is tracking the record properly, part of each tracking beam is focused on the pit circumference; the other part covers the mirrored area between pit tracks. The three beams are reflected back through the quarter wave plate and polarizing beam splitter; the main beam strikes the four quadrant photodiode, and the two tracking beams strike two separate photodiodes mounted to either side of the main photodiode.
As the three spots drift to either side of the pit track, the amount of light reflected from the tracking beams varies. There is less average light intensity reflected by the beam which encounters more pit area and greater reflected light intensity from the beam which encounters less (Fig. 1). The relative output voltages from the two tracking photodiodes thus form an error-correction signal.
The electronic system used to convert the photodiode outputs to a meaningful correction signal utilizes current to-voltage converters, comparators, amplifiers, delay lines, level shifters, and a driver stage. As shown in Fig. 2, the outputs of both tracking photodiodes are applied to operational amplifiers Al and A2 to convert the current outputs to voltages. The gain of the left error-tracking signal is adjusted by a trimmer in the feedback loop. The right error-tracking signal is not adjustable; however, its output voltage is delayed by 30 µS. This is necessary because the left tracking beam reads a given pit 30 µS after the right beam does, due to the displacement of the two beams on the rotating disc. Delaying the right tracking signal just this amount allows both signals to be compared on the basis of the same pit-as if they were reading the pit simultaneously.
The difference between the tracking signals is determined at the comparator. If tracking is precisely aligned, the difference here is 0 V. If the beams drift, a difference signal is generated, varying positively for a left drift and negatively for a right drift, thus creating an S-curve tracking-error signal. That signal is buffered and amplified by amplifier A3 before being sent to the gain control and servo-drive circuits.
The gain-control circuits maintain relatively constant tracking-signal voltages, despite differences in disc reflectivity due to such causes as manufacturing differences and soiling of the disc or the player's optics. Keeping these voltages at the proper levels ensures proper data recovery and minimizes actuator noise during playback.
The gain of the level-control amplifier stage is varied according to the intensity of the laser beam sensed during the initial reading of the disc's "Table of Contents." A microprocessor varies the gain ± 10 dB by switching resistors into the amplifier's circuits. The initial gain adjustment for each disc is maintained throughout its playing, but the system reverts to a nominal gain level during searches or jumps.
The error signal is gated by control from the microprocessor and other sources that verify proper tracking.
One of these control signals comes from a damage-detection circuit, which alerts the tracking servo system to scratched or defective discs.
If the gating circuit passes it, the tracking circuit is then applied to a push-pull transistor circuit, which drives the actuator coil. The two-axis actuator assembly contains a permanent magnet and the focus/tracking coil. When the driving voltage is applied to the coil, the bobbin swings around a shaft to move the objective lens laterally, so the main laser spot is again centered and the tracking-error signal is again zeroed.
Aside from the tracking accuracy needed to keep the laser beams on track, a motor must properly move the pickup across the disc surface to track the entire pit sequence. Also, the pickup must be able to jump from one place on the disc to another, find the desired place on the spiral, and resume tracking. These functions are handled by separate circuits, primarily using previously generated control signals. A coreless-type slide motor is used to provide constant tracking of the pit circumference. The tracking-error signal used for auto-tracking is sent to other drive transistors by an amplifier for fine control of the slide motor.
It is important to note that the movement of a stylus in the groove of an LP is much like a CD player's operation: With the aid of the auto-tracking system, the signal path "pulls" the pickup across the disc. In the search mode, the microprocessor takes command to provide faster motion than is possible during normal tracking. Control pulses are directed to the drive transistors for accelerated movement in the forward or reverse direction.
For forward or reverse jumps to programmed locations on the disc, the tracking-error signal is disabled by a flip-flop, and signals from the micro processor drive the slide motor. When the correct location is reached, the S curve generated by the tracking-error signal is referenced to a microprocessor-generated control signal, the flip flop is switched, and the circuitry is informed that proper track alignment is occurring. Just prior to alignment, a brake pulse is generated to compensate for the pickup's inertia. The actuator comes to rest on the correct track, and normal auto-tracking is resumed.
A Compact Disc player uses a lot more technology than an analog re cord player to achieve tracking, but most would agree that the result is much more satisfactory. Compact Discs might cost a little more than phonograph records, but the longevity of CD recordings, thanks to the laser tracking system, makes them a worth while investment. The optical pickup, auto-focus, and auto-tracking are certainly some of the niftiest engineering elements in the Compact Disc system.
But there are still a number of tricks in the players, and in the discs them selves, to be explored in upcoming columns.
(adapted from Audio magazine, Mar. 1985; KEN POHLMANN )
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