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The basic requirement for reading a Compact Disc is converting the physical pattern of all the transitions along the spiral track to an equivalent electrical signal. If a coherent monochromatic light beam is bounced off the silvered surface of the disc, and the height of the bumps is equal to one quarter the wavelength of the light, the intensity of the reflected light will fall to near zero when the beam traverses a bump. This is because the light reflected from each side of the track is delayed 180 degrees in phase, and cancellation occurs.
The light source actually used is a compact solid-state laser diode, with a power of about 5 milliwatts and a wavelength of 780 nanometres (nm) in air. The height of the bumps on the disc is 110 nm, which is about one quarter wavelength in the transparent polycarbonate coating. The laser light is in the infra-red band, making it invisible to the human eye, which responds from 430 nm to 700 nm. It has an activating region made of AlGaAs (Aluminium Gallium Arsenic) sandwiched between layers of refracting material, forming a light resonator. A photo-diode near the laser diode monitors its output power and compensates for temperature.
The beam reflected from the disc is detected by an array of photo-diodes, which drive the player's electronics. In between the laser and the photo-diodes, the light beam follows a tortuous path through an opto-mechanical system that focusses the beam and keeps it following the track as it whizzes by on the disc. At the outside surface of the disc, the beam is about 0.8 mm in diameter, which means imperfections of particles on the surface must be 0.5 mm or more is size to seriously disturb the beam and defeat the error correction systems. The beam must travel through 1.2 mm of polycarbonate, which has a refractive index of 1.55, and focus as a round spot only 1.7 micrometers (um) in diameter at the reflective signal surface. The beam must remain focussed and move less than 1.0 um horizontally and vertically away from the track, even though the CD specification allows a vertical disc warp of 1.2 mm.
One of the few things not rigidly defined by the CD standard is the pickup mechanism. It must be able to read the signal from the disc, detect out-of-focus conditions due to minute warps or marring of the disc, and detect lateral tracking errors due to the disc not being perfectly centred. The pickup must also have the ability to physically compensate for these two types of dynamic error.
Of the many types of mechanism possible, two have been accepted by the industry. One-beam push-pull tracking with Foucault focussing, and Three-beam tracking with astigmatic focussing. Each system has certain advantages and either may be argued as the better. One-beam is simpler to engineer and is more compact. Three-beam has more optical components and requires more factory adjustments to align. It must also be mounted on a linear tracking sled driven by a motorised screw thread, whereas one- beam will work successfully on a coil driven radial arm that swings an arc across the disc. Minute variations in tracking are performed by a small magnetically positioned mirror in the path of the beam. A comparison made on a player for player basis reveals there is negligible difference between the two systems in the areas of audio performance and tracking reliability.
Figure 1: Single-beam pickup
The top half of Fig. 1 is a basic diagram of the one-beam system optics. The laser beam first bounces off a Semi-Transparent Mirror. It then passes through a Collimator, which forces all the light rays to be parallel. The Objective Lens focusses it to a tiny spot on the disc, which reflects off the track surface and back through the Objective Lens and Collimator. Some of the beam then passes straight through the mirror and an Optical Wedge, which divides the beam into two parts. Each of the beams hits two photo-diodes, which by the signal processing circuit in the bottom half of Fig. 1, the track correction, focus correction and data signals are derived. The mirror is an inefficiency which loses some of the transmitted beam's energy.
Figure 2: Focus servo for single-beam pickup
As the disc focus varies, the pair of knife edged prisms making up the Optical Wedge varies the spacing of the two beams incident on the photo-diode array, due to an effect first observed by J.B.L. Foucault in the 1850s; hence its name. (No link to the speaker mob) Focussing is performed by moving the objective lens coaxially along the optic path using an electromagnetic drive not unlike a moving coil loudspeaker; see Fig. 2.
Figure 3: Three-beam pickup
Fig. 3 represents the more complex three-beam system. The laser beam first passes through a diffraction grating, which is a screen with slits spaced only a few wavelengths apart. Many beams emerge from the other side, diffracted at different angles. The central beam and the first either side go on to pass through a Polarization Beam Splitter (PBS). This consists of two prisms seperated by a one-way mirror membrane, at 45 degrees to the central beam. The three beams go straight through the PBS and are aligned by a Collimator. Between the Collimator and the Objective Lens, although not shown in Fig. 2, there is a Quarter Wave Plate (QWP) which rotates the plane of polarisation of the three beams by 45 degrees.
The beams then pass through the movable Objective Lens and strike the disc. The central beam follows the spiral track, the other two just straddle either side of it, about 20 um forward and aft. On the return path, the QWP rotates the polarisation of the beams a further 45 degrees so that when they strike the PBS, they are deflected off to a Cylindrical Lens at one side. Emerging from this lens, the beams finally encounter a photo- diode array.
The two side beams are monitored by two photo-diodes, which generate a radial tracking error signal. The central beam produces a circular dot on four photo-diodes when focus is correct. The sum of their outputs is the data signal. Due to the astigmatic properties of the Cylindrical Lens, a focus error will elongate the spot in either of two perpendicular directions, depending apon near or far focus error. The lower half of Fig. 2 describes the focus correction drive logic.
Figure 4: Three-beam mechanism
The Foucault focus method has a range of about 1 mm. Astigmatic has a capture range of only 40 um, so it requires a search mechanism to get an initial focus. Fig. 4 is a full diagram of the three-beam mechanism that shows a two axis device with two coils being used to provide both focus and fine tracking.
There are various other methods that may be used for focus and tracking, including knife edges, split sensors, critical angle focussing, differential phase detection and tracking dither. These are not common and have disadvantages for CD players. In the next installment I will discuss Digital to Analog Conversion (shudder).
Figure 1 & 3 - Ken Pohlmann, Digital Domain - Aural Arguments, Audio magazine, July 1986, p19
Figure 2 & 4 - J.R. Watkinson, Principles of optical storage - 2 - Focus and tracking mechanisms, Electronics & Wireless World magazine, April 1985, p43 & 46
Originally published in MAC Audio News No. 161, May 1988, pp 16-20.