Read/Write speeds

  Graham ® 19:27 28 Nov 2005

What are the speeds referring to when they say a DVD writer is 8X or 20X? I assume all drives spin at the same speed, so how can they be different?

Along the same lines, how do they get the lasers to move such tiny amounts?

  Skyver 19:55 28 Nov 2005
  Fruit Bat /\0/\ 21:02 28 Nov 2005

It is often stated that the laser beam in a CD player is like the stylus of a phonograph turntable. While this is a true statement, the actual magnitude of this achievement is usually overlooked. Consider that the phonograph stylus is electromechanical. Stylus positioning - analogous to tracking and focus in an optical pickup - is based on the stylus riding in the record's grooves controlled by the suspension of the pickup cartridge and tone arm. The analog audio is sensed most often by electromagnetic induction produced by the stylus's minute movements wiggling a magnet within a pair of sense coils.

The optical pickup must perform all of these functions without any mechanical assistance from the CD. It is guided only be a fraction of a mW of laser light and a few milligrams of silicon based electronic circuitry.

Furthermore, the precision involved is easily more than 2 orders of magnitude finer compared to a phonograph. Sophisticated servo systems maintain focus and tracking to within a fraction of a micrometer of optimal. (1 um is equal to 1/25,400 of an inch). Data is read out by detecting the difference in depth of pits and lands of 1/4 wavelength of laser light (about 0.15 um in the CD)!

  Fruit Bat /\0/\ 21:02 28 Nov 2005

* The laser beam is generated by a solid state laser diode emitting at 780 nm (near IR). Optical power from the laser diode is no more than a couple of mW and exits in a wedge shaped beam with a typical divergence of 10x30 degrees in the X and Y directions respectively.

Note that despite what some people believe, the laser diode in a CD or DVD player is a true laser and not just a glorified LED. It has a gain medium (the semiconductor), mirrors (on the cleaved parallel ends of the crystal), and an means of excitation (electric current). Its nearly monochromatic single spatial mode (TEM00) beam can be focused to a spot less than 2 um in diameter. No LED or other non-laser light source is capable of this kind of performance.

* A diffraction grating splits the beam into a main beam and two (first order) side beams. (The higher order beams are not used). Note that the diffraction grating is used to generate multiple beams, not for its more common function of splitting up light into its constituent colors. The side beams are used for tracking and straddle the track which is being read. The tracking servo maintains this centering by keeping the amplitude of the two return beams equalized.)

* Next, the laser beam passes through a polarizing beam splitter (a type of prism or mirror which redirects the return beam to the photodiode array), a collimating lens, a quarter wave plate, a turning mirror, and the objective lens before finally reaching the disc.

* The collimating lens converts the diverging beam from the laser into a parallel beam.

* The quarter wave plate converts the linearly polarized beam into a circularly polarized beam. The purpose of this obscure step will become clear below.

* A turning mirror (optional depending on the specific optical path used) then reflects the laser light up to the objective lens and focus/tracking actuators.

* The objective lens is similar in many ways to a high quality microscope objective lens. It is mounted on a platform which provides for movement in two directions. The actuators operate similarly to the voice coils in loudspeakers. Fixed permanent magnets provide the magnetic fields which the coils act upon. The focus actuator moves the lens up and down. The tracking actuator moves the coil in and out with respect to the disc center.

  Fruit Bat /\0/\ 21:03 28 Nov 2005

* The collimated laser beams (including the 2 side beams) pass through the objective lens and are focused to diffraction limited spots on the information - pits - layer of the disc (after passing through the 1.2 millimeters of clear polycarbonate plastic which forms the bulk of the disc).

* The reflected beams retrace the original path back through the quarter wave plate and polarizing beam splitter. However, the reflected circularly polarized beam in passing through the quarter wave plate gets converted back into a linearly polarized beam but at an orientation 90 degrees to the original! So, the polarizing beam splitter is able to divert it to the photodiode array. To repeat in different words: The polarizing beam splitter passes the (horizontally polarized) laser beams straight through. However, two passes (out and back) through the quarter wave plate rotates the polarization of the return beam to be vertical instead and it is reflected by the polarizing beam splitter toward the photodiode array. Sorry, an explaination of why quarter wave plates and polarized laser beams behave this way is somewhat beyond the scope of this document.

The return beams from the disc's information layer are used for servo control of focus and tracking and for data recovery.

* A cylindrical lens slightly alters the horizontal and vertical focal distances of the resulting spot on the photodiode array. The spot will then be perfectly circular only when the lens is positioned correctly. To close or to far and it will be elliptical (e.g., elongated on the 45 degree axis if too close but on the 135 degree axis if too far).

The central part of the photodiode array is divided into 4 equal quadrants labeled A,B,C,D. Focus is perfect when the signal = (A+C)-(B+D) = 0.

The actual implementation may use a thick beam splitter mirror (which adds astigmatism) or an astigmatic objective lens rather than a separate cylindrical lens to reduce cost but the effect is the same. Since the objective lens is molded plastic, it costs no more to mold an astigmat (though grinding the original molds may have been a treat!). It is even possible that in some cases, the natural astigmatism of the laser diode itself plays a part in this process.

* The side beams created by the diffraction grating are positioned forward and back of the main beam straddling the track of pits being followed (not directly on either side as shown in the diagram - but that was easier to draw!).

Segments on either side of the photodiode array designated E and F monitor the side beams. Tracking is perfect when the E and F signals are equal.

* The data signal is the sum of A+B+C+D.

In essence, the optical pickup is an electronically steered and stabilized microscope which is extracting information from tracks 1/20 the width of a human red blood cell while flying along at a linear velocity of 1.2 meters per second!

  Graham ® 21:35 28 Nov 2005

Thanks. Blimey Charlie, I will have to digest that slowly in the morning.

  ade.h 21:42 28 Nov 2005

I think we've found Fruitbat's specialist subject ;-)

  DieSse 23:49 28 Nov 2005

"I assume all drives spin at the same speed, so how can they be different?"

No - they don't all spin at the same speed - the 8x literally means 8 times the speed (the spin sped actually varies over the surface of the DVD - so the 8x is a maximum.).

Ogf course this doesn't mean that a film, say, wil play at 8x the normal speed - the speed can be controlled to prevent this, and a data buffer can be used to slow the effective data rate to that for films, etc.

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