Laser Tracking on the CD Scaled Views of a Compact Disc

More documents

Recommendations

Info

Detection of CD Pits The tracking laser beam sees the pits as raised areas which are about a quarter-wavelength high for the laser light. The reflected light from the pit is then 180° out of phase with the reflection from the flat area, so the reflected light intensity drops as the beam moves over a pit. The threshold of the photodiode detector can be adjusted to switch on this light level change. CD Response to Defects The signal from a compact disc is relatively insensitive to the presence of small defects such as dust or fine scratches on the bottom surface of the CD because the laser beam is fairly large at that point, about 0.8 mm. As illustrated below, typical dust particles are much smaller than that. As the laser is further focused down to about 1.7 micrometers at the depth of the pits, any shadow from the small defects is blurred and indistinct and does not cause a read error. Larger defects are handled by errorcorrecting codes in the handling of the digital data. Error-Correction of CD Signals The data on a compact disc is encoded in such a way that some well- developed error-correction schemes can be used. A sophisticated error- correction code known as CIRC (cross interleave Reed- Solomon code) is used to deal with both burst errors from dirt and scratches and random errors from inaccurate cutting of the disc. The data on the disc are formatted in frames which contain 408 bits of audio data and another 180 bits of data which include parity and sync bits and a subcode. A given frame can contain information from other frames and the correlation between frames can be used to minimize errors. Errors on the disc could lead to some output frequencies above 22kHz (half the sampling frequency of 44.1 kHz) which could cause serious problems by "aliasing" down to audible frequencies. A technique called oversampling is used to reduce such noise. Using a digital filter to sample four times and average provides a 6-decibel improvement in signal-to-noise ratio. For more details, see the references.
Data Encoding on Compact Discs When the laser in a compact disc player sweeps over the track of pits which represents the data, a transition from a flat area to a pit area or vice versa is interpreted as a binary 1, and the absence of a transition in a time interval called a clock cycle is interpreted as a binary 0. This kind of detection is called an NRZI code. The particular NRZI code used with compact discs is EFM (eight-to-fourteen modulation) in which eight bits of data are represented by fourteen channel bits. In addition to the actual digital sound data, parity and sync bits and a subcode are also recorded on the disc in "frames" . In a given frame, 408 bits of audio data are recorded with another 180 bits of data which permit a sophisticated error-correction code to be used. A given frame can contain information from other frames and the correlation between frames can be used to minimize errors. In addition to detection, a significant amount of computation must be done to decode the signal and prepare it for conversion back to analog form with a DAC. Detection of Compact Disc Data The pits which encode the digital data on a compact disc are tracked by a laser. The reflected light from the pits is out of phase with that from the surrounding area, so the reflected light intensity drops when the laser moves over a pit area. The nature of a photodiode is such that it can be used as the sensing element in a light-activated switch. It conducts an electric current which is proportional to the light falling on it. The photodiode and switch can be adjusted so that a transition to a pit area will switch it off, and a transition from a pit area will switch it on. Either transition is interpreted as a binary 1, while the absence of a transition in a given clock cycle is interpreted as a binary zero. The data on the disc is encoded in a sophisticated way, so that decoding is necessary before sending the digital signal representing the sound to a digital-to-analog converter (DAC) for reconversion to analog form. Cylindrical Lens for Positioning A clever use of a cylindrical lens generates a correction signal to position the main focusing lens for the detector in a compact disc player. The combination of a symmetric lens and a cylindrical lens produces a circular beam at only one distance past the cylindrical lens. A segmented photodiode arrangement can detect whether the beam is circular and generate an error voltage to reposition the main lens so that it is. The error voltage drives a coil which can rapidly reposition the lens in response to changes in distance to the CD as it rotates.
Page 1: Laser Trac
Page 5 and 6: espectively. This gives total inter
Page 7 and 8: The calcite gem above is 52.3 carat
Page 9 and 10: This huge single crystal of calcite
Page 11 and 12: of n 1 =1.5 upon a medium of n 2 =1
Page 13 and 14: For light from a medium of index =
Page 15 and 16: = and = The transmission coefficien
Page 17 and 18: Rayleigh scattering can be consider
Page 19 and 20: There are many different mixtures o
Page 21: The images above from the commercia

Laser Tracking on the CD Scaled Views of a Compact Disc

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?