DUAL DATA RECORDED COMPACT DISC

Abstract: The optical recording device including a reading head (11) and a reading laser (7) for encoding optical data (3) on a CD (1) as two dimensional (i.e. pit length and pit depths (13, 15)) optical data (3) along a spiral track, one of the data streams may be used to implement a hologram on the data recording surface, the preferred form of the invention uses a uniform pit depth within each pit, causing pit depth to vary only from pit to pit and not within a single pit, the second data stream relies on the primary data stream for much of the necessary error correction required in dense data transmission media.

Description: DUAL DATA RECORDED COMPACT DISC BACKGROUND OF THE INVENTION Compact discs are typically composite objects having a series of pits impressed into a plastic surface, -usually, dur- - ing an injection molding process. These pits and the lands between them are arranged in a spiral pattern that can be tracked by a laser beam which advances slowly along a radius while the disc is spun about an axis through the center of the disc. The pit/land surface is coated with a thin reflective metal layer. The laser beam is reflected from the metal layer coating the pits and lands and the reflection is analyzed to observe the change in reflectivity associated with a transi- tion from a land to a pit or vice versa. To enhance the change in reflectivity the wavelength of light is chosen to cause interference between light reflected from the bottom of a pit and light reflected from a land. In common practice, the light is incident from the side that sees the pit as a bump, and the reference to light reflected from the bottom of a pit herein may be understood as light reflected from the pit seen from beneath as a bump. The wavelength of light is not chosen arbitrarily, but is related to the depth of the pits. By detecting the change in reflectivity, the length of the pit and or land is measured. Data is encoded onto the compact disc as varying lengths of the pits and lands. The data may be analog or digital data. For example a voltage level may correspond directly to a pit length in an analog recording, or the length of the pits/lands may represent strings of O's or l's for the recording of digi- tal data. The amount of data that can be encoded onto a disc is - limited by the density of pits and lands that can be accurate- ly reproduced by the injection molding procedure and that can be accurately read by the scanning laser device. Some error rate can be tolerated if the data is placed on the disc in a somewhat redundant format that allows errors to be sensed and corrected. Thus as the pits and lands are decreased in size it becomes necessary to provide greater redundancy in the data until a limit is reached in the amount of data that can be stored on the disc surface. BRIEF DESCRIPTION OF THE INVENTION The present invention stores two separate streams of optical data on a CD as two dimensional (i.e. pit length and pit depth) data along the spiral track. One of the data streams corresponds to pit depth and may be used to implement a hologram on the data recording surface. The preferred form of the invention uses a uniform pit depth within each pit, causing pit depth to vary only from pit to pit and not within a single pit. The second data stream requires less error correction data encoded as pit depth; it relies on the primary data stream encoded as pit length for much of the necessary error correction required in the compact disc. The invention also comprises a novel method for manufac- turing such a two dimensionally encoded compact disc and a novel method for reading the data using level detectors to separate the two data streams. The invention relies on the unexpected discovery that it is not necessary to fix pit depth at one quarter of a wave- length of the reading laser in order to effectively read data from a compact disc. An alternative embodiment of the present invention does not record a separate secondary data bit in each pit/bump but rather uses a clock to record a bit in each equal time inter- val corresponding to at least the largest time interval occu- pied by a pit/bump under the reading head. In that case one of the level detectors used to read the data is replaced by a clock. This records less data than the preferred embodiment, but has the advantage of simplifying the system electronics. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 represents a schematic view of a CD and its reading components. Fig. 2 represents a cross section view of the data recording surface of a CD according to the present invention. Fig. 3 represents the intensity level diagram for the output voltage signal at the reader head of a CD player for the present invention. Fig. 4 represents a schematic block diagram of the algorithmic logic of the data interpreter of the present invention. Fig. 5 represents a diagram of the intensity level for the output voltage signal showing the voltage levels trigger- ing the logic of Fig. 4. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT The present invention implements an unexpected discovery concerning data recorded as pits on a compact disc, namely that the depth of the pits is, contrary to the published lit- erature, not a critical parameter in the manufacture of such discs. For purposes of this disclosure the compact discs will be referred to as CDs, but it should be understood that the term CD is intended to include multiple sided compact discs and to the DVD format or other optical recording formats as well. In particular, it has been unexpectedly discovered that a laser reading beam need not have a wavelength that is approxi- mately 4 times the depth of a pit or bump. Indeed it has been unexpectedly discovered that a wavelength approximately 2 times the depth of a pit or bump is most effective for reading data from the compact disc. This directly contradicts the simplistic view of the interference/diffraction/scattering effects at the surface of the data region of a compact disc and is believed to be caused by the complexity of the inter- ference/diffraction of light from an irregularly pitted sur- face. This unexpected discovery opens up literally a new di en- sion for the storage of digital data on compact discs. Where- as data has been stored one dimensionally as pit lengths along a spiral track, the present invention enables one to store the data two dimensionally as both pit length and pit depth along the spiral track. The preferred embodiment of this invention " uses a uniform pit depth for each pit, causing pit depth to vary only from pit to pit and not within a single pit. This provides two advantages. First it enhances quality control because it is possible to recognize unintended pit variation when it occurs within a single pit. Second it enables the use of conventional data recording equipment in the compact disc recording field. Figure 1 depicts a compact disc 1 or single surface of a two sided disc with the data pits 3 . greatly enlarged. The pits are arranged in a spiral pattern with a land area 5 be- tween adjacent spirals so that they may be read in sequence by a reading laser 7. Figure 2 depicts a cross section of the information bearing surface 9 of the compact disc, preferably a metallic film capable of reflecting laser light, the film being embedded in a plastic protective layer through which laser light reflected from the data surface may be detected by a reading head 11. The surfaces are formed with pits of vary- ing depth, by a method to be described below, and the reading head contains a peak detecting means that is capable of sens- ing the depth of pit. Figure 3 shows a typical voltage signal as a function of time corresponding to the intensity of the light received in the photodetector of the reading head of the compact disc player. Since the disc rotates at constant linear velocity under the reading head Figure 3 also shows an indication of the depth of the pit along the data track. As shown in Figure 3 , a low intensity signal 13 . represents a pit having a wave- " length of approximately λ/2. A medium intensity signal 15 represents a pit having a wavelength of approximately λ/4. A high intensity signal 17 represents a land area between pits on the data recording surface. In operation the reading head of the player senses a variation of intensity of the signal reflected from the data containing surface. The intensity encodes two data signals that must be unfolded from each other. This is accomplished by the algorithm schematically represented in Fig. 4. The signal is first passed through a signal splitter and one por- tion is sent to a logical analyzer 21 to perform the normal CD interpretation of the pit/bump length data. The second por- tion is sent to logical analyzer 23., i.e., a level detector which recognizes the crossover from the land to a pit bump (see point 25 in Fig.5). The level detector 23 . then enables the AND gate 27 . and passes the signal to a high level detector which causes the AND to go high only if the pit/bump depth is sufficiently deep. In the preferred embodiment each pit pre- sents the opportunity to encode a single secondary data bit regardless of the length of the pit/bump. The logic presented recognizes the termination of a single pit/bump in order to provide for the situation where two identical bits occur in succession. An alternative embodiment of the present invention does not record a separate secondary data bit in each pit/bump but rather uses a clock to record a bit in each equal time inter- val corresponding to the largest time interval occupied by a " pit/bump under the reading head. In that case the first level detector is simply replaced by a clock. This has the disad- vantage of recording less data, but the advantage of simpli- fying the system by eliminating the need to continuously locate the edge of a pit/bump. One advantage of the present invention is that it is not necessary to provide error correction in as much detail for the secondary data as for the primary data, because the pri- mary data can contain such information as is necessary to locate the origin of a data word in the event that foreign particles obscure the data. In effect, checksum data or its equivalent which is provided in the primary data will show when reading errors have become excessive and the reading head can assume that such errors are not affecting the secondary data if they have not affected the primary data. This piggy- back effect, is not to my knowledge implemented in any other form of data communication. The process for manufacturing a disc according the present invention is as follows: The normal manufacturing process for a CD is altered by taking the normal laser writing signal that is modulated in an on/off manner to encode the pits and further modulating the intensity of the laser to create different exposure levels in the photoresist or in the non-photoresist coating on a glass master. In an ablative process the same variation in laser intensity is performed. Typical wavelengths for the reading light are 780 nm for CD's and 650 n for DVD's. One use for the present invention is to superpose a hologram onto the data portion of the data recording surface of a CD. This is accomplished by taking a conventional holo- gram from a surface where one is impressed in plastic and re- cording the height of each pixel element and expressing that data in a digital form for recording on the surface of a CD. This causes a slight distortion of the hologram since the pre- sent invention is restricted to pixels that correspond in dimension to the length of the available pits. It has unex- pectedly been discovered, however, that in view of the small size of the pits/bumps on the surface a conventional hologram, that this slight distortion is not a significant detriment to the operation of this method. This effect is minimized in the embodiment where a clock is used to define the pixel size rather than relying on the individual pit/bump length.

Claims: WHAT IS CLAIMED IS : 1. A disc medium for the optical recording first and second digital data for readout by a laser having a wavelength " λ comprising a series of pits arranged in a spiral pattern in a reflective material having a predetermined plurality of lengths encoding first digital data and a separate predetermined plurality of pit depths encoding second digital data, wherein said plurality of pit depths comprises at least the depths λ/2 and λ/4. 2. The disc medium for optical recording of claim 1 wherein said second digital data comprises a bit pattern which when implemented in variable pit depths forms a hologram observable through a transparent surface of said disc medium. 3. The disc medium for optical recording of claim 1 wherein said second digital data comprises a bit pattern which when implemented in variable pit depths forms a security encoded area on said disc. 4. The disc medium for the optical recording of claim 1 wherein the second digital data is encoded by a pit of greater depth (λ/2) representing a digital 1 and a pit of lesser depth (λ/4) representing a digital 0 or vice versa. 5. The disc medium for the optical recording of claim 1 wherein the second digital is encoded by a transition from a pit of greater depth (λ/2) to a pit of lesser depth (λ/4) representing a digital 1 and the absence of such a transition representing a digital 0, or vice versa. 6. The disc medium for the optical recording of claim 1 " wherein said pit depths include n-2 additional depths and each such pit depth represents a digit in a base n system, wherein the density of data represented by said pit depths is enhanced by a factor of (n/2) x where x is the word length. 7. A system for reading dual digital data from a com- pact disc having data pits with variable length and variable pit depth comprising peak detector means for detecting the depth of a pit. 8. A method for separating two data signals from a CD having variable pit/bump length and pit/bump depth, comprising sensing a variation of intensity of a signal reflected from a data containing surface, wherein the intensity encodes two data signals, that must be unfolded from each other, splitting the signal and sending one portion to a logical analyzer, performing the normal CD interpretation of the pit/bump length data, sending a second portion of the signal to a second logical analyzer comprising a level detector, recognizing the crossover from the land to a pit bump implementing logical circuitry to determine whether the pit/bump depth is sufficiently deep. 9. A method for separating two data signals from a CD " having variable pit/bump length and pit/bump depth, comprising sensing a variation of intensity of a signal reflected from a data containing surface, wherein the intensity encodes two data signals, that must be unfolded from each other, splitting the signal and sending one portion to a logical analyzer, performing the normal CD interpretation of the pit/bump length data, sending a second portion of the signal to a second logical analyzer comprising a level detector, clocking a signal to indicate the start of a new depth encoded data bit, implementing logical circuitry to determine whether the pit/bump depth is sufficiently deep. 10. A disc medium for the optical recording first and second digital data for readout by a laser having a wavelength λ comprising a series of pits arranged in a spiral pattern in a reflective material having a predetermined plurality of lengths encoding first digital data and a separate predetermined plurality of pit depths encoding second digital data, wherein said plurality of pit depths comprises at least two different predetermined pit depths.

Inventors: MECCA CHARLES, MECCA, CHARLES
Applicants: WEA MFG INC, WEA MANUFACTURING, INC.
Application year: 1997
Main classification: G11B7
IPC codes: G11B7