High density compact disc

A high density compact disc having tracks for recording large amounts of information reproducible for a long interval of time is disclosed. In order to increase the track density, a recording surface on a substrate is divided into grooves and lands to thereby form tracks, and pits representing information signals are arranged in both the grooves and the lands. Here, the groove is formed at an optimized predetermined depth, depending on the wavelength of light used to read the information recorded on the disc and the refractive index of a disc substrate, to thereby reduce crosstalk caused by pits of an adjacent groove or land during reproduction. Accordingly, the present invention is effective in enhancing the practicality of the compact disc as a medium for preserving large amounts of information.

This application claims the benefit of copending Provisional Application 
Ser. No. 60/019891 entitled "High Density Compact Disk," filed on Jun. 13, 
1996. 
BACKGROUND OF THE INVENTION 
The present invention relates to a compact disc for reproduction of 
information recorded thereon, and more particularly, to a compact disc 
having a high track density. 
An example of a compact disc currently on the market is a video compact 
disc (video-CD) on which sound, text and images are recorded. The video 
compact disc, compared to a magnetic recording video cassette tape, has a 
shorter seek time and is readable with a light source and thus does is not 
require wearing contact between the medium and the reproducing mechanism 
during reproduction. This prevents deterioration of information recorded 
on the disc. However, since the capacity of the video-CD is limited, i.e., 
approximately 650 MB in the case of a 12 cm disc, the disc cannot store 
enough image information for long intervals of reproduction. Accordingly, 
in order to record enough image information for long term reproduction, a 
high density disc is required. 
FIG. 1 shows the structure of a conventional compact disc and a recording 
format thereof. 
The conventional compact disc has a substrate 1, a reflective film 2 and a 
protective layer 3. A multiplicity of pits 4 comprises units of 
information and are formed on the substrate 1. Here, the pits 4 are 
arranged in concentric circles or in a spiral with respect to the center 
of the disc on tracks having a land portion. When a laser beam 5 from an 
optical pickup (not shown) is focused onto the reflective film 2 through 
the substrate 1, a light spot 6 is formed on a recording surface (pit 
formation surface). When the light spot 6 moves along the pits 4, the 
reflectivity on the reflective film 2 changes. Accordingly, an electric 
signal is detected from the change in the reflected light so that it is 
possible to read both the information recorded on the compact disc and the 
position information for tracking and focusing of the light spot. 
The pitch between the tracks, on which the pits 4 are arranged, is usually 
1.6 .mu.m so that the light spot 6 does not overlap the pits of adjacent 
tracks. Meanwhile, the depth of the pit 4 is approximately 1/5-1/4 the 
wavelength of the light used to read the data from the medium. 
In the conventional compact disc, in order to increase the recording 
density, there is known a method of narrowing the pitch between the 
tracks, thereby increasing the track density. However, in the case where 
only the pitch between tracks is narrowed in order to increase the track 
density, crosstalk occurs due to light interference caused by the pits in 
adjacent tracks, generating noise in the reproduction signal. Also, in the 
recording format of the conventional compact disc, since the pits of a 
track adjacent to a target track are arranged in a random manner, the 
crosstalk cannot be suppressed. Accordingly, in order to increase the 
track density, it is necessary to change the recording format of the 
conventional compact disc. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a high density compact 
disc having a high track density for recording information for long 
periods of reproduction. 
Also, it is another object to provide a high density compact disc having a 
high track density in which crosstalk does not occur during reproduction. 
To accomplish the above objects of the present invention, there is provided 
a high density compact disc comprising a recording surface on which pits 
representing information signals are arranged along a predetermined track, 
wherein the recording surface comprises grooves and lands alternately 
arranged along the tracks, and the pits are formed on both the grooves and 
the lands. 
Preferably, the depth of the grooves is within the range of 
##EQU1## 
where .lambda. is the wavelength of light used to read information 
recorded on the disc and .eta. is the refractive index of a disc substrate 
.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the drawings, a preferred embodiment for realizing a high 
density compact disc according to the present invention will be described 
in detail. 
Referring to FIGS. 2 and 3, reference numeral 10 denotes a transparent 
substrate having a predetermined refractive index, which transmits the 
laser light for reading information recorded on the disc. The substrate 10 
has a recording surface formed on the upper surface thereof. The recording 
surface is formed with a plurality of grooves 11 and lands 12 which are 
adjacent to each other along tracks formed in concentric circles or in a 
spiral with respect to the center of the disc. Information is recorded on 
a series of pits 13 formed in the grooves 11 and the lands 12, each pit 
representing a unit information signal. A reflective film 14, such as a 
metal lamina for reflecting the laser light, is coated on the recording 
surface of the substrate 10 on the grooves 11 and the lands 12. A 
protective layer 15 is formed on the reflective film 14. 
If the interval between the adjacent grooves 11 or the adjacent lands 12 is 
the same as the pitch between the tracks of the conventional compact disc, 
the track density obtained with the present invention is twice as dense as 
that of the conventional format. In this state, when a laser beam is 
projected and focused onto a target groove from the optical pickup, a 
light spot formed on the recording surface partially hits two lands 12 at 
both sides of the target groove 11 (see FIGS. 4A and 4B). 
Here, crosstalk due to pits 13 in the land 12 may occur. However, according 
to the present invention, the level of crosstalk can be reduced by 
optimizing the recording format by adjusting the depth d of the groove 11, 
based on conditions to be described below. 
Crosstalk can be illustrated by interference in light reflected from the 
groove region and light reflected from the land region. For example, as 
shown in FIGS. 4A and 4B, if the pits 13 are arranged only in the grooves 
11 by predetermined size and in intervals of q, and no information is 
recorded on the land region 12, and the light spot 16 moves along a center 
of the land 12 region, the intensity of the reflected light changes 
according to the pit 13 formed on the adjacent groove 11 and at a period 
of the interval q. That is, the amplitude of the change in the intensity 
can be defined as the crosstalk. 
In the case that the light spots 16 are located at positions shown in FIGS. 
4A and 4B, respectively, the phase differences between the reflected light 
in the groove 11 region and the land 12 region can be expressed as: 
##EQU2## 
respectively, wherein .lambda. is the wavelength of the laser beam, and d 
is the depth of the groove 11. 
The conditions for reducing the degree of interference in the phase 
differences are as follows. 
##EQU3## 
Also, in the case where pits 13 are arranged only in the lands 12 and the 
light spot 16 is focused on the center of a groove 11, the conditions for 
reducing the degree of interference in the phase differences are as 
follows, where the result is the same as that of the above case. 
That is, when the depth d of the groove 11 is approximately 1/8 the 
wavelength .lambda. of the light, crosstalk is at a minimum. 
##EQU4## 
The above result is a simplified example of the recording formats of the 
high density compact disc, where the refractive index of the 
above-mentioned substrate is not considered. 
The following are actual conditions which may be applied. These are data 
for measuring the level of crosstalk according to the depth of the groove 
by a computer simulator, based on the theory of diffraction by Hopkins. 
wavelength .lambda. of laser: 680 nm 
aperture of an object lens: 0.6 
width of groove and land: 0.7 .mu.m 
track pitch: 1.4 .mu.m 
interval of pits: 1.1 .mu.m 
pit depth: .lambda./4.eta. (.eta.=1.51, the refractive index of substrate) 
pit length: 0.55 .mu.m 
pit width: 0.5 .mu.m 
The level of crosstalk measured as a function of the groove depth is shown 
by the graph of FIG. 5. On the graph, the unit along the horizontal axis 
shows the groove depth in nm, and the unit along the vertical axis shows 
the level of crosstalk in dB. According to the graph, when the groove 
depth is approximately 70 nm, i.e. within the range 
.lambda./7.eta..about..lambda./6.eta., the crosstalk is reduced 
significantly. In this range, .eta. represents the refractive index of the 
disc substrate. Here, the level of crosstalk changes depending on the form 
of the pits of an adjacent track, however, the level of crosstalk is small 
enough that the variations are insignificant. 
As described above, according to the present invention, tracks are divided 
into grooves and lands, where pits representing information signals are 
formed in both the grooves and lands, to thereby double the track density 
over the conventional compact disc. The present invention provides a high 
density compact disc reproducible for a long interval without noise due to 
crosstalk. Accordingly, the present invention is effective in enhancing 
the practicality of the compact disc as a medium for preserving large 
amounts of sound, character and image information. 
Also, in the high density compact disc according to the present invention, 
tracking signals can be simply detected from changes in the light 
intensity due to the depth of both grooves and lands. Thus, the tracking 
control by a push-pull method, which could not be adopted for the 
conventional compact disc, is possible, and a light pickup optical system 
can be simplified.