Disc-shaped information carrier provided with two portions having different read-out speeds

A disc-shaped information carrier with a substrate and an information layer which includes optically readable elementary marks having a substantially uniform density. The information layer includes a first portion suitable for being read out at a first read-out speed and a second portion suitable for being read out at a second read-out speed higher than the first read-out speed. According to the invention, the second portion of the information layer is at a greater distance to a center of the disc-shaped information carrier than is the first portion of the information layer. A range of rotational speeds which an optical read-out unit must have for a full and correct read-out of the information carrier is reduced in this manner. In a special embodiment, the first portion of the information layer includes a first version of an audio recording provided on the information layer by the DSD transformation method, whereas the second portion of the information layer includes a second version of this same audio recording provided on the information layer by the PCM transformation method.

FIELD OF THE INVENTION 
The invention relates to a disc-shaped information carrier with a substrate 
and an information layer which comprises optically readable elementary 
marks with a substantially uniform density, said information layer being 
provided with a first portion comprising marks to be read out at a first 
read-out speed and a second portion comprising marks to be read out at a 
second read-out speed higher than the first read-out speed. 
BACKGROUND OF THE INVENTION 
A known optically readable disc-shaped information carrier is the CD. The 
elementary marks of the information layer of the CD comprise tiny pits 
which are provided along tracks in the information layer. The density of 
the marks is defined by an interspacing present between the tracks and an 
interspacing present between the marks on a track. Besides the standard 
CD, which can contain an audio program of well over an hour, a 
high-density CD (HDCD) is known having an information layer where the 
interspacings present between the tracks, the interspacings present 
between the marks on a track, and the pits themselves are substantially 
smaller than on a standard CD. The amount of information which the 
high-density CD can accommodate is thus substantially greater than the 
amount of information on the standard CD. 
Since the available space on a high-density CD is comparatively large, the 
information layer of a high-density CD can be provided with more than one 
portion, such as, for example, a first portion and a second portion, the 
information of the first portion being suitable, for example, for being 
read out at a first read-out speed and the information of the second 
portion being suitable, for example, for being read out at a second 
read-out speed which is substantially higher than the first read-out 
speed. The term "read-out speed" is understood to mean the number of 
elementary marks to be read out per unit time, also called bit rate in the 
case of a CD. The read-out speed of each of the two portions is determined 
inter alia by the nature of the information present on each portion of the 
information layer. 
A disadvantage of a disc-shaped information carrier of the kind mentioned 
in the opening paragraph is that the information carrier can only be fully 
read out by means of an optical read-out unit which has a comparatively 
wide range of rotational speeds. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a disc-shaped information 
carrier of the kind mentioned in the opening paragraph which is capable of 
complete read-out by means of a read-out unit whose range of rotational 
speeds is as limited as possible. 
According to the invention, the disc-shaped information carrier is for this 
purpose characterized in that the second portion of the information layer 
is at a greater distance to the center of the disc-shaped information 
carrier than is the first portion of the information layer. The rotational 
speed of the disc-shaped information carrier is determined by the quotient 
of the read-out speed with which the information is to be read and the 
distance between the information to be read and the center of the 
disc-shaped information carrier. As a result, the rotational speed of the 
information carrier is comparatively high during read-out of information 
present close to the center of the information carrier and comparatively 
low during read-out of information present adjacent the outer edge of the 
information carrier. Since the second portion of the information layer 
with the comparatively high, second read-out speed is at a greater 
distance to the center of the information carrier than is the first 
portion of the information layer having the comparatively low, first 
read-out speed, according to the invention, the difference between the 
rotational speed during read-out of the first portion and rotational speed 
during the read-out of the second portion is substantially reduced. As a 
result, the information carrier according to the invention is fully 
readable by means of a read-out unit having a range of rotational speeds 
which is as limited as possible, so that the read-out unit can be provided 
with an optimized drive unit. 
A special embodiment of a disc-shaped information carrier according to the 
invention is characterized in that the first portion comprises a first 
version of an audio recording and the second portion comprises a second 
version of said audio recording different from the first version. In this 
special embodiment, in which the available space on the information layer 
is utilized in an effective manner, the two portions of the information 
layer comprise two different versions of one and the same audio recording. 
A user of the information carrier thus has the possibility, for example, 
of choosing between two different technical versions of the audio 
recording, or the information carrier is suitable, for example, for use in 
two different types of read-out units. 
A further embodiment of a disc-shaped information carrier according to the 
invention is characterized in that the first version comprises a 
two-channel version of the audio recording, while the second version is a 
six-channel version of the audio recording. The result of this is that a 
user of the disc-shaped information carrier has an interesting choice, for 
example, between a two-channel playback and a six-channel playback of the 
audio recording. 
A yet further embodiment of a disc-shaped information carrier according to 
the invention is characterized in that the first portion and the second 
portion are provided on the information carrier by means of mutually 
differing transformation methods. The first portion comprises, for 
example, information transformed by the DSD method (bit stream method), 
whereas the second portion comprises, for example, information transformed 
by the PCM method (pulse code modulation method). As a result, the 
information carrier is suitable, for example, for use in two different 
kinds of read-out units. 
A special embodiment of a disc-shaped information carrier according to the 
invention is characterized in that the first portion and the second 
portion are provided on the information carrier by mutually corresponding 
transformation methods.

DETAILED DESCRIPTION OF THE DRAWINGS 
The disc-shaped information carrier 1 according to the invention 
diagrammatically shown in FIG. 1 comprises a disc-shaped, transparent, 
synthetic-resin substrate 3 on which a comparatively thin reflecting 
information layer 5 is provided, which layer comprises optically readable 
elementary marks. The information layer 5 is readable by means of an 
optical beam from a side of the substrate 3 facing away from the 
information layer 5. The elementary marks of the information layer 5 
comprise tiny pits which are provided along substantially circular tracks 
in the information layer 5. The information layer 5 has a substantially 
uniform density which is defined by an interspacing present between the 
consecutive tracks and an interspacing present between the elementary 
marks on a track. The information layer 5 is readable by means of a 
read-out unit suitable for this purpose, which unit rotates the 
information carrier 1 about an axis of rotation 9 which passes through a 
center 7 of the information carrier 1, an optical beam generated by the 
read-out unit following the tracks of the information layer 5. 
As is shown diagrammatically in FIG. 1, the information layer 5 comprises a 
first portion 11 of optically readable marks and a second portion 13 of 
optically readable marks. The first portion 11 of the information layer 5 
is suitable for being read out at a first read-out speed, whereas the 
second portion 13 of the information layer 5 is suitable for being read 
out at a second read-out speed which is substantially higher than the 
first read-out speed. The term "read-out speed" is understood to mean the 
number of elementary marks to be read out per unit time in order to 
achieve a correct playback of the information present on the information 
layer 5, this being subject inter alia to the nature of the information to 
be read out. 
As was mentioned above, the information carrier 1 is rotated about the axis 
of rotation 9 during read-out. The rotational speed .omega. is determined 
by the quotient of the read-out speed and the distance r between the track 
to be read and the center 7 of the information carrier 1. As a result, the 
rotational speed .omega. is comparatively high during the read-out of 
information close to the center of the CD (comparatively small value of r) 
and comparatively low during the read-out of information adjacent an outer 
edge of the CD (comparatively high value of r) in the case of a standard 
CD having a single, constant read-out speed. FIG. 2 diagrammatically shows 
the relation between the rotational speed .omega. of such a standard CD 
and distance r to the center of the CD. The difference between the maximum 
rotational speed .omega..sub.max and the minimum rotational speed 
.omega..sub.MIN is comparatively great, so that it is only possible to 
read out the standard CD fully by means of a read-out unit having a 
sufficiently wide range of rotational speeds. 
As FIG. 1 shows, the second portion 13 of the information layer 5 of the 
information carrier 1 according to the invention is at a greater distance 
to the center 7 of the information carrier 1 than is the first portion 11 
of the information carrier 1. Since the read-out speed of the second 
portion 13 is substantially higher than the read-out speed of the first 
portion 11, and the rotational speed of the information carrier 1 is 
determined by the quotient of the read-out speed and the distance between 
the information to be read and the center 7 of the track carrier 1, it 
follows that the difference between the rotational speed of the 
information carrier 1 during reading of the first portion 11 and the 
rotational speed of the information carrier 1 during reading of the second 
portion 13 is comparatively small. FIG. 3 diagrammatically shows the 
relation between the rotational speed .omega.' of the information carrier 
1 according to the invention and the distance r to the center 7 of the 
information carrier 1. The difference between the maximum rotational speed 
.omega.'.sub.MAX and the minimum rotational speed .omega.'.sub.MIN is 
considerably smaller than the difference shown in FIG. 2 between the 
maximum rotational speed .omega..sub.MAX and the minimum rotational speed 
.omega..sub.MIN for the standard CD. As a result, the information carrier 
1 can be fully read out by means of a read-out unit which has only a 
limited range of rotational speeds. 
The first portion 11 and the second portion 13 of the information layer 5 
comprise, for example, a first version and a second version of one and the 
same audio recording, respectively. The first version corresponds, for 
example, to a two-channel version of the audio recording provided on the 
first portion 11 by the DSD transformation method (bit stream method or 
1-bit oversampled audio signal method), whereas the second version 
corresponds, for example, to a six-channel version of the audio recording 
provided on the second portion 13 by the PCM transformation method (pulse 
code modulation method). A user of the information carrier 1 thus has a 
choice, for example, between the two versions of the audio recording 
mentioned above, or the information carrier 1 can be read out, for 
example, by means of two different kinds of read-out units, i.e. by means 
of a read-out unit suitable for reading out information transformed by the 
DSD transformation method and a read-out unit suitable for reading out 
information transformed by the PCM transformation method. 
In the disc-shaped information carrier 1 according to the invention as 
described above, the first portion 11 is suitable for being read out at a 
first read-out speed, while the second portion 13 is suitable for being 
read out at a second read-out speed which is substantially higher than the 
first read-out speed. It is noted that the first read-out speed and the 
second read-out speed themselves need not have constant or substantially 
constant values. Thus, for example, the first read-out speed and the 
second read-out speed may each have an average value, the values of the 
two read-out speeds being allowed to deviate from the respective average 
values by, for example, at most 10%. 
It is further noted that a disc-shaped information carrier according to the 
invention may alternatively be provided with more than two portions. Thus 
an information carrier according to the invention may be provided, for 
example, with three portions, a first portion provided adjacent the center 
of the information carrier having a comparatively low read-out speed, a 
third portion provided adjacent an outer edge of the information carrier 
having a comparatively high read-out speed, and a second portion provided 
between the first portion and the third portion having a read-out speed 
which is higher than the read-out speed of the first portion and lower 
than the read-out speed of the third portion. 
The first portion 11 of the information carrier 1 according to the 
invention as described above comprises a first version of an audio 
recording, while the second portion 13 comprises a second version of the 
same audio recording. It is noted that the two portions of an information 
carrier according to the invention may also be assigned in a different 
manner such as, for example, an arrangement in which the first portion 
comprises a version of a first audio recording and the second portion a 
version of a second audio recording. 
It is finally noted that the two portions of the information carrier 
according to the invention may also be provided on the information carrier 
by one and the same transformation method such as, for example, the DSD 
transformation method or the PCM transformation method. 
The invention has been disclosed with reference to specific preferred 
embodiments, to enable those skilled in the art to make and use the 
invention, and to describe the best mode contemplated for carrying out the 
invention. Those skilled in the art may modify or add to these embodiments 
or provide other embodiments without departing from the spirit of the 
invention. Thus, the scope of the invention is only limited by the 
following claims: