 |
Description  |
|
|
The invention relates to an apparatus for reading a disc-shaped record
carrier in which signals are recorded in optically coded form in parallel
extending tracks, which apparatus comprises a radiation source for
emitting a radiation beam and a detector unit for converting the
information which is contained in said radiation beam after interaction
with the record carrier into an electric signal, which detector unit
comprises two read detectors, which occupy positions which are offset
relative to each other in a direction parallel to the image of the track
direction of the record carrier on the detector unit, and a first
combination circuit for subtracting the signals supplied by the two read
detectors from each other, which first combination circuit is connected to
a first signal processing channel for processing the information contained
in the output signal of said first combination circuit.
Such an apparatus is described in the German Pat. App. No. 2,342,906 which
has been laid open for public inspection. The apparatus described in said
Patent Application is employed for reading record carriers on which the
information is recorded in a relief pattern, which for example consists of
a sequence of recesses and intermediate areas in the surface of a record
carrier. This type of record carriers is most suited for mass production,
because said record carriers can be manufactured rapidly and inexpensively
by means of pressing techniques.
In an embodiment of the read apparatus for such a record carrier shown in
said German Patent Application a detector unit is employed which consists
of four read detectors. Two of said read detectors are disposed
symmetrically relative to a plane perpendicular to the record carrier and
parallel to the track direction and they are used for measuring the radial
position of the scanning spot on the record carrier relative to the track
in order to obtain a control signal for the radial tracking system. The
two other read detectors are disposed symmetrically relative to a plane
which is both perpendicular to the record carrier and perpendicular to the
track direction and they are used for reading the information which is
recorded on the record carrier.
Reading is effected by determining the difference between the amounts of
light incident at the two read detectors with the aid of a differential
amplifier, so-called differential reading. Said method of reading is based
on the insight that when the scanning spot produced by the road beam of
radiation is completely situated at a recess or completely at an
intermediate area, the two read detectors receive the same amount of light
and the output signal of the differential amplifier is consequently zero.
When said scanning spot hits the transition between a recess and an
intermediate area, this is no longer the case and the amounts of light
received by the two read detectors will differ as a result of diffraction
of the beam. In this manner the transitions of the recesses and the
intermediate areas are detected, thus enabling the recorded signal to be
reproduced.
It has been found that below a certain limit frequency whose magnitude
varies with the read diameter the sensitivity of said reading system
decreases with decreasing frequency. The frequency characteristic of this
system is consequently not flat at lower frequencies, but has a certain
roll-off in that region. As a result of this, said part of the frequency
spectrum of comparatively low frequency cannot readily be used for signal
transmission, so that the available frequency band for signal transmission
is limited. It will be obvious that this may be a limitation especially
for the transmission of a video signal, when a large band width is
desirable.
It is an object of the invention to mitigate said drawback and the
invention is characterized in that the detector unit comprises a second
combination circuit, which adds the signals supplied by the two read
detectors and is connected to a second signal processing channel, the
first signal processing channel being equipped to process signal
components which occupy a part of the frequency spectrum of comparatively
high frequency and the second signal processing channel being equipped to
process signal components which occupy a part of said frequency spectrum
of comparatively low frequency.
The addition of the second combination circuit as it were introduces a
second read method. The addition of the signals supplied by the two
detectors basically has no other effect than the detection of the full
information contained in the beam which is used for reading. Said method
of reading is known per se and an embodiment thereof is for example
described in U.S. Pat. Application Ser. No. 344,866, filed Mar. 26, 1973
and now abandoned in favor of U.S. Ser. No. 536,161, filed Dec. 24, 1974.
Said method is readily suitable for reading the information which is
recorded on the record carrier. The transfer function of said read method
in respect of the part of comparatively high frequency substantially
corresponds to the transfer function of the first-mentioned read method.
However, in the part of the transfer function of comparatively low
frequency said read method does not have the drawback of the
first-mentioned read method, but on the contrary it has a comparatively
flat characteristic. This means that by means of this read method signal
components which are contained in the part of comparatively low frequency
of the frequency spectrum can be read.
The step according to the invention thus provides a read method which
combines the advantages of the two read methods and which allows a
frequency band for the storage of the desired signals to be used which is
essentially not limited at the low frequency end and which extends down to
zero frequency. These and other advantages of the apparatus according to
the invention will be described in more detail with reference to the
Figures in which
FIG. 1 schematically shows an embodiment of the apparatus according to the
invention.
FIG. 2 shows the associated signal waveforms and
FIGS. 3 and 4 show two frequency spectra.
FIG. 5 by way of example shows the two signal processing channels, and
FIG. 6 shows an alternative arrangement of the detector unit.
The disc-shaped record carrier 1 in FIG. 1 is rotated in a direction V by
means of a motor 3 and a spindle 2 which extends through a central hole of
the record carrier 1. On said record carrier information is recorded in
tangentially extending tracks 4. In this respect the recording and reading
of a video signal is first of all thought of, which signal consists of
luminance information, chrominance information and sound information. Said
information may be recorded both in a single spiral track and in a
multiplicity of concentric tracks. In the embodiment shown the information
is recorded in the form of recesses 5 in the surface of the record
carrier, so that the track 4 consists of a sequence of recesses 5 and
intermediate areas 6.
The information which is recorded in said record carrier 1 is read with the
aid of an optical read system, which first of all comprises a radiation
source 7. Said radiation source 7 emits a beam 8 which by means a lens 9
is converted into a converging beam 10. Said beam 10 hits the record
carrier 1 in point S, which is called the scanning spot, and interacts
with the information at the location of said scanning spot. S. Eventually,
the radiation beam 11 which is modulated by said information hits a
detector unit 12, on which the scanning spot S is thus imaged as a
radiation spot S'.
The detector unit 12, in accordance with the detector unit shown in the
said German Pat. Application 2,342,906, consists of four read detectors
13a, 13b, 13c and 13d which together occupy a square surface area. The two
read detectors 13a and 13c are then disposed symmetrically relative to a
plane which is perpendicular to the imaged track direction p', while the
two read detectors 13b and 13d are disposed symmetrically relative to a
plane which is perpendicular to said first-mentioned plane of symmetry. As
is described in said German Patent Specification the read detectors 13b
and 13d may be used for obtaining information about the radial position of
the scanning spot S on the record carrier. As long as the scanning spot S
correctly follows the track 4 the amounts of light received by the read
detectors 13b and 13d are equal. If a deviation occurs from the radial
position of the scanning spot S, the image S' is moved perpendicularly to
the direction p' owing to diffraction phenomena, so that the amounts of
light received by the two read detectors differ. By determining this
difference with the aid of a differential amplifier 14 a signal is
obtained at a terminal 15, which is suited to serve as a control signal
for a radial tracking system, not shown, which controls the radial
position of the scanning spot S.
The information in the track 4 of the record carrier is read in accordance
with the said German Patent Application with the aid of a differential
amplifier 16 whose two inputs are connected to the read detectors 13a and
13c and which consequently supplies a signal at its output terminal 17
which is proportional to the difference between the amounts of light
received by the two read detectors. The operation of said read method is
further explained with reference to FIG. 2. FIG. 2a shows a part of the
track 4, with recesses 5 and intermediate areas 6, which are scanned by
the scanning spot 5. When said scanning spot S is completely projected on
a recess or completely on an intermediate area the two read detectors
receive the same amount of light and the output signal of the differential
amplifier 16 is zero. However, as soon as the scanning spot impinges on a
transition between a recess and an intermediate area there will be a
difference between the amounts of light received by the two read detectors
13a and 13c owing to diffraction, and the differential amplifier 16
supplies a non-zero output signal. The pattern shown in FIG. 2a in theory
yields an output signal of the differential amplifier 16 in accordance
with the signal which is indicated by the dotted line in FIG. 2b. However,
since the bandwidth of the transmission system is limited, the actual
output signal of said differential amplifier will have a variation as
indicated by the uninterrupted line.
The transfer function F.sub.1 of a read system which is based on said
method of reading is shown in FIG. 3. Said transfer function F.sub.1
obviously has a limited bandwidth and consequently exhibits a roll-off at
increasing frequency above a specific frequency of for example
approximately 5 MHz. Moreover, said transfer function exhibits a roll-off
at decreasing frequency below a certain frequency of for example approx. 4
MHz. This means that only a limited part of the total frequency band is
suited for signal transmission. The Figure shows by way of example in
which manner the various signal components of a color television signal
can be positioned within said transfer function.
The luminance signal is recorded as a frequency modulation of a carrier
wave f.sub.y. It is assumed that said modulated carrier wave has a
frequency swing from f.sub.z = 6 MHz (synchronizing pulse) to f.sub.w = 8
MHz (maximum white) and that as maximum bandwidth of the luminance signal
3 MHz is selected, so that the first-order side bands of said modulated
carrier wave at one side have 3.6 MHz, i.e. 6.6. MHz (maximum black) minus
3 MHz, as limit frequency, and at the other side 11 MHz. Thus, the
luminance information in total covers the frequency band E.sub.y from
3.6MHz to 11 MHz. The chrominance information is recorded as a modulation
of a carrier wave f.sub.c = 3 MHz and covers the frequency band E.sub.c
from 2.5 MHz to 3.5 MHz. The shape of said chrominance signal and the
method of modulation are irrelevant in this respect, so that these aspects
will not be discussed any further. Finally, a sound signal is modulated on
a carrier wave f.sub.g = 2 MHz, resulting in the frequency band E.sub.g.
Owing to the roll-off of the transfer function F.sub.1 at lower frequencies
the frequency band below 1.5 to 2 MHz is less suitable for signal
transmission, because at these low frequencies the signal-noise ratio has
deteriorated too far. It will be evident that this imposes limitations on
the bandwidth available for the various signal components and the choice
of the various carrier frequencies.
The invention overcomes this by means of the addition to the known read
system shown in FIG. 1. According to the invention the signals from the
read detectors 13a and 13c are also applied to an amplifier 18, which
supplies the sum of these two signals at a terminal 19. This step results
in a read method, known per se, described in the previously mentioned U.S.
Patent application, in which reading is effected with only one read
detector. Basically, it makes no difference whether reading is effected
with one read detector, or with two read detectors whose output signals
are added.
The signal waveform obtained by means of said method of reading is shown in
FIG. 2c, again related to the track pattern shown in FIG. 2a. The amount
of light received by a read detector will differ depending on whether the
scanning spot impinges on a recess 5 or an intermediate area 6, which
results in a signal variation as represented by the dashed line in FIG.
2c. Here, said a.c. signal is superimposed on average d.c. value V=, which
corresponds to the average incident light level. When said signal is
amplified using a.c. coupling, it is obvious that an a.c. signal is
obtained which is symmetrical relative to zero. Owing to the bandwidth
limitation the actual signal will be as represented by the continuous line
in FIG. 2c.
The signal thus obtained again contains the full information recorded on
the record carrier in analogy to the read method described hereinbefore.
The transfer function F.sub.2 of a read system based on this method of
reading, however, differs from the transfer function F.sub.1 of the first
system described and is shown in FIG. 4. To facilitate a comparison of the
two transmission systems the transfer function F.sub.1 is shown again in
said Figure.
The high-frequency parts (above approx. 5 MHz) of the two transfer
functions F.sub.1 and F.sub.2 are substantially identical. However, in the
low-frequency part there is a substantial and basic difference. The
transfer function F.sub.2 has no roll-off at lower frequencies but is
substantially flat down to zero frequency. This means that for the
last-mentioned method of reading the frequency band which is available for
signal transmission is substantially unlimited at the low-frequency end.
To clarify the step according to the invention an example of positioning
the various signal components of a color television signal is shown in the
frequency spectrum of FIG. 4 in analogy with FIG. 3, and FIG. 5 shows the
processing of said signal components. The luminance signal is again
frequency modulated on a carrier wave f.sub.y of 7.3 MHz, which
corresponds to grey level, the same frequency swing being selected as in
FIG. 3, namely from f.sub.z = 6 MHz (synchronizing pulse) to f.sub.w = 8
MHz (maximum white). However, for the first-order side band a frequency
spacing of 3.6 MHz is now reserved relative to black level (6.6 MHz), so
that said lower side band extends to 3 MHz. The bandwidth of the
eventually reproduced luminance signal is thus extended substantially,
viz. 3.6 - 30= 0.6 MHz.
By means of a band-pass filter 21 (see FIG. 5) said luminance signal is
extracted from the signal which is available at terminal 17. Terminal 17
in this case is the output of the differential amplifier 16 (FIG. 1) and
consequently provides a signal which is derived from the difference
between the amounts of light incident at the two read detectors 13a and
13c. Said extracted luminance signal E.sub.y is demodulated with the aid
of the demodulator circuit 25.
The chrominance signal E.sub.c (FIG. 4) as modulation of a carrier wave
f.sub.c = 2 MHz covers a frequency band which extends from 1.3 to 2.7
MHz. Consequently, said chrominance signal also has a larger bandwidth
than in the situation of FIG. 3. Finally, two sound signals E.sub.g.sbsb.1
and E.sub.g.sbsb.2 are recorded on two carrier waves f.sub.g.sbsb.1 = 0.7
MHz and f.sub.g.sbsb.2 = 0.4 MHz. These signal components E.sub.c,
E.sub.g.sbsb.1 and E.sub.g.sbsb.2 are located in a frequency range which
can no longer be transmitted correctly using the first-mentioned read
system with transfer function F.sub.1, because said transfer function
F.sub.1 already has a substantial roll-off in this frequency range.
Recording of such signals is described in U.S. Pat. No. 3,893,163.
Said signal components E.sub.g.sbsb.1, E.sub.g.sbsb.2 and E.sub.c are
therefore extracted from the signal which is available at terminal 19 with
the aid of band filters 22, 23 and 24 (FIG. 5). Said terminal 19 is the
output of amplifier 18 (FIG. 1) and supplies as signal the sum of the
signals supplied by the two read detectors 13a and 13c, for which the
transfer function F.sub.2 applies which is readily usable in the relevant
frequency range. The extracted sound signals E.sub.g.sbsb.1 and
E.sub.g.sbsb.2 are demodulated with the aid of FM demodulators 26 and 27
(FIG. 5) which supply the signals G.sub.1 and G.sub.2 at their respective
outputs. The chrominance signal E.sub.c is supplied to a transformation
unit 28, which transforms said chrominance signal E.sub.c to a chrominance
signal C which is suited for reproduction by a standard color television
receiver. It is obvious that the embodiment of said unit 28 depends on the
manner in which the color signal is coded, for which many modifications
are known. However, the method of coding is immaterial for the principle
of the invention. The signals Y, G.sub.1, G.sub.2 and C are finally
applied to a circuit 29, in which said signals are for example modulated
on a high-frequency carrier wave, so that a HF television signal is
available at output terminal 30, which may be applied directly to the
aerial input of a television receiver.
By means of the steps according to the invention the frequency band
available for signal transmission is extended substantially, while
maintaining the advantages of the first-mentioned read system. Both for
the luminance signal E.sub.y and the chrominance signal E.sub.c a wider
frequency band is available. An advantage of the use of the
first-mentioned read system is the fact that with this method of reading
the frequency modulated luminance signal is less susceptible to
interference owing to the occurrence of cross-products with the other
signal components, especially when said other signal components are added
to said frequency modulated luminance signal as a pulse-width modulation.
This can be illustrated simple by means of FIG. 2a. A pulse width
modulation in said frequency modulated luminance signal manifests itself
as an oppositely directed shift of two successive signal pulses, which
means that the two signal pulses in FIG. 2b, which denote the two
transitions from a recess 5 to an intermediate area 6 are shifted in
opposite directions, as is illustrated by means of the pulses P and Q
(dash-dot lines). The resulting signal is denoted by the dash-dot line R.
When the luminance signal is frequency modulated the applied signal is
limited, so that a pulse-shaped signal is obtained, which means that as
information solely the position of zero crossings of the applied signal is
used. However, FIG. 2b shows that initially the position of the zero
crossing Z is not influenced by the pulse-width modulation, so that said
pulse-width modulation initially does not influence the limited frequency
modulated luminance signal in the case of said differential read method in
accordance with the first read system.
FIG. 6 shows an alternative arrangement of the detector unit 12. The read
detectors 13a and 13b are then disposed in line parallel to the imaged
track direction P' and the read detectors 13d and 13c are also disposed in
this manner. To obtain the desired two read methods the signals of the
read detectors 13a and 13d may first of all be added by means of an
amplifier 31 and the signals of the read detectors 13b and 13c by means of
amplifier 32. The output signals of said two amplifiers 31 and 32 may then
be applied to the amplifiers 16 and 18 at whose outputs 17 and 19 the
desired signals are then available.
It is to be noted that the depth of the recesses in the record carrier
should preferably selected so that the two read systems operate in an
optimum manner. In the case of a record carrier which reflects the beam of
radiation, said depth will for example be the eighth part of the
wavelength of the light that is used. However, a different choice is
equally possible.
* * * * *
|
|
 |
|
 |
Description |
|
