JPH0785524A - Magneto-optical recorder - Google Patents

Abstract

PURPOSE:To initialize only the region where recording is executed and to increase writing power by adding an initializing magnetic field of the magnitude to the extent of a track width only to the track to be recorded just before recording. CONSTITUTION:An electromagnet which is an initializing magnetic field impressing means is on-off controlled according to the control of the initializing magnetic field control means 10 in such a manner that only the recording region of a disk where recording is going to be executed is initialized. The medium region initialized in such a manner passes a position to be irradiated with a beam as the disk is rotated by a rotating the laser beam to be irradiated is demodulated according to the information to be recorded by a pulse demodulating means 6. The beam emitted from a laser beam source 7 is reflected by a beam splitter 8 and the initialized part of the medium under impression of a recording magnetic field Hb is irradiated with this beam. As a result, recording is completed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magneto-optical recording apparatus which does not modulate the direction of a recording magnetic field but overwrites only by modulating the intensity of a light beam.

[0002]

2. Description of the Related Art Light modulation overwrite is a recording method in which only a light beam is modulated according to information without modulating a recording magnetic field. Overwriting refers to the act of rewriting recorded information by overwriting new information without erasing previous information. When overwriting, the previously recorded information must be erased in the overwritten recording area, in other words, only the newly recorded information when reproducing (information reading) Must be played and the previous information must not be played. The term "overwrite" as used in this specification means that the laser beam is irradiated while being pulse-modulated according to the information to be recorded, without modulating the direction and intensity of the recording magnetic field Hb (i.
rradiate) means to overwrite.

Recently, an optical recording / reproducing method satisfying various requirements including high density, large capacity, high access speed, and high recording / reproducing speed, and a recording device, reproducing device and recording medium used therein have been developed. Efforts are being made to try. Among a wide range of optical recording / reproducing methods, the magneto-optical recording / reproducing method has a unique advantage that information can be recorded and then erased, and new information can be recorded again and again. For being full of the greatest attraction.

A recording medium used in this magneto-optical recording / reproducing method has a perpendicular magnetic layer (perpendicular magnetic layer or layers) consisting of one layer or multiple layers as a layer for recording. This magnetic film is made of, for example, amorphous GdFe or Gd.
It is composed of Co, GdFeCo, TbFe, TbCo, TbFeCo and the like. The perpendicular magnetic film generally has concentric or spiral tracks, and information is recorded on the tracks. There are two types of tracks, explicit and implicit.

[Explicit Track] The magneto-optical recording medium has a disk shape. In a disc having explicit tracks, tracks for recording information are formed in a spiral shape or a concentric circle shape when viewed in a direction perpendicular to the disk plane. There is a groove for tracking and separation between two adjacent tracks. Conversely, the space between the grooves is called a land. In reality, the land and groove are reversed on the front and back of the disc. Therefore, looking at the disk in the same way as the beam enters, the front side is called a groove and the back side is called a land. Since the perpendicularly magnetized film is formed over both the groove and the land, the groove portion may be the track or the land portion may be the track. There is no particular relationship between the width of the groove and the width of the land.
In order to form such a land and a groove, the substrate generally has a land formed in a spiral or concentric shape on the surface and a groove sandwiched between two adjacent lands. A thin perpendicular magnetic film is formed on such a substrate. As a result, the perpendicular magnetization film has lands and grooves.

[Mark] In the present specification, one of “upward” and “downward” with respect to the film surface is defined as “A direction” and the other is defined as “reverse A direction”. The information to be recorded is binarized in advance, and this information has two signals: a mark (B ₁ ) having a magnetization in the “A direction” and a mark (B ₀ ) having a magnetization in the “reverse A direction”. Recorded in. These marks B ₁ and B ₀ correspond to either one or the other of the digital signals 1 and 0, respectively. However, generally, the magnetization of the track to be recorded is aligned in the "reverse A direction" by applying a strong external magnetic field before recording. Act to align the magnetization direction is referred to as the "initialization ^* (initialize ^*)" in the old sense. Then, a mark (B ₁ ) having "A direction" magnetization is formed on the track. Information is represented by the presence or absence of the mark (B ₁ ), the position, the front end position, the rear end position of the mark, the mark length, and the like. In particular, a method in which the edge position of a mark represents information is called mark length recording. A mark has been called a pit or a bit in the past, but is recently called a mark.

By the way, to re-use a recorded medium (1) to reinitialize the medium ^* device initialization ^* either by,
Or (2) it is necessary to add an erasing head similar to the recording head to the recording device, or (3) it is necessary to erase the recorded information in advance by using the recording device or the erasing device as a pre-stage process. Therefore, in the magneto-optical recording system, it has hitherto been impossible to perform overwriting capable of recording new information on the spot regardless of the presence or absence of recorded information. However,
If the direction of the recording magnetic field Hb can be freely modulated between the "A direction" and the "reverse A direction" as necessary,
Overwriting is possible. However, it is impossible to modulate the direction of the recording magnetic field Hb at high speed. For example, when the recording magnetic field Hb is a permanent magnet, it is necessary to mechanically reverse the direction of the magnet. However, it is impossible to reverse the direction of the magnet at high speed. Recording magnetic field Hb
Even if is an electromagnet, it is not possible to modulate the direction of a large capacity current at such a high speed.

However, the technical progress is remarkable, and the intensity of the irradiation light beam should be recorded without modulating the intensity of the recording magnetic field Hb (including ON and OFF) or without modulating the direction of the recording magnetic field Hb. A magneto-optical recording method capable of overwriting, a magneto-optical recording medium capable of being overwritten, and an overwritable recording apparatus also employed therefor have been invented only by modulating in accordance with binary information. A patent application has been filed (JP-A-62-175948 = DE3,619,61)
8A1 = US patent pending Ser. No. 453,255). Hereinafter, this invention is referred to as a "basic invention".

[Explanation of Basic Invention] In the basic invention, "a recording / reproducing layer record basically composed of a magnetic thin film capable of perpendicular magnetization is used.
ing layer (this recording / reproducing layer is referred to as a memory layer Memory layer or M layer in this specification) and a recording auxiliary layer consisting of a perpendicularly magnetizable magnetic thin film reference layer (in this specification, this recording auxiliary layer is written. Layer Writing la
yer or W layer), and both layers are exchange coupled (exch
ange-coupled) and uses an overwritable multi-layer magneto-optical recording medium capable of keeping only the magnetization of the W layer in a predetermined direction without changing the magnetization direction of the M layer at room temperature. To do. Then, the information is recorded in the M layer (and in some cases also in the W layer) with a mark having “A direction” magnetization and “inverse A direction”.
It is represented by a mark having "direction" magnetization and recorded.

In this medium, the W layer can have its magnetization direction aligned in the "A direction" by an external means (for example, the initial auxiliary magnetic field Hini.). Moreover, at that time, the magnetization direction of the M layer is not reversed, and further, the magnetization direction of the W layer once aligned in the “A direction” is not reversed even when the exchange coupling force from the M layer is received. On the contrary, the magnetization direction of the M layer is not reversed even when receiving the exchange coupling force from the W layer aligned in the “A direction”. The W layer has a lower coercive force H _C than the M layer.
And has a high Curie point T _C. According to the recording method of the basic invention, only the magnetization direction of the W layer of the recording medium is aligned in the “A direction” by external means before recording. This act is specifically referred to herein as "initialize".
This "initialization" is unique to overwritable media.

Then, the medium is irradiated with a laser beam pulse-modulated according to the binarized information. The intensity of the laser beam has a high level P _H and a low level P _L , which correspond to the high level and low level of the pulse. This low level is higher than the reproduction level P _R that illuminates the medium during reproduction. As is already known, the laser beam may be turned on at a <very low level> even when recording is not performed, for example, to access a predetermined recording position on the medium. This <very low level> is also the same as or close to the reproduction level P _R.

For example, "initialize" for "A direction"
When the medium subjected to e) ”is irradiated with a laser beam of a low level P _L , the temperature of the medium is increased and the coercive force H _{c1 of the} M layer is increased.
Becomes very small or extremely zero. It becomes zero when the temperature of the medium is equal to or higher than the Curie point of the M layer. At this time, the coercive force H _c2 of the W layer is sufficiently large,
It is not reversed by the recording magnetic field Hb in the "reverse A direction".
Then, the force of the W layer reaches the M layer via the exchange coupling force. M
Layers and W layers are generally heavy rare earth metals.
et al: hereinafter abbreviated as RE) -transition metal (transition met
al: hereinafter abbreviated as TM). The exchange coupling force is composed of a force that aligns the RE magnetic moments of both layers and a force that aligns the TM magnetic moments of both layers. In the alloy, the sublattice magnetization of RE and the sublattice magnetization of TM have opposite directions, and the direction of the larger sublattice magnetization determines the direction of magnetization of the alloy. When both sublattice magnetizations are equal, the composition is called a compensation composition and the temperature is called a compensation temperature. Above the compensation temperature, the TM sublattice magnetization is stronger,
Below the compensation temperature, the RE sublattice magnetization is stronger.

The state of the mark before the irradiation of the laser beam is a state in which an interface domain wall exists between the M layer and the W layer,
There are two types: the nonexistent state. The mark which does not exist corresponds to the mark to be formed. The existing mark does not match the mark to be formed.
In the latter case, as a result of the force of the W layer exerting on the M layer via the exchange coupling force, the magnetization of the M layer having a coercive force H _c1 which has become extremely small is the predetermined direction (eg,
"A direction"). As a result, a mark having no interface wall between the M layer and the W layer (target mark)
Is formed.

Even if the magnetization of the M layer is zero (T _c1 or more), the irradiation of the laser beam is stopped, and the temperature of the medium naturally lowers to slightly lower than the Curie point T _c1.
Magnetization appears in the layer. At this time, similarly, the force of the W layer reaches the M layer via the exchange coupling force. Therefore, the magnetization appearing in the M layer has a predetermined direction (for example, “A
Direction "). From this state, the temperature returns to room temperature, but the predetermined orientation is maintained. However, if there is a compensation temperature in the M layer and the W layer during returning to room temperature, the magnetization direction of the layer is reversed when the temperature exceeds the compensation temperature. This process is called a cold cycle or cold process.

On the other hand, for example, "Initialization (ini
tialize) "is media receives the irradiation of the laser beam of high level P _H, the coercive force H _c1 of the M layer is improved temperature of the medium is zero, the coercive force H _c2 of W layer is very Therefore, the magnetization of the W layer, which has a very small coercive force H _c2 , loses the recording magnetic field Hb and faces a predetermined direction (for example, the “reverse A direction”). Even if the magnetization of the W layer is zero, the irradiation of the laser beam is stopped and the temperature of the medium is naturally lowered to the Curie point T _c2.
When it goes down a little, magnetization appears in the W layer, but at this time,
Similarly, when the recording magnetic field Hb is lost, the magnetization of the W layer is oriented in a predetermined direction (for example, "reverse A direction"). Further, when the temperature of the medium cools and falls slightly below the Curie point T _c1 , magnetization appears in the M layer. At this time, the force of the W layer reaches the M layer via the exchange coupling force. Therefore, the magnetization appearing in the M layer is oriented in a predetermined direction (for example, “reverse A direction”) dominated by the W layer.
From this state, the temperature returns to room temperature, but the predetermined orientation is maintained. However, if there is a compensation temperature in the M and W layers on the way back to room temperature,
When it exceeds that, the magnetization directions of the M layer and the W layer are reversed. This process is called a high temperature cycle or high temperature process.

The above low temperature cycle and high temperature cycle are M
It occurs regardless of the direction of magnetization of the layer and the W layer. anyway,
Before the laser beam irradiation, the W layer is “initialized (initializ
e) ”, so it is possible to overwrite.

In the basic invention, the laser beam is pulse-modulated according to the information to be recorded. However, this itself is also performed in the conventional magneto-optical recording, and the means for modulating the beam intensity in a pulse shape according to the binary information to be recorded is a known means. For example, THE BELL SYSTEM
This is described in detail in TECHNICAL JOURNAL, Vol.62 (1983), 1923-1936. Therefore, given the required high and low levels of beam intensity, they are readily available with some modifications to conventional modulation means. For those skilled in the art, such modification is easy given the high and low levels of beam intensity.

One of the features of the basic invention is
There are high and low levels of beam intensity. That is, when the beam intensity is at a high level, the "A direction" magnetization of the W layer is reversed (reverse A direction) by the recording magnetic field Hb or other external means (re
verse), and the "reverse A direction" magnetization of the W layer forms a mark having "reverse A direction" magnetization (or "A direction" magnetization) in the M layer. When the beam intensity is low, the magnetization direction of the W layer is the same as in the "initialized" state, and
By the action of the layer (this action is transmitted to the M layer through the exchange coupling force), the M layer is magnetized "A direction" [or "reverse A direction"].
Magnetization] is formed. In this specification,
The expression ○○○ [or △△△] means the following ○○○ [or △△△ ] when you read ○○○ outside [] first.
In case of, I will read ○○○ outside []. On the other hand, if you select the △△△ in [] without reading ○○○ first and read it, the following ○○○ [or △△
Even in the case of △ ], △ △ △ in [] should be read without reading ○○○.

The medium used in the basic invention is roughly classified into a first embodiment and a second embodiment. In any of the embodiments, the recording medium has a multilayer structure including an M layer and a W layer. The M layer is a magnetic layer having a high coercive force and a low magnetization reversal temperature at room temperature. The W layer is a magnetic layer having a lower coercive force and a higher magnetization reversal temperature at room temperature than the M layer. Both the M layer and the W layer may themselves be composed of a multilayer film. In some cases, an intermediate layer (for example, an exchange coupling force σ _W adjusting layer ..., This layer is abbreviated as an Int. Layer hereinafter) may be present between the M layer and the W layer. For the Int. Layer, see
See 64-50257 and JP-A-1-273248. Also, regarding overwritable magneto-optical recording, many other materials such as JP-A-4-123339 and JP-A-4-134741 have been published, and therefore, further explanation is omitted here. The disc having a four-layer structure disclosed in Japanese Patent Laid-Open No. 4-123339 has an "initialization" layer (In
Itializing layer: hereinafter referred to as I layer) and a switching layer (Swithing layer: hereinafter referred to as S layer) for turning on / off exchange coupling between the I layer and the W layer.

In order to increase the C / N ratio, a read-out layer (Readout layer: below) having a higher Curie point and a higher Kerr effect than the M layer on the M layer (that is, on the laser beam incident side):
A laminate of R layers) is also proposed. For example, see JP-A-63-64651 and JP-A-63-48637. The proposed R layer is also composed of a RE-TM based alloy. In the light modulation type overwrite described above, the binary value is recorded only by modulating the light beam, so that the medium is initialized (that is, "initialization in which the magnetization direction of the writing layer is aligned in a predetermined direction before recording"). )). In the conventional optical modulation type overwrite magneto-optical recording / reproducing apparatus, in order to perform this initialization,
An initializing magnet was placed over the entire radial length of the disc.

[0021]

In the above conventional recording / reproducing apparatus, almost the entire recording area of the recording medium is always initialized. However, when reproducing the initialized media, if the reproducing power is set too high, the temperature of the memory layer will reach near the Curie point during reproduction, so the magnetization direction of the writing layer will be reversed by the initialization. At the recorded position, the direction of magnetization of the memory layer is also reversed during reproduction due to the influence of the writing layer (that is, a low temperature process occurs). As described above, in the initialized medium, the reproduction power is limited due to the problem that the magnetization of the writing layer is transferred in the vicinity of the Curie point of the memory layer and the recorded information is lost. The higher the reproduction power, the higher the S / N ratio obtained. Therefore, the upper limit of the reproduction power should be large. However, due to this limitation, the power cannot be made too large. Also when performing high-level write-Initialized medium (writing with high beam P _H), the Curie point near the writing layer, near the Curie point of the temperature memory layer of the adjacent areas of the area irradiated with the light beam There is a problem that a low temperature process occurs in the adjacent area and destroys the information in the adjacent area. Due to this problem, the laser beam power P _{H for} high level writing is limited. The margin of the high level write power (the allowable range of the power) is preferably as wide as possible because the mark becomes thin and the S / N ratio decreases when the power of the high level write is small. The current situation is that it cannot be made too large.

[0022]

In order to solve the above problems, in a recording / reproducing apparatus of the optical modulation overwrite type according to the present invention, initialization is performed just before recording only a region of a recording medium where recording is performed. As a method, an initializing magnetic field having a size of about the track width is applied to only the track to be recorded immediately before recording.

[0023]

When recording is performed by the apparatus of the present invention, only the track to be recorded is initialized, and the adjacent tracks are not initialized. Therefore, the write power is set within a range where a high temperature process does not occur in the adjacent tracks. Can be increased. That is, in the conventional device, the adjacent track is low.
In contrast to the conventional device, the write power can be increased because the adjacent track does not have to reach the temperature at which the high temperature process occurs, whereas the temperature at which the warm process should not occur. Because the adjacent track is not initialized, the magnetization of the writing layer of the adjacent track is not reversed, and therefore the magnetization of the memory layer is affected by the magnetization of the writing layer even when the adjacent track reaches the temperature at which the low temperature process occurs. This is because the information of the adjacent track is retained because it is not inverted by.

Further, at the time of reproduction, the reproduction power can be increased within a range where a high temperature process does not occur in the reproduced track. That is, in the conventional device, the track to be regenerated should not be at the temperature at which the low temperature process occurs, whereas in the device of the present invention, the track to be regenerated should not be at the temperature at which the high temperature process occurs.
The reproduction power can be increased as compared with the conventional device. Because the track to be played is not initialized, the magnetization of the writing layer of the track is not reversed,
Therefore, even if the temperature of the track reaches the temperature at which the low temperature process occurs, the magnetization of the memory layer is not reversed by the action of the magnetization of the writing layer, and the track information is retained.

[0025]

FIG. 1 is a schematic view showing a recording / reproducing apparatus of this embodiment. Here, 1 is a recording medium, which is rotated by a rotating means 3 via a rotating shaft 2. The apparatus includes an initializing magnetic field applying means 4 for applying an initializing magnetic field H _ini in a manner which will be described later in detail during recording, a recording magnetic field applying means 5 for applying a recording magnetic field Hb to a portion where recording is performed on the medium during recording, and an input. Pulse modulation means 6 for pulse-modulating the intensity of the laser beam between a high level P _H and a low level P _L according to the recorded information to be recorded, a laser beam light source 7 for emitting a pulse-modulated laser beam, and a beam splitter. 8, a detector 9 for detecting signal light during reproduction, and an initialization magnetic field control means 10 for controlling the operation timing of the initialization magnetic field application means.

FIG. 2 is a schematic sectional view showing an example of a magnetic field generator which can be used as the initializing magnetic field applying means in the present invention. The magnetic field generated by this magnetic field generator is also schematically shown. Recording of information on the medium in the above apparatus is performed as follows. The initializing magnetic field applying means 4 interlocks with the beam irradiation position of the disk 1 so that the initializing magnetic field is applied to the same track as the track irradiated with the laser beam on the medium by a means (not shown) during recording. Moved radially. The initialization magnetic field applying means of the present embodiment is an electromagnet, and on / off control is performed under the control of the initialization magnetic field control means 10 so that only the recording area of the disk where recording is to be performed is initialized. The medium area thus initialized passes through the beam irradiation position as the disk is rotated by the rotating means.
The emitted laser beam is modulated according to the information recorded by the pulse modulation means 6. The beam emitted from the laser light source 7 is reflected by the beam splitter 8 and applied to the initialized portion of the medium under the application of the recording magnetic field Hb. This completes the recording.

When the information recorded on the medium is reproduced by this apparatus, a reproducing laser beam which is not modulated is irradiated. The beam emitted from the laser light source 7 is reflected by the beam splitter 8 and guided to the medium surface. At this time, the recording magnetic field applying means is off. The beam reflected by the medium surface passes through the beam splitter 8 and enters the detector 9. The detector 9 converts the optical signal into an electric signal and outputs it. In this way, the information recorded on the medium is reproduced.

-Comparative test-Duty on a sample disk at 4MHz, dut using the device according to the invention
Fig. 3 shows the frequency characteristic spectrum reproduced at 1mW after writing the y50% mark. Here the linear velocity is 11.3 m / s. From this figure, it can be seen that the C / N ratio is 58.7db.
Next, this sample was reproduced for 1 minute with a reproducing power of 3.5 mW by the reproducing apparatus according to the present invention and a conventional reproducing apparatus (the initializing magnetic field was applied also at the time of reproducing), and then the frequency characteristics and C / N were again measured. Graphs obtained by measuring the ratio with a reproducing power of 1 mW are shown in FIGS. FIG. 4 shows a device according to the present invention, and FIG. 5 shows a device according to the related art. From the figure, it can be seen that the reproducing apparatus according to the present invention has a C / N ratio of 58.7db, which is not reduced, but the conventional reproducing apparatus has a C / N ratio of 37.8db, which is about 20db. C / in this conventional playback device
The lowering of the N ratio is due to the fact that the playback area of the conventional device has been initialized, and the 3.5 mW playback power causes a low-temperature process, which lowers the C / N ratio. it is conceivable that. On the other hand, according to the present invention, since the reproduced area is not initialized, the C / N ratio does not decrease even if the reproducing power is increased, and a recording / reproducing apparatus having a high reproducing power becomes possible.

The layer structure of the sample disk used for the test is schematically shown in FIG. The sample disc is
A SiN protective layer 22 having a thickness of 70 nm, a GdFeCo reproducing layer 23 having a thickness of 30 nm, and a T layer having a thickness of 25 nm are formed on a glass substrate 21 having a resin layer on which a tracking guide groove is formed.
bFeCo memory layer 24, 15 nm thick GdFeC
An intermediate layer 25, a DyFeCo writing layer 26 having a thickness of 50 nm, a GdFeCo initialization magnetic field assisting layer 27 having a thickness of 100 nm, and a SiN protective layer 28 having a thickness of 70 nm were sequentially laminated, and used.

[0030]

According to the present invention, since the minimum reproducing power can be increased, the range from the minimum recording power to the H recording power becomes small and the device can be manufactured by using the laser having the small dynamic range. Further, according to the present invention, since the tracks other than the recording track are not initialized, so-called thermal crosstalk that affects the recording on the adjacent tracks cannot occur, and therefore the high-level beam power at the time of recording is also large. I can.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a recording / reproducing apparatus according to the present invention.

FIG. 2 is a schematic cross-sectional view of a magnetic field generator used in this example.

FIG. 3 is a frequency spectrum of a reproduction signal immediately after writing a mark on a sample disc.

FIG. 4 is a frequency spectrum of a reproduced signal after the sample disk is reproduced for 1 minute by the apparatus of the embodiment.

FIG. 5 is a frequency spectrum of a reproduced signal after the sample disk is reproduced for 1 minute by the apparatus of the embodiment.

FIG. 6 is a diagram schematically showing the layer structure of a sample disc.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 recording medium 2 rotating shaft 3 rotating means 4 initializing magnetic field applying means 5 recording magnetic field applying means 6 pulse modulating means 7 laser light source 8 beam splitter 9 detector 10 initializing magnetic field controlling means

Claims (3)

[Claims] 1. Initializing for aligning the magnetization direction of a writing layer to a predetermined direction without changing the magnetization direction of the memory layer, which includes at least two layers of a memory layer and a writing layer, each of which is composed of a perpendicular magnetization film. A recording / reproducing apparatus of an optical modulation overwrite system for a magneto-optical recording medium capable of overwriting by an optical modulation system capable of performing: an initialization magnetic field applying means for performing the initialization of the medium;
High level P according to the binary information to be recorded at the time of recording
A laser beam irradiating means for irradiating a medium with a recording laser beam pulse-modulated between _H and a low level P _L to record information, and irradiating a reproducing laser beam with a constant intensity which is not modulated during reproduction. In the recording / reproducing apparatus having the above, the recording / reproducing apparatus for an overwritable magneto-optical recording medium, wherein the initialization magnetic field applying means initializes only a region on the medium where recording is to be performed immediately before recording. 2. The initialization magnetic field applying means applies a magnetic field having a magnitude substantially equal to the width of a recording track of the medium only to a track where recording is to be performed, and at the time of recording, the initialization magnetic field applying means 2. The recording / reproducing according to claim 1, wherein the laser beam irradiating means is moved in conjunction with the radial direction of the medium so that the track to which the initialization magnetic field is applied and the track to which the beam is applied are the same. apparatus. 3. The initialization magnetic field applying means is an electromagnet, and the initialization magnetic field applying means controls the on / off of the initialization magnetic field applying means so that the initialization magnetic field is applied only to a region of the medium where recording is to be performed. The recording / reproducing apparatus according to claim 2, further comprising control means.