GB2157876A

GB2157876A - High resolution reproduction of signals from phase transformable discs

Info

Publication number: GB2157876A
Application number: GB08509112A
Authority: GB
Inventors: Ichiro Ueno
Original assignee: Victor Company of Japan Ltd; Nippon Victor KK
Current assignee: Victor Company of Japan Ltd; Nippon Victor KK
Priority date: 1984-04-09
Filing date: 1985-04-09
Publication date: 1985-10-30
Also published as: DE3512715A1; DE3512715C2; GB2157876B; GB8509112D0

Abstract

A method is disclosed wherein electromagnetic radiant energy, either microwave or optical, is modulated in frequency with a signal to be recorded. The modulated energy is concentrated on a layer of a material which is transformable in phase structure between crystalline and amorphous, to cause a phase structural change to occur therein. The concentrated energy is moved along a predetermined path to form a series of regions of phase structural change. On reproduction, microwave energy of a constant intensity is applied to an electrode, which is moved in contact with the layer along said path to cause the microwave energy to be modulated in frequency with varying conductivity of said regions. The frequency-modulated microwave energy is subject to demodulation to recover the original signal. On playback, microwave energy is preferably modulated in frequency with the signal to be recorded and applied to the same moving electrode as used in the reproduction operation. <IMAGE>

Description

SPECIFICATION High resolution reproduction of signals from phase transformable discs BACKGROUND OF THE INVENTION The present invention relates generally to recording and reproduction of signals on storage mediums, and more particularly to a method and apparatus for recording a signal on and reproducing it from an erasable disc of the type in which the signal is recorded as changes in phase structure between crystalline and amorphous states.

One of the known eraseable optical discs is of the type in which signals are recorded in the form of a series of regions of amorphous state in a crystalline structure. The disc comprises a material such as TeOx which is made to change states from crystalline to amorphous when it is locally heated by an incident laser beam to a temperature higher than a threshold value.

Since the size of a light spot is determined by the wavelength of laser, current attempts with laser to achieve higher resolution reproduction and/or higher density recording have met with difficulty.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to permit high resolution reproduction of signals from such discs with the exclusive use of microwave energy.

This object is obtained by the application of microwave energy to a stylus electrode and injecting it to a microscopic area to read information which has been recorded in an eraseable disc in the form of changes in phase structure.

Preferably, the information is recorded by modulating it upon microwave energy and concentrating the modulated energy on the eraseable disc using the stylus electrode in common with playback operation.

According to the present invention, electromagnetic radiant energy, either microwave or optical, is modulated in frequency with a signal to be recorded and the modulated energy is concentrated on a layer of a material which is transformable in phase structure between crystalline and amorphous to cause a phase structural change to occur therein. The concentrated energy is moved along a predetermined path to form a series of regions of phase structural change. On reproduction, microwave energy of a constant intensity is applied to an electrode and the electrode is moved in contact with the layer along said path to cause the microwave energy to be modulated in frequency with varying conductivity of said regions. The frequency-modulated microwave energy is subject to demodulation to recover the original signal.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be described in further detail with reference to the accompanying drawings, in which: Figures 1A and 1B are block diagrams of a first embodiment of the present invention; Figures 2A to 2Fare side views of embodiments of the recording disc which can be used to advantage in the present invention; Figure 3 is a partially enlarged view of the surface of a disc according to the invention revealing the pattern of pits; Figure 4 is a cross-sectional view taken along the lines 4-4 of Fig. 3; Figure 5 is a block diagram of an optical arrangement used for erasing recorded information; Figures 6 and 7 are further embodiments of the invention which permit recorded signals to be monitored simultaneously with recording operation; and Figure 8 is a perspective view of a recording/playback stylus for application of microwave energy.

DETAILED DESCRIPTION Referring now to Figs. 1 A and 1 B, there is shown a preferred embodiment of the recording and reproducing apparatus of the present invention.

During recording mode, mode select switches 16a and 16b are in the recording position. A video signal is applied through an input terminal 11 to a frequency modulator 10. A switching signal generator 15 is synchronized with a rotation of a recording disc 30, Fig. 1 B, to generate a switching pulse burst and a switching pulse at every vertical retrace period of the input signal for purposes of switching tracking control signals to be described later. The fed to an adder 14 where it is combined with a switching pulse burst from the generator 15. The output of adder 14 is supplied to an amplitude modulator 13 to modulate a 1-GHz carrier supplied from a microwave oscillator 12 via switch 16a. The amplitude-modulated 1-GHz carrier is supplied through a coaxial resonator 42 to a terminal 17 and thence to a stylus 20.The switching pulse from generator 15 is applied on line 27 to terminal 57.

As shown in Fig. 8, the stylus comprises an insulative support 1, usually diamond, and an electrode 2 attached to the rear wall of the diamond support. The diamond support 1 is coupled to a metal shank 3 to which the modulated signal on terminal 17 is applied.

The electrode is a narrow strip of conductor extending vertically from the shank 3 down to the bottom of the diamond. Diamond support 1 has a contact area at the bottom thereof having a width of several tracks on which it rests. At the downward tip, the electrode has an area measuring 10,000 and 2000 Angstroms in transverse and longitudinal dimen sions, respectively. The transverse dimension of the stylus electrode 2 is substantially equal to the width of the recording track on a disc 30.

The stylus 20 is secured to a stylus actuator 21 which is in turn mounted on a stylus carrier 22. The stylus carrier 22 has an internally threaded throughbore which engages an externally threaded drive shaft 23 coupled to the rotor of a motor 24. The drive shaft 23 extends in a radial direction of a turntable 26 on which an eraseable disc 30 is mounted.

With the rotation of drive shaft 23, stylus 20 is moved across the disc 30 from the outermost track to the innermost track. A guide rod 25 supports the carrier 22 as it travels on drive shaft 23. In a manner as will be detailed later, the stylus actuator 21 is controlled by a tracking signal which is developed in a servo control circuit generally shown at 19 in Fig.

1A and supplied through terminal 18.

Before proceeding with further description of the embodiment of Figs. 1 A and 1 B, it is appropriate to describe embodiments of the eraseable disc which are illustrated in Figs. 2A to 2F. In Fig. 2A, an eraseable disc 30a is generally similar to conventional optical disc of the phase transformable type in which the application of laser beam causes a change in phase structure between crystalline and amorphous state. The disc 30a comprises a substrate 31, preferably, of transparent material.

Suitable materials for the substrate 31 are acrylic resin, polycarbonate resin or vinyl chloride resin. Conductive particles may be added to the substrate material to render the substrate 31 conductive. On the substrate 31 is deposited a layer 32 of a phase transformable material such as TeOxGeSn or CUAI. Sputtering or vacuum evaporation technique is used to form the recording layer 32 until it attains a thickness in a range between 1000 Angstroms and 1500 Angstroms.

The initial state of the phase structure depends on what heat treatment is performed on the transformable layer. Recording layer 32 can initially be made amorphous by heating the whole area to a level higher than its melting point and then cooling it rapidly. For practical applications, it is preferable that transformable layer 32 be initially made crystalline by heating the whole area to a level higher than its softening or melting point temperature and then allowing it to cool gradually.

According to one embodiment of the invention the phase structure is made to change in response to the application of microwave energy and the phase structural change is detected as a variation in electrical conductivity.

According to another embodiment, a laser beam is used for recording signals, while microwave energy is used for detecting the recorded signals. In either embodiment, the density of signal elements recorded can be made higher than is available with current techniques in which laser beam is exclusively used for recording and playback operations.

High density recording is achieved by applying microwave energy to the electrode of stylus 20 to inject the energy to the transformable layer 32. The applied energy, which is at a highest level when the input signal is at a high amplitude level, raises the contact region of layer 32 to a level at which the phase structure of that region changes from crystalline phase to amorphous phase. Since the contact area of the stylus electrode is smaller than the beam spot size, the stylus can provide a higher recording density than is available with the current optical recording technique and is capable of higher resolution on playback than is available with the current optical reading technique.

A recording disc 30b, shown at Fig. 2B, is provided with a layer 33 of a dielectric material such as styrene over the surface of transformable layer 32. This dielectric layer is deposited by the spinner or sputtering method until it attains a thickness smaller than onehalf the minimum interval between successively recorded signal elements. Dielectric layer 33 is heated by the microwave energy in proportion to its dielectric loss. The thermal energy generated in dielectric layer 33 raises the temperature of the underlying recording layer 32 which is also heated by the injected microwave energy. The provision of the dielectric layer serves to increase the heating efficiency of the recording layer as well as to serve as a protection cover. It was confirmed that a dielectric layer having a thickness larger than the above-noted value tends to decrease the heating efficiency.

Recording discs 30c and 30d, shown respectively at Figs. 2C and 2D, are modifications of the discs of Figs. 2A and 2B, respectively. In these embodiments a conductive layer 34 is sandwiched between layer 32 and substrate 31. The thickness of conductive layer 34 is determined so that it presents a surface resistivity in a range from 50 ohms/cm2 to 100 ohms/cm2. The conductive layer forms with the stylus electrode a capacitor with the intermediate layer or layers therebetween serving as a dielectric material with a variable dielectric constant. The provision of this conductive layer improves the efficiency with which the recorded signal is detected on playback mode.

Recording disc 30e shown at Fig. 2F is similar to disc 30d but differs therefrom by the inclusion of a second dielectric layer 35 with a thickness of about 500 Angstroms between transformable layer 32 and conductive layer 34. Dielectric layer 35 is formed of a material such as barium titanate which is more efficient in terms of dielectric heating than the protective dielectric coating 33 because its dielectric loss higher than that of styrene.

A recording disc 30f shown at Fig. 2F comprises a conductive substrate 36 and a transformable layer 37. The substrate 36 is formed of a mixture of transparent resinous material and conductive particles and the recording layer 37 is composed of a mixture of phase transformable material such as TeOx GeSn or CuAI and dielectric material such as barium titanate. Obviously, transformable layer 32 of the mixed compound is preferably provided with overlying dielectric layer 33 and underlying conductive layer 34.

Each of the various forms of the recording disc 30 is formed with spiral turns of recording and servo tracks. As shown in Figs. 3 and 4 in an enlarged form, the servo track turns are created by a spiral groove 38 having a trapezoidal profile. A series of rectangular pits 39 is formed on the floor of each of alternate groove turns 38 and a series of rectangular pits 40 of different frequency is formed on the floor of groove turns which are alternate with those in which pits 39 are provided. Between successive turns of groove are created recording tracks 41 along which the stylus 20 is moved under control of motor 24.

Shorter pits 39 occur at a higher frequency fp, and longer pits 40 occur at a lower frequency fp2. Each of these frequencies is much lower than the frequency of the modulating signal. During recording and playback operations, tracking servo control signals are derived from the pits on each side of the recording track. As the stylus 20 travels along a given track turn, the tracking servo frequencies are at fp1 on one side of the recording track and at fp, on the other side and when the stylus advances to the next recording track turn the servo frequencies are reversed on opposite sides. For this reason, the pulse burst generated by switching pulse generator 15 is recorded at a point where the stylus encounters a change in servo control frequencies.Tracking servo pits 39 and 40 may be provided only in a prescribed angular position of the disc which corresponds to the vertical retrace period of the input video signal.

It is seen therefore that during recording modes, the modulated carrier generates a high frequency field at the tip of the stylus electrode and produces a series of regions of amorphous state along each turn of the recording track 41.

The coaxial resonator 42 acts as a frequency demodulator for detecting tracking servo signals during recording operation and detecting the recorded information and tracking servo signals on playback. For this purpose, the resonator 42 has a resonant peak at a frequency somewhat displaced from the 1 GHz oscillator frequency. During recording mode, the amplitude-modulated microwave energy is injected to the disc 30 and at the same time it is detected by the resonator 42.

More specifically, geometric variations created by pits 39 and 40 cause the injected energy to deviate in frequency from the resonant peak of the coaxial resonator 42 at frequencies fp, and fp2. The output of resonator 42 is therefore a high-frequency signal whose envelope varies with the geometric variations of pits 39 and 40. Band-pass filters 43 and 44 of a tracking servo control circuit 19 receive the output of coaxial resonator 42 to pass the envelopes at frequencies fp, and fp2, respectively, through a switch 45 to detectors 46 and 47. After removing the high-frequency components, the different frequency envelopes are derived from detectors 46 and 47 and fed to the input terminals of a differential amplifier 48.The output of differential amplifier 48 thus represents the amount and direction of deviation of stylus 20 from the center line of the recording track which the electrode 2 is following. The output of differential amplifier 48 is passed through a known phase compensator 49, amplifier 50 and terminal 18 to stylus actuator 21 to compensate for tracking errors. Switching control pulse on line 27 is supplied to a T flip-flop 53. The output of flip-flop 53 is at a high level at alternate turns of recording track to reverse the connections from the outputs of band-pass filters 43 and 44 to the inputs of detectos 46 and 47.

On playback, switches 16a and 1 6b are switched to the playback position to disable the switching signal generator 15 and apply a 1-GHz constant-amplitude carrier to the stylus 20 through resonator 42. As in the recording mode described above, geometric variations of pits 39 and 40 are detected by coaxial resonator 42 and fed to the tracking servo control circuit 19. The recorded switching pulse burst is passed through a band-pass filter 51 and the envelope of this signal is detected by a detector 52 to trigger the flip-flop 53.

According to the present invention, the constant-intensity high frequency energy generated at the stylus tip is modulated in frequency with conductivity variations created by a series of amorphous regions on the cystalline transformable layer 32 and further modulated in frequency with geometric variations of tracking pits 39 and 40. After being frequency demodulated by coaxial resonator 42, the former component of the frequency modulation is detected by a circuit including a highpass filter 54 and a detector 55 for delivery to an output terminal 56 and the latter component is detected by the tracking control circuit 19 in a manner described above in connection with the recording operation.

On reproduction it is preferable that the power level of the 1-GHz oscillator 12 be lower than that required during recording. The difference in power level between the recording and playback modes can be compensated for by applying bias optical energy during recording mode to the disc to produce a light spot which would raise the temperature of the transformable layer 32 to a level somewhat lower than the threshold at which the phase structure of layer 32 is made to change in the absence of the microwave energy injected by stylus 20. With an optical bias being applied, the injection of microwave energy through stylus electrode 2 raises the temperature of the contact region of layer 32 to a level higher than the phase change threshold. An optical bias supply 60 shown in Fig. 1 B provides the optical energy needed to bias the disc 30.

The bias supply 60 is mounted on a suitable carrier, not shown, which is located below disc 30 and driven by motor 24 through a mechanical linkage schematically indicated by a broken-line 61 at the same speed with the stylus carrier 22. A laser 62 in the bias suppy 60 provides the optical bias energy whose intensity is slightly lower than the threshold. A laser beam directed from light source 62 is a horizontally polarized light which is collimated by a lens 63 and passes through a prism 64 and a quarterwave plate 65. The beam emerges the quarterwave plate as a circular polarized light which is reflected by a mirror 66 to a focusing lens 67 where it is focused on the disc 30 to form a light spot in a position opposed to the stylus 20. The diameter of the light spot is somewhat larger the width of the recording track.The recording disc 30 is thus injected with microwave energy on the upper side and with optical energy on the lower side.

Disc 30 partially reflects the incident light through lens 67 to mirror 66 and thence to quarterwave plate 65. After passing through it, the circularly polarized returning light emerges as a beam having a vertically polarized plane. Because of the vertical polarizatin, the beam incident on prism 54 is reflected to light detector 68. The light detector 68 converts the optical signal to a corresponding electrical signal to enable the light spot to follow the same recording track with stylus 20. The signal from detector 68 is fed to a tracking servo control circuit 70 to derive a servo control signal. The input from detector 68 is passed through a low pass filter 71 to band-pass filters 72 and 73.The frequency components fp1 and fp, of the optical output are respectively detected by band-pass filters 72 and 73 and fed through a switch 74 to detectors 75 and 76 whose outputs are applied to a differential amplifier 77. The deviation of the light spot is represented by the output of amplifier 77. After being passed through phase compensator 78 and amplifier 79, the servo control signal is applied to a mirror actuator 69 which controls the angular position of mirror 66 so that the deviation is reduced substantially to zero. The switch 74 is controlled by the output of a flip-flop 80 which receives the switching pulse supplied through terminal 57.

Fig. 5 is an illustration of an optical arrangement by which previously recorded signal is erased immediately prior to the recording of a new program. In Fig. 5, parts corresponding to those in Fig. 1 B are marked with the same numerals as in Fig. 1 B. A bias laser beam having a wavelength of 780 nanometers is emitted by a semiconductor laser 181 and directed to a lens 182 where the beam is polarized to have a parallel polarization plane.

A prism 183 transforms the cross-section of the beam into a substantially circular shape.

The light leaving the prism 183 passes through a dichroic mirror 184, prism 185, quarterwave plate 186 and focusing lens 187 to the disc 30 and follows a return path including lens 187 and quarterwave plate 186 and reflects off the prism 185 to the light detector 68. An erasing laser beam is emitted by a semiconductor laser 82. This laser beam has a wavelength of 830 nanometers and an intensity sufficient to revert the phase structure of signal elements to crystalline state. The erasing beam is passed through a lens 83 and leaves it as a parallelly polarized light. A halfwave plate 84 is located in the beam path to impart a phase shift of a half wavelength of the incident light so that it elongates as it passes through a prism 85 in the longitudinal direction of the recording track to illuminate an elliptical area measuring 1.3 micometers in width and 10 micrometers in length.The elliptically shaped beam is reflected on dichroic mirror 184 and passed through prism 185, quarterwave plate 186 and lens 187 and falls on the disc 30 at a position slightly ahead of the light spot produced by the biasing beam. Both laser beams are controlled in response to the same servo control signal supplied from the circuit 70.

Since the actual condition of recorded signal elements on medium 30 is not only effected by the energy level of the recording signal but by the material of the recording medium, it is advantageous to provide a system which allows recorded signal elements to be reproduced as they are being recorded to thereby permit users to monitor the actual recording condition.

Fig. 6 is an illustration of a second embodiment of the present invention which allows such monitoring operation to be effected using microwave energy for recording and playback operations and optical energy for monitoring purposes. In Fig. 6, like numerals are used to designate parts corresponding to those in Figs. 1 A and 1 B. In this embodiment, the apparatus comprises a recording/ playback circuit 90, which is identical to the circuit shown in Fig. 1A, and a monitor circuit 91 which is similar to the circuit of Fig. 1 B with the exception that a high-pass filter 92 and a frequency demodulator 93 are con nected in series from the output of light detector 68. The stylus 20 and the beam spot are moved at the same speed by motor 24 as in the previous embodiment.The beam incident on disc 30 serves as a bias light as in the previous embodiment and is modulated at the same time in frequency with the phase changes just recorded with the microwave energy. The incident light is partially reflected from the disc and returns to light detector 68.

The purpose of high-pass filter 92 is to reject the servo control signals contained in the output of detector 68 and pass the frequency modulated video signal. Frequency demodulator 93 demodulates the output of high-pass filter 92 and delivers the original signal to a monitor terminal 94.

Fig. 7 is a modification of the embodiment of Fig. 6. In this modification, the optical energy is used on recording mode and the microwave energy is used for monitoring and playback operations. This embodiment comprises a monitor/playback circuit 100 and an optical recording circuit 200.

During recording mode, switches 116 and 220 are in recording position to active switching signal generator 115 and laser 201. Video input signal at terminal 111 is modulated by frequency modulator 110 and combined with the switching pulse burst in adder 114 and fed to the control terminal of a light modulator 203 of an optical unit 260. Light modulator 203 is located in the path of a laser beam emitted from laser 201. The laser beam, which is intensity modulated with the frequency-modulated video signal, passes through collimating lens 204, prism 205 and quarterwave plate 206 and bends its way at mirror 207, and is focused by lens 208 on disc 30. The recording laser beam produces a light spot of the same size as in the previous embodiment. The intensity of the laser is sufficient to cause phase changes in disc 30.

Partially reflected light from disc is detected by light detector 210 and fed to tracking servo control circuit 270 to keep the light spot on recording track.

In the monitor/playback circuit, oscillator 112 supplies reading 1-GHz microwave energy to stylus 120 to cause it to be modulated in frequency with the conductivity variations recorded by optical recording circuit 200.

Coaxial resonator 142 demodulates the modulated microwave energy and supplies it to high-pass filter 154 and tracking servo control circuit 119. Detector 155 removes the high frequency carrier component and delivers the recorded signal to terminal 156. Stylus actuator 21 is controlled by servo circuit 119 to keep the stylus on track.

On playback, switches 116 and 202 are open to deactivate switching signal generator 115 and the recording circuit 200. Monitor/playback circuit 100 performs the same function as in the monitor mode just described. Since the contact area of the electrode stylus is smaller than the shortest wavelength of light, the stylus is capable of higher resolution than is achieved by the use of a laser beam, an therefore it is possible to provide the same recording density with the optical energy as that obtained with the microwave energy in so far as the stylus 20 is used on reproduction.

Claims

1. A method for recording a signal on and reproducing it from a disc having a layer of material transformable in phase structure between crystalline state and amorphous state in response to the application of heat-producing energy, comprising the steps of: a) modulating electromagnetic radiant energy in frequency with said signal; b) concentrating the modulated energy on said layer to cause a phase structural change to occur therein; c) moving the concentrated energy along a predetermined path on said layer to form a series of regions of phase structural change along said path; d) on reproduction, applying microwave energy of a constant intensity to an electrode and moving the electrode in contact with said layer along said path to cause the microwave energy to be modulated in frequency with varying conductivity of said regions; and e) demodulating the frequency-modulated microwave energy to recover said signal.

2. A method for recording a signal on and reproducing it from a disc having a layer of material transformable in phase structure between crystalline state and amorphous state in response to the application of heat-producing energy, comprising the steps of: a) modulating microwave energy in frequency with said signal; b) applying the modulated microwave energy to an electrode in contact with said layer to cause a phase structural change to occur therein; c) moving said electrode along a predetermined path on said layer to form a series of regions of phase structural change along said path; d) on reproduction, moving said electrode in contact with said layer along said path; e) applying microwave energy of constant intensity to the electrode to cause the microwave energy to be modulated in frequency with varying conductivity of said regions; and f) demodulating the frequency-modulated microwave energy to recover said signal.

3. A method as claimed in claim 2, further comprising the steps of: A) forming a light spot on said layer in a position opposed to said electrode, said light spot having an intensity which, in the absence of said modulated microwave energy, would raise the temperature of a portion of said layer to a level slightly below a threshold at which said phase structural change takes place; and B) moving the light spot simultaneously with said electrode along said path at the same speed as the speed of movement of the electrode.

4. A method as claimed in claim 3, further comprising: detecting light reflecting from said disc; and demodulating the detected light to recover said signal to allow same to be monitored.

5. A method as claimed in claim 2, further comprising the steps of: A) forming a light spot on said layer in a position opposed to said electrode of step (b); B) moving the light spot simultaneously with said electrode of step (b) along said path at the same speed as the speed of movement of the electrode to cause the light incident on said layer to be modulated with said regions; C) detecting light reflecting from said disc; and D) demodulating the detected light to recover said signal to allow same to be monitored.

6. A method for recording a signal on and reproducing it from a disc having a layer of material transformable in phase structure between crystalline state and amorphous state in response to the application of heat-producing energy, comprising the steps of: a) modulating a laser beam with said signal; b) forming a light spot of said modulated laser beam on said layer; c) moving the light spot along a predetermined path on said layer to form said series of regions of phase structural change; d) on reproduction, applying microwave energy of constant intensity to an electrode in contact with said layer in a position opposite to said light spot; e) moving the electrode simultaneously with said light spot along said path at the same speed as the speed of movement of said light spot to cause the microwave energy to be modulated with varying conductivity of said regions; and f) demodulating the modulated energy to recover said signal to allow same to be monitored.

7. A method as claimed in any one of the preceding claims, wherein said electrode has a contact area with which it makes contact with said layer, said contact area having dimensions each being smaller than the shortest wavelength of light.

8. An apparatus for recording a signal on and reproducing it from a disc having a layer of material transformable in phase structure between crystalline state and amorphous state in response to the application of heat-producing energy, comprising: a motor-driven turntable on which said disc is placed; a source for generating a microwave energy; a frequency modulator for modulating said microwave energy with said signal; a stylus having an electrode connectable to said frequency modulator during recording mode and to said source during playback mode;; means for moving said stylus on said layer so that said electrode follows a predetermined path, whereby during recording mode said modulated microwave energy is injected to said layer to cause a phase structural change to occur in a region which is contact with said electrode and during playback mode the microwave energy from said source is injected to said layer to cause the injected energy to be modulated in frequency with varying conductivity of said regions; and means for demodulating said frequencymodulated energy to recover said signal during playback mode.

9. An apparatus as claimed in claim 8, further comprising optical biasing means operable during recording mode, the biasing means comprising: means for directing a laser beam and forming a laser beam spot on said disc in a position opposed to said electrode, said beam spot having an intensity which, in the absence of said modulated microwave energy, would raise the temperature of said region to a level slightly below a threshold at which said phase structural change takes place; and means for moving said laser beam spot along said path at the same speed as the speed of movement of said stylus.

10. An apparatus as claimed in claim 9, further comprising means for detecting light reflecting from said disc and converting it to a corresponding electrical signal and second frequency demodulating means for demodulating said electrical signal.

11. An apparatus as claimed in claim 8, further comprising optical monitoring means operable during recording mode, said monitoring means comprising: means for directing a laser beam and forming a laser beam spot on said disc in a position opposed to said electrode; means for moving said laser beam spot along said path at the same speed as the speed of movement of said stylus to cause the light incident on said disc to be modulated in frequency with said phase structural change; means for detecting light reflecting from said disc and converting it to a corresponding electrical signal; and second frequency demodulating means for demodulating said electrical signal to allow the demodulated signal to be monitored.

12. An apparatus for recording a signal on and reproducing it from a disc having a layer of material transformable in phase structure between crystalline state and amorphous state in response to the application of heat-producing energy: a motor-driven turntable on which said disc is placed; a source for generating a microwave energy; a frequency modulator for modulating said microwave energy with said signal; means including laser for modulating a laser beam from said laser with said frequency modulated energy and directing the modulated laser beam to said disc to form a light spot, the light spot having an intensity sufficient to cause a portion of said layer to change in phase structure; means for moving said light spot on said layer so that it follows a predetermined path to thereby form a series of regions of phase structural change along said path;; a stylus having an electrode connected to said source; means for moving said stylus on said layer so that said electrode follows said predetermined path, whereby during recording and playback modes the microwave energy from said source is injected to said layer to cause the injected energy to be modulated in frequency with varying conductivity of said regions; and means for demodulating said frequencymodulated energy to recover said signal during playback mode.

13. An apparatus as claimed in any one of claims 8, 9, 10, 11 or 12, wherein said disc includes a first series of pits along a first guide path and a second series of pits along a second guide path, said predetermined path being located between said first and second guide path, further comprising servo control means responsive to said frequency demodulated energy to keep said stylus on said predetermined path, said servo control means including means for deriving tracking signals from said first and second series of pits and moving said stylus in proportion to the difference between said tracking signals.

14. An apparatus as claimed in claim 13, further comprising second servo control means responsive to the output of said second frequency demodulating means to keep said beam spot on said predetermined path, said second servo control means including means for deriving tracking signals from said first and second series of pits and moving said beam spot in proportion to the difference between the last-mentioned tracking signals.

1 5. An apparatus as claimed in claim 8, 9, 10, 11, 12, 13 or 14, wherein the firstmentioned frequency demodulating means comprises a resonator having a resonant peak slightly displaced from the frequency of said microwave energy.

16. An apparatus as claimed in claim 8, 9, 10, 11, 12, 13, 14 or 15, wherein said electrode has a contact area with which it makes contact with said layer, said contact area having dimensions each being smaller than the shortest wavelength of light.

17. An apparatus as claimed in claim 8, 9, 10, 11, 12, 13, 14, 15 or 16, further comprising an erasing means comprising: means including a laser beam and forming a light spot of the laser beam on said layer in a position adjacent said electrode, said light spot having an intensity sufficient to erase said phase structural change; and means for moving the last-mentioned light spot along said path at the same speed as the speed of movement of said stylus.

18. A recording disc for use in the apparatus as claimed in any one of claims 8 to 17, wherein the disc comprises: a substrate; a first layer of conductive material on said substrate; and a second layer on said first layer, the second layer being composed of a material which is transformable in phase structure between crystalline and amorphous states when it is heated to a predetermined temperature.

19. A recording disc as claimed in claim 18, further comprising a third layer of dielectric material on said second layer, said third layer having a thickness smaller than one-half the spacing between signal elements to be recorded.

20. A recording disc as claimed in claim 18 or 19, wherein said second layer contains a dielectric material to induce heat in the second layer in proportion to the dielectric loss thereof.

21. A recording disc as claimed in claim 18, 19 or 20, wherein said second layer comprises an upper layer portion composed of said transformable material and a lower layer portion composed of said dielectric material.

22. A recording disc as claimed in claim 21, wherein the dielectric material of said second layer comprises barium titanate.

23. A recording disc as claimed in claim 18, 19, 20, 21 or 22, further comprising a plurality of radially spaced apart recording track turns, a plurality of first servo track turns each including a first series of pits which occur at a first frequency, and a plurality of second servo track turns each including a second series of pits which occur at a second frequency, each of said recording track turns being located between adjacent first and second servo track turns.

24. A recording disc as claimed in claim 18, 19, 20, 21, 22 or 23, wherein said substrate comprises a light transmissive material.

25. A recording disc as claimed in claim 24, wherein said disc is provided with turns of groove, and wherein said first and second series of pits are formed on the floor of the groove turns.

26. A recording disc as claimed in claim 18, 19, 20, 21, 22, 23, 24 or 25, wherein said substrate is conductive.

27. A recording disc as claimed in claim 18, 19, 20, 21, 22, 23, 24 or 25, wherein said substrate is insulative.

28. A method for recording a signal on and reproducing it from a disc substantially as shown and described with reference to the accompanying drawings.

29 An apparatus for recording a signal on and reproducing it from a disc constructed substantially as shown and described with reference to the accompanying drawings.

30. A recording disc constructured substantially as shown and described with reference to Figs. 2B to 2F.

31. A method of recording a signal on a disc having a layer of material transformable in phase structure between crystalline state and amorphous state in response to the application of heat-producing energy, comprising the steps of: (a) modulating electromagnetic radiant energy in frequency with said signal; (b) concentrating modulated energy on said layer to cause a phase structural change to occur therein; and (c) moving the concentrated energy along a predetermined path on said layer to form a series of regions of phase structural change along said path.

32. A method of reproducing a signal from a disc having a layer of material transformable in phase structure between crystalline state and amorphous state in response to the application of heat-producing energy, comprising the steps of: applying microwave energy of a constant intensity to an electrode and moving the electrode in contact with said layer along said path to cause the microwave energy to be modulated in frequency with varying conductivity of said regions; and demodulating the frequency-modulated microwave energy to recover said signal.

33. A method of recording a signal on a disc having a layer of material transformable in phase structure between crystalline state and amorphous state in response to the application of heat-producing energy, comprising the steps of: (a) modulating microwave energy in frequency with said signal; (b) applying the modulated microwave energy to an electrode in contact with said layer to cause a phase structural change to occur therein; and (c) moving said electrode along a predetermined path on said layer to form a series of regions of phase structural change along said path.

34. A method of reproducing a signal from a disc having a layer of material transformable in phase structure between crystalline state and amorphous state in response to the application of heat-producing energy, comprising the steps of: moving said electrode in contact with said layer along said path; applying microwave energy of constant intensity to the electrode to cause the microwave energy to be modulated in frequency with varying conductivity of said regions; and demodulating the frequency-modulated microwave energy to recover said signal.

35. A method of recording a signal on disc having a layer of material transformable in phase structure between crystalline state and amorphous state in response to the application of heat-producing energy, comprising the steps of: (a) modulating a laser beam with said signal; (b) forming a light spot of said modulated layer beam on said layer; and (c) moving the light spot along a predetermined path on said layer to form said series of regions of phase structural change.

36. A method of reproducing a signal from a disc having a layer of material transformable in phase structure between crystalline state and amorphouse state in response to the application of heat-producing energy, comprising the steps of: modulating a laser beam with said signal; forming a light spot of said modulated layer beam on said layer; applying microwave energy of constant intensity to an electrode in contact with said layer in a position opposite to said light spot; moving the electrode simultaneously with said light spot along said path at the same speed as the speed of movement of said light spot to cause the microwave energy to be modulated with varying conductivity of said region; and demodulating the modulated energy to recover said signal to allow same to be monitored.

37. An apparatus for recording a signal on a disc having a layer of material transformable in phase structure between crystalline state and amorphous state in response to the application of heat-producing energy, comprising: a motor-driven turntable on which said disc is placed; a source for generating a microwave energy; a frequency modulator for modulating said microwave energy with said signal; a stylus having an electrode connectable to said frequency modulator during recording mode; means for moving said stylus on said layer so that said electrode follows a predetermined path, whereby during recording mode said modulated microwave energy is injected to said layer to cause a phase structural change to occur in a region which is contact with said electrode.

38. An apparatus for reproducing a signal from a disc having a layer of material transfor mable in phase structure between crystalline state and amorphous state in response to the application of heat-producing energy, comprising: a motor-driven turntable on which said disc is motor-drivenplaced; a source for generating a microwave energy; a stylus having an electrode connectable to said frequency modulator during playback mode; means for moving said stylus on said layer so that said electrode follows a predetermined path, whereby during playback mode the microwave energy from said source is injected to said layer to cause the injected energy to be modulated in frequency with varying conductivity of said regions; and means for demodulating said frequencymodulated energy to recover said signal during playback mode.

39. An apparatus for recording a signal on a disc having a layer of material transformable in phase structure between crystalline state and amorphous state in response to the application of heat-producing energy: a motor-driven turntable on which said disc is placed; a source for generating a microwave energy; a frequency modulator for modulating said microwave energy with said signal; means including a laser for modulating a laser beam from said laser with said frequency modulated energy and directing the modulated laser beam to said disc to form a light spot, the light spot having an intensity sufficient to cause a portion of said layer to change in phase structure; means for moving said light spot on said layer so that it follows a predetermined path to thereby form a series of regions of phase structural change along said path;; a stylus having an electrode connected to said source; and means for moving said stylus on said layer so that said electrode follows said predetermined path, whereby during recording, the microwave energy from said source is injected to said layer to cause the injected energy to be modulated in frequency with varying conductivity of said regions.

40. An apparatus for reproducing a signal from a disc having a layer of material transformable in phase structure between crystalline state and amorphous state in response to the application of heat-producing energy: a motor-driven turntable on which said disc is placed; a a source for generating a microwave energy; a frequency modulator for modulating said microwave energy with said signal; means including a laser for modulating a laser beam from said laser with said frequency modulated energy and directing the modulated laser beam to sid disc to form a light spot, the light spot having an intensity sufficient to cause a portion of said layer to change in phase structure; means for moving said light spot on said layer so that it follows a predetermined path to thereby form a series of regions of phase structural change along said path; ; a stylus having an electrode connected to said source; means for moving said stylus on said layer so that said electrode follows said predetermined path, whereby the microwave energy from said source is injected to said layer to cause the injected energy to be modulated in frequency with varying conductivity of said regions; and means for demodulating said frequencymodulated energy to recover said signal.