HK81796A

HK81796A - Detector circuit

Info

Publication number: HK81796A
Application number: HK81796A
Authority: HK
Inventors: Gleim Guenter; Fueldner Friedrich; Rekla Bernd
Original assignee: Deutsche Thomson-Brandt Gmbh
Priority date: 1989-09-30
Filing date: 1996-05-09
Publication date: 1996-05-17
Also published as: EP0494892B1; TR24612A; HUT61118A; EP0494892A1; KR920704281A; ATE119706T1; WO1991005340A1; DD298325A5; AU6418690A; JP2854131B2; CN1050637A; CN1020009C; FI921365A0; FI921365L; DE3932831A1; KR100227668B1; ES2071829T3; DE59008679D1; JPH05500577A; MY107309A

Abstract

A light beam which scans the data on an optical recording support, generates an r.f. signal in the recording and/or playback device. As the r.f. signal is interrupted when the light beam leaves a data track, the detector circuit recognises from the r.f. signal level whether the beam is passing from one data track to the next. As the interruptions in the r.f. signal can also be caused by dirt, damage or errors on the recording support, it is impossible to be certain whether such an interruption has been caused by a track change or by dirt, damage or errors on the recording support. In order to recognise track changes reliably, the detector circuit does not generate a signal indicating a track change until the r.f. signal (HF) is below a first predetermined threshold (R1) and at the same time a track error signal (TE) is above a second predetermined threshold (R2). Detector circuit which recognises when a scanner leaves a mark or data track, e.g. in CD players, video record players, DRAW-disc players and magneto-optical recording and/or playback devices.

Description

The invention relates to a detection circuit for a track circuit which directs the beam of light produced by an optical scanner along the data traces of a recording medium, whereby the beam of light which scans the data produces an RF signal and a trace error signal and whereby it is tested whether the enclosure of the RF signal drops below an initial threshold value.

CD players, VCR players, DRAW disc players or magneto-optical recording and playback devices are equipped, for example, with a track circuit and an optical scanning device.

The structure and function of an optical scanning device, called an optical pick-up, are described in Electronic Components & Applications, Vol. 6, No. 4, 1984, pp. 209 - 215.

The light beam emitted by a laser diode is focused on the CD-ROM by means of lenses and reflected from there on a photodetector. From the output signal of the photodetector, the data stored on the CD-ROM and the current value for the focus and for the track circuit are obtained.

The focus control circuit is a coil whose magnetic field moves a lens along the optical axis, and which, by moving the lens, causes the light beam emitted by the laser diode to always be focused on the CD-ROM. The optical scanning device can be moved radially in relation to the CD-ROM by means of the track control circuit, often called radial drive.

In some devices, the radial drive is made up of a so-called coarse and a so-called fine drive. The coarse drive is, for example, made as a spindle, by means of which the entire optical scanning device from the laser diode, the lens, the prism beam separator and the photodetector can be radially moved. With the fine drive, the beam of light can additionally be moved in a radial direction or, for example, tilted by a predefined small angle.

In order to obtain a proper reproduction of data, be it image and sound on a VCR or just sound on a CD or the data on a magneto-optical disc, precise guidance along the data traces of the disc is required in addition to a precise focusing of the light beam on the disc.

The photodetector PD is the optical scanning device of a CD player, where three laser beams L1, L2 and L3 are focused on the CD. The laser beams L2 and L3 are the refractive rays +1. and -1. order.

The photodetector has four square photodiodes A, B, C and D, which are joined together to form a square. In front of the square formed by the four photodiodes A, B, C and D, there is a rectangular photodiode E; behind the square there is another photodiode F. The middle laser beam L1, focused on the four photodiodes A, B, C and D, generates the data signal HF = AS + BS + CS + DS and the focus error signal FE = (AS + CS) - (BS + DS). The two outer light beams L2 and L3, of which the front L3 falls on the photodiode E, the rear L2 on the photodiode F, generates the error signal - FS, FS, FS, CS, D, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F, F,

In Figure 1, the middle laser beam L1 follows exactly the centre of a track S. The TE track signal has the value zero.

TE = ES - FS = 0.

When the middle ray of light deviates from the center of a trace S, one of the deflection rays moves more towards the center of the trace, while the other deflection ray radiates to the space between two traces S. But because the reflection properties of a trace and an intermediate space are different, one deflection ray is reflected more strongly than the other.

In Figure 2 the case is shown where the laser beams L1, L2 and L3 are shifted to the right of the track S. The track error signal assumes a negative value:

<formula-text> <formula-text> <formula-text> <math><mrow><mtext>TE = ES - FS < 0.</mtext></mrow></math> </formula-text> </formula-text> </formula-text>

The control of the track circuit now moves the optical scanning device to the left until the TE track error signal becomes zero.

In the opposite case, if the laser beams are shifted to the left of the track, the trace error signal is positive: TE = ES - FS > 0.

If the beam L1 and the associated diffraction beams L2 and L3 intersect several data tracks, the trace error signal TE takes the sine-shaped path shown in Figure 4.

JP-OS 60 10429 provides a track circuit in which the lower and upper envelope curves of the RF signal detect whether the beam of light crosses data tracks.

To determine the number of paths crossed by the beam, the envelope curve of the RF signal is formed and converted into an impulse-shaped signal fed to the counting input of a forward-reverse counter.

To determine in which direction the beam is moving, radially inward or outward, a so-called directional logic is required, which evaluates the phase shift between the TE trace error signal and the RF signal envelopes.

GB-A 2 073 543 provides a track circuit for checking whether the RF signal is below a specified threshold; if this is the case, a positive or negative voltage is applied to the data-reading optical scanner's switch, depending on the signal characteristics, to guide it to the correct data track.

However, since dust, dirt, fingerprints or scratches on a CD-ROM can also cause a breakdown of the RF signal, measures must be taken to distinguish RF breakdowns caused by dirt or damage on the recording medium from such RF breakdowns caused by a change in the trace of the light beam.

EP-A 0 183 303 is a CD player in which the locking-in of the light speck is performed at the point of greatest eccentricity of the plate, because at the point of greatest eccentricity the relative speed between the light beam and the data track is the lowest. To determine the number of data tracks crossed by the light beam, the envelope of the RF signal is generated and compared with a threshold value.

To make the CD player safer from so-called drop-outs - which are audible disturbances in the sound reproduction due to a faulty, scratched or dirty compact disc - the envelope of the RF signal can be compared to a second threshold.

WO-A-88/09988 is known to have a CD player with a drop-out detector that evaluates the RF signal.

The invention is therefore intended to design a detection circuit in accordance with the general concept of claim 1 in such a way that dirt or scratches on the recording medium can no longer simulate a change in the trace of the light beam being detected.

The invention solves this problem by the fact that the detection circuit only emits a signal indicating that the beam of light scanning the data leaves a data trace when the RF signal envelope is below the first threshold and the trace error signal is above a second threshold.

The present invention is based on the finding that, while the level of the RF signal decreases when contaminated or damaged areas of the recording medium are examined, the trace error signal is not significantly disturbed by dirt and damage on the recording medium. If, therefore, only the RF signal breaks down, the trace error signal, the deviation of the value from the set value but maintains a value close to zero, it is assumed that the RF burst was caused by dirt, damage or error on the recording medium, but not by a trace change in the beam.

Show it Figure 5a first example of implementationFigure 6a second example of implementationFigure 7 and 8impulse diagrams to illustrate the second example of implementation.

The first example of the invention in Figure 5 is now used to describe and explain the invention.

In Figure 5, the envelope EH of the HF signal is fed to the first input of a comparator V1 at the second input of which a reference voltage R1 is located. The sine-shaped track error signal TE is fed to the second input of a comparator V2 at the second input of which a reference voltage R2 is located. The outputs of the two comparators V1 and V2 are connected to the inputs of a UND gate U, which emits a logical one at its output if the level of the HF signal HF is below the reference voltage R1 and at the same time the track error signal TE is above the reference voltage R2.

The second example of implementation shown in Figure 6 is now described.

The input of a falling-flank triggered monoflop M1 is fed an impulse-formed casing curve signal HP, derived from the casing curve of the RF signal. An impulse-formed trace error signal TZ, generated from the sine-formed trace error signal TE, is fed to the first input of an ODER gate O1 and a falling-flank triggered monoflop M2. The inverted impulse-formed trace error signal TZ is fed to the first input of an ODER gate O2 and a falling-flank triggered monoflop M3. The outputs of the monoflops M2 and M3 are connected to the inputs of an ODER gate E3, whose output is connected to the output of an ODER gate RSF9 of a monoflop M1.one output of a directional logic RL, which, depending on the direction of movement of the beam, gives a logical one at either of its two outputs, is connected to the second input of the ODER gate O1, while the other output of the directional logic RL is connected to the second input of the ODER gate O2. The outputs of the ODER gates O1 and O2 are connected to the inputs of a UND gate U10, the output of which is connected to the second input of the UND gate U9. When the light beam is reflected back onto the two tracks lying between the two tracks, the RS-F-F loop is placed; it is placed between the two tracks,when the beam of light is directed towards a track.

The monoflop M1 emits a logical one at its output at each falling flank of the pulsed envelope signal HP. Because the pulsed envelope signal HP is obtained from the envelope signal HF, a falling flank in the pulsed envelope signal occurs whenever the level of the envelope curve falls below a predefined threshold. When the light beam is moved in one direction, the UND gate U10 emits a logical one at each positive pulse of the pulsed monoflop signal TZ; the RS-flip-flip F6 is only set when the monoflop is set at the same time and the pulsed envelope signal TZ is set at the same time. The light is then reversed in the direction of the other one, giving a positive impulse, when the pulsed envelope signal TZ is set at the same time and the RS-flip-flip signal TZ is set at the same time. The light is reversed in the direction of the other one, so the UND gate U10 emits a positive impulse when the pulsed envelope signal TZ is set at each inverse of the RS-flip signal TZ1 and the pulsed envelope signal TZ1 is set at the same time.

A falling flank in the pulsed track-fault signal TZ or a rising flank in the inverted pulsed track-fault signal TZ shall reset the RS flip-flop F6.

Figures 7 and 8 show the RF signal HF, the resulting pulsed envelope signal HP, the sine-shaped track error signal TE, the pulsed track error signal TZ, the inverted pulsed track error signal TZ, the monoflop output signals M1, M2 and M3, the signal at the exit of the AND gates U9 and U10, the signal at the exit of the OOR gate O3 and the RS-flip flop output F6.

The invention is generally applicable to counting devices for counting markings or to control circuits for positioning a unit, e.g. a scanner, in which the unit is positioned by scanning markings. The method of scanning, whether mechanical or non-contact, is irrelevant. In particular, the invention is applicable to track control circuits, such as those found in CD players, VCRs, DRAW-Disc players or magneto-optical recording and/or playback devices.

Claims

Detector circuit for a tracking control loop which guides the light beam produced by an optical scanner along the data tracks of a recording medium, wherein the light beam scanning the items of data generates a HF signal (HF) and a tracking error signal (TE) and wherein it is checked as to whether the envelope (EH) of the HF signal (HF) falls below a first predetermined threshold value (R1), characterised in that, the detector circuit only emits a signal, which indicates that the light beam scanning the items of data is departing from a data track, if the envelope (EH) of the HF signal (HF) lies below the first predetermined threshold value (R1) and if, simultaneously, the tracking error signal (TE) is above a second predetermined threshold value (R2).
Detector circuit in accordance with Claim 1, characterised in that, the envelope (EH) of the HF signal (HF) is supplied to the first input of a first comparator (V1), on the second input of which, there is a first reference voltage (R1), that the tracking error signal (TE) is supplied to the first input of a second comparator (V2), on the second input of which, there is a second reference voltage (R2), and that the outputs of the comparators (V1, V2) are connected to the inputs of an AND gate (U) whose output signal indicates that the light beam is departing from a data track.
Detector circuit for a tracking control loop which guides the light beam produced by an optical scanner along the data tracks of a recording medium, wherein the light beam scanning the items of data generates a HF signal (HF) and a tracking error signal (TE) and wherein a pulse-shaped signal (HP) corresponding to the curve of the envelope is generated from the envelope (EH) of the HF signal (HF), characterised in that, a pulse-shaped tracking error signal (TZ) is produced from the tracking error signal (TE), that the pulse-shaped envelope-curve signal (HP) is supplied to a first monoflop (M1) which can be triggered by a trailing edge and the output of which is connected to the first input of a first AND gate (U9), that the pulse-shaped tracking error signal (TZ) is supplied to a second monoflop (M2) which can be triggered by a trailing edge and to the first input of a first OR gate (O1), that the inverted pulse-shaped tracking error signal (TZ) is supplied to a third monoflop (M3) which can be triggered by a trailing edge and to the first input of a second OR gate (O2), that the outputs of the second and third monoflops (M2, M3) are connected to the inputs of a third OR gate (O3) whose output is connected to the reset input of an RS flip-flop (F6), that the output of the first AND gate (U9) is connected to the set input of the RS flip-flop (F6), that the outputs of the first and second OR gates (O1, O2) are connected to the inputs of a second AND gate (U10) whose output is connected to the second input of the first AND gate (U9), that each output of a directional logic circuit (RL) is respectively connected to the second input of the first and second OR gates (O1, O2), that the directional logic circuit (RL) emits a logical one at either its one or other output in dependence on the direction of movement of the light beam and that the signal at the Q output of the RS flip-flop (F6) indicates whether the light beam is shining on a data track or on the space between two tracks.