Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Patent
1992-04-27
1993-11-23
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
307311, G01J 132
Patent
active
052646920
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates generally to CD readers and, more particularly, to control circuits which minimize read errors by controlling laser output.
BACKGROUND
When reading data from optical recording media, in particular from so-called compact discs (CDs), to achieve as high a quality as possible of the signal generated by the opto-electric receiver an optimum setting of the power of the laser is required. U.S. Pat. No. 4,698,797, Komatsky, discloses a CD reading device in which the information signal reflected from the CD and received by the reading device is evaluated by a controlling circuit. In this case, the controlling circuit controls the output power of a laser diode, acting as transmitter, in such a way that the level of the received information signal is kept at a constant value, corresponding to a predetermined, fixed reference voltage. In this case, due to temperature changes, ageing and soiling, a signal of poorer signal-to-noise ratio may be produced, which results in a higher error rate of the read-out data.
SUMMARY OF THE INVENTION
The object of the present circuit arrangement is to ensure the satisfactory function of a CD reading device irrespective of temperature and other ambient influences and to guarantee as low an error rate as possible.
The circuit arrangement according to the invention has the advantage that the output power of the laser assumes an optimum value irrespective of the above-mentioned influences.
Further developments of the invention permit, inter alia, a determination of that direction in which the output power of the laser is in each case to be changed. The invention may be specifically carried out with the aid of analog or digital circuits or with suitable microcomputers and signal processors as well as corresponding programs.
DRAWINGS
FIG. 1 shows a first exemplary embodiment, in which a quality signal is derived with the aid of an envelope detector,
FIG. 2 shows a second exemplary embodiment, in which a quality signal is derived from an error signal,
FIG. 3 shows a third exemplary embodiment, in which the circuit according to the invention is realised with the aid of a microcomputer, and
FIG. 4 shows an exemplary embodiment in which the function of the circuit arrangement according to the invention is assumed by a signal processor, which also carries out the processing of the output signals of the opto-electric receiver, including error correction.
The same parts are provided with the same reference numerals in the figures .
DETAILED DESCRIPTION
In FIG. 1, of a known CD reading device only a laser 1, an output stage 2, a receiver diode 3, which serves as opto-electric receiver, a preamplifier 4 and a signal processor 5 are shown, from the output 6 of the latter the read data can be taken. Further details of CD reading devices are known per se and need not be explained in any more detail in the context of the present invention.
In the case of the exemplary embodiment according to FIG. 1, the output signal of the preamplifier 4 is passed via an envelope detector 7 for the generation of a quality signal. With optimum setting of the power of the amplifier 1, the output signal of the preamplifier 4 has the greatest amplitude, so that the quality signal G assumes the maximum value. Under less favourable conditions, the quality signal G becomes correspondingly smaller. With a delay circuit 8 and a comparison circuit 9, the change .delta.G in the quality signal G is determined. The output signal of the comparison circuit 9 is referred to hereinafter as quality-change signal.
The quality-change signal passes via a multiplier 10 to a controller 11, which is essentially an integralaction controller. The output signal of the controller 11 is fed via an adder 12 to the input of the output stage 2. Both the multiplier 10 and the adder 12 receive from a generator 13 an alternating (square-wave) voltage (modulation voltage M), the frequency of which is considerably less than the scanning frequency on which the recorded data are based.
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REFERENCES:
patent: 4580044 (1986-04-01), Hongo et al.
patent: 4689795 (1987-08-01), Yoshimoto et al.
patent: 4692606 (1987-09-01), Sakai et al.
patent: 4698797 (1987-10-01), Komatsu
patent: 4733398 (1988-03-01), Shibagaki et al.
patent: 4827116 (1989-05-01), Takagi et al.
Notzel Gunter
Rosinger Andreas
Schmid Helmut
Nelms David C.
Robert & Bosch GmbH
Shami K.
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