Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Rectangular or pulse waveform width control
Reexamination Certificate
2001-02-22
2002-10-01
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Rectangular or pulse waveform width control
C327S100000, C327S345000
Reexamination Certificate
active
06459315
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a waveform reforming circuit for reforming a waveform of an input signal, more particularly relates to a waveform reforming circuit for reforming a signal read from a recording medium etc. with a temporal mean value fluctuating relative with respect to a predetermined value due to an external disturbance component to a binary signal having the predetermined temporal mean value.
The recording medium known as an optical disc is constituted by a transparent plastic substrate having laterally long holes in a circumferential direction referred to as “pits” formed corresponding to the signal, a thin metal film deposited thereon, and a hard resin layer for protecting the thin metal film.
The information recorded on the optical disc is read from the recording medium by focusing light such as a laser beam to the surface of the transparent plastic substrate and converting the light reflected by the thin metal film to an electric signal by an opto-electric conversion element. Namely, at the spot on the circumference of the optical disc on which the light is focused, the intensity of the light reflected from the thin metal film changes between a case where there is a pit and a case where there is no pit, therefore the information recorded based by the pit on the optical disc is converted to a strong or weak electric signal by detecting the intensity of the reflected light by the opto-electric conversion element.
The information recorded on the optical disc by the pit is recorded by a modulation method referred to as eight-to-fourteen modulation (EFM modulation or 8-14 modulation). According to this EFM modulation, what had been an 8-bit code before modulation is converted to a 14-bit code based on an EFM modulation table. The conversion table is selected so that a pulse width of a pulse train resulting from the created code becomes 3T to 11T where one cycle of the pulse is T.
Further, a 3-bit code is added between one 14-bit code and another separately from them. The value of this code is selected for every interval of 14-bit codes so that the probability of “1” or “0” arising in the created pulse train becomes 50%. Accordingly, the electric signal obtained by reading the information on the optical disc modulated by the EFM modulation method ideally becomes constant in temporal mean value.
In the process referred to as “mastering” for converting an electric signal to pits to prepare a master of an optical disc, light such as a laser beam modulated in accordance with the electric signal is focused on to a photosensitive substance such as a photoresist uniformly coated on for example a polished glass plate, then this is developed to prepare a metal mask forming the master by using the uneven surface of the photoresist formed by the focusing of the light. The pits prepared at this time finely change in shape and size according to various conditions such as the power of the laser used for the mastering and the development time. For example, according to the various conditions, the lengths of the pits change so become slightly longer or shorter by substantially the same amounts even among pits having different lengths.
Such fluctuation of the length of the pits becomes the fluctuation of the pulse width of the electric signal read from the optical disc as it is, therefore the temporal mean value of the electric signal, which ideally should become constant as mentioned above, will fluctuate relative to the ideal value. The phenomenon of the temporal mean value of the read electric signal deviating according to the variance in the lengths of the pits in this way is referred to as “asymmetry”.
The RF signal directly output from an optical signal reading unit (optical pickup) of the optical disc is not a rectangular wave, but a waveform resembling a sine wave. In order to process this as a digital signal, this sine wave-shaped signal must be converted to a binary pulse signal. However, when the asymmetry of the read signal becomes large, in the process of converting the sine wave-shaped RF signal to a binary pulse signal, the threshold value for the binary coding fluctuates, so erroneous binary coding results and the inconvenience that the error rate of the data is increased occurs.
In order to avoid such an inconvenience, conventionally a waveform reforming circuit as shown in
FIG. 1
has been used.
FIG. 1
is a circuit diagram of a conventional waveform reforming circuit for correction of asymmetry.
In
FIG. 1
,
10
denotes a comparator,
11
a DC bias circuit,
20
a smoothing circuit,
40
a voltage amplifier, R
11
, R
12
, R
21
, R
22
, and R
41
to R
43
denote resistors, C
11
, C
21
, and C
22
denote capacitors, U
3
and U
4
denote inversion gates, and U
40
denotes an operation amplifier. Further, VDD denotes a power supply voltage of the circuit.
The DC bias circuit eliminates the DC component from the RF signal output from the optical pickup, gives a DC bias voltage of a half of the power supply voltage (VDD/2), and outputs the same to the comparator
10
.
Specifically, one terminal of the capacitor C
11
receives the RP signal output from the optical pickup, while the other terminal of the capacitor C
11
is connected to a node of the resistor R
11
and the resistor R
12
having equal resistance values cascade connected between the power supply voltage and a ground potential. The RF signal is output from this node to the comparator
10
.
The comparator
10
compares the RF signal output from the DC bias circuit
10
and the threshold voltage output from the voltage amplifier
40
and outputs an output signal CDATA binary coded to a high level equal to the power supply voltage and a low level equal to the ground potential.
The smoothing circuit
20
receives the output signal CDATA via the cascade connected inversion gates U
3
and U
4
and outputs the temporal mean value smoothing the output signal CDATA to the voltage amplifier circuit
40
.
The voltage amplifier
40
amplifies a difference voltage between the temporal mean value of the output signal CDATA received from the smoothing circuit
20
and the DC bias voltage (VDD/2) and outputs the amplified difference voltage to the comparator
10
as the threshold voltage for the binary coding.
Specifically, a positive side input terminal of the operation amplifier U
40
receives the temporal mean value of the output signal CDATA from the smoothing circuit
20
, while a negative side input terminal of the operation amplifier U
40
is connected to the node of the resistor R
41
and the resistor R
42
having equal resistance values cascade connected between the power supply voltage and the ground potential. The output voltage of the operation amplifier U
40
is fed back via the resistor R
43
to the negative side input terminal of the operation amplifier U
40
and, at the same time, output to the comparator
10
.
Next, an explanation will be made of the operation of the conventional waveform reforming circuit having the above configuration.
The RF signal input from a not illustrated optical pickup circuit to the DC bias circuit
11
is cleared of its DC component by the capacitor C
11
and, at the same time, given the DC bias voltage (VDD/2) at the node of the resistor R
11
and the resistor R
12
and output to the comparator
10
.
FIG. 2
is a view of the waveforms of the RF signal in the input and output of the DC bias circuit
11
.
In
FIG. 2
, A denotes the voltage waveform of the RF signal in the input of the DC bias circuit, B denotes the temporal mean value of the voltage waveform A, C denotes the voltage waveform of the RF signal in the output of the DC bias circuit, and D denotes the temporal mean value of the voltage waveform C. Further, the broken lines in the figure represent the temporal mean values in an ideal state free from asymmetry.
As shown in
FIG. 2
, when a fluctuation of “a” occurs in the temporal mean value of the input RF signal due to the asymmetry, the ideal value of the temporal mean value of the RF signal in the output of th
Kananen, Esq. Ronald P.
Rader & Fishman & Grauer, PLLC
Wells Kenneth B.
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