JPH0445910B2 - - Google Patents

Description

[Detailed Description of the Invention] [A. Field of Application] The present invention provides a method for detecting signals detected from a recording medium.
The present invention relates to a reference level setting circuit for digital signal detection configured to automatically change a reference level that separates high-level and low-level periods of a detection signal so that an accurate original digital signal is detected.

[B Background of the invention]

Recently, audio signals (analog signals)
Convert to PCM signal and optically record to disk,
A digital audio disc for playback has been proposed. In this case, the digital data is modulated using an appropriate digital modulation method, such as 8/14 modulation (converts 8-bit data to 14-bit data.EFM
It has been considered that data can be recorded on a disk by modulation (called modulation). In order to obtain the original digital signal from the recorded disc in this manner, the high level period and low level period can be determined by comparing the signal picked up by the pickup means with an appropriate reference level. . However, if this reference level is not set accurately, the obtained (detected) digital signal will be an inaccurate signal, and the error rate of digital data reproduction will increase. It is not preferable to set this reference level uniquely.
This is because the actual signal obtained from the disk is
This is because there is temporal modulation between disks and even within the same disk due to the influence of variations in disk writing conditions, disk fluctuations at the time of final storage, and the like.

[C Prior art]

In consideration of the above points, a configuration in which the reference level is automatically changed so that a digital signal with accurate high-level and low-level periods can always be detected has already been disclosed in Japanese Patent Application No. 177468/1983. Proposed.

[D Disadvantages of conventional technology]

If the above-mentioned automatic change of the reference level is always performed, there is a risk that such automatic change will malfunction when the signal itself detected from the recording medium is missing.

[E Objective of the present invention]

The present invention prevents malfunctions by automatically changing the reference level only when no signal is missing.

[F Examples of the present invention]

First, a reference level automatic setting circuit, which is a premise of the present invention, will be explained.

In the currently proposed digital audio disks, digital data is EFM modulated and recorded. With EFM modulation, 8 data bits of data are converted to 14 channel bits of data, but since the number of data that can be expressed with 14 bits is naturally greater than the number of data that can be expressed with 8 bits, 14 Some constraints can be placed on the channel bit data. For example, 14 channel bits of data
When recording in the NRZ system, a restriction can be imposed that the signal inversion interval (high level and low level period) is 3 channel bits or more and 11 channel bits or less. Therefore, in the embodiment described below, the reference level is automatically changed so that the minimum signal inversion interval becomes the interval of three regular channel bits.

Comparator 1 contains the data obtained from the pick-up means.
The EFM signal (Figure 2 a) is input with its DC component cut off by a capacitor 2, and a reference level signal (Figure 2 g), which is normally at 0 level, is also input, so that the reference level The high level and low level periods of the EFM signal a are determined with respect to g, and an EFM digital signal as shown in FIG. 2b is created. The reference level signal g is output from a reference level changing circuit 3 composed of an up/down counter 4 and a D/A converter 5, and the output of the counter 4 is normally 0. EFM signal a obtained from pick-up means
Since the waveform is blunted, the obtained EFM digital signal b varies greatly depending on the position of the reference level g.

The EFM digital signal b is input to the judgment circuit 6 ,
It is determined whether the minimum period of the high level and low level periods is a period corresponding to three regular channel bits (approximately 694 ns).

The monostable multivibrator 7 and the D-type flip-flop 8 have a low level period of a predetermined period (approximately
694ns) or less, it is determined whether the reference level g is below the appropriate level h. The monostable multivibrator 7 is triggered by the fall of the signal b, and the Q output (FIG. 2c) is at a high level for the predetermined period. flip prop 8
is in a reset state when the reset input is at a low level, and reads the data input (signal c) in response to the rise of the T input (signal b) when the reset input is at a high level. According to Fig. 2, the low level period of the EFM digital signals (b1, b2) is less than the predetermined period, and at this time, the judgment pulse (d1, d2) is obtained from the Q output of flip-flop 8. . Therefore, the counter 4 is counted up by this judgment pulse, and the output of the D/A converter 5 is increased.
Push the reference level g upward to bring it closer to the appropriate level h.

The monostable multivibrator 9 and the D-type flip-flop 10 determine whether the reference level g is higher than the appropriate level h by determining whether the high level period is shorter than a predetermined period. (See Figure 3). In this case, a signal obtained by inverting the signal b by the inverter 11 is used.In that case, if the reference level g
is shifted upward, the judgment pulse (f1, f2,
f3) is obtained, counter 4 is counted down,
The output of the D/A converter 5 decreases, pushing the reference level g downward and bringing it closer to the appropriate level h.

Although the explanation so far has been based on the use of the minimum signal inversion interval, the maximum signal inversion interval (equivalent to 11 channel bits) may also be used.

Now, the gist of the present invention is that when a signal to be detected from a recording medium is missing due to dropout or the like, this is detected by the signal loss detection circuit 20 , and this detection output j is used to change the reference level change circuit 3.
The purpose is to drive a prohibition circuit 30 that prohibits the operation of the device. That is, when a signal loss is detected in the signal loss detection circuit 20 and its output j becomes L level, the two gates 3 constituting the inhibition circuit 30
0a and 30b are both turned off, and the operation of the reference level changing circuit 3 is stopped.

FIG. 4 shows an example of the signal loss detection circuit 20 . In this circuit, first, diodes 21, 21, capacitors 22, 22, and resistors 23, 23 generate a DC voltage equal to the peak-to-peak value (P, -P) of signal a across a variable resistor 24. That is, FIG. 5a shows the envelope of the signal a, and there are two terminals at both ends of the variable resistor 24, as shown in FIG. 5a''.
A DC voltage of a value obtained by subtracting the voltage drop D across the diode 21 from the peak value is applied. This DC voltage is appropriately divided by a variable resistor 24 to a level k, and is inputted to one terminal of a comparator 25 (see FIG. 5a). On the other hand, the signal a is input as is to the + terminal of the comparator 25. Then, when the signal a is present, the output i of the comparator 25 becomes H level - L
Although the level is repeated (shaded area i in FIG. 5), the signal remains at the L level during the period T during which the signal is lost. The output i of the comparator 25 is input to a retriggerable one-shot circuit 26. Therefore, the Q output of this one-shot circuit 26 becomes L level during the signal drop period (FIG. 5j). That is, during this period, the inhibition circuit 30 is driven and the reference level changing circuit 3 is in an inactive state. Note that the metastable period of the one-shot circuit 26 is set to be slightly longer than the maximum period included in the signal a.

[G effect]

If there is a signal dropout that is longer than the maximum period of the signal, the reference level is not changed, so there will be no malfunction.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a reference level setting circuit for digital signal detection according to the present invention, Figs. 2 and 3 are its operating waveform diagrams, Fig. 4 is a diagram showing a signal dropout detection circuit, and Fig. 5 is its operating waveform diagram. 3 is a reference level change circuit, 20 is a signal loss detection circuit, and 30 is a prohibition circuit.

Claims (1)

[Claims] 1. In detecting the original digital signal by determining the high level and low level periods of the signal by comparing the signal detected from the recording medium with a reference level, A reference level change circuit that automatically changes the reference level based on a signal indicating whether the low level period is less than (or more than) the regular minimum (or maximum) period, and a reference level change circuit that is detected from the recording medium. A reference level setting circuit for a digital signal, comprising a prohibition circuit that prohibits the reference level changing circuit from changing the reference level when a signal is missing.