JPH0439953B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention provides a method for converting a received teletext analog signal into a digital signal by adjusting the reference voltage (slice level) to the most appropriate voltage for the fluctuating text signal. This invention relates to a correction circuit for correcting.

“Prior art” In teletext broadcasting, general
Since the teletext signal A that is transmitted superimposed on the vertical period of the TV broadcast signal is an analog signal,
It is necessary to A/D convert it into a digital signal before inputting it to a teletext playback device, but the size of the analog teletext signal A (high ) fluctuates and is not converted into an accurate digital teletext signal. That is, if the slice level is too low, it will be converted into a wide pulse, and if it is too high, it will be converted into a narrow pulse.

As a device for correcting this fluctuating slice level to an appropriate level, there is a slice level correction device shown in FIG. The correction device shown in FIG. 5 includes reference pulse generation circuits 11 and 1, respectively, which set the upper and lower limits of the allowable range of the clock pulse width of the A/D-converted digital teletext signal.
2, pulse width comparison circuits 13 and 14 that compare the width of the pulse set by the reference pulse generation circuits 11 and 12 with the width of the clock pulse of the digital teletext signal, and A flip-flop 15 operates at the rising edge and starts up the reference pulse generating circuits 11 and 12 at the same time as the input of the clock pulse of the digital teletext signal, thereby forming a circuit for comparing the clock pulse of the digital teletext signal and the reference pulse. There is. Furthermore, a shift register 16 that operates according to the outputs of the pulse width comparison circuits 13 and 14, and a D/A conversion circuit 17 that converts the state of the shift register 16 into an analog signal,
This is designed to generate a signal for setting the slice level. Note that 18 is an input terminal for a digital teletext signal, 19 is an output terminal for a signal for correcting the slice level, and 20 is an input terminal for a clock signal.

In this device, when a clock pulse of a digital teletext signal is input to an input terminal 18, this clock pulse is input to pulse width comparison circuits 13 and 14, and a flip-flop 15 converts the reference pulse generation circuits 11 and 12. rise, generate reference pulses with respective set widths, and pass these reference pulses to the pulse width comparison circuits 13 and 14.
Enter. The pulse width comparison circuit 13 compares the clock pulse width of the digital teletext signal with the reference pulse width at the upper limit of the allowable range, and when the width of the clock pulse of the digital teletext signal is wider than the reference pulse width, the clock pulse width of the digital teletext signal is input to the shift register 16. send a signal.
The pulse width comparison circuit 14 compares the clock pulse width of the digital teletext signal with the reference pulse width at the lower limit of the allowable range, and when the clock pulse width of the digital teletext signal is narrower than the reference pulse width, the shift register 16 send a signal to. The shift register 16 shifts one bit to the right when it receives a signal from the pulse width comparison circuit 13, and shifts it one bit to the left when it receives a signal from the pulse width comparison circuit 14. Then, the state of this shift register 16 is converted into an analog signal for slice level correction by a D/A conversion circuit and output.

"Problems to be Solved by the Invention" The above-mentioned correction device compares the width of the clock pulse of the digital teletext signal with the width of the reference pulse.
Whether the value is above or below the allowable range is input to the shift register, and when it is above, the shift register is shifted one bit to the right, and when it is below, it is shifted to the left by one bit. It shifts one bit at a time. In other words, one comparison corrects the slice level by one step, and if the analog character transmission signal A suddenly fluctuates, for example, by four steps, the slice level is corrected four times. - Since the correction is performed gradually by shifting, there is a problem that A/D conversion cannot be performed at an appropriate slice level during this period.

"Means for Solving the Problems" The present invention has been made to solve these problems, and includes a reference pulse generation circuit that generates a reference pulse based on the rising edge of a clock pulse of a teletext signal, and a first counter for counting the difference in time width between the rising edge and the falling edge of the clock pulse of the teletext signal when the pulse time width is smaller than the time width of the reference pulse; A second counter that counts the difference in time width between the two when the time width of the pulse until falling is larger than the time width of the reference pulse, and when the time width of the clock pulse of the teletext signal is smaller than the time width of the reference pulse. The first counter is selected and the slice level correction is set in the negative direction, and when the time width of the clock pulse of the teletext signal is larger than the time width of the reference pulse, the second counter is selected and the slice level correction is set in the negative direction. a selection circuit for setting the direction, and a digital circuit for correcting the slice level in the negative or positive direction by an amount corresponding to the data of the first counter or the second counter selected by this selection circuit.
It consists of an analog converter.

``Operation'' The reference pulse generation circuit generates a reference pulse at an appropriate slice level using the rising edge of the clock pulse of the digital teletext signal as a reference. With respect to this reference pulse, it is detected whether the time width of the clock pulse of the actual teletext signal is larger or smaller, and the difference between the two is determined as a digital value using a first counter or a second counter. Count. If the clock pulse of the teletext signal is smaller than the reference pulse, the slice level is too high, so it is corrected to lower it by the value obtained by the first counter. On the other hand, if it is large, it is too low, so the correction is made to increase it by the value obtained by the second counter.

“Embodiment” An embodiment of the present invention will be described below with reference to the drawings.

1 is a pulse signal input terminal that inputs the clock pulse of the A/D converted digital teletext signal. Specifically, when A/D converted at the appropriate slice level (Bo) in Figure 3a, Accurate clock pulse as shown in Figure 3b, narrow clock pulse as shown in Figure 3c if A/D conversion is performed at a too high slice level (Bu) in Figure 3a, and narrow clock pulse as in Figure 3a.
If A/D conversion is performed at a slice level Bd that is too low, a wide clock pulse as shown in FIG. 3e will be input. Reference numeral 2 denotes an input terminal for a clock signal (for example, 46 MHz) serving as a reference for this correction circuit. 3 is
A reference pulse generation circuit consisting of a monostable multivibrator outputs a reference pulse having the same width as the clock pulse width (W ₀ ) (175 ns) when A/D converted at an appropriate slice level. A first counter 4 outputs a digital value corresponding to the difference when a clock pulse having a width (W ₁ ) smaller than the reference pulse width (W ₀ ) is input. A second counter 5 outputs a digital value corresponding to the difference when a clock pulse having a width (W ₂ ) larger than the reference pulse width (W ₀ ) is input. Reference numeral 6 denotes a selection circuit, which selects the first counter 4 when the width of the clock pulse is smaller than the reference pulse width (W ₀ ), and selects the second counter 5 when it is larger. It is. 7 and 8 are buffer circuits for sending the outputs of the first and second counters 4 and 5 to the next stage, respectively. 9 is a digital-to-analog converter for correcting the slice level, which includes a terminal D for setting whether to correct the slice level in the positive direction or in the negative direction, and a terminal A for setting the correction amount.
It has B and C. 10 is an output terminal for the corrected slice level.

In the above configuration, when the slice level is appropriate as shown by Bo in Figure 3a, the clock pulse signal of the digital teletext signal input to input terminal 1 is as shown in Figure 3b. The pulse widths (W ₀ ) from the rising edge to the falling edge and from the falling edge to the rising edge are both 175 ns, and the output of the digital-to-analog converter 9 is "0000" in FIG. 2, which is not corrected.

When the slice level is too high as shown in Bu in FIG. 3a, a pulse signal having a pulse width (W ₁ ) from the rising edge to the falling edge as shown in FIG. 3c is input to the input terminal 1. It is assumed that the slice level (Bu) at this time is "0011" in FIG. Using the rising edge (t ₁ ) of the pulse shown in FIG. 3c, the reference pulse generating circuit 3 generates a reference pulse as shown in FIG. 3d. When the pulse shown in FIG. 3c falls (t ₂ ), the first counter 4 starts counting the clock signal input from the input terminal 2, and stops counting when the reference pulse falls. Let this count value be "M". Here, as shown in FIG. 2, when the fourth digit of the truth table is too high, it is "1" and when it is too low, it is "0". 7 is selected. Then, a signal instructing the slice level to be lowered from this buffer circuit is sent to the input terminal D, and an output "M" corresponding to the correction amount of the buffer circuit 7 is sent to the input terminals A, B, and C.
The analog converter 9 corrects the slice level to lower it by this value.

Similarly, if the slice level is too low as Bd in Figure 3a, a pulse as shown in Figure 3e is applied.
W ₂ is input to the input terminal 1, and in the same way, the second counter 5 obtains the correction amount shown in the perspective part N of FIG.
It is sent to analog converter 9. Since the signal to the input terminal D of the selection circuit 6 is inverted, the slice level is corrected to increase by "N".

"Effects of the Invention" Since the present invention is configured as described above, the slice level can always be corrected to an appropriate position, and a stable digital signal can always be detected during analog-to-digital conversion. In addition, by comparing the clock pulse of the digital teletext signal with the reference pulse to obtain the slice level correction amount, multiple levels of correction can be made at once, so even sudden changes in the analog teletext signal can be quickly and appropriately corrected. This has the effect that correction can be made to the slice level.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of a slice level correction circuit according to the present invention. FIG. 2 is an explanatory diagram of slice levels and analog signals, FIG. 3 is an operational waveform diagram of each part, FIG. 4 is a teletext waveform diagram, and FIG. 5 is a block diagram showing a conventional example. 1...Character signal input terminal, 2...Clock signal input terminal, 3...Reference pulse generation circuit, 4, 5
... Counter, 6 ... Selection circuit, 7, 8 ... Buffer circuit, 9 ... Digital-to-analog conversion circuit,
10...Output terminal.

Claims (1)

[Scope of Claims] 1. A reference pulse generation circuit that generates a reference pulse based on the rising edge of a clock pulse of a teletext signal, and a pulse time width from the rise to the fall of the clock pulse of the teletext signal is the time of the reference pulse. a first counter that counts the difference in time width between the two when the time width is smaller than the time width of the clock pulse of the teletext signal; A second counter that counts the difference in width, and a second counter that selects the first counter when the time width of the clock pulse of the teletext signal is smaller than the time width of the reference pulse and sets the slice level correction in the negative direction, a selection circuit for selecting a second counter and setting slice level correction in the positive direction when the time width of the clock pulse of the signal is larger than the time width of the reference pulse; and a first counter selected by the selection circuit; 1. A slice level correction circuit for teletext broadcasting, comprising: a second converter; and a digital-to-analog converter that corrects the slice level in a negative or positive direction by an amount corresponding to the data of the counter. 2. The slice level correction circuit in teletext broadcasting according to claim 1, wherein the reference pulse generation circuit is composed of a monostable multivibrator, and the reference pulse is set to be the same as the pulse width at the appropriate slice level.