JPS6248410B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an AD conversion device, which converts an angle-modulated wave signal obtained by performing angle modulation such as frequency modulation on an analog signal into a digital signal, thereby obtaining a digital signal. The amount of data can be reduced,
Another object of the present invention is to provide an AD converter that has a simple and inexpensive circuit structure, has a short period of malfunction due to noise contamination, and can output correctly converted digital signals.

BACKGROUND OF THE INVENTION Conventionally, AD conversion devices have been used to convert analog signals such as video signals into digital signals for supplying the reproduced signal to a memory for correcting time axis fluctuations of a VTR and for various other purposes. Conventionally, for high-efficiency encoding of such AD conversion devices,
Differential pulse code modulation (DPCM) as shown in Figure 1
I found a device to do this. In the figure, an analog signal e _i inputted to an input terminal 1 is supplied to a subtracter 2, where it is subtracted from the analog signal e _p of one clock before the decoder 9 and converted into a difference signal e _D. After that, the signal is supplied to the encoder 3, where it is encoded.

Number of encoded bits extracted from encoder 3 n
The digital signal E _D is supplied to the adder 4 and output to the output terminal 5. The adder 4 adds this digital signal E _D to a signal E ₁ ' obtained by dividing the addition output of the digital signal E _D up to one clock ago by a predetermined coefficient by the divider 7, and then performs the addition. The output signal E ₀ is supplied to latch 6 to cause it to latch. The digital signal E _D and the timed signal _E1 are taken out from the latch 6 and supplied to the divider 7, while being supplied to the latch 8, which generates the digital signal E _D and the timed signal E1. E ₂ is output. this signal
E ₂ is supplied to the decoder 9 and decoded into an analog signal e _p , and then supplied to the subtracter 2 .

In the above-mentioned conventional apparatus, an input analog signal e is applied to a sample clock having a frequency c shown in FIGS. When _i is a triangular wave with a frequency of 1/8c as shown in FIG. 2A, the output signal e _D of the subtracter 2 becomes a trapezoidal wave as shown in FIG. 2C. On the other hand, if the input analog signal e _i is a triangular wave with a frequency of 1/4c and the same amplitude as the triangular wave shown in FIG. 2A, as shown in FIG. 3A, the output signal waveform of the subtracter 2 becomes a trapezoidal wave as shown in FIG. 3C, and its amplitude is V twice that of the trapezoidal wave shown in FIG. 2C.

That is, if the number of encoded bits n is set to be maximum when the amplitude of the trapezoidal wave shown in FIG. 3C is V, then
The number of code bits for the trapezoidal wave shown in ^FIG . The higher the frequency of _i , the better. In other words, the utilization efficiency for the number of encoded bits of the output signal of subtracter 2 is
Shows the characteristics of 6d _B /oct.

Therefore, in the encoding of television signals that has already been published, the apparatus shown in Figure 1 is used, taking advantage of the fact that the level value for the frequency of the television signal is statistically _-6dB /oct. Ta. However, when a television signal that is not based on the above-mentioned statistical prediction is received, the above-mentioned conventional apparatus cannot accurately convert the digital signal and obtain a correct reproduced signal e _p . That is, the input analog signal e _i is a television signal as shown in FIG. , the difference signal e _D obtained from the subtracter 2 is B
The waveform is not ideal as shown in Figure C, but the waveform is as shown in Figure C. Therefore, the reproduced signal e _p extracted from the decoder 9 becomes as shown in Figure D, which is significantly different from the original signal a. This results in a different waveform.

Further, in the conventional device described above, if an error occurs during its operation, the error is added, so a means for periodically correcting the error is required, and the circuit configuration is complicated. Furthermore, as mentioned above, the above conventional device
With respect to the frequency, the utilization efficiency with respect to the number of encoded bits of the output signal of the subtracter 2 is 6d _B /oct, which has a disadvantage of poor utilization efficiency.

This invention eliminates all of the above-mentioned drawbacks of the present invention, and one embodiment thereof will be described with reference to the drawings from FIG. 5 onwards.

FIG. 5 shows a block system diagram of an embodiment of the AD converter according to the present invention. In the figure, the same components as in FIG. 1 are denoted by the same reference numerals, and their explanations will be omitted. The analog signal e _i that enters the input terminal 1 is supplied to the frequency modulator 10, where it is frequency modulated to become a frequency modulated wave signal e _FM , and then the low pass filter 11 converts it into a frequency modulated wave signal e FM at a rate of -6d _B /oct. The higher the frequency, the more attenuated the frequency modulated wave signal e _FM ' is, which is supplied to one input terminal of the subtracter 2.

FIG. 10 shows a circuit diagram of one embodiment of the low-pass filter 11. As shown in the figure, the low-pass filter 11 includes an NPN transistor Q ₁ whose base is connected to the input terminal 12, and the collector resistance (resistance value R) and emitter resistance (resistance value R') of the transistor Q ₁ .
, a field effect transistor (FET) Q ₂ , and a capacitor (capacitance value C) connected between the base of transistor Q ₁ , the gate of FET Q ₂ , and ground, and the input signal amplified from the collector of transistor Q _1. is taken out and supplied to a capacitor having a capacitance of C, and the resulting voltage across the capacitor is impedance-converted by FETQ ₂ forming a source follower and output to an output terminal 13. Now, the input voltage of input terminal 12 is
Vi, the output voltage taken out from output terminal 13
Assuming V ₀ , the output voltage V ₀ is expressed by the following equation.

V ₀ = Vi' (R・1/jωC)/(1/jωC+R) = Vi'/(jωC+1/R) However, in the above equation, Vi'=Vi/R', ω=2π, is the frequency of the input voltage Vi It is. If jωC≫1/R in the above formula, the output voltage V ₀ will approximately be Vi'/jωC, so
This is equivalent to passing through a low-pass filter of −6d _B /oct. Therefore, the impedance of the capacitor jωC
By setting d to a sufficiently large value compared to 1/R, it is possible to configure the low-pass filter 11 with a simple R and C configuration.

The frequency modulated wave signal e _FM ' extracted from the low-pass filter 11 is converted into a digital signal E _D by a circuit having substantially the same configuration as the AD converter shown in FIG. 1, and is outputted from the output terminal 5. Note that since the signal supplied to the divider 7 and the subtracter 2 shown in FIG. 1 is a simple waveform rather than a composite waveform of each frequency like a television signal, this is not particularly necessary and can be omitted.

In this way, after frequency modulating the analog signal to be AD converted to obtain a frequency modulated wave signal e _FM with a constant amplitude, the signal is attenuated by -6d _B /oct in advance and then supplied to the subtracter 2. Therefore, a difference signal e _D whose level does not change even if the frequency changes can be obtained. For example, when the repetition frequency of the sample clock is c as shown in FIGS. 6B and 7B, the output frequency modulated wave signal e of the low-pass filter 11 is
When the frequency of _FM ' is c/8, the amplitude is large as shown in Fig. 6A, and on the other hand, when the frequency of the frequency modulated wave signal e _FM ' is c/4, the amplitude is c as shown in Fig. 7A. /8
It is 1/2 of the case of . Therefore, as explained in conjunction with FIGS. 2A to 3C and 3A to C, the amplitude of the difference signal e _D taken out from the subtractor 2 has a characteristic of 6d _B /oct with respect to the input signal frequency of the subtractor 2. Therefore, depending on the frequencies shown in Figures 6A and 7A, -
Frequency modulated wave signal e _FM with an amplitude of 6d _B /oct
In the case of this embodiment in which the signal ' is supplied to the subtracter 2, the amplitude of the difference signal e _D is the same amplitude V as shown in FIGS. 6C and 7C. Therefore, according to this embodiment, since the maximum number of encoded bits can always be used, the utilization efficiency is greater than that of the conventional apparatus.

Furthermore, in the present invention, since the signal to be converted into a digital signal is the frequency modulated wave signal e _FM ', for example, the analog signal e _i is a television signal, and under normal conditions, the decoder 9 of the conventional device
A normal television signal _as shown in FIG. Of course, when a malfunction occurs, the reproduced signal e of the conventional device
While _p is a television signal having an erroneous signal part b and a clip part _c as shown in FIG. As shown by the solid line in B, does it generate an erroneous signal part b' or a clip part c'?
The effect is short because the frequency modulated wave signal e _p has a high frequency. Incidentally, if the device of the present invention is used as part of the time base collector of a current VTR, the frequency bands of the frequency modulated wave signals e _FM , e _FM ′, and e _p will be approximately the same as the frequency modulated video signal band of the current VTR. Yes, the frequency is higher than the baseband of the video signal. Further, the cutoff frequency of the low-pass filter 11 is selected, for example, to be approximately the lower limit frequency of a band including the primary sideband of the frequency modulated wave signal _eFM .

Note that the analog signal e _i converted into a digital signal by the apparatus of the present invention is not limited to a television signal, but can of course be applied to audio signals and other analog signals. Further, a phase modulator may be used instead of the frequency modulator 10.

As described above, the AD conversion device according to the present invention includes an angle modulator that angle modulates an analog signal, and attenuates the high frequency of the angle modulated wave signal output from the angle modulator and supplies it to the differential pulse code modulation circuit. Since the present invention is equipped with a low-pass filter, the output reproduction signal of the decoder in the differential pulse code modulation circuit can be obtained accurately with a simple circuit configuration, instead of being based on statistical values as in conventional devices. In addition, since the amplitude of the differential signal can be kept approximately constant regardless of changes in frequency, the efficiency of using the number of encoded bits can be greatly improved compared to conventional methods, and the amount of data when digitized can be reduced. Even if an erroneous signal is mixed in, the erroneous operation period is short, so the erroneous operation can be corrected in a short time.

[Brief explanation of the drawing]

Fig. 1 is a block system diagram showing an example of a conventional device, Fig. 2 A to C, Fig. 3 A to C, and Fig. 4 A to
D is a signal waveform diagram for explaining the operation of FIG. 1, and FIG. 5 is a block system diagram showing an embodiment of the device of the present invention.
6A-C and 7A-C are signal waveform diagrams for explaining the operation of FIG. 5, FIGS. 8A and B, and FIG. 9A, respectively.
B is a diagram showing reproduced signal waveforms during normal operation and malfunction in FIGS. 1 and 5, respectively, and FIG. 10 is a circuit diagram showing an embodiment of the main part of FIG. 5. 1...Analog signal input terminal, 2...Subtractor,
3... Encoder, 4... Adder, 5... Digital signal output terminal, 9... Decoder, 10... Frequency modulator, 11... Low pass filter.

Claims (1)

[Claims] 1. In an AD conversion device that converts an analog signal into a digital signal using a differential pulse code modulation circuit and outputs the digital signal, an angle modulator that angle-modulates the analog signal and a high frequency of the angle-modulated wave signal output from the angle modulator are provided. and a low-pass filter that attenuates and supplies the attenuated signal to the differential pulse code modulation circuit.