JPS6146898B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a digital signal demodulation method.

The first object of the present invention is to
The purpose of this project is to improve the demodulation method for digital signals recorded using the phase shift method so that they can be read accurately.

A second object of the present invention is to enable accurate demodulation even when the duty of the digital signal recorded by the above method deviates from 50%.

A third object of the present invention is to enable accurate reading even if the card feeding speed is uneven when used, for example, to read a magnetic card.

Two-frequency coherent phases method (F2F
The demodulation method for digital signals recorded using the above method is often used, for example, to read magnetic cards. It needs to be readable.

To explain the demodulation method conventionally used in Fig. 1, the time of bit frame B ₁ is measured, and the time from the start of B ₂ is B ₁ × 3/4 (this value is theoretically an appropriate value). Bitframes elapsed
It is checked whether there is a polarity change in _B2 , and in this case, since there is no polarity change, an output of "0" is output. In parallel with these operations, measure the time of bit frame B ₂ and check whether there is a change in polarity from the start of B ₃ until the time of B ₂ × 3/4 has elapsed. Therefore, it outputs “1”. By repeating this process, the F ₂ F signal is demodulated.

Such conventional demodulation methods have the following drawbacks. That is, when the F ₂ F signal is warmed and amplified by a magnetic head, a waveform as shown in FIG. 2 is obtained. If the level of this waveform is detected at slice levels ○A and ○B, the waveform will be as shown below, but if the upper and lower slice levels differ as shown in ○A and ○B, the bit frame B ₁ = B ₂ , B ₃ = B ₄ , B ₃ = B ₁ , B ₄ = B ₂ , and the waveform data does not become 50%.

If the data deviates from 50% in this way, the conventional method of checking whether there is a change in polarity within the time period of 3/4 times the previous bit frame as described above is prone to errors. It is.

The present invention improves this point, so that accurate demodulation can be performed even if the data deviates from 50%.
Based on the preceding pulse width of the same polarity (if the pulse width corresponds to a bit frame, multiply that value by 3/4, or if the pulse width corresponds to a part of a bit frame, multiply the value by 3/4) (2 x 3/4 multiplied by 2 x 3/4 as the reference time), determine whether there is a polarity change in the subsequent bit frame within this reference time,
It is designed to demodulate the F ₂ F signal.

Note that the value K of 3/4 (75%) may be in the range of 0.5<K<1, but since there is a variation of about ±10% when encoding the F ₂ F signal, it is not practical in practice.
It is sufficient to set it as 0.6≦K≦0.9. The conventional 75% was set at a midpoint between 0.5 and 1.

An embodiment of the present invention will be described below with reference to FIG. 3. A signal recorded in the F ₂ F system is read out by a magnetic head and has a waveform as shown in FIG. If we detect this level using slice levels ○ha and ○d,
The waveform will look like this. In this case, the waveform data deviates from 50%.

Here, perform the following as initial settings. That is, at least H level and L level "0" signals (with no polarity change in the bit frame) are inserted at the beginning of the F ₂ F data, and in the flowchart of FIG. 5, the first two bits are A time period of 3/4 times the pulse width is entered into the level storage circuit M _H and the L level storage circuit M _L so that no data is output.

Returning to Figure 3 again to explain the action, if it comes from T ₁ after initial setting, determine the level →
H level. Count time → T ₁ . Compare T ₁ with the time T _H1 stored in the memory circuit M _H (determining whether or not there is a polarity change in the bit frame T ₁ within the time T _H1 )→T ₁ >T _H1 . Store T ₁ ×3/4=T _H2 in M _H. This generates "0" data and a clock signal.

Next, when T ₂ comes, the level is determined → L level. Counting time → T ₂ , T ₂ and memory circuit M _H
Compare the time T _L1 stored in →T ₂ >
T _L1 . Store in T ₂ × 3/4T _L. Generates data “0” and clock. The same operation as before is performed at T ₃ and T ₄ as well.

Next, when T ₅ comes, the level is determined → H level. Count the time → T ₅ . Compare T ₅ and time T _H3 stored in M _H →T ₅ ≦T _H3 . _T5
x 3/2 (2 x 3/4) = Store T _H4 in M _H. Count the time of T ₆ → T ₆ . Store T ₆ ×3/2=T _L4 in M _L. Generates data “1” and clock. Repeat the same process below. In addition, in FIG. 5, M _H is H
A memory circuit for level, M _L is a memory circuit for L level,
T _H is the time stored in M _H , T _L is the time stored in M _L , and A, B, C, and D are pulse widths.

What is shown in FIG. 4 is a circuit for producing the results shown in FIG. Namely, 1 is a magnetic card with information recorded on a magnetic stripe (not shown) provided on a part of the card, 2 is a magnetic head that reads information from the traveling magnetic card 1, and 3 is a magnetic head 2. 4 is a level detection circuit that detects the level of the output waveform of the amplifier 3 at a slice level. 5 is an LSI that processes the output signal of the level detection circuit 4 and generates demodulated data and clock pulses. It is. Indicated by the arrow in the center of this circuit, ,, in Fig. 3,
This results in a waveform of

Next, another embodiment of the present invention will be explained with reference to FIG. 6. In this case, when there is a polarity change in the bit frame like T ₅ and T ₆ in FIG. 3, only the time of T ₅ is stored. However, when demodulating _T8 , _T4 is used as a reference, and the rest is the same as the flowchart in FIG.

FIG. 7 shows still another embodiment of the present invention. If there is a polarity change in the bit frame like T ₅ and T ₆ in Figure 3, the times of T ₅ and T ₆ are not stored in the memory circuit and only judgments of “0” and “1” are made. The rest is the same as the flowchart in FIG.

Although these two embodiments have some problems in accuracy compared to the first embodiment described above, there is no problem in practical use.

As described above, in the demodulation method of a digital signal recorded by the F ₂ F method, the present invention determines the polarity of the pulse of the digital signal, and determines the reference time for determination based on the pulse width of the pulse. The polarity of the pulse is stored in different storage means, and during demodulation, the determination reference time obtained from the pulse of the same polarity as the bit frame stored in the storage means is used as a reference, using the start end of the reproduced bit frame as a reference. The digital signal is demodulated by determining whether or not there is a polarity change within the bit frame.

Therefore, even if the date deviates from 50%,
The property that the time widths of the H level pulse and L level pulse of F ₂ F data are repeated with a certain proportional relationship, and the time width of the pulse at a certain location is approximately equal to the time width of the previous pulse at the same level. is utilized and the subsequent bit frame is demodulated based on the time width of the preceding pulse of the same level, so accurate demodulation can be achieved. Furthermore, since the time width of the preceding pulse is used as the reference, there is an advantage that accurate demodulation can be performed even if the length of the bit frame changes due to fluctuations in card speed.

[Brief explanation of the drawing]

Figure 1 is a waveform diagram for explaining the conventional example;
The figure is a waveform diagram when processing the one in Figure 1, and the waveform diagram in Figure 3.
4 is a block diagram showing a circuit example of the present invention, FIG. 5 is a flowchart of one embodiment of the present invention, and FIG. 6 is a waveform diagram for explaining the present invention. Flowchart of an Embodiment FIG. 7 is a flowchart of yet another embodiment of the present invention. 1...Magnetic card, 2...Magnetic head, 3...Amplifier, 4...Level detection circuit, 5...LSI.

Claims (1)

[Claims] 1. In a method of demodulating a digital signal recorded by the two-frequency coherent phase method, the polarity of the pulse of the digital signal is determined, and the reference time for determination is determined based on the pulse width of the pulse. , the pulses are stored in different storage means depending on the polarity of the pulses, and during demodulation, a criterion for determination is obtained from pulses of the same polarity as the bit frames stored in the storage means, using the start end of the reproduced bit frame as a reference. A demodulation method that demodulates a digital signal by determining whether or not there is a polarity change within the bit frame over time. 2. As the reference time for judgment, measure the width of two consecutive pulses of different polarity preceding the bit frame to be demodulated, and if the pulse widths each correspond to a bit frame, multiply each pulse width by an appropriate constant K. If the two pulse widths together correspond to a bit frame, use the time that is twice the width of each pulse multiplied by the constant K, and set the range of the constant K to 0.5<K. The demodulation method according to claim 1, wherein <1.