Data demodulation

Details Data demodulation Data demodulation

2000-03-30

2004-06-29

Phu, Phoung (Department: 2631)

Pulse or digital communications

Receivers

Angle modulation

C375S372000, C375S342000, C341S071000

Reexamination Certificate

active

06757342

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a technique for demodulating data signals recorded on, for example, magnetic cards, and more specifically relates to a highly reliable data demodulation technique for binary data recorded by a pulse code modulating method in which data is recorded by a combination of two types of pulses, F and 2F.
General recording and reproducing devices such as magnetic card readers use a modulation method by which binary data signal are identified by one of two types of pulses, F and 2F. Binary data recorded by this modulation technique is reproduced in the following manner.
A magnetic head and a magnetic stripe on a magnetic card are moved relative to one another to reproduce the magnetically recorded data in the form of an analog signal. Based on the signal waveform of the analog signal, the binary data is determined.
FIG. 10
illustrates a general functional block diagram of a conventional data demodulation of this kind and
FIG. 11
shows a signal waveform for each block. The recorded signal reproduction at
FIG. 11
a
illustrates the pulse code modulation which under the Japanese Industrial Standard (JIS) is labeled: F and 2F frequency modulation.
In
FIG. 10
, an output signal of the magnetic head
11
, which is obtained when a magnetic card
10
moves relative to the magnetic head, is amplified by two amplifiers
12
and
15
. An output signal of the amplifier
12
is supplied to a peak detecting circuit
13
for peak detection, and a peak detection signal of the peak detecting circuit
13
is compared to zero level by a comparator
14
to detect zero crossing points thereof. An output signal of the other amplifier
15
is compared to zero level by a comparator
16
to detect zero crossing points thereof, and its output is input to a timing generation circuit
17
. The timing generation circuit
17
changes the output level of the comparator
16
according to the level of the output signal of the comparator
16
which is observed at changing positions of the output signal of the comparator
14
. The output signal of the timing generation circuit
17
is input to a data discriminating circuit or CPU
18
for a predetermined signal process to identify the character.
A magnetic stripe of a general magnetic card has not only a significant data region in which a recorded data is substantially stored, but also a sync bit region that comes before the significant data region, an STX code region that indicates the beginning of the recorded data, an ETX code region that comes after the significant data region and indicates the end of the data, LRC code region, and a sync bit region.
The operation of the functional block diagram illustrated in
FIG. 10
will be described more specifically referring to
FIG. 11
as well. FIG.
11
(
a
) illustrates an example of a signal recorded on the magnetic card
10
. The recorded signal is a binary data signal composed of a combination of two kinds of frequencies, F and 2F, and expresses the bit by “0” or “1” according to the existence of inversion of signal polarity within a time interval (distance) T equal to one bit. The example of FIG.
11
(
a
) expresses “01101”. FIG.
11
(
b
) shows an example of the recorded signal of FIG.
11
(
a
) which is read by the magnetic head
11
and amplified by the amplifiers
12
and
15
. The output frequency of the amplifier
12
and
15
which corresponds to the recorded signal “1” is twice as long as that which corresponds to the recorded signal “0”.
The peak detecting circuit
13
is composed of a differentiating circuit. Therefore, the peak detection output provides a signal waveform in which the zero crossing points appear at the peak positions of the output signal of the amplifier
12
, as illustrated in FIG.
11
(
c
). This signal is compared to zero level by the comparator
14
and converted to a digital signal which inverts at the zero crossing positions in the peak detection waveform as illustrated in FIG.
11
(
d
). The output waveform of the amplifier
15
is compared to zero level by the comparator
16
and converted to a digital signal which inverts at the zero crossing positions thereof, as illustrated in FIG.
11
(
e
). The timing generation circuit
17
outputs the signal as illustrated in FIG.
11
(
f
). In other words, the timing generation circuit
17
changes the output level of the comparator
16
according to the level of the output signal of the comparator
16
which is observed the comparator
16
at changing positions of the output signal of the comparator
14
. The signal illustrated in FIG.
11
(
f
) is the same digital signal expressing “01101” as the signal of FIG.
11
(
a
). Thus, it is understood that the data signal recorded on the magnetic card is demodulated.
Problems To Be Solved
The above mentioned performance of reading data recorded on magnetic cards is affected by the condition of card, contamination and wear of the magnetic head, electric noise or mechanical noise from a motor, etc. In other words, a recording medium such as magnetic cards receives various stresses over repetitive use; as a result, the contamination or scratches on the recording medium may cause signals that originally did not exist. Also, basic information once written on the recording medium will not be overwritten even with repetitive use; over the time that the recording medium makes repeated contacts with the magnetic head, the magnetic force decreases, and therefore signal intensity necessary for reproduction becomes insufficient, degrading accuracy of data reading. Further, the resolution power of the magnetic head is decreased due to wear on the magnetic head, causing peak shift.
If error occurs in reading data as above, the performance of reading data recorded on the medium may be degraded, affecting correct data identification.
FIG. 4
illustrates an example of a false reading caused by peak shift. In
FIG. 4
, the correct binary data constituting one character is “1011101” where the second bit within the character time interval is originally “0”. However, the length of the second bit within the character time interval, which is currently under decoding, is narrower than the original distance between peaks due to peak shift. Consequently, when this second bit is demodulated by a conventional method illustrated in FIG.
10
and
FIG. 11
, it is falsely decoded as “1” and accordingly the bit line is falsely determined as “1111111”. In addition, the boundaries between the bits after the third bit are shifted due to peak shift, affecting the successive character interval (distance) and causing a false reading therein.
FIG. 14
illustrates another example of the waveform that contains peaks which originally do not appear or do not appear at expected positions. In this waveform, only one peak should appear between the second bit and third bit; although the original is “10011”, three peaks are generated between the two bits for some reasons. If this waveform is demodulated by a conventional method shown in FIG.
10
and
FIG. 11
, the bit line will be falsely read as “11111”.
In the aforementioned FM modulating method, as illustrated in
FIG. 3
, a constant reference time &agr;T (where 0≦&agr;≦1) is set with respect to a time interval (distance) T for one bit, and “0” or “1” is allocated to the bit by observing polarity inversion in the read signal within the reference time&agr;T. In other words, if there is no polarity inversion within the reference time&agr;T, the bit is defined as “0” by the frequency F; if there is a polarity inversion within the reference time&agr;T, the bit is defined as “1” by the frequency 2F. With this, false reading caused by peak shift can be prevented to some extent.
However, like the example of
FIG. 3
, even if the reference time&agr;T is set and the bit is identified by observing polarity inversion of the read signal within the reference time&agr;T, the aforementioned factors may cause a false reading; even when a false reading occurs in only one bit in the bit line, the false reading affects the successive bi

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