Storing information through magnetic patterns was demonstrated to record audio. Since then, this idea has become applied for different goods like floppy disks, audio/video tapes, hard disks, and magnetic stripe cards. This post targets Magnetic stripe cards used extensively for financial transactions and access control throughout the world.
Reading magnetic stripe cards requires significant analog circuitry besides digital logic to decode data. Recording of information around the magnetic cards is digital and is also done by magnetizing particles along the size of the stripe. Reading the magnetic card successfully can be a challenge mainly because that this amplitude of sensor signal varies using the speed in which card is swiped, the grade of the credit card, and also the sensitivity of magnetic read head. Moreover, frequency also varies together with the swipe speed. This involves passport reader to evolve to such changes and process the sensor signal without distortion. This article explains mechanisms for handling variations within the sensor signal.
In order to be aware of the outcomes of card swipe speed, the standard of the credit card, and sensitivity from the sensor, it is important to know how information is stored with a card and also the way is sensed from the read head. In magnetic-based storage systems, information is represented by pole patterns on the magnetizing material like iron oxide. Figure 1 shows a magnetic stripe coated with magnetizing material. The particles inside a magnetizing material probably have some specific alignment or might be in random directions if it is not previously put through a magnetic field by using a particular orientation. However, when subjected to another magnetic field, particles around the stripe are aligned with the external applied field.
In practical systems, a magnetic write head is commonly used which is outright a coil wound around a core. The magnetic field orientation can be easily programmed by manipulating the current direction from the coil. This assists to create north-south pole patterns around the card. The narrower air gaps in between the poles, the higher the density of information, that may be programmed around the card.
In F2F encoding, in case a pole transition occurs in the middle the bit period, it is actually logic 1 else it can be logic . For example, as shown in Figure 3, when the bit period is ? and if a transition happens at ?/2, then its logic 1, else it can be logic . Notice that the length occupied by logic 1 and logic on the card is same. However, the bit period ? varies with the swipe speed which should be made up when reading the credit card.
Now the reading process is just reverse. It requires a read head which is just like the passport scanner arrangement shown in Figure 2. Be aware that you will see one sensor for every track. Once the card is swiped, the magnetic field from the stripe induces voltage from the read head coil. Figure 5 shows the waveform from the read head.
The signal peaks at each and every flux transition. This is because of the high density of magnetic flux in the pole edges. As you can see, facts are represented from the location of signal peaks. A peak detector circuit can decode this signal or a hysteresis comparator with the thresholds kept not far from the signal peak. However, additional processing is necessary before we can easily give this signal towards the detector circuit for that following reasons:
Swipe speed: Swipe speed is specified in inches/sec (IPS). Generally, a magnetic card reader is required to function properly within the swipe speed range of 5 IPS to 50 IPS. The amplitude from the sensor signal varies using the swipe speed: a rise in swipe speed brings about a heightened rate of change of flux cut with the coil inside the 89dexlpky head, leading to increased amplitude of the signal. As opposed, once the swipe speed is slow, the signal amplitude is lower which could cause difficulty in reading the data.
Quality of the card: Over time and depending on the usage, card quality degrades with decreased magnetic field strength and distortion as a result of dust and scratches on the card. Together, these decrease the amplitude of your sensor signal.
Due to each one of these parameters, magnetic stripe card reader could be between several 100s of uV to 10s of mV. This range could be compensated employing an amplifier. However, it should not be a fixed gain amplifier. When the swipe speed is high as well as the card quality is useful, the amplifier output can saturate for the rails. And when the signal saturates, information, which is the time distinction between two successive peaks, is lost. Thus, it is very important faithfully amplify the sensor signal without saturating or altering the wave shape. This requires a configurable gain amplifier to ensure we can tune the gain about the fly. To do this, the device must be capable of sense once the signal is weak.