Storing information through magnetic patterns was basically demonstrated to record audio. Ever since then, this idea has been applied for different goods like floppy disks, audio/video tapes, hard disks, and magnetic stripe cards. This informative article targets Magnetic stripe cards used extensively for financial transactions and access control across the globe.
Reading magnetic stripe cards requires significant analog circuitry besides digital logic to decode data. Recording of data in the magnetic cards is digital and it is completed by magnetizing particles along the size of the stripe. Reading the magnetic card successfully is actually a challenge because of the fact how the amplitude of sensor signal varies with all the speed in which card is swiped, the standard of the credit card, and the sensitivity of magnetic read head. Moreover, frequency also varies together with the swipe speed. This requires magnetic card reader to adapt to such changes and process the sensor signal without distortion. This short article explains mechanisms for handling variations in the sensor signal.
As a way to know the effects of card swipe speed, the grade of the card, and sensitivity in the sensor, it is important to learn how information is stored with a card along with the way it is sensed with the read head. In magnetic-based storage systems, details are represented by pole patterns with a magnetizing material like iron oxide. Figure 1 shows a magnetic stripe coated with magnetizing material. The particles in the magnetizing material might have some specific alignment or may be in random directions if it has not been previously subjected to a magnetic field by using a particular orientation. However, when exposed to an external magnetic field, particles about the stripe are aligned with all the external applied field.
In practical systems, a magnetic write head is utilized which is nothing but a coil wound around a core. The magnetic field orientation can be simply programmed by controlling the current direction in the coil. It will help to create north-south pole patterns on the card. The narrower the air gaps between your poles, the greater the density of data, which can be programmed on the card.
In F2F encoding, when a pole transition happens in between the bit period, it can be logic 1 else it really is logic . By way of example, as shown in Figure 3, if the bit period is ? and if a transition transpires at ?/2, then it is logic 1, else it really is logic . Realize that the length occupied by logic 1 and logic on the card is same. However, the bit period ? varies with all the swipe speed and also this needs to be made up when reading the credit card.
Now the reading process is precisely reverse. It will require a read head which is similar to the passport scanner arrangement shown in Figure 2. Remember that you will find one sensor for every track. Once the card is swiped, the magnetic field through the stripe induces voltage inside the read head coil. Figure 5 shows the waveform taken from the read head.
The signal peaks at each flux transition. This is due to the high density of magnetic flux at the pole edges. As you have seen, information is represented through the location of signal peaks. A peak detector circuit can decode this signal or even a hysteresis comparator using the thresholds kept not far from the signal peak. However, additional processing is needed before we could give this signal to the detector circuit for your following reasons:
Swipe speed: Swipe speed is specified in inches/sec (IPS). Generally, a magnetic card reader must function properly inside the swipe speed array of 5 IPS to 50 IPS. The amplitude in the sensor signal varies using the swipe speed: a rise in swipe speed brings about an increased rate of change of flux cut from the coil inside the 89dexlpky head, resulting in increased amplitude of the signal. In contrast, as soon as the swipe speed is slow, the signal amplitude is less which could cause difficulty in reading the information.
Excellence of the card: With time and in accordance with the usage, card quality degrades with decreased magnetic field strength and distortion because of dust and scratches on the card. Together, these lower the amplitude of the sensor signal.
As a result of all these parameters, TTL magnetic card reader might be anywhere between several 100s of uV to 10s of mV. This range might be compensated having an amplifier. However, it should not be a set gain amplifier. Once the swipe speed is high and also the card quality is good, the amplifier output can saturate on the rails. And when the signal saturates, information, the time distinction between two successive peaks, is lost. Thus, it is essential to faithfully amplify the sensor signal without saturating or altering the wave shape. This requires a configurable gain amplifier to ensure that we could tune the gain around the fly. To get this done, the system must be able to sense if the signal is weak.