Storing information through magnetic patterns was first demonstrated to record audio. Since that time, this idea continues to be applied for different items like floppy disks, audio/video tapes, hard disks, and magnetic stripe cards. This short article targets Magnetic stripe cards used extensively for financial transactions and access control across the world.
Reading magnetic stripe cards requires significant analog circuitry besides digital logic to decode data. Recording of information in the magnetic cards is digital which is completed by magnetizing particles along the size of the stripe. Reading the magnetic card successfully can be a challenge due to the fact how the amplitude of sensor signal varies together with the speed where card is swiped, the quality of the credit card, as well as the sensitivity of magnetic read head. Moreover, frequency also varies together with the swipe speed. This calls for card dispenser to adapt to the changes and process the sensor signal without distortion. This article explains mechanisms for handling variations in the sensor signal.
As a way to be aware of the outcomes of card swipe speed, the quality of the credit card, and sensitivity in the sensor, you should understand how facts are stored with a card and also the actual way it is sensed by the read head. In magnetic-based storage systems, information and facts are represented by pole patterns over a magnetizing material like iron oxide. Figure 1 shows a magnetic stripe coated with magnetizing material. The particles in a magnetizing material may have some specific alignment or may be in random directions if this has not been previously put through a magnetic field with a particular orientation. However, when subjected to an external magnetic field, particles around the stripe are aligned using the external applied field.
In practical systems, a magnetic write head is used which can be nothing but a coil wound around a core. The magnetic field orientation can be easily programmed by manipulating the current direction in the coil. This assists to produce north-south pole patterns on the card. The narrower air gaps between your poles, the higher the density of web data, that may be programmed around the card.
In F2F encoding, if your pole transition happens in the middle the bit period, it really is logic 1 else it is logic . As an example, as shown in Figure 3, in the event the bit period is ? and in case a transition happens at ?/2, then its logic 1, else it is logic . Realize that the length occupied by logic 1 and logic about the card is same. However, the bit period ? varies with the swipe speed and this should be accounted for when reading the credit card.
Now the reading process is precisely reverse. It takes a read head which is comparable to the passport reader arrangement shown in Figure 2. Note that you will have one sensor for each and every track. As soon as the card is swiped, the magnetic field through the stripe induces voltage within the read head coil. Figure 5 shows the waveform from the read head.
The signal peaks at each flux transition. This is due to the top density of magnetic flux with the pole edges. As you have seen, facts are represented with the location of signal peaks. A peak detector circuit can decode this signal or a hysteresis comparator with all the thresholds kept not far from the signal peak. However, additional processing is needed before we can give this signal for the detector circuit for the following reasons:
Swipe speed: Swipe speed is specified in inches/sec (IPS). Generally, a magnetic card reader is required to function properly from the swipe speed range of 5 IPS to 50 IPS. The amplitude in the sensor signal varies with the swipe speed: a rise in swipe speed contributes to an elevated rate of change of flux cut from the coil within the 89dexlpky head, ultimately causing increased amplitude of your signal. In contrast, if the swipe speed is slow, the signal amplitude is less which could cause difficulty in reading the information.
Expertise of the card: With time and in line with the usage, card quality degrades with decreased magnetic field strength and distortion on account of dust and scratches about the card. Together, these lower the amplitude of your sensor signal.
Because of each one of these parameters, magnetic stripe card reader could be between several 100s of uV to 10s of mV. This range may be compensated using an amplifier. However, it cannot be a set gain amplifier. When the swipe speed is high as well as the card quality is great, the amplifier output can saturate towards the rails. And when the signal saturates, information, which is 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 calls for a configurable gain amplifier in order that we are able to tune the gain about the fly. To do this, the system must have the ability to sense once the signal is weak.