A passport reader requires reliable and accurate performance across many different different environments and card swipe speeds; irrespective of the health in the card. There are three essential elements of a magnetic card that must be implemented to make sure this performance:
(1) automatic gain control (AGC) to automatically adjust the amplitude of your input waveform to maximize dynamic range;
(2) accurate peak detection and raw data decoding; and
(3)preventing noise from the system from causing erroneous readings.
By using a PGA plus an ADC, the input waveform could be measured and scaled to increase the dynamic variety of the system;which will allow a wide range of input waveforms to be detected. This information will also show what techniques can be used to accurately detect peaks within the input waveform to learn the primary information in the magnetic card.
The initial essential component of passport scanner is definitely the automatic gain control (AGC), which automatically adjusts the amplitude from the input waveform to improve the dynamic array of the device. The amplitude from the waveform is extremely determined by the credit card swipe speed. Faster swipe speeds produce waveforms with peaks of greater amplitude, and slower swipe speeds produce waveforms with peaks of smaller amplitude. The voltage created by the magnetic read head is small, but may vary by greater than 25 dB across all swipe speeds. A set gain could be used to bring this voltage to some usable level, but to ensure the signal is in the optimum level in any way swipe speeds, AGC can be a necessity. During the given swipe, an individual will inadvertently change their swipe speed a few times. As such, the gain of the circuit should be adjusted through the swipe to be sure any alterations in signal amplitude are taken into account.
There are 2 essential components required to implement AGC: an ADC as well as a PGA. As a way to determine what gain needs to be placed on the PGA at any time, we must be aware of current amplitude of our input waveform. The ADC could be used to monitor the input signal level and adjust the PGA if needed. In case the input signal passes below a set minimum threshold, the gain is increased. When the input signal passes above a set maximum threshold and approaches saturation, the gain is decreased.
Ever since the peaks of a magnetic card signal are incredibly pronounced, it can be hard on an ADC to sample the input signal in a sufficient rate to guarantee the amplitude of the peaks from the waveform are accurately measured. To help lessen the load in the ADC, a peak and hold circuit could be used to contain the amplitude for each peak. The 17dexbpky time where the amplitude is sampled is irrelevant, provided that the sampling and updating from the PGA occur regularly.
As a way to decode the information contained in the waveform, the peaks of the input waveform should be detected. You can do this in a multitude of ways, each way having positives and negatives. Constructing an elementary peak detection circuit is relatively easy, but making a peak detector for magnetic stripe card reader can be tough for several reasons:
1.The velocity from the incoming peaks can vary anywhere from a few hundred bits per second to over 10 kb/s, based on the swipe speed, card and card channel.
2.The amplitude of the peaks can vary greatly. This could be partially remedied with the aid of AGC, yet still needs to be considered for precise peak detection.
3.The peaks in the magnetic card waveform are pronounced, but the regions between each peak can be quite flat – which could cause noise issues in comparator or differentiator based designs.