A passport reader requires reliable and accurate performance across many different different environments and card swipe speeds; regardless of the wellbeing of the card. There are three essential aspects of a magnetic card that must be implemented to guarantee 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 inside the system from causing erroneous readings.
By using a PGA as well as an ADC, the input waveform might be measured and scaled to maximize the dynamic variety of the program;that can allow an array of input waveforms to become detected. This short article will also show what techniques can be used to accurately detect peaks in the input waveform to learn the essential information through the magnetic card.
The very first essential aspect of card dispenser is the automatic gain control (AGC), which automatically adjusts the amplitude of your input waveform to improve the dynamic array of the machine. The amplitude of your waveform is tremendously dependent on the 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 produced by the magnetic read head is small, but will vary by over 25 dB across all swipe speeds. A fixed gain may be used to bring this voltage to your usable level, but to guarantee the signal reaches the optimum level by any means swipe speeds, AGC is a necessity. During the given swipe, a person will inadvertently change their swipe speed repeatedly. As a result, the gain in the circuit needs to be adjusted through the entire swipe to make sure any alterations in signal amplitude are accounted for.
There are two essential components needed to implement AGC: an ADC along with a PGA. As a way to understand what gain should be used on the PGA at any given time, we must know the current amplitude of our own input waveform. The ADC can be used to monitor the input signal level and adjust the PGA as required. When 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.
Since the peaks of a magnetic card signal are really pronounced, it can be difficult on an ADC to sample the input signal at the high enough rate to guarantee the amplitude in the peaks inside the waveform are accurately measured. To help reduce the load about the ADC, a peak and hold circuit could be used to support the amplitude of each peak. The 17dexbpky time from which the amplitude is sampled is not important, so long as the sampling and updating in the PGA occur regularly.
To be able to decode the info contained in the waveform, the peaks in the input waveform has to be detected. This can be achieved in numerous ways, each way having positives and negatives. Constructing a fundamental peak detection circuit is relatively easy, but creating a peak detector for TTL magnetic card reader can be tough for several reasons:
1.The rate of the incoming peaks can vary anywhere from a couple of hundred bits per second to over 10 kb/s, dependant upon the swipe speed, card and card channel.
2.The amplitude of your peaks can differ greatly. This is often partially remedied if you use AGC, but still needs to be considered for precise peak detection.
3.The peaks of the magnetic card waveform are pronounced, although the regions between each peak can be extremely flat – which may cause noise issues in comparator or differentiator based designs.