A magnetic card reader requires reliable and accurate performance across various different environments and card swipe speeds; regardless of the physical condition of your card. You can find three essential elements of a magnetic card that really must be implemented in order to ensure this performance:
(1) automatic gain control (AGC) to automatically adjust the amplitude of the input waveform to improve dynamic range;
(2) accurate peak detection and raw data decoding; and
(3)preventing noise within the system from causing erroneous readings.
Using a PGA as well as an ADC, the input waveform could be measured and scaled to maximize the dynamic selection of the program;which will allow an array of input waveforms to be detected. This information will also show what techniques can be used to accurately detect peaks from the input waveform to see the fundamental information through the magnetic card.
The 1st essential aspect of passport reader is the automatic gain control (AGC), which automatically adjusts the amplitude from the input waveform to optimize the dynamic range of the program. The amplitude of your waveform is tremendously determined by 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 made by the magnetic read head is small, but could vary by more than 25 dB across all swipe speeds. A fixed gain enables you to bring this voltage to some usable level, but so that the signal is at the optimum level in any way swipe speeds, AGC is a necessity. In a given swipe, an individual will inadvertently change their swipe speed repeatedly. Consequently, the gain from the circuit needs to be adjusted through the swipe to be sure any variations in signal amplitude are taken into account.
There are two essential components necessary to implement AGC: an ADC as well as a PGA. To be able to understand what gain needs to be used on the PGA at any given time, we have to 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 needed. In the event the input signal passes below a set minimum threshold, the gain is increased. In the event the input signal passes above a set maximum threshold and approaches saturation, the gain is decreased.
Because the peaks of your magnetic card signal are extremely pronounced, it can be hard for the ADC to sample the input signal with a high enough rate to ensure the amplitude of the peaks in the waveform are accurately measured. To help lessen the stress about the ADC, a peak and hold circuit enables you to support the amplitude of each peak. The 17dexbpky time from which the amplitude is sampled is not important, provided that the sampling and updating of the PGA occur regularly.
So that you can decode the data contained within the waveform, the peaks of your input waveform needs to be detected. You can do this in a multitude of ways, each way having benefits and drawbacks. Constructing a basic peak detection circuit is pretty easy, but developing a peak detector for TTL magnetic card reader can be challenging for a lot of reasons:
1.The rate of your incoming peaks can differ from a few hundred bits per second to in excess of 10 kb/s, according to the swipe speed, card and card channel.
2.The amplitude of the peaks can vary greatly. This could be partially remedied if you use AGC, but nevertheless should 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 can cause noise issues in comparator or differentiator based designs.