The simplest magnetic stripe reader

Setup

Using the software

• -t <track> : Type of track to read, valid values are 1 to 4, default is 2. This option selects several parameters appropriated for each track, some of which may be changed through other options. Values 1 to 3 correspond to the three standard tracks and value 4 is experimental, to be used with non standard tracks. Using this track 4 the program does not look for start and end sentinel and it does not check parity and LRC. It also decodes the bits in a slightly different manner than the standard: 0s are the same, but 1s are only one short flux reversal, and not two short consecutive reversals like in the standard. I found this non standard coding of bits in at least one card (photocopier card).
  
  
• -c <clk_bits> : Number of leading clocking bits, default value depends on the selected track. This is the number of initial bits that are forced to be 0s (or 1s if -i option is given, see below) and their mean duration is used as reference to decode the rest of the bits.
  
  
• -p <peak_points> : Minimum number of consecutive data points which a data peak should have, default value depends on the selected track. This is to avoid noise peaks, which are usually small, however a value too large may have an undesirable effect because in the presence of abundant noise good data peaks may not be found.
  
  
• -s <stop_points> : Minimum number of consecutive data points under noise threshold (see below) before program stops taking data and starts decoding bits. Default value is 500, which is around 10 milliseconds. Some non standard cards have tracks with several records separated by a pause without data, you may have to change this parameter if you find one of these cards with a big pause.
  
  
• -l <threshold> : Minimum level of signal to be considered data, below this value we consider it noise. Default is to use about the first 100 milliseconds of data (in which we expect no swipe yet) as a measure of the noise level. With noisy sound cards it may be better to set threshold by hand using this option. Don't use this option the first time you run my program and pay attention to the value labeled 'noise mean' on screen (see example below), if it is not zero it means there is a DC current and it might be enough to erase your card while trying to read it. So better check this before reading any card and if you find you have a DC current then add a capacitor in series with the magnetic head to avoid it (see above). Note that you might have a DC current even in case 'noise mean' is zero if the sound card amplifier is capacitor coupled, so better check anyway using a voltmeter.
  
  
• -m <s|p|z> : Method to recognize bits, i.e. their start and end so that their duration, and hence their nature, can be established. I developed first the slope (s) method, then I moved to the newer peak (p) algorithm which I find more robust and left the older method for historical reasons. There is a third method, the zero crossing (z) algorithm, which I had to implement to decode cards recorded with my mobile. I don't know why different amplifiers see magnetic stripes in a different way. If you have problems reading cards with this program, first try to determine which method works better with your sound card. Peak method (p) is the default.
  
  
• -d : Print raw data to stdout (to use the card as a digital oscilloscope) which can be used with data analyzing tools to view what's going on, this helps to find the coding scheme of non standard tracks. If you use this option you will have to redirect output in order to save data.
  
  
• -b : Print decoded bits to stdout. Using this option you can count the number of leading clocking bits. Next time you read that card you can use this number for the clocking bits parameter, but it only makes sense if it is lower than the default number or much higher. Setting the right number of clocking bits improves the success rate in card decoding.
  
  
• -n : Don't stop decoding bytes after LRC. The program assumes that LRC is the last byte of the data stream, but some cards like boarding passes have two or three records separated by clocking bits.
  
  
• -i : Inverse clocking bits, they will be 1s instead of 0s. I found this non standard behavior in at least one type of cards, Madrid Underground tickets.
  
  
• -x : Strict detection, by default data peaks are not required to be above noise threshold, this level is only used to find out when data stream begins and ends. This is done to avoid some clipping effects which sometimes happen hiding true data peaks below threshold, specially if the latter is high. However in the presence of abundant noise it may be convenient to use this option to decrease the probability of taking noise peaks as data peaks. In these conditions the use of the -p parameter may also help (see above).

Some results

Bad or no clocking bits parameter, using default: 60
Bad or no peak duration parameter, using default: 4
Bad or no stop duration parameter, using default: 500
No method parameter, using default: peak
Reading track 1
Bad or no threshold level parameter, using data...
Noise max./mean/rms amplitude: 0.003/0.000/0.001
Threshold: 0.012
Swipe time: 0.267 s
Data max./mean/rms amplitude: 0.628/-0.000/0.210
Bits: 490, individual duration: 0.545 ms
0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000101000111101100000001000000100000011100111000011111000100101010110101100000010001101100010101101000000001000000110110110000001010010110001011001100000000101001010000100101001000000010000001000000100000010000001000000100000010000001000000100000010000001000000100000010000001000000100000011111100110000100000000000000000000000000000000000000000000000000000000000000000000000000000000
Start sentinel found
%W   SPCJK 816  M 219 20E                ?
End sentinel found
LRC OK.

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