In recent years, through telephone scams elderly liar more and more, almost to the extent of flooding. Liar usually fabricate a lie, then called the old man enter the bank cards in hands-free mode using landline phone, and after a moment later, fooled bank card money is transferred away. Why on the telephone by bank card password would be a liar to steal bank card password?
There is actually a scientific reason: liar by touch-tone telephone to know which key you pressed. Dial phone There are several ways, the most common form of dial signal is DTMF (ie Dual Tone Multi Frequency, dual-tone multi-frequency), which was originally developed by Bell Labs, you call dial button is pressed DTMF tone sent is voice signals, each digit is composed of two sine waves of different frequencies are added together to form, is Asin (f_H) + Bsin (f_L), which can be seen from the following table (Common phone is not A, B, C, D key). DTMF keypad frequency table as follows:
DTMF decoding is easy, cheats Here you enter the number when you press, each time you press a number, the phone will emit a key tone, liar put this voice can be recorded and analyzed by a voice analysis software, you can get two fundamental frequencies (in fact, the sound signal to do Fourier analysis), such as it was is 852Hz and 1477Hz these two frequencies, then from the above table that can be found, the number keys you press 9.
So, if there is a stranger to hear from you by telephone keys, you must be careful, the other party can know what key you press, in particular to enter the bank card number and password for this operation, make sure not to be bugged or recording. Of course, we go to the bank ATMs press time password, the keyboard will sound, but these sounds with different touch-tone telephone, there is no corresponding keyboard frequency table can be viewed, the next person can not know your password, this is the ATM keyboard than the telephone keypad to advanced some reasons. Further reading: Why use magnetic stripe bank card deposits and withdrawals may not be secure?
(Author: Zhang Xuan)
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