Melody behind Phone Numbers
Have you ever found that the dialing sound of each number is different when you make a call?
At the beginning of last month (September, 2012), a Chinese college student Liu decoded the mobile phone number of Mr. Zhou (the boss of China’s IT giant 360) through listening and analyzing the dialing sound from a video in which Mr. Zhou was called. Mr. Zhou subsequently confirmed that the truth of this issue and praised that student for his well-done. Moreover, Li Kaifu, a former CEO of Google China encouraged that student to join his current company.
How could a college student without professional knowledge of sound analysis and decode do this?
We can see from movies or cartoons that someone who has absolute pitch can identify the telephone number when hearing the dialing sounds. Some people even can make a phone call by singing. Amazing, isn’t it?
In fact, the telecommunication signaling uses the technology of Dual-tone multi-frequency signaling (DTMF). That means when we press a button on phone keyboard, the sound sent out has a high frequency and a low frequency. In other words, if you make a sound with a related high and low frequency, the related number can be called. Although it is hard to achieve singing dialing in our daily life, some kinds of software like CoolEdit has been invented to make a call and do not need to press the keypads.
The keypad of DTMF is laid out in a 4*4 matrix as the following table.
DTMF Keypad Frequencies (with sound clips)
|1209 HZ||1366 HZ||1477HZ||1633 HZ|
|Low Frequency||697 HZ||1||2||3||A|
According to this knowledge, Liu firstly collected the dialing sound from the video, then changed it to a PCM (pulse-code modulation) signed 24 bits mono wav. After that, he opened that wav in the audio analysis software such as Adobe Soundbooth to get a waveform. Finally changed the waveform to the sound spectrum, from which can easily identified the numbers.
Picture 1: Sound spectrum with the frequencies and numbers (From: Liu’s Blog)
(To protect the privacy, some parts of this picture are mosaics.)
The whole process is easy to learn and imitate, and the audio analysis software also can be down or bought from Internet. A college student of TV editing and directing major said he can decode the number from the sound in 5 minutes and every one can achieve it once know the principle and processes. Now, we have to worry about our privacy and information security considering the development of high-technology.
Please be careful when you make a call, because if someone records it, the phone number an be decoded by sound analysis.
However, do not worry too much about your passwords of credit cards, because the sound of each number on ATM or POS is the same, so this sound analysis technology cannot be used to decoded your passwords. But you still need to take care in case of other stealing methods.