Analog devices, such as tape recorders, not only capture the intended signal, but also characteristics of the recorder itself. Tape recorders leave signatures from their record and erase heads which then can be used to identify the specific recorder used.
It can also be seen if another recorder has been used to change the original contents and then it is possible to come to the conclusion that the recording has been manipulated. Nowadays, the recordings are increasingly made by digital devices and these do not leave physical traces in the same way as their analog counterparts. Therefore a new approach is needed to determine the authenticity of digital recordings.
In forensic investigations, verification of digital recordings is an important aspect. There are numerous methods to verify authentication of recordings, but it is difficult to determine when the media was recorded. By studying the electrical network frequency, one can find a unique signature and then match the recording to this signature. By matching a recorded signal to a database, which contains all necessary information, one can find the time when the recording was made.
Source: Linköping University
Author: Andersson, Fredrik