Audio forensics
[1][2][3][4] Audio forensic evidence may come from a criminal investigation by law enforcement or as part of an official inquiry into an accident, fraud, accusation of slander, or some other civil incident.
[2] Modern audio forensics makes extensive use of digital signal processing, with the former use of analog filters now being obsolete.
[3] Recent advances in audio forensics techniques include voice biometrics and electrical network frequency analysis.
[7] For the first time, the judge in the McKeever case was asked to determine the legal admissibility of the conversation recorded that involved the defendant.
In a second step, the properties of the retrievable traces are analysed to determine if they support or oppose the hypothesis that the recording has been modified.
[14] In case the stationary noise bandwidth occupies the same frequency range of the desired signal, a simple separation filter will not be helpful.
[4] Audio enhancement is realized with both time-domain, automatic gain control, and frequency-domain methods, frequency selective filters and spectral subtraction.
[15] Time-domain enhancement usually involves gain adjustments to normalize the amplitude envelope of the recorded audio signal.
Typically is used the automatic gain control technique, or gain compression/expansion technique, that tries to reach a constant sound level during the playback: portions of the recording referable only to noise are made quieter, low-amplitude signal passages are amplified, and loud passages are attenuated or left alone.
The role of the examiner is to adjust the threshold level so that the speech can pass through the gate while the noise signal, that occurs in the silence parts, is blocked.
These advanced systems help the examiner to remove particular types of noise and hiss present in the audio recording.
The principle behind this technique is to enhance the quality of a recording by selectively attenuating tonal components in the spectrum, such as power-related hum and buzz signals.
The estimate is usually obtained from an input signal frame that is known to contain only the background noise, such as a pause between sentences in a recorded conversation.
[15] For example in the case of a speech recording this means preparing a transcription of the audio content, identifying the talkers, interpreting the background sounds, and so on.
The judgment needs to be made at the time the case is heard, so the court needs to weigh the various pieces of evidence and assess whatever level of doubt there may be.
