Audio and Video Forensic Filtering Overview

Audio and Video Forensic Filtering Overview WHAT IS FORENSIC FILTERING? According to dictionary.com, the definition of the word "forensics" is "the use of science and technology to investigate and establish facts in criminal or civil courts of law." A "filter", according to the same source, is defined as "any of various electric, electronic, acoustic, or optical devices used to reject signals, vibrations, or radiations of certain frequencies while allowing others to pass." Combining these two definitions conveys the general meaning of audio and video forensic filtering - using technical devices to reject noise (or conversely, to restore or enhance the audio or image) to assist a law enforcement agency's (LEA) criminal investigation and the court process.	. Admittedly, this definition broadly encompasses filtering at any stage of the investigative or court process - whether it is accomplished by a forensic examiner in a laboratory, a special agent or technical surveillance specialist in the field, a linguist at a workstation, or is simply a built-in feature of the collection, recording, or transcription equipment. Regardless of whether one considers filtering that happens at the time of collection to be "forensic" filtering or not, it is useful for the purposes of this discussion to include it as many of the issues are the same or similar. In fact, to properly address filtering one has to talk about "noise" and "interference" and they are introduced into the evidence, among other places, at the time of collection.

NOISE & INTERFERENCE The noise and interference that make it onto the evidence recording may occur in the acoustic or visual environment that the microphone or camera, respectively, are located in. Some common examples of acoustic noise and interference include: ••• resonance ••• echo ••• engines and other machinery ••• air conditioning and fan hum ••• live music ••• radio and TV ••• wind ••• other talkers ••• aircraft ••• vehicular traffic and road noise	In addition, the problems of noise and interference may enter via the surveillance, recording, and playback equipment, which might include anything from a body wire on a police informant or undercover officer, a telephone tap (or "intercept"), a night vision camera, a wireless transmitter, a closed circuit television (CCTV) camera, a 911 (or other emergency services) phone recorder, a memo recorder, security video time lapse recorder or multiplexer, thermal imager in a helicopter, shotgun microphone or parabolic dish, video tape recorder (VTR), or the like. All are potential entry points for noise and interference, whether self-generated (e.g. tape wear, multiplexing, and compression effects), conducted (e.g. AC mains hum), or inducted (e.g. cell phone interference).

USING FORENSIC FILTERING Once the evidence recording has been made, with all its accompanying problems, forensic filtering can be used in the laboratory to reduce or eliminate the noise and interference, as well as to clarify or enhance the audio and video information - the process otherwise known as restoration and enhancement. Of course, a technician or forensic examiner, sometimes in conjunction with other professionals such as linguists or phoneticians, may also perform other tasks, such as evaluation, repair (of the tape cassette or the tape itself), recording analysis and authentication, transcription, voice identification, duplication, still printing, and, eventually, testimony.	As mentioned previously, the types of noises and interferences that can be present in the evidence recording can be strong, subtle, and/or varied, the filtering techniques must be powerful, precise, and adaptive. Various devices (processors) for filtering are available, including consumer and professional grade varieties of hardware boxes and PC (personal computer) software packages or plug-ins. The professionals operating them must also have appropriate training in their function and use, which varies depending on the task to be performed and the complexity of the processor. The goal, of course, is to remove the noise and interference and then recover the needed information, while in the process not introducing any new problems (processing artifacts) which either cover up information, annoy the viewer/listener, or otherwise make the evidence seem unnatural (or tampered with).

FORENSIC FILTERING METHODS The types of audio filtering that may be employed in the restoration and enhancement may include, but are not limited to, the following: ••• bandlimiting ••• (e.g. highpass, lowpass, ••• bandpass, etc.) ••• deconvolution ••• (a.k.a. "one channel adaptive" ••• or "LMS" filtering) ••• equalization ••• (i.e. graphic, parametric, ••• spectral) ••• gain ••• mixing and beamforming ••• (e.g. lobing, summing, combining, ••• mic steering, array processing) ••• dynamic range control ••• (i.e. noise gate, compression, ••• expansion, limit) ••• combing	Similarly, for video (to include still images, by definition): ••• bandlimiting ••• (e.g. highpass, lowpass, ••• bandpass, etc.) ••• brightness ••• contrast ••• sharpening (vertical and horizontal) ••• frame averaging ••• image stabilization ••• median ••• security demultiplexing ••• deinterlacing ••• deblur ••• (motion and focus, ••• a.k.a. deconvolution) ••• equalization (histogram and ••• homomorphic)

LOOKING FORWARD It should be noted that for video processing, recent advances in PC computing power and storage capabilities now allow live filtering of not only audio, but also of full motion video. This has opened the door for many new and powerful techniques for restoring and enhancing video (hence, the references to frame averaging and image stabilization above). No longer must one select a single frame to process in isolation of all the other information on the surrounding frames. The increase in computing power has also made advanced spatial filtering techniques possible (i.e. beamforming) for suppressing noise sources and recovering remote audio.	In addition, improvements in algorithms and our understanding of human perception have also led to more powerful and effective filters, such as adaptive spectral equalization/inversion and (true) motion and focus deblur, and more embedded intelligence, thereby making them easier to employ. The field of forensic filtering continues to grow along with the advancements in technology, becoming ever more powerful, complex, and subtle.

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