Audio in Video Evidence

Audio EvidenceThough there are many ways to detect an edit within video evidence, sometimes a critical ear can be just as important to a video forensic expert. Though they say “A picture is worth a thousand words,” an audio file can be worth just the same. You just need to know what to look (or listen) for.

Many CCTV systems now have the capability to capture audio, and this audio could be crucial to the legitimacy of the evidence. Audio is one of the easiest ways to detect an edit when watching a piece of footage, but to do so there’s a bit you need to learn about audio, in itself.

Audio is comprised of “sound pressure waves,” which are waveforms representative of the change in air pressure in a recording. One characteristic of sound pressure waves is that they are always smooth and continuous, no matter what.

Let’s say, for example, you’re recording in an open, quiet room. While you’re recording, a rebellious teenager comes in the room and blows off his air-horn. Even though that loud sound completely changed the overall sound in the room, the wave that represents the pressure change will always be smooth and continuous.

The only time that a wave is not smooth and continuous is when an edit is made. Keeping this in mind will give you more of an idea of what you’re listening for.

When an edit is made to a recording, this disturbs the waveform. This makes it temporarily rigid and inconsistent. Now, remember; all sound pressure waves should be the opposite of that. So, the second that you hear a sound that is outside of that smooth, uninterrupted audio file, you know you have an edit.

How is that disturbance represented? Well, it will usually manifest itself in the form of a ‘pop’. In the context of video, it usually will only last a frame, but it will be there. If you hear anything out of the already established waveform, you know that the evidence has been edited.

Adobe (as always in the world of forensics) has other software that can allow you to more accurately detect these edits. For example, Adobe’s “Audition” uses what’s known as a “spectrogram,” that detects something known as the “noise floor”. As with audio waves, your noise floor will always be level and uninterrupted. In regards to a spectrogram, the spectrum recorded for a noise floor will always be consistent in visual characteristics. When you see a disturbance in that consistency, just as if you hear one in the audio file, you can tell that the audio, and sub-sequentially the video, has been altered.

Obviously there are many ways to detect edits visually. However, being a forensic expert means heightening your senses to a critical level, and though sight is the most prominent sense for a video forensic expert, having a critical sense of audio is just as important. The two work hand in hand to present the evidence, so having some knowledge of both will only excel your skills as an expert.

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