When performing an?audio and video authentication and analysis,a trained audio video forensic expert will utilize several methods in an attempt to detect an edit in a video recording that is used as evidence. Some of the time a critical ear can be just as important to a video forensic expert as the scientific community accepted software tools and an established chain of custody.
Though it is said that ?A picture is worth a thousand words,? an audio file can be worth even more in a video forensic laboratory. A trained video forensic expert knows what to look for (or listen to) during a forensic video authentication and analysis investigation.
Many CCTV systems now have the capability to record audio, and this audio portion of the surveillance video recording can be crucial to the legitimacy of the digital video evidence. Audio is a great tool to investigate and detect an anomaly or edit when investigating a video recording. To do so there?s a process and protocol we follow at Primeau Forensics.?
Audio is comprised of ?sound pressure waves,? which are waveform representative of the change in air pressure in a recording. One characteristic of sound pressure waves is that they are always smooth and continuous.
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. All sound pressure waves should be the opposite of that. So, when I am critically listening and hear a sound that is outside of that smooth, uninterrupted audio file, I know I have an anomaly that may be 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 the sound will be there. If you hear anything that deviates from the already established waveform, you know that the evidence may have been edited.
Adobe and Izatope RX has software that allows a forensic experts to more accurately detect these edits. For example, a ?spectrogram,? detects the ?noise floor? in a recording. The spectrum recorded for a noise floor should be consistent in visual characteristics as long as nothing changes with the ambiance in a recording. When you see a deviation in that consistency, just as if you hear one in the dialogue, you can tell that the audio, and sub-sequentially the video, may have been altered.
There are many ways to detect edits visually when reviewing digital video evidence. Beginning with an established chain of custody and performing forensic video authentication and analysis will reveal integrity in your video recording or anomalies that reveal scientifically that the CCTV video recording may have been compromised and is unreliable.