Video Authentication – This task description details some of the basics of videotape recording, the editing of tapes, and its detection by the forensic expert.
VCRs present instabilities during both the recording and playback of a videotape. These instabilities cause timing differences or time-based errors in the video signal. It is the mechanical instabilities of the recording and playback process of both camcorders and VCRs that cause the timing differences to produce anomalies in the horizontal, vertical, and color synchronization pulses. Instrumental analysis of these various instabilities produce “signatures” which help the forensic expert to identify whether the tape is an original or a copy, and also whether it has been edited or tampered with. In most cases the original recorder is also requested for testing purposes.
Audiotape Tampering – This service is utilized to distinguish original recordings from copies, to identify equipment used to make recordings, and to detect intended or unintended alterations such as physical or electronic edits, erasures and over-recordings.
Obtaining a Voice Exemplar –
Instructions for the proper acquisition of a verbatim voice sample.
Questionable CD Recordings – Many CD copies are corrupted in that they contain heavily compressed and missing audio data which is not suitable for tape authentication, tape enhancement, or voice identification examinations.
It is important that forensic specialists understand how CDs work, the problems surrounding the use of CD recordings, and how these problems impact the forensic examination of CD copies of tape recordings. The forensic examiner is always focused on attempting to determine if there were any suspicious record events (i.e. anomalies) during the recording or playback process. In addition to signal losses, there also can be dirty optics related to the laser playback mechanism. Mechanical problems associated with CD playback include: dirty optics, broken parts within the CD player, a need for electronic servo adjustments, bad connections, defective motors, weak or dead laser diodes, and also a missing optical pick-up shield.
Voice Identification/Elimination – Voice identification is used to identify or eliminate individual speakers based upon aural (auditory) and spectrographic (visual) comparisons of recorded material. Transcripts can be reviewed for clarification of disputed conversations.
Corrupted DVDs – Evidentiary DVDs often contain some type of corruption.
– Enhances speech intelligibility to clarify identification of “near” and “far” voices and analyze non-speech sounds important to criminal and civil cases.
Examples of such corruption include: visible artifacts such as color banding, pixelization, missing detail, and other suspicious visual events which are not consistent with a properly manufactured DVD. The forensic examiner is continually attempting to identify the fundamental causes and factors for poor DVD creation as well as what forensic artifacts are associated with flawed DVD production. In addition, he is concerned with out of sync audio and video components which may not accurately depict events that occurred. The forensic examiner must also be aware of the many digital recording systems in use as the files associated with these systems are many times proprietary and incompatible with common playback equipment and software.
Testing of Security and Recording Equipment
– Security VCRs have a number of additional features
specifically designed for the industrial marketplace or for
those institutions which require 24 hour a day video and/or
This task description will focus on the operational characteristics associated with both analog and digital CCTV time lapse VCRs and the forensic tools utilized in their testing and evaluation.