NCJ Number
218450
Date Published
July 2006
Length
21 pages
Annotation
After reviewing recording principles for VHS video cassettes, this paper explains the theory related to recording over ("overrecording") previous recordings on VHS tapes, reports the procedures and results for test overrecordings, presents examples, and discusses forensic authenticity applications.
Abstract
The authors first explain the mechanical, electronic, and magnetic processes that occur in the course of recording images and sounds with a VHS cassette inserted into a standard VCR or camcorder. The features of VHS recorders that make possible such sound and image transfers to VHS tapes are also explained. Detailed descriptions are provided of VCR video and audio heads, as well as the control-track head and erase heads. The latter can erase all previously recorded information on the tape. The second major section of the paper discusses general overrecording theory. An overrecording occurs when new information is written over a previous VHS recording, erasing a segment of the existing video, audio, and control-track information while replacing it with new video, audio, and control-track data. This discussion is followed by a report on results from a series of test recordings prepared on four VCR's that ranged from consumer-level to professional-quality units. A review of the test recordings found two general classes of results related to the length of the overrecordings. The longer test overrecordings were of sufficient length to achieve complete erasure of the portion of the tape between the full-track erase head and the beginning of the helical-scan heads (the previous erased area). The short overrecording, however, did not completely erase that segment. The paper concludes with recommendations on procedures for analyzing VHS cassettes for suspected overrecordings, along with a list of observations that can have forensic significance. 4 tables, 4 figures, and 16 references