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A 1996 publication of Video Maker Magazine
cites that approximately 50 million people have purchased consumer-type
camcorders and that this number is increasing exponentially and greatly exceeds
the number of professional cameras involved in the broadcast industries (York,
1996). As this availability of consumer analog and dig ital camcorder equipment
continues to increase, even larger numbers of VCRs will be purchased not only
for playback of the video image but more recently to provide both non-linear
(non-computer-based) and on line (computer based) editing of original
videotapes. These VCRs can also selectively record, edit, title and even provide
special digital effects, which are added onto the original tape during its
production. This article details some of the basics of videotape recording, the
editing of tape and its detection by the forensic expert, and the continuing and
increasing threat posed by both linear and non-linear editing to both
professional and consumer videographer and the attorney requiring forensic
videotape analysis.
TECHNICAL ISSUES
Analog Recording vs. Digital
Recording
Storage of information for both audio and
videotape recordings is accomplished using either analog or digital magnetic
tape or in the case of the computer, high-density digital computer disks (or
diskettes) (Miles, 1997). An analog video signal transmits light and color
information by means of both measuring and displaying voltage amplitude and
frequency differences of the signal presented (MacCauley, 1988). Ana log
camcorders are relatively inexpensive and cost between $400 and $1,000, and many
possess both in-camera special effects and editing capabilities.
The most popular videotapes presently in use
include VI-IS, S-VHS, VHS-C, 8 mm, Hi-8, and DV (digital video), although
greater resolution and quality of film can be obtained in the Hi-8, SVHS and DV
formats. The consumer market lately has been adopting "multi-media" or "desktop"
video work stations into the recording and playback of video information. The
more popular workstations provide for a combination of both analog recording and
digital editing at a fraction of the cost that was possible as little as two
years ago.
In digital recording, the combinations of
differing high and low voltages are stored as a series of zeros and ones of
binary code understandable to computers. Due to its excel lent reproducibility
and the tapeless medium aspect of digital audio and video recordings, this
technology will soon become the predominant form of recording (Levine, 1996).
The latest digital camcorders provide the highest resolution and lack of
generation loss during the "dubbing' process but can cost between $1,000 to
$3,500.
Mechanical and Electrical "Signatures" of
Editing
N u m e r o us
VCRs present instabilities during both the
recording and playback of the videotape, which often causes timing differences
or time- based errors in the video signal.
Essentially it is the mechanical instabilities
of the recording and play back camcorders and VCRs that cause timing differences
that produce anomalies in the horizontal, vertical and color synchronization
pulses. Slight variations in the synchronization pulses tend to cause vertical
jumping, horizontal wavering, and color and hue saturation differences in the
video signal. Added to these difficulties are tape stretching and tension
difficulties as the tape wraps around the respective video heads. Often these
"time- based' errors in synchronization and other signal anomalies permit the
forensic expert to help individualize the camcorder and/or VCR recorders
(Suretow, 1996).
DETECTION OF VIDEO TAPE
EDITING
Analog (Linear) vs. Computer Based
(Non-Linear) Editing
The two major categories of alteration
include: 1) that associated with re-recording where changes do occur to the
original source recording and; 2) when an altered copy or an enhanced copy
(either audio or video) is re-recorded with no physical change done to the
source recording.
In the simplest type of tape editing, one need
only use a camcorder, a VCR, and the appropriate connecting cable to affect the
edit. Most inexpensive editing is accomplished
through connecting the VCR to the camera which has most of
the editing functions, i.e. flying erase head, special effects, time-code and
other analog and digital camera options.
"Assemble" editing injects not only new video, audio, and
time code data, but also control track pulses are added.
A different type of editing is known as "insert" editing.
Unlike the aforementioned "assemble" editing does affect the audio portion of
the program. In this form of editing, part of previously shot footage is
replaced with new video, but basically leaves the audio portion intact. Both of
these types of simple editing can result in errors including timing, phase and
synchronization problems, which would produce glitches, smears, rolls or other
artifacts consistent with consumer-type editing.
Many videotape formats have at least two separate audio
tracks, which lend themselves to fairly easy manipulation during any tampering
effort. Tell-tale signs of the editing process may include poor synchronization
with a new narration existing with the video component and ambient noise levels.
The audio signal in videotape is generally much less complex and contains less
information than the video signal and is much easier to manipulate or fabricate.
Using audio dubbing technology, it is possible to completely rearrange words,
sounds and sentences, or to produce audio segments with unintended, opposite and
legally detrimental meanings. Audible signs that may reflect editing would
include significant changes in volume, content, or continuity with either the
main speaker's words or background sounds; sudden or strange sounds; and the
audio component not fully synchronized with the relevant video
picture.
Computer-Based (Non-Linear) Editing
Systems
For approximately $200 one can now purchase a video capture
card while the more expensive varieties provide for the reading of a variety of
time codes to include additional effects as a tilter, audio mixer, and special
effects generator (Nedph, 1996).
Using digital technology, the consumer
videographer is now more capable of falsification and fabrication of both still
and video motion images. The tell-tale glitches or other edit artifacts often
associated with cut and paste editing or assemble editing are rapidly being
overcome as the digital video work station becomes more powerful and accessible
to the community. Obviously this type of "seamless" editing poses an increasing
threat to tape admissibility, especially if authenticity questions arise out of
its manufacture.
Instrumental Analysis of Edited
Tapes
With the advent of more sophisticated analog
and digital editing devices, it is possible that detection of intentional edits
will become more difficult if not impossible to identify. To aid the forensic
expert in the detection process, a variety of instrumental tools are available.
These include a cross- pulse monitor, frequency generators, video signal
generators, oscilloscopes, waveform monitors/vectorscopes, alignment tapes, and
magnetic developing solutions.
The waveform analyzer/vectorscope can assist
in troubleshooting problems involving luminance, chrominance and audio stages of
the VCR that allegedly were used to record the original tape. For example,
certain waveform monitors can measure anomalies in the servo motors; detect
capstan speed errors; measure capstan jitter; drum speed errors; and calculate
inconsistencies in the drum speed movements (i.e. drum jitter). If the camcorder
is available for inspection, additional video analyzing devices allow for
measurement of deficiencies in the camera's video signals, (luminance and
chrominance) and also color aberrations. Additional tests include the
measurement of power adapter/power supply problems, video and chroma (noise)
artifacts, "burst frequency," and "frequency error problems."
Legal Issues
Material alterations to videotape can be done
either intentionally or accidentally. Authorized judicial "editing" of the tape
often involves the removal of irrelevant or prejudicial content materials,
privileged and confidential conversations; and enhancing the image for jury
review. If the editing is accomplished intentionally or surreptitiously, then it
is within the discretion the trial judge to allow the evidence, although many
courts are viewing this type of evidence more and more unfavorably (IBID
#6).
If the original recording equipment and tape
is available for examination, the expert should be able to make the following
determinations:
1. If the recording is an original or copy
2. Which record
buttons were depressed in the process of making the recording and where they are
relative to the
video images being presented.
3. If part of the original
recording was masked by over- recording.
4. If the recorder presented is
actually the one used to
make the tape evaluated.
5. Which of a limited
number of recorders was used to
make a video recording.
6. Other
case-specific questions that may be appropriate.
As the ability to manipulate video/audio
evidence becomes more widespread and effective, the historical admissibility
standards will be rendered inadequate. Likewise as "undetectable" video editing
becomes more prevalent, it is likely that future video evidence may be excluded
if the slightest hint of falsifications exists (IBID #6).
References
1. Gruber,
G. Eledronic Evidence, Lawyers Cooperative Publishing, 1995, pages
551-552.
2. IBID. No. 6, page 506.
3. IBID, No. 6, page 561.
4. IBID,
No. 6, page 924-925.
5. Levine, B. VideoMaker Magazine, "Bits and Bytes,"
June 1996, pages 90-91.
6. MacCauly, D. The Way Things Work,
Houghton-Mifflin, 1988, pages 260-261.
7. Miles, DH. Audio Production
Techniques for Video, Focal Press, 1987, pages 12-13.
8. Nedph, R. VideoMaker
Magazine, "Linear vs. Non-Linear Editing Systems," June 1996, page 62.
9.
Stinson, J. VideoMaker Magazine, "Simple Camcorder Editing," June 1995, pages
16-18.
10. Suretow, S. VideoMaker Magazine, "A TUC Tale," June 1996,
pages 63-66.
11. York, M. VideoMaker Magazine, (#3), published by Patricia
York, New York, June 1996, page 5.
