Expert Article Library
The Forensic Examination Of Video Tape--Technical, Integrity And Legal Issues
by Steve Cain
By: Steve Cain E-mail: firstname.lastname@example.org
A recent publication of VideoMaker Magazine cites that approximately 30 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.(1)
As this ready availability of consumer camcorder equipment continues to increase, the likelihood exists that even larger numbers of VCRs will be purchased not only for playback of the video image but more recently to provide off-line (non-computer based) editing of original videotapes. The purpose of this article is to review 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 (on-line or computer based) editing to both the professional and consumer videographer.
Analog Recording vs. Digital Recording
Storage of information for both audio and video tape recording is accomplished using either analog magnetic tape or in the case of the computer, high density digital computer disks (or diskettes).(2) During playback, the recorder senses the different voltage changes, converts the electrical signals, and selectively filters the resulting electrical signals which actuate the electron guns or amplifiers of televisions or video monitors. The actual conversion of the electrical output signals occurs by passing the videotape in front of a rapidly rotating helically scanning magnetic head. 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.(3) 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. A digital recorder reproduces the audio and video signals through their binary code conversions. Due to its excellent reproducibility and the tapeless medium aspect of digital audio and video recordings, this technology will soon become the predominant form of recording.(4)
The most popular videotapes presently in use include VHS, VHS-C, and 8 mm although greater resolution and quality of film can be obtained in the Hi-8 and S-VHS varieties of film. The consumer market lately has been adopting "multi-media" or "desk-top" video work stations into the recording and playback of video information. The more popular work stations provide for a combination of both analog recording and digital editing at a fraction of the cost that was possible as little as five years ago. The computer can also be used as a "edit controller" which instructs which analog tape decks to play and record thus allowing a composite videotape to be produced which incorporates portions of different source recordings.
Time Code and Timing Errors
Numerous 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. Time code is an electrical signal (usually eight digits in length) which tracks individual frames for the purpose of editing. A troublesome feature of time code is that its appearance on the tape does not mean that it was originally generated by the original equipment. Time code can also be employed to "stripe" the tape which means basically over-recording or erasing the original time code and then add additional time codes through the use of an external time code generator. As digital editing becomes more widespread, time codes of all types will become more vulnerable to tampering since like all information on the videotape they can be reduced to easily manipulate binary code.
Essentially it is the mechanical instabilities of the recording and playback camcorders and VCRs that cause timing differences which 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. (5)
Analog (Linear) vs. Computer Based (Non-Linear) Editing
Editing normally involves: 1) real-time editing which is done simultaneously with the making of the video footage, and also commonly known as "in-camera" editing; 2) editing done subsequently to the production of the video tape which does not affect the source original tape, and 3) post-production editing which is done subsequent to the original shooting but which does affect the original source recording. Contemporaneous or real-time editing normally involves the videographers subjective decisions as to who, what, when, and where to record. This is achieved by interrupting the record events through stopping, pausing, restarting and rewinding or over-recording a portion of the original tape. Depending upon the type of camcorder used, this form of editing can be seen as a natural part of the recording process and not necessarily used to distort or falsify evidence recordings. Post-production editing can also be legitimately accomplished through a variety of analog and digital recorders, especially if this type of editing has been authorized by appropriate judicial authorities. It is the context in which the editing occurs which determines whether intentional falsification occurs. Falsification normally has two distinct meanings: 1) intentional alteration wherein the videographer intends to deceive the viewer is known as "falsification" or 2) "alteration" which is any subsequent change made to the original recording. The latter category may not be suspicious, however, if it involves real-time editing or the re-recording of unaltered copies, especially at the request of the attorneys.
The two major categories of alteration include: 1) that associated with splicing, erasure and 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 other words, the actual alteration that is observed occurs always in the re-recorded tape or "edited master", and not on the original source recording. Examples of non-source affected changes include deletion of prejudicial or objectionable material; rearrangement of the video content for demonstrative purposes, or edition of video material not present in the source recording.(6)
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 , ie. Flying erase head, special effects, time-code and other analog and digital camera options.
"Assemble Editing" basically involves laying down one scene after another. It requires that you locate the last frame if that event you wish to keep and hitting the pause button on the VCR at the edit point. With the camcorder in pause mode, you would then press the record button to toggle the record/pause mode thus allowing the camcorder to begin assembling respective edits.
A different type of editing is known as "insert" editing and 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.
Audio-dubbing is the opposite of video-dubbing in that it replaces the audio without touching the video. If your camcorder does not contain an "audio dub" feature for insert dubbing, you can attach an audio mixer between the audio connection of your source VCR and the recording camcorder. Lately many VCRs also include both audio and video dubbing features together with other editing options. If glitch-free edits are desired, it is necessary for the videographer to use flying erase heads, audio dubbing keys, video insert features and a jog/shuttle control knob to provide accurate methods of locating precise frame locations.(7)
Many videotape formats have at least two separate audio tracks which lend themselves to fairly easy manipulation during any tampering effort. Telltale signs of the editing process may include poor synchronization with a new narration seen to exist 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 speakers 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 $400 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 titler, audio mixer, and special effects generator. Currently a complete windows-based non-linear editing system costs approximately $2500 to $3000 which includes a 133 mhz CPU, 16 megabyte of RAM, and a 1.6 gb hard drive together with capture and display of video images.(8)
Using digital technology, the consumer videographer is now more capable of falsification and fabrication of both still and video motion images. The telltale 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.
Detection of Editing
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 aide 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 which allegedly was 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 (ie. drum jitter). If the camcorder is available for inspection, additional video analyzing devices allow for measurement of deficiencies in the cameras 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." Lastly, magnetic development of the original videotape can often visualize important audio and control track timing and misalignment information. For example if an 8mm original tape was subsequently subjected to the full-width erase head action of a VCR, the resultant erase head signature could be visualized on the video microscope and documented if authenticity questions subsequently arose.
Material alterations to video tape can either be done intentionally or accidentally. If accidental, the videographer should provide a detailed description of the circumstances surrounding his unintentional editing of the original tape. If his actions to not affect the reliability or trustworthiness of the evidence, the recording can still be admitted. Authorized judicial "editing" of the tape often involves the removal of irrelevant or prejudicial content materials; privileged and confidential conversations; enhancing the image for jury review. If the editing is accomplished intentionally or surreptitiously, then it is within the trial judges discretion to allow the evidence, although many courts are looking more and more at this type of evidence unfavorably.(9)
If questions still arise concerning the recordings authenticity, then discovery actions and preliminary interrogations would normally ensue by the attorneys and would include anything that bears on the relevance, authenticity, and possible incompetency of the video evidence including:
- When and how the videotape was produced
- Was the opposing party in control of its production
- How and when did the tape come into their possession
- The identity of the videographer and his credentials
- Identity of authentication witnesses
- Whether in-camera or post-production editing was accomplished and how
- Does opposing party plan to use it for substantive or illustrative purposes
- Attempt to obtain all relevant footage including "out takes".
- Request all recording equipment including VCRs, cameras, cabling materials, microphones, and any other materials used in the taping process.
NOTE: IF THE ORIGINAL RECORDING EQUIPMENT AND TAPE IS AVAILABLE FOR EXAMINATION, THE EXPERT SHOULD BE ABLE TO MAKE THE FOLLOWING DETERMINATIONS:
- If the recording is an original or copy.
- Which record buttons were depressed in the process of making the
- recording and where they are relative to the video images being presented.
- If part of the original recording was masked by over-recording.
- If the recorder presented is actually the one used to make the tape evaluated.
- Which of a limited number of recorders was used to make a video recording.
- 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 falsification exists.(10)
Attorney Jordan Gruber (a noted legal and technical authority on videotape evidence) has suggested the following Foundation Requirements for Videotape Evidence:
- Relevance to a material issue
- formal offer of proof by counsel if necessary
- Competency and qualifications of foundational witness
- Identification of subject matter
- Accuracy and authenticity of the representation
experts determination that video recording was not altered in any way, built-up or
chain of custody established
- --Video camera or
camcorder was checked and properly operating
- --Video recording is
same as what witness saw on playback immediately after recording
--No material alterations, surreptitious editing, or fabrications have taken place
--Under "pictoral communication" theory, testimony by a percipient witness, not necessarily the videographer, that the video recording is a "fair and accurate portrayal" of what it is purported to represent.
--Under "silent witness" theory, testimony, establishing authenticity, integrity, and competency of video recording.
The following elements may be necessary in some cases and are at least helpful in others:
- Continuous chain of custody
- Accuracy of processes used to duplicate video recording, and number of generations from original "camera master to actual video exhibit
- Presence of timecode or time/date window dub
- Appropriateness of, and disclosure of, all editing
- Appropriateness of titles or narration, if any
- Usefulness of video recording to trier of fact
- Presence of witness to overcome hearsay objection
- Qualifications, competency, and experience of videographer
- Relationship of videographer to offering party, if any
- Features, format, and type of video equipment used
- Physical circumstances and conditions at time video recording was made
- Matthew York, VideoMaker Magazine, (#3), published by Patricia York, New York, June 1996, page 5.
- David Huber Miles, Audio Production Techniques for Video, Focal Press, 1987, pages 12-13.
- D. MacCauly, The Way Things Work, Houghton-Mifflin, 1988, pages 260-261
- Bob Levine, VideoMaker Magazine, "Bits and Bytes," June 1996, pages 90-91
- Stuart Suretow, VideoMaker Magazine, "A TBC Tale," June 1996, pages 63-66
- Gordon S. Gruber, Electronic Evidence, Lawyers Cooperative Publishing, 1995, pages 551-552
- Jim Stinson, VideoMaker Magazine,"Simple Camcorder Editing," June 1995, pages 16-18
- Robert Nedph, VideoMaker Magazine, "Linear vs. Non-Linear Editing Systems," June 1996, page 62
- Ibid #6, page506
- Ibid #6, page 561
- Ibid #6, pages 924-925