The mission of the SWGIT Forensic Video Subcommittee is to provide guidelines regarding video images and their capture, playback, processing, enhancement, storage, and output in a forensic environment.
The purpose of this document is to provide definitions, recommendations, and guidelines for the use of video capture and processing to ensure the successful introduction of video imagery as evidence in a court of law. This section includes definitions, equipment used, guidelines for standard operating procedures, and general procedures for video processing.
CD/DVD (compact disc/digital versatile disc): Optical disc formats designed to function as data storage media.
Camcorder: A self-contained video camera and recording device.
Capture card/frame grabber: A piece of computer hardware that converts an analog video signal to digital data.
Deinterlacing: Separating an interlaced frame into two separate fields.
Enhancement: Any process intended to improve the visual appearance of an image.
Field: An element of a video signal containing alternate horizontal lines. In NTSC-interlaced video, the scanning pattern is divided into two sets (odd and even) of spaced lines that are displayed sequentially. Each set of lines is called a field, and the interlaced set of the two sets of lines is a frame.
Forensic: Relating to or dealing with the application of scientific knowledge to legal problems, especially in relation to the detection of crime.
Forensic video analysis: The scientific examination, comparison, and/or evaluation of video in legal matters.
Frame: Lines of spatial information of a video signal. For interlaced video, a frame consists of two fields, one of odd lines and one of even lines, displayed in sequence. For progressive scan (noninterlaced) video, the frame contains samples starting from one instant and continuing through successive lines to the bottom of the frame.
Interlaced scan: Video image format. The video frame consists of two fields. The first field contains all the odd-numbered horizontal lines and the second field all the even-numbered lines.
Monitor: An electronic device used to display live or recorded images.
Multiplexer/demultiplexer (muxer/demuxer): A device used to combine multiple video signals into a single signal or separate a combined signal. These devices are frequently used in security and law enforcement applications for recording and/or displaying multiple camera images simultaneously or in succession.
NTSC: National Television Standards Committee.
Output: The means by which an image is presented for examination or observation.
Playback: Recorded material viewed and heard as recorded, facilitated by camcorder, videocassette recorder, or other device.
Processing: Any activity that transforms an input image and/or signal into an output image and /or signal.
Production switcher: A device used to mix video signals from two or more sources (e.g., cameras, videocassette recorder/players, time-base correctors, character generators) for dissolves, wipes, and other transition effects.
Routing switcher: A device used to direct the path of one or more signals into one or more devices.
Time-base corrector (TBC): An electronic device used to correct timing inconsistencies and stabilize the playback of the video signal for optimum quality. It also synchronizes video sources allowing image mixing.
Time-lapse videocassette recorder: A type of video recorder that can be set to record continuously over long periods, achieved by the tape moving in steps and recording one frame or field at a time. These devices are frequently used in security and law enforcement applications.
Vectorscope: An electronic device that measures a video signal's chrominance (color) performance.
Video: The electronic representation of a sequence of images, depicting either stationary or moving scenes. It may include audio.
Video distribution amplifier: A device used to divide single video signals, while boosting their strength for delivery to multiple video devices.
Videocassette recorder (VCR): Multifunction machine intended primarily for playback and recording of videotape stored in cassettes.
Waveform monitor: An electronic device that provides a graphic display of a video signal's strength.
The purpose of this section is to identify some of the equipment used to view, process, and enhance video images.
must be capable of producing an accurate representation of the recorded
image. Playback devices include but are not limited to the following:
- Videocassette recorders, including time-lapse recorders
- CD/DVD players
- Digital media players
- Time-base corrector
- Capture card
- Frame grabber or other image-capturing device
- Waveform monitor/vectorscope
- Editing system
- Routing switcher
- Production switcher
- Distribution amplifier
- Videocassette recorder
- Storage devices (e.g., computer, CD/DVD recorder, internal or removable memory storage devices)
Guidelines for Video Processing Standard Operating Procedures
Processing Standard Operating Procedures
PURPOSE: To apply processing techniques intended to enhance and/or analyze video images. Note: The successful processing of video images should be made with regard to the four legal tests: reliability, reproducibility, security, and discovery.
agency should address the following minimum equipment requirements:
- Playback devices
- Image-processing systems
- Output devices
- Image management
- Image processing
Agencies should establish specific step-by-step procedures for video
processing in accordance with SWGIT guidelines and agency requirements.
These procedures should address the following, as a minimum:
- Image management
CALIBRATION: If necessary, agencies should develop procedures specific to their needs.
CALCULATIONS: If necessary, agencies should develop procedures specific to their needs.
LIMITATIONS: Agencies should take into consideration budget, equipment, management, and accrediting agency requirements.
SAFETY: Agencies should develop procedures specific to their needs.
REFERENCES: Agency-specific documentation, manufacturers' manuals, and SWGIT guidelines.
TRAINING: Agencies should document procedures to ensure sufficient training to afford competence and proficiency with applicable video processing. (See Guidelines and Recommendations for Training in Imaging Technologies in the Criminal Justice System at www.fbi.gov/hq/lab/fsc/backissu/april2002/swgittraining.htm).
Steps for Processing Videotape
Agencies should establish specific step-by-step procedures for video processing in accordance with SWGIT guidelines and agency requirements. These procedures should address the following, as a minimum.
- Do a visual inspection of the tape and cassette housing to
- Ensure housing is intact
- Inspect tape for damage (e.g., twisting, separation)
- If damage is found, take corrective action and document.
- Enable any record-protection device (e.g., punch-out tab, slide record tab, remove record button).
- Determine if the submitted tape is an original or a copy. If it is an original, proceed to
Step 4. If it is a copy, contact the submitter and request that original tape be submitted, and terminate examination of the copy. If the copy is the best available, proceed with the examination.
- Determine the make, model, and settings of the device used to record the submitted video, if possible. These settings may include the recording format and speed.
- Select the appropriate playback device(s) to achieve optimal signal quality.
- Using the selected devices and settings, review the submitted video to locate the pertinent segment(s).
- Determine the appropriate playback speed for processing.
- A time-base corrector may be used to stabilize the signal for playback and permit adjustment of video, chroma, pedestal levels, or other signals.
- At the examiner's discretion, a working copy of the pertinent segment may be generated using an available analog or digital device.
- The images may be enhanced using a number of processing operations
that may include but are not limited to histogram equalization,
multiframe averaging, contrast and levels adjustments, and sharpening.
(See Recommendations and Guidelines for the Use of Digital Image
Processing in the Criminal Justice System at http://www.fbi.gov/hq/lab/fsc/backiussu/jan2003/swgitdigital.htm)
- Once enhanced, the final images are output to appropriate media.