Equipping the Modern Audio-Video Forensic Laboratory, Bruce E. Koenig, Douglas S. Lacey, Noel Herold, FSC, April 2003
April 2003 - Volume 5 - Number 2
Research and Technology
Equipping the Modern Audio-Video Forensic Laboratory
Bruce E. Koenig
Audio-Video Forensics Expert
Douglas S. Lacey
Forensic Audio, Video, and Image Analysis Unit
Federal Bureau of Investigation
Video Forensics Expert
Introduction | Audio-Video Examinations | Equipment and Costs | Basic Playback, Duplication, and Repair | Audio Enhancement | Voice Identification | Video Image Duplication and Enhancement | Signal Analysis and Authenticity | Digital Data Analysis and Retrieval | Laboratory Space Considerations | References
Establishing or increasing the capability of a modern forensic audio-video laboratory draws on diverse disciplines including physics, electrical and electronic engineering, computer science, analog and digital theory, acoustics, digital signal analysis, digital imaging, and other related fields. These disciplines require a wide variety of electronic equipment to enable a forensic examiner to conduct reliable examinations. This paper is directed toward established laboratories that have limited audio-video capability but may be required to increase audio-video forensic support.
The following factors should be considered when establishing or increasing the capability of a modern audio/video forensic laboratory:
- Identifying potential employees
- Requiring a lengthy apprenticeship or equivalent experience of personnel in certain audio and video analyses fields
- Obtaining specialized training in such areas as digital signal analysis, recording theory, sound measurement, and video imaging
- Seeking support and guidance from other established laboratories
- Equipping the laboratory to play back and record in numerous analog and digital audio and video formats, and then providing the capability to improve voice intelligibility, compare voices, identify non-voice signals, authenticate recordings, enhance video images, and conduct other related analyses
- Procuring professional audio, video, enhancement, signal analysis, imaging, and other related equipment
- Identifying physical space for the laboratory
This paper will concentrate on the last three items in the list by discussing eight audio-video examinations, the identification and costs of equipment needed to examine audio-video evidence, and laboratory-space considerations. Examination procedures and personnel matters will not be addressed.
Modern audio-video forensic laboratories are capable of analyzing analog or digital audio-video recordings to support criminal investigations, governmental intelligence, civil litigation, personnel and administrative matters, and other related issues. Some laboratories conduct a wide range of examinations on audio and video recordings, whereas others provide only duplication and a few other commonly requested analyses.
The eight most commonly conducted audio-video examinations are listed below, with the level of complexity from lowest to highest:
- Playback and duplication: The capability to play back recordings and provide high-quality, readily useable duplicates on standard formats. A fully equipped laboratory can work with most audio-video formats as well as specialized recordings (e.g., miniature law enforcement formats, 911 logging recordings, time-lapse video).
- Repair: The capability to repair torn or stretched tapes in standard audio-video analog and digital formats
- Audio enhancement: The capability to improve the voice intelligibility of recordings and to prepare enhanced copies of audio recordings and the audio information on video formats
- Voice identification: The capability to compare unknown recorded voices to known voice exemplars by identifying similar and dissimilar characteristics
- Video image duplication and enhancement: The capability to duplicate or improve video recordings
- Signal analysis: The capability to quantify, identify, and compare non-voice signals to determine origin and characteristics (i.e., telephone signaling, gunshot sounds)
- Authenticity: The capability to resolve the authenticity of audio-video recordings by determining originality, continuity, and integrity
- Digital data analysis and retrieval: The capability to extract and analyze digital audio-video recordings on commercial and proprietary recording devices
|Figure 1 An Audio Workstation. Click to enlarge image.||Figure 2 A Magnetic Imaging Workstation. Click to enlarge image.|
The equipment requirements for the eight audio-video forensic examinations are listed below. The phase numbers represent the most basic through the more sophisticated equipment requirements. The equipment costs reflect the prices at the time of publication and do not include personnel, training, and laboratory space.
Equipment: To allow high-quality playback and standard-format duplication of audio and video recordings, a laboratory should have the following equipment (Koenig 1987; Luther and Inglis 1999; Pohlmann 2000; Watkinson 2001; Watkinson 2000):
- Standard professional audiocassette decks with heavy-duty transports, a +10 percent or greater speed adjustment around each standard speed, a three-head design, Dolby-noise-reduction systems (B, C, and S), and real-time counters. Useful additions include set speeds of 15/16, 17/8, and 3¾ inches per second (ips), a continuous playback speed adjustment from at least 15/32 to 3¾ ips, and a headphone jack with its own volume control.
- Professional audio microcassette playback decks optimized for forensic applications, including continuous playback speeds of 0.7 to 2.4 centimeters per second, accurate metering, headphone volume controls, and professional-quality transport and electronic systems. Laboratory personnel normally do not make audio microcassette copies due to their inherently lower quality.
- Specialized audio playback systems including open-reel, minicassette, miniature on-the-body, NT digital, and other analog and digital devices with playback speed, track configuration, and other features that match the type of recordings expected to be received in the laboratory. Playback of selected recordings may require specialized devices and/or software available only to law enforcement agencies.
- Video recorders/playback units with formats ranging from analog U-Matic to the latest digital configurations, including VHS, VHS-C, SVHS, 8mm, Hi8, time-lapse, 6mm digital, and 8mm digital. High-quality or professional decks ensure the best possible playback and/or recording. In video playback, emphasis must be placed on the need for each head (field) to precisely follow its respective track. This may require procuring a number of video playback units of similar type in order to optimize the output results. A professional analog or digital format or a semiprofessional format (i.e., SVHS) unit should be available when interim copies are needed to minimize inherent losses in video duplication.
- A professional digital time-base corrector to remedy timing errors for the variety of video formats expected in the laboratory and to capture and hold separate fields and frames in its memory
- A video monitor to accurately display the full bandwidth and resolution of the highest quality format that will be played back in the laboratory. A hand-held screen degausser should also be available to ensure long-term color purity.
- Equipment racks of the correct width for the equipment, usually 19 inches. They should include professional-quality accessories including patch bays, spacer panels, and wire runs.
- Tables for workspace and equipment that are sturdy and at least 30 by 60 inches in size to allow ample work and equipment space. Avoid folding tables, except for temporary use.
- High-quality wiring, connectors, and adapters to manage all required equipment connections
- Tape repair supplies including appropriate splicing blocks, edit-tabs, razor blades, small knives to cut open housings, nonmagnetic-leader tape, tape-cleaning solvent, and miniature, non-magnetized tool kits
- Ferrofluid solution with particle sizes in the 0.2 to 1.5 micron range. Smaller particle sizes cannot consistently be removed from analog tapes.
- A ten-power loupe to check for a tape’s track configuration and azimuth alignment, after application of the ferrofluid solution
- Well-padded, heavy-duty, professional-quality headphones that completely seal around the ears. Cheaper consumer headphones can be sonically equal to the professional models, but they usually do not hold up to the heavy-duty use in the laboratory.
- Blank high-quality audio and video recordable media for preparing copies
- Accurate test recordings for calibration of playback and recording equipment
Cost: A basic audio system with professional or high-end consumer equipment would cost between $8,000 and $11,000 and would allow playback, duplication, and repair of audiocassettes and microcassettes.
A basic video system would add between $9,000 and $12,000 to the basic audio system of Phase I and would allow playback, duplication, and repair of VHS, SVHS, VHS-C, 8 mm, Hi8, and time-lapse formats. A complete Phase I audio-video setup would cost between $17,000 and $23,000.
Equipment: To allow meaningful improvements in the voice intelligibility of recorded audio information, an audio-video laboratory should have the following equipment (Bellanger 2001; Davis 2002; Koenig 1988):
- A digital-adaptive enhancement processor that allows the operator to access and implement a number of different filter algorithms, including one- and two-channel adaptive, bandpass, comb, notch, spectral inverse, parametric, and graphic equalizer. The digital-adaptive enhancement processor should also permit the use of compression and limiting functions. The best configurations are hardware implementations with software control, because purely software programs may not implement complex filters in real-time, and the audio signals are susceptible to corruption when the computer is doing other operations simultaneously. All of these filtering systems should contain high-quality 16-bit, or greater, analog-to-digital and digital-to-analog converters, at least two input channels, and a range of sampling rates to at least 32 kHz.
- A separate compressor/limiter with at least two channels, adjustable and automatic compression ratios, attack-release times, and gain reduction of at least 40 dB
- A spectrum analyzer that provides a detailed visual representation of the audio signal. The unit should be a fast-Fourier transform design (not a real-time analyzer) with single-channel capability. If the laboratory also conducts signal analysis and/or authenticity examinations, a dual-channel or larger unit would be needed. The fast-Fourier transform device should have 16-bit or greater resolution, frequency display ranges adjustable from at least 0-100 Hz up to 0-20 kHz, a variable number of averages, and at least 800 lines of resolution on any frequency range. A zoom capability can also be a useful feature for some applications (Bracewell 2000).
- A modern computer system that can be used for a number of applications including controlling the digital enhancement devices and running other appropriate software.
- A computer printer that provides hard copies of fast-Fourier transform plots, filter settings of the digital enhancement devices, and other applicable information.
- Non-real-time software programs for precise, non-linear time and amplitude processing of audio recordings. Some of the functions applicable to the forensic field include pitch shifting for general or localized correction of playback speed, amplitude adjustments, and normalization. These software programs also allow redactions of specific portions of a recording to be performed with more ease and accuracy.
Cost: An audio enhancement capability added to Phase I (audio and video) would cost between $28,000 and $53,000 and would allow intelligibility enhancement of audio recording formats and the audio track on video recordings.
Equipment: Present laboratory spectrographic and/or computer voice comparison systems do not produce conclusive results, but meaningful findings are possible with careful analysis of speech samples collected under forensic conditions. The minimum requirements include the following equipment (IAI Voice Identification and Acoustic Analysis Subcommittee 1991; Committee on Evaluation of Sound Spectrograms, National Research Council, National Academy of Sciences 1979; Tosi 1979; Koenig 1993; Koenig 1986):
- An analog sound spectrograph that produces excellent voice spectrograms, especially under noisy recording conditions. It is being quickly replaced with specialized spectrographic software.
- Specialized spectrogram software that produces digitally calculated spectrograms that have been optimized for the speech and forensic communities. This software should be user-friendly and allow the operator to control all the important time and frequency characteristics of the graphic representation.
- Specialized forensic voice identification algorithms that are presently being developed (Nakasone and Beck 2001; Reynolds et al. 2000). When fully developed, this specialized, computer-based software will allow automated and/or operator-assisted voice comparisons between different voice samples.
- Editing software that allows two or more recorded voice samples to be selectively isolated and combined into a new recording.
- A headphone-switching box that allows the rapid toggling between two input signals containing separate voice samples for aural comparison.
Cost: Adding spectrographic and computer-based voice identification capability to Phases I and II would cost between $12,000 and $25,000 and would allow comparisons between unknown recorded voices and known voice exemplars.
Equipment: To allow the most accurate duplication and enhancement of video recordings, an audio-video forensic laboratory should have the following equipment (Blitzer and Jacobia 2002; Bovik 2000; Russ 2002; Russ 2001):
- A video-capture device that accurately captures the entire resolution of a single field/frame and continuous video for any analog format expected in the laboratory. Inputs should include at least composite and S-Video.
- A high-speed computer system with a fast processor, a large monitor, sufficient random access memory, an input for digital video (i.e., FireWire), and extensive hard-drive storage to handle the video applications of the laboratory. Some manufacturers provide computer equipment that has already been optimized for this task.
- Color and black-and-white computer and video printers that provide hard-copy images from video evidence. Like other components in the system, the printers should have sufficient resolution to reproduce the entire captured and/or enhanced fields/frames.
- Software programs for continuous video and still video enhancement, with appropriate algorithms to sharpen, enlarge, enhance, edit, and correct visual details
Cost: A video imaging capability added to Phases I and II would cost between $4,000 and $50,000. The wide range in cost is due to the options selected, the computer platform, the resolution, and the specifications.
Equipment: This phase includes signal analysis of audio signals and authenticity analyses of audio and video formats. The minimum requirements include the following equipment (Bellamy 2000; Bolt et al. 1974; Hodges 2001; Koenig 1990; Koenig et al. 1998; Rappaport 1996; Reeve 1995; Reeve 1992):
- The Phase IV computer system
- Professional video monitors that include pulse-cross and under-scan modes, component and composite inputs, and at least 600 lines of horizontal resolution. They should be able to fully reproduce all anticipated digital formats up to the highest level resolution. Multiple monitors may be needed to manage all of the analog and digital recordings.
- A dual-channel spectrum analyzer of the fast-Fourier transform design, not a real-time analyzer. The required features include zoom, two-channel comparison algorithms, and long-term averaging.
- A macro-photographic or digital camera system that allows the production of accurate pictures of the tape surface, ranging from 0.0125 up to 2 inches in width. If a digital camera is used, it should have a resolution of at least 2000 by 2000 pixels.
- A high-quality laboratory computer sound card that has a 16-bit or higher resolution, sample rates to at least 44.1 kHz, two or more input and output channels, and low-noise and distortion characteristics.
- Signal analysis software with the capability to perform waveform, narrow-band spectrum, spectrographic, and other analyses on .wav, as well as on other computer-formatted audio files. It should allow aural review of selected portions of the displayed data, permit high-quality printouts of the various analyses, and have the capability to display multiple windows on the screen with time correlation between the various displays.
- Professional audio and video digital recorders that permit the duplication of original evidence without any loss of visual or aural information.
Cost: A signal analysis and authenticity capability added to Phases I through IV would cost between $10,000 and $60,000. The wide range in cost is due primarily to the selected imaging system and software.
Equipment: This phase involves the transfer, analysis, and retrieval (when necessary) of digital data contained on the various audio and video recorders used in the investigative field. Digital forensic recordings may exist on a variety of media types including tape (DAT, NT-2, DDS), optical (CD-R/RW, DVD-R/+R/-RW/+RW/RAM), magneto-optical (MiniDisc), and random access (FLASH memory chips, hard drives). These recordings may have been recorded in a standard format or on proprietary recorders that require specialized hardware and/or software to play back and analyze the recorded data. Most of the equipment needed for this phase was discussed in Phases I through V; however, there are a few additional requirements include the following items:
- Software that allows the user to view the digital data down to the bit level. The proprietary recorders may require additional hardware and/or advanced software that often are available only to law enforcement agencies.
- A removable, writable digital medium that has sufficient storage capability to archive large data files that exist on higher-capacity formats and on solid-state body recorders.
Cost: Digital data analysis and retrieval capability added to Phases I through V would cost between $1,000 and $5,000 depending on the archival storage media chosen.
The laboratory should be configured or constructed to decrease outside noise and vibration, dampen laboratory equipment sounds, and minimize radio-frequency interference and magnetic fields. Equipment should be optimally arranged for sufficient appropriate workspace including separate electrical circuits, proper lighting, and secure and adequate storage (Berger et al. 2000; Harris 1991; Salter 1998).
- The following strategies should be implemented to decrease outside noise and vibration:
- Eliminate as much of the outside noise as possible by locating the laboratory away from noisy outdoor environments like train tracks, airports, playgrounds, and busy streets.
- Increase the distance between building noise and the laboratory by avoiding shipping docks, cafeterias, or machine shops. Distancing the laboratory may mean moving the laboratory space to a quieter location in the same building or to a different building. If laboratory relocation is not possible, noise relocation may be an option. For example, request that most of the dock shipments be loaded or unloaded as far as possible from the laboratory or arrange with the cafeteria so that the lunch tables and the noisiest equipment are farthest from the common wall with the laboratory. Machine shop noise can be reduced by placing sound-absorbing materials around the equipment and installing soundproofing material on the walls and ceiling.
- The laboratory walls can be isolated and soundproofed. However, building new walls inside the present laboratory, isolating the new walls from the old outside walls, and treating the space between the two walls reduces laboratory space, increases construction costs, and creates temporary noise and dust.
- Decrease the noise generated in the laboratory by purchasing equipment and computers with low-noise fans. Place noisy equipment in soundproof isolation compartments. Whenever possible, avoid fluorescent lighting to prevent the audible buzz the lighting produces and the low-level noise it can induce in certain electronic equipment. Limit the speed of the heating and air conditioning fans, and correct noisy air vents.
- Minimize radio-frequency interference by locating laboratories away from commercial radio and TV transmitting stations or police and other radio-transmitter towers. Some equipment is more susceptible to outside fields than others, so place that equipment in different locations in the laboratory or at different heights in the racks. Placing the laboratory space below ground usually minimizes radio-frequency problems. Interference from the building’s electrical wiring can often be removed with specialized AC power filters. Large transformers or loudspeakers with large magnets should not be placed near any audio or video evidence. A portable gauss meter can determine if there are problematic magnetic fields in the area. To avoid electrical spikes and other irregularities, the laboratory should have dedicated electrical circuits.
- Equipment may be placed on tables, but sturdy equipment racks are preferred. Racks allow higher density with better cooling, provide better protection, facilitate wiring and rapid changes in connections, are moveable, and look professional.
- The workspace should be flexible enough to handle reorganization. Floor space should be planned to accommodate current and future equipment. The aisles should allow easy access for bulky evidence, large equipment, and equipment repair. There should be sufficient table space to visually examine, mark, and lay out evidence. The workspace should have sufficient lighting, preferably incandescent track lighting.
- A separate, secured area containing a safe or safe file for evidence storage should be available with controlled entry and an alarm. The storage area should be free of magnetic fields and be temperature- and humidity-controlled.
Bellamy, J. C. Digital Telephone. 3rd ed., Wiley, New York, 2000.
Bellanger, M. G. Adaptive Digital Filters. 2nd ed., Dekker, New York, 2001.
Berger, E. H., Royster, L. H., Royster, J. D., Driscoll, D. P., and Layne, M. eds. Noise Manual. 5th ed., AIHA, Fairfax, Virginia, 2000.
Blitzer, H. L. and Jacobia, J. Forensic Digital Imaging and Photography. Academic, San Diego, California, 2002.
Bolt, R. H., Cooper, F. S., Flanagan, J. L., McKnight, J. G., Stockham, T. G., and Weiss, M. R. Report on a Technical Investigation Conducted for the U.S. District Court for the District of Columbia by the Advisory Panel on White House Tapes. U.S. Government Printing Office, Washington, DC, 1974.
Bovik, A., ed. Handbook of Image and Video Processing. Academic, San Diego, California, 2000.
Bracewell, R. N. Fourier Transform and Its Applications. 3rd ed., McGraw-Hill, Boston, Massachusetts, 2000.
Committee on Evaluation of Sound Spectrograms, National Research Council, National Academy of Sciences. On the Theory and Practice of Voice Identification. National Academy of Sciences, Washington, DC, 1979.
Davis, G. M., ed. Noise Reduction in Speech Applications. CRC, Boca Raton, Florida, 2002.
Harris, C. M. Handbook of Acoustical Measurements and Noise Control. 3rd ed., McGraw-Hill, New York, 1991.
Hodges, P. Introduction to Video Measurement. 2nd ed., Focal Press, Oxford, England, 2001.
Koenig, B. E. Selected topics in forensic voice identification, Crime Laboratory Digest (1993) 20(4):78-81.
Koenig, B. E. Authentication of forensic audio recordings, Journal of the Audio Engineering Society (1990) 38(1-2):3-33.
Koenig, B. E. Enhancement of forensic audio recordings, Journal of the Audio Engineering Society (1988) 36(11):884-894.
Koenig, B. E. Making effective forensic audio tape recordings, FBI Law Enforcement Bulletin (1987) 56(5):10-18.
Koenig, B. E. Spectrographic voice identification, Crime Laboratory Digest (1986) 13(4):105-118.
Koenig, B. E., Hoffman, S. M., Nakasone, H., and Beck, S. D. Signal convolution of recorded free-field gunshot sounds, Journal of the Audio Engineering Society (1998) 46(7-8):634-653.
IAI Voice Identification and Acoustic Analysis Subcommittee. Voice comparison standards, Journal of Forensic Identification (1991) 41(5):373-392.
Luther, A. and Inglis, A. Video Engineering. 3rd ed., McGraw-Hill, New York, 1999.
Nakasone, H. and Beck, S. D. Forensic automatic speaker recognition, Speaker Recognition Workshop: 2001 A Speaker Odyssey (2001).
Pohlmann, K. C. Principles of Digital Audio. 4th ed., McGraw-Hill, New York, 2000.
Rappaport, T. S. Wireless Communications: Principles and Practices. IEEE, New York, 1996.
Reeve, W. D. Subscriber Loop Signaling and Transmission Handbook: Digital. IEEE, New York, 1995.
Reeve, W. D. Subscriber Loop Signaling and Transmission Handbook: Analog. IEEE, New York, 1992.
Reynolds, D. A., Quatieri, T. F., and Dunn, R. B. Speaker verification using adapted gaussian mixture models, Digital Signal Processing (2000) 10:19-41.
Russ, J. C. Image Processing Handbook. 4th ed., CRC, Boca Raton, Florida, 2002.
Russ, J. C. Forensic Uses of Digital Imaging. CRC, Boca Raton, Florida, 2001.
Salter, C. M. Associates. Acoustics: Architecture, Engineering, the Environment. William Stout, San Francisco, California, 1998.
Tosi, O. Voice Identification: Theory and Legal Applications. University Park, Baltimore, Maryland, 1979.
Watkinson, J. Art of Digital Audio. 3rd ed., Focal, Oxford, England, 2001.
Watkinson, J. Art of Digital Video. 3rd ed., Focal, Oxford, England, 2000.