Research and Technology - Forensic Science Communications - October 2005
October 2005 - Volume 7 - Number 4
Research and Technology
Validation of a Forensic Method for Analog VHS Video Recorder Identification
Audio Visual Unit
Victoria Police Forensic Services Department
Macleod, Victoria, Australia
Audio Visual Unit
Victoria Police Forensic Services Department
Macleod, Victoria, Australia
Audio Visual Unit
Victoria Police Forensic Services Department
Macleod, Victoria, Australia
Introduction | Video-Signal Recording and Reproduction | Replay Radio-Frequency Waveform | Replay Tracking Position | Audio-Track Configuration | Blind Validation Trials | Materials and Methods | Results and Discussion | ConclusionsAcknowledgments | References |
Forensic audio-recording authenticity analysis became prominent following a 1974 report prepared by the advisory committee appointed to examine the erased portion of an audiotape recording between former U.S. president Richard M. Nixon and his chief of staff, H. R. Haldeman (Koenig 1990). The tapes became known as the “Watergate tapes.”
Today, many organizations conduct forensic video authenticity analysis; however, little documented evidence exists of research conducted in this area (Aldridge 1995). Although digital communication and information technologies are both emerging and indeed converging, many people still own analog systems (Catoggio 2001).
This paper outlines a method for determining whether a particular videotape was recorded in a particular video recorder. The method uses three techniques that compare replay radio-frequency waveform, audio-track configuration, and replay-tracking position. The techniques require the use of a storage oscilloscope, plotter, video monitor, and video player.
An investigator submitted to the Victoria, Australia, Police Forensic Services Department a number of illicit video home system (VHS) recordings and recorders. The copies had been sold publicly without classification and without the consent of the copyright owner. In Australia, films and videotapes must be classified according to the Commonwealth Classification (Publications, Films and Computer Games) Act 1995. The Commonwealth Copyright Act 1968 protects copyright ownership of literary, dramatic, musical, and artistic works. The investigator requested that the department determine whether any of the copies had been made in any of the video recorders that had been seized from the suspects. Before this could be done, it was necessary to determine whether VHS video recorders impart any electronic or physical identifying features onto videotapes. A subsequent review of the available literature and the results of early experiments confirmed this phenomenon.
When a video recording is replayed, an examination of the video head-switching transients (Aldridge 1995), signal dropouts, and picture content (Owen 1989) can help to identify the recorder used. Traditional audio authentication concepts can also be applied to the examination of video machines (Koenig 1990).
This paper is principally concerned with three other features that can be used for identification purposes: replay radio-frequency waveform (Klasen 1998), audio-track configuration, and replay-tracking position (Catoggio et al. 2002). To validate and test the robustness of the examination techniques, the authors conducted two blind trials using 50 recorded videotapes.
This section provides a basic explanation of the aspects of video-signal theory that are important to understanding the analysis techniques discussed in this paper.
The Video Picture
The transmission of sound and picture information requires a signal bandwidth of up to 7 megahertz (MHz) (Botto 1992). In Australia and many countries in Europe, video signals are transmitted using the Phase-Alternation Line (PAL) standard. A transmission speed of 25 picture frames per second gives the illusion of continuous motion. Each frame has 625 horizontal lines, which are divided into odd and even fields as shown in Figure 1 (Mee and Daniel 1989). Each line represents the picture information, which varies in color and brightness along the length of the line. During image capture, a camera scans one line at a time. A line-sync pulse marks the end of each line. When the signal is displayed by a television, this pulse tells its cathode-ray beam to begin tracing the next line. The Séquential Couleur avec Mémoire (“Sequential Color with Memory,” SECAM) standard is used in France and similarly employs 625 lines and 25 frames per second. The National Television System Committee (NTSC) standard employs 525 lines at 29.97 frames per second and is widely used across the United States.
Figure 1: Portion of One Picture Frame Consisting of Odd and Even Fields
Two-Head Helical-Scan Recording System
The Japan Victor Company (JVC Group, Yokohama, Japan) introduced the VHS video-recording format in 1976 (Botto 1992; Mee and Daniel 1989). It uses a two-head helical-scan method to magnetically record a video signal to tape. Figure 2 (Botto 1992) depicts how, using this method, tape is transported from a supply reel to a take-up reel by audio- and video-record heads and tape guides. Tension is applied to the take-up spool, while a rubber pinch roller and capstan regulate the tape speed.
Figure 2: Illustration of VHS Tape Threading and Head Wrap
A high video-head-to-videotape speed is required to record a signal bandwidth approaching 7 MHz. This speed is achieved by placing two oppositely set heads into a rotating drum as demonstrated in Figure 2. The drum is tilted by approximately six degrees and rotates in the same direction as the linear movement of the tape (International Electrotechnical Commission 1994). The picture information contained in each field is recorded diagonally as the head rotates across the tape.
In the context of recording, the Phase-Alternation Line video signal is transmitted directly through frequency modulation (FM) of a 4.43-MHz subcarrier (Mee and Daniel 1989). FM is used to transmit all picture information, including color and brightness.
Audio information is recorded using a stationary head on a longitudinal track at the top portion of the tape. In addition, high-fidelity stereo audio is recorded through careful integration with the picture information transmitted on the FM subcarrier. Figure 3 (Mee and Daniel 1989) depicts the audio- and video-track configurations of a helical-scan system. A stationary full-track erase head precedes the drum to ensure that the tape is erased prior to recording.
Figure 3: Illustration of a Two-Head Helical-Scan Video-Recording System and Recording-Track Configuration
The tape guides and slant pins cause the tape to wrap more than 180 degrees around the video drum. This allows the second video head to begin its scan of the tape just as the first video head is ending its scan. Figures 2 and 3 illustrate how take-up and supply guides and pins are used to ensure proper alignment of the tape to the heads.
To guarantee optimum reproduction of the video picture between machines, the tracks must be scanned during playback in precisely the same way as when they are recorded. Incorrect tracking may cause picture flicker, loss of color, high-fidelity audio distortion, and other symptomatic picture and audio degradation.
During recording, the phase of the drum rotation is detected and adjusted according to the vertical frame sync pulse. This signal also causes a constant capstan speed. A split from this signal then becomes a reference signal, which is recorded to a control track on the bottom portion of the videotape.
During playback, a sync signal is generated and compared with the detected drum rotation, which, in turn, is controlled so that it remains constant and in phase. Comparing the sync signal with the control signal regulates the capstan rotation and, hence, the speed of the tape. Controlling the tape speed causes the video head to trace precisely along the recorded track.
Most domestic video players allow a degree of manual or automatic tracking adjustment to ensure compatibility among machines (Mee and Daniel 1989). Manual adjustment causes a time delay between the replay of the control track and the generation of the head-switching pulses (Aldridge 1995).
Although beyond the scope of this paper, other tracking systems exist. Some of these systems use digital processes (Mee and Daniel 1989).
Frequency modulation of a 4.43-MHz subcarrier transmits the picture information. This signal is converted to magnetic energy by the video head, which, in turn, magnetizes the videotape. During playback, the video heads alternately scan the diagonal signal tracks corresponding to the odd and even field lines. As they scan, the heads detect a change in magnetic flux, which induces a weak signal. This weak signal is directly proportional to the recorded FM 4.43-MHz subcarrier. Figure 4 (Botto 1992) shows that the signal produced by each video head is preamplified and eventually combined by a switching amplifier. The combined signal is often referred to as the replay radio-frequency waveform or frequency-modulation waveform. Radio frequency, commonly referred to as RF, is used to indicate the complete range of frequencies used for the transmission of information by electromagnetic waves (Spottiswoode 1978). When viewed on a 100-millisecond (ms) oscilloscope trace, the waveform looks similar to the one depicted in Figure 4.
Figure 4: Diagram Showing Video-Signal Reproduction
The waveform analysis discussed in this paper is based on a common technique for servicing video recorders. Examination of the waveform envelope is used to check the recorder’s tape alignment and the amount of head wear and tear. Adjusting the tape guides alters the tape alignment. Assuming that the combination of head wear and the position of the tape guides might be characteristic of specific recorders, the authors conducted some early experiments. Figures 5.1 to 5.10 present a summary of findings when comparing the effects of tape-guide position, tape wear, and head wear on the waveform. The waveform shapes are exaggerated to aid explanation.
Figures 5.1-5.10: Comparison of Replay Radio-Frequency Output Waveform Shapes and Causes
Figure 5.1 depicts the ideal replay waveform of a tape. This assumes that the recorder has no head wear and that it is properly aligned. Figures 5.2 to 5.10 depict replay waveforms in which the recorder is suffering from head wear, misalignment, or other problems. As expected, the individuality of each recorder becomes more distinct the further the waveform deviates from the ideal. During these experiments, all tapes were replayed in a device set for optimum tracking when playing a National Panasonic VHS alignment tape (No. VFM8180HADH, Matsushita Electric Industrial, Osaka, Japan).
Experimentation also has shown that although the general waveform shape is consistent throughout the replay of a recording, fluctuations can occur depending on changes in image content and television-channel reception. Examples include moving from a predominantly white scene to a black scene and intermittent dropouts, respectively.
Over time, machine use can cause head wear and tape misalignment, altering the waveform. The effects of time and use should be considered when determining whether a particular recorder was used to make a tape recording. The effect of dirt buildup on the video heads and the quality of the videotape also should be considered.
Tracking is adjusted to ensure compatibility among machines. Typically, tracking is adjusted using a rotary dial or a keypad system. A measure of tracking adjustment may be displayed on the screen or on the machine display. Figure 6 provides an example of an on-screen tracking indicator. A tape that is recorded and replayed on the same machine will display a central tracking position. Experimentation has shown that optimum tracking can vary among machines. In most cases, the deviation from the central tracking position is consistent for the duration of the recording. Mechanical faults may cause variations in tracking over time.
Figure 6: Example of an On-Screen Tracking Indicator
The very coarse tracking indicator shown in Figure 6 can be useful in comparing tapes recorded on different machines. It would be expected that, similar to the replay radio-frequency waveform, the individuality of each machine will become more distinct the further the tracking deviates from the ideal central position.
It is important to note that adjusting the replay tracking will cause a noticeable change in the shape of the replay radio-frequency waveform. Therefore, it is only valid to compare waveforms displayed at the same replay tracking (nominally central position) or at their respective optimum settings.
Audio information is recorded using a linear track, and high-fidelity audio is modulated with the video signal along the diagonal tracks. Examining the audio-track configuration can provide a simple clue in matching a tape to a recorder.
Recorders fit into one of two audio categories: either they possess a linear track only, or they possess both a linear and a high-fidelity track. The authors do not know of any video recorder that records only high-fidelity audio.
The absence or presence of a high-fidelity track usually is indicated on the control display of a machine capable of playing back both linear and high-fidelity tracks. Some video players also allow the user to select playback of either the linear track or the high-fidelity track.
Some recorders, particularly professional or high-end consumer products, allow the user to manually turn off the high-fidelity recording capability. Therefore, if two tapes are presented for comparison, the absence of a high-fidelity track on one does not discount that the tapes share a common origin. Conversely, if a questioned recorder does not possess high-fidelity recording capabilities, yet the tape in question contains a high-fidelity recording, then this recorder could not have been used. Indeed, this finding would negate the need to conduct tracking and radio-frequency waveform analyses.
A typical forensic case would involve the comparison of an exhibit tape and a sample recording from an exhibit machine. However, the validation trials were not based on this case scenario because of the 50 percent chance of a random match. Instead, the authors decided to test the robustness of the technique by comparing multiple items recorded on multiple machines.
The first of two trials was conducted to familiarize the authors with the limitations of the examination techniques and to determine accuracy. With the aid of hindsight, the authors conducted a second trial approximately four years later to refine the examination techniques.
Unique identifier codes were placed on each of 50 TDK VHS videotapes (No. HS180, TDK Corporation, Tokyo, Japan). An independent assistant then distributed the tapes to 27 volunteers, as depicted in Table 1. Some volunteers were given one tape, whereas others were given two or three. Each of the volunteers then recorded approximately 60 minutes of an Australian television broadcast on each of the tapes. The volunteers were specifically asked to use standard record speed and not long-play mode. The independent assistant then collated the recorded tapes and submitted them to the authors, who were unaware of how the tapes had been distributed.
Table 1: Distribution of Videotapes to Volunteers
To provide a balance between an adequate sample size and a manageable number of items for comparison, the trial was limited to 50 tapes recorded in 27 different machines. The task was to use the techniques discussed in this paper to determine which of the 50 tapes were recorded on the same machine. Single tapes recorded on different machines were introduced so that the authors would not expect to find a balanced number of matched pairs or groups of three.
In the second trial four years later, the process was repeated and included some volunteers from the first trial who had retained their original machines. Trial 2 tapes also were allocated as described in Table 1. During Trial 2, the authors were again unaware that the distribution profile was the same as in Trial 1.
Initially, an attempt was made to compare only replay radio-frequency waveform to identify groups. In this case, comparing the radio-frequency waveform of each tape with the radio-frequency waveform of the other 49 tapes proved very difficult and tedious. Therefore, comparison of replay-tracking position and audio-track configuration was incorporated into the testing regime. The testing procedure for Trial 1 is presented in Table 2.
Table 2: Trial 1 Videotape-Testing Procedure
The effects of the tape brand and age were not considered during this trial. However, a poor-quality or well-used tape might cause some instability in the replay radio-frequency waveform.
The Sony SLV-X827A (Sony Corporation, Tokyo, Japan) is a domestic VHS Phase-Alternation Line video machine and was used to replay all tapes. The machine was tested for functionality, ease of access to circuit-board test points, and ability to consistently reproduce a waveform approaching the ideal for a National Panasonic VHS Phase-Alternation Line alignment tape (No. VFM8180HADH). The device was not calibrated or aligned according to an international standard such as those published by the International Electro-Technical Commission (Geneva, Switzerland). The replay machine was used only in these trials; therefore, the possible waveform-altering effects of time and usage were minimized. The heads and guides were cleaned before each tape was tested. As is the case for the comparison of audiotapes, under these conditions, the precision of the replay device is considered acceptable (Catoggio 1998).
Determining Audio-Track Configuration and Replay-Tracking Position
A number of tapes did not possess a high-fidelity audio track. This was indicated simply by the absence of a “HiFi” light on the machine display.
The replay machine provided, on a Panasonic BT-H1490Y monitor, a pictorial indication of replay-tracking position similar to the one depicted in Figure 6. The automatic replay tracking was activated, and a pictorial note was made of the tracking display. For the purposes of this trial, all other picture and audio content was ignored.
Comparing Replay Radio-Frequency Waveform
A Roland DXY-1300 plotter (Roland Corporation, Osaka, Japan) was connected to a Tektronix 2230 100-MHz digital storage oscilloscope (Tektronix, Beaverton, Oregon). The replay machine provided a test point for both the radio-frequency waveform and the switching-head signal. The origin of these signals is depicted in Figure 4. The signal from the radio-frequency waveform test point was displayed, using the switching-head signal as a trigger to stabilize the image. When displayed at 10 ms per division (i.e., a 100-ms trace), a sequence of four complete waveform patterns is evident. These correspond to consecutive odd and even fields. The Tektronix 2230 allows the user to store and overlay waveforms, which is particularly useful in comparing small differences in waveforms.
Guidelines for Comparison for Validation Trial 1
Visual comparisons were made of radio-frequency waveforms and replay-tracking position. To aid this subjective process, the following guidelines were used to critically compare tapes:
- The two tapes had a common origin if an assessment was made that there were only minor differences in the features listed in Table 3. It was expected that radio-frequency waveforms of the same origin would display some minor differences due to mechanical and electronic intravariability.
- The two tapes did not have a common origin if an assessment was made that there was a noticeable difference in at least one feature listed in Table 3. The noticeable difference was not expected to be due to mechanical and electronic intravariability.
- It was inconclusive as to whether two tapes had the same origin if the features of Table 3 were similar but there was uncertainty as to whether they constituted minor or noticeable differences.
Table 3: Waveform Features: Points of Comparison
Examples of minor and noticeable differences in radio-frequency waveforms are displayed and discussed in Figure 7.
Figure 7: Comparison of Replay Radio-Frequency Waveforms
Results of Trial 1
Based on early experimentation, the authors had an expectation of the range of differences in waveforms and the ability to distinguish them. In part, this trial tested these expectations, which turned out to be incorrect. Based on the guidelines presented in the previous section, tapes were grouped according to similar waveforms, audio-track configuration, and replay-tracking position. These groupings were then compared with the actual groupings listed in Table 1. Table 4 presents a summary of findings for Trial 1, which was conducted in 1998. Twenty-nine tapes were matched correctly; however, six tapes were matched incorrectly. A wide variety of waveform patterns was displayed. It was easy to assess the significance of obvious differences; however, it was difficult to detect minor differences and assess their significance.
Table 4: Summary of Findings for Trial 1 Conducted in 1998
Guidelines for Comparison for Trial 2
After Trial 1, the authors had a far better understanding of the significance of differences in radio-frequency waveforms. With this experience and further experimentation, the authors made improvements to the test procedure in an attempt to decrease the number of incorrect matches. These improvements are shown in Table 5.
Table 5: Improvements to the Test Procedure Made for Trial 2
Results of Trial 2
The number of incorrectly matched tapes was reduced to zero in the second trial. The number of correctly matched tapes was reduced, whereas the number of inconclusive results increased. Table 6 provides a summary of findings for Trial 2, which was conducted in 2002.
Table 6: Summary of Findings for Trial 2 Conducted in 2002
Comparison of replay radio-frequency waveform, audio-track configuration, and replay-tracking position is a useful technique to aid video recorder identification. An understanding of the significance of minor or noticeable differences between waveforms is essential to the accuracy of this technique. The authors developed an understanding through an investigation of intra- and inter-machine waveform variations. Consistency in determining differences can be facilitated by using a guide, such as the examples in Figure 7. Viewing waveforms at different replay-tracking settings highlights differences and improves the robustness of the technique.
An expected improvement occurred in the results of the second of two blind trials. This was attributed to changes in the test procedure and interpretations that produced an increase in inconclusive results but a decrease in false-positive results.
This method can be useful for screening a large quantity of items for examination. Other video-identification methods can then be applied to items that display a positive match or inconclusive result. Other techniques include the examination of head-switching transients, picture information, and audio-switching transients. This method also can be adapted for the identification of other helical-scan recording devices, such as digital videotape, digital audiotape, and both analog and digital 8-mm video formats. The radio-frequency waveform of a digital audiotape machine is known to alter according to guide heights (Issacs 2000).
The authors hope to determine the effects of time and machine usage on the replay radio-frequency waveform produced by video recorders. This can be achieved by comparing tapes of the same origin from Trials 1 and 2, which were recorded four years apart. It also may be useful to compare the waveforms produced by recorders of the same brand and model.
Increasingly since the 1960s, law enforcement agencies, business owners, and even members of the public have used audiovisual devices to capture evidence of serious crimes, such as murder, assault, or extortion. Although new technologies continue to emerge, significant use of analog devices remains. Meanwhile, information and communication technologies are converging, resulting in the advent of new devices, such as multifunctional cellular phones. As occurred with the Watergate tapes, the authenticity of recordings produced by such technologies may be questioned, giving rise to the need for new methodologies and, hence, new research opportunities. Given the lack of literature relating to forensic video analysis, the method presented in this paper may form a useful part of the forensic practitioner’s examination tool kit.
This paper was adapted from a presentation to the 16th International Symposium on the Forensic Sciences, Canberra, Australia (Catoggio et al. 2002).
The authors thank Julia Caluzzi, Peter Woodman, Chantel Marise, Michael Liddy, and Jim Pearson for their support and suggestions; Greg Cornel and Layton Moss for assistance in early experiments; and Bruce Koenig, Doug Lacey, and Graeme Kinraid for their technical reviews.
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