Introduction
| VHS Recording Methodology
| General Overrecording Theory
| Test Overrecordings | Examinations of Test Recordings
| Test-Recording Results
| Examples | Forensic Applications | Acknowledgments
| References
Introduction
Authenticity examinations of VHS (video home system) cassettes
are commonly performed in forensic laboratories and can usually
determine whether a submitted recording is original, continuous,
and unaltered. One of the important determinations of this analysis
is identifying any portions that have been recorded over. An overrrecording
occurs by placing a cassette in a VCR (video cassette recorder)
or a camcorder (camera-recorder) and recording over previously written
information. This paper reviews the appropriate VHS recording principles,
explains overrecording theory, details the procedures and results
of test overrecordings, provides examples, and discusses forensic
authenticity applications.
This paper is concerned with the normal overrecording process,
not insert or other editing functions available on some VCRs and
camcorders. All of the video recorders tested for this research
article employed the National Television System (or Standards) Committee
(NTSC) standard, which is used in the United States, Canada, Korea,
Japan, and some other countries; however, the methodology can be
easily adapted to other related video standards, such as PAL (Phase-Alternation
Line Standard) and SECAM (Séquentiel Couleur avec Mémoire,
which is French for Sequential Color with Memory).
VHS Recording Methodology
When a VHS cassette is inserted into a standard VCR or camcorder,
which is then placed in the record mode, the following mechanical,
electronic, and magnetic processes occur (Epstein 2003; Goodman
1996; Grob and Herndon 1999; Luther and Inglis 1999; Trundle 1996):
- The tape is pulled from its cassette housing by threading guides
and then wound over a series of rollers, spindles, guides, stationary
erase and record heads, a pinch roller/capstan mechanism, and
a helical-scan head drum. Figures 1a (without a VHS cassette)
and 1b (with a cassette) show the inside of a typical consumer
VCR with the major components labeled.
- When the record function is activated, the erase heads start
erasing any previously recorded material on the VHS tape.
- Simultaneously with the erase function, the helical-scan video
heads start recording any new video being provided to the recorder;
additionally, units with helical-scan high-fidelity (hi-fi) audio
heads start recording any new audio.
- Simultaneously with the erase, video record, and hi-fi audio
record functions, the linear audio head starts recording any new
audio being provided to the recorder.
- Simultaneously with the erase, video record, and audio record
functions, the control-track record head starts recording 29.97-hertz
(Hz) pulses.
Figure
1a: Inside of a typical consumer VCR with no VHS cassette inserted.
This figure is in Adobe Portable Document Format. To view it, you
will need to have the Adobe Acrobat Reader plug-in installed on
your computer. The Reader can be downloaded at no cost from http://www.adobe.com/products/acrobat/readermain.html.
Figure
1b: Inside of a typical consumer VCR with a VHS cassette inserted
and the front cover removed. This figure is in Adobe Portable Document
Format.
VHS Format
All VHS recorders have helical-scan video, and many have hi-fi
audio heads, which record information at a shallow angle across
the 12.65-millimeter (mm)-wide tape surface. Simultaneously, linear
control-track and audio heads record longitudinally on the tape
edges. As seen in Figure 2, which is a photograph of a recorded
segment of a VHS tape at SP (Standard Play) speed (see below), the
narrow linear audio track is at the top, the wider helical-scan
video and hi-fi audio tracks are in the middle, and the narrow control
track is at the bottom. The tracks in Figure 2 were made visible
by coating the tape with a ferrofluid (that is, magnetically developing),
which contains small iron particles in the 1- to 3-micron range,
which are suspended in Freon CFC-113, or chemically, 1,1,2-Trichloro-1,2,2-trifluoroethane
(Koenig 1990). Ferrofluids normally should not be applied to
evidential VHS tape surfaces because even with proper cleaning,
they can cause playback problems, including tracking errors; degraded
audio and video; and clogging of the video, audio, erase, and control-track
heads. However, if magnetic development is required, application
of a ferrofluid should be the final step in the examination process.
Table 1 (International Electrotechnical Commission [IEC] 1993; IEC
1994; IEC 1999) sets forth many of the standards involved in recording
and playing back the video and audio information onto VHS cassettes,
including the helical-scan angles and track widths.
Figure
2: A recorded segment of a VHS tape at SP speed that has been
magnetically developed with a ferrofluid. The angle and direction
of the helical-scan lines and the length of a single line of one
field are represented. This figure is in Adobe Portable Document
Format.
Table 1: NTSC VHS tape and track standards (all dimensions
in mm, except as noted)
FEATURE |
SP |
EP/SLP |
Videotape Width |
12.65 ±
0.01 |
Linear Tape Speed (mm/s) |
33.35 |
11.12 |
Video Tracks:
Total Vertical Height
|
10.60
|
Effective Vertical Height |
10.07 |
Width |
0.058 |
0.019 |
Helical-Scan Angle (Stationary) |
5°56′07.4″
|
Helical-Scan Angle (Moving) |
5°58′09.9″
|
5°56′48.1″
|
Azimuth Angles |
+6° ± 10′ (field 1)
–6° ± 10′ (field 2)
|
Linear Width (Stationary) |
96.86 |
Linear Width (Moving) |
96.30 |
96.67 |
Hi-Fi Audio Tracks:
Width
|
0.010 →
0.029 |
Azimuth Angles |
–30° ± 30′ (channel
1)
+30° ± 30′ (channel 2) |
Control-Track Width |
0.75 ±
0.10 |
Linear Audio Track Widths:
Monaural
|
1.00 ±
0.10 |
Each Stereo |
0.35 ±
0.05 |
Stereo Guard Band |
0.30 ±
0.05 |
Linear Audio/Control-Track
Distance from End of Field 2
|
79.244 |
79.253 |
Sources: Beeching 2001; International Electrotechnical Commission
1993, 1994, and 1999; Ryan 1992
Based on the NTSC nomenclature, each video picture (frame)
has a total of 525 scan lines of information, with approximately
485 lines containing visible information (usually lines 32.5 through
517.5). Each video frame is composed of two interlaced fields,
with the first field containing the odd-numbered scan lines and
the second field, the even-numbered lines. The playback and recording
rates are always 29.97 frames, or 59.94 fields per second. There
are only two standardized linear record/playback speeds, SP (33.35
mm/second) and EP (Extended Play, at 11.12 mm/second). EP is often
referred to as SLP (Super Long Play), and many VCRs have an interim
speed of LP (Long Play, at 22.24 mm/second), which, however, has
not been standardized (Grob and Herndon 1999; Luther 1999; Weise
and Weynand 2004).
Video Heads
To record the high frequencies required for VHS video, VCRs normally
use a spinning 62.0-mm-diameter helical-scan drum with a tape wrap
of slightly more than 180 degrees. The drum contains two embedded
video heads, 180 degrees apart on the drum, with each head recording
one of the two fields in a frame. The drum spins at 1,798.2 revolutions
per minute (rpm) in the same direction as the linear movement of
the videotape, so that each recorded field lasts 0.01668 second
(1 ÷ 59.94). Many camcorders use a smaller helical-scan mechanism
with a 41.3-mm-diameter drum, a 270-degree wrap, and a 2,250-rpm
spin speed (Trundle 1996). The helical-scan drums are placed at
a slight angle to the horizontal alignment of the tape, so that
each track starts near the lower edge of the tape and finishes near
the upper edge in a right-to-left configuration. This produces a
series of parallel diagonal tracks, each containing a recorded field.
Each video field comprises 262.5 scan lines of information, recorded
along the length of the diagonal track (see Figure 2). Although
the standards reflect that the track widths are 0.058 mm for the
SP speed and 0.019 mm for EP, in reality, on most VCRs, the actual
widths are usually narrower for SP and wider for EP. To avoid interference
between adjacent tracks, the two field tracks in each frame are
recorded at different azimuth angles, as listed in Table 1 (Beeching
2001; IEC 1994; Ryan 1992).
For illustration purposes, Figure 3 is a drawing of the curved
VHS tape path represented in a straight-line, linear mode, for a
tape at the SP speed. In this figure, the magnetic side of the tape
is shown moving right to left from a viewpoint behind the erase,
video, audio, and control-track heads. The vertical dimensions have
been enlarged four times compared to the horizontal to provide increased
detail. The video heads travel diagonally across the tape at a shallow
angle (see Figures 2 and 3), which is directly affected by the linear
tape speed. As noted in Table 1, the heads in the helical-scan drum
are at a “stationary” angle of 5°56′07.4″
(5 degrees, 56 minutes, and 7.4 seconds of arc); however, when the
tape is transported during recording or playback, the effective
angles are changed slightly, to 5°58′09.9″ for SP
speed and 5°56′48.1″ for EP. These changes in angle
affect the length of each field track on the videotape, including
its linear width, as noted in Table 1. The linear width of the tracks
recorded on the tape change from 96.86 mm for the stationary drum
to 96.30 mm for SP speed (see Figure 3) and 96.67 mm for EP.
Figure
3: A linear representation of the magnetic side of a segment
of VHS tape, aligned with the video, audio, and erase heads, in
a typical VCR at SP speed. The actual tape path is not straight,
but curved, and the helical-scan drum is circular, not flat, as
shown in Figures 1a and 1b. The vertical dimensions have been enlarged
four times compared to the horizontal to allow for increased detail.
This figure is in Adobe Portable Document Format.
The helical-scan drums often contain additional heads for better
video recording and playback at different VCR speeds, for video
special effects, for hi-fi audio, and for “flying erase”
capability.
Audio Heads
Most modern VHS recorders and camcorders have two types of audio
heads: linear and hi‑fi. The stationary linear audio head
is located exactly 79.244 mm from the very end of the second field
track of a frame at SP speed (79.253 mm at EP) and continuously
records the incoming audio information. It is usually in the same
head block as the control-track head, as shown in Figure 3. The
quality of the linear audio is directly dependent upon the linear
record speed of the recorder; thus information at the SP speed will
be of higher fidelity than at EP. The linear audio track is usually
monaural on newer VHS units, but some older recorders can have a
stereo track configuration (IEC 1993; IEC 1994; Luther 1999).
The hi-fi, or frequency-modulation (FM), audio stereo heads are
located on the helical-scan drum, often 60 degrees out of phase
with the video heads. The hi-fi audio is recorded during the 0.0334-second
time period just prior to, but in the same tape location as, the
corresponding video information. To allow differentiation of the
audio and video signals and to avoid complete erasure of the audio
track by the subsequent overlying video track, the signals have
different azimuth angles and track widths and record at different
tape depths. The video head partially erases the hi-fi audio, usually
dropping its amplitude about 12 decibels (dB). The two channels
of stereo audio are recorded with different FM carrier frequencies
for better record and playback characteristics. Because of the recording
methods, the signal-to-noise ratio, frequency response, and other
specifications are always better for hi-fi than linear audio. Hi-fi
audio quality is virtually the same at both SP and EP speeds; however,
not all VCRs have hi-fi audio capability (Beeching 2001; IEC 1999;
Trundle 1999).
Control-Track Head
The stationary control-track record head is located exactly 79.244
mm from the very end of the second field track of a frame at SP
speed (79.253 mm at EP) and is usually located in the same head
block as the linear audio head, as shown in Figure 3. The control-track
head records and reads the 29.97-Hz pulses placed on the track at
the bottom edge of the VHS tape, designating the beginning of the
first field in the two-field frame. Each synchronization pulse is
composed of a rectangular signal consisting of:
- A full-amplitude positive impulse lasting less than 200 microseconds
(µs).
- A direct-current (DC) segment lasting about 0.0222 second.
- A full-amplitude negative impulse lasting less than 200 µs.
- A DC segment lasting until the start of the next synchronization
pulse.
Each positive pulse is physically recorded on the tape at the
very end of each frame (see the bottom of Figure 2), where
the positive and negative impulses are recorded as a series of short
vertical lines. Only the positive pulses are used by the playback
VCR, where the magnetically developed control track is aligned with
the rectangular pulse signal, as reflected in Figure 4. The control
track is used to determine playback speed, to maintain proper playback
speed even with tape stretching or shrinkage, to ensure that the
playback heads properly read the recorded video tracks, and to update
the elapsed time on VCR real-time counters (IEC 1993; IEC 1994;
McComb 1995).
Figure
4: A portion of the control track from Figure 2, which has been
aligned with the rectangular signal that produced it. This figure
is in Adobe Portable Document Format.
Erase Heads
VHS recorders can contain up to four separate erase heads: full-track,
flying, linear audio, and control-track. The full-track erase head
precedes all of the record, playback, and other erase heads; it
erases all previously recorded information on the tape, including
the control, linear audio, hi-fi audio, and video tracks. Some VCR
units and many camcorders also contain a flying erase head (sometimes
two), located on the helical-scan drum. The flying erase head allows
the start of a video recording over previously recorded information
with little or no distortion or degradation of the ensuing video
images and hi-fi audio. A properly functioning flying erase head
allows for a recording with an uncorrupted image of the last frame
of the underlying recording, followed by an uncorrupted frame of
the new recording. The linear audio and control-track erase heads
normally are located in the same housing and erase their respective
tracks when the record mode is activated. Most units do not have
a separate control-track erase head and instead use the erasing
effect of the record head and the full-track erase head to delete
any underlying information (Epstein 2003; Luther 1999; Luther and
Inglis 1999; McComb 1995).
General Overrecording Theory
An overrecording occurs when new information is written over a
previous VHS recording, erasing a segment of the existing video,
audio, and control-track information, while replacing it with new
video, audio, and control-track data.
Video Changes
When a VHS video recorder is placed in the record mode over an
existing recording, the full-track erase head starts erasing all
of the information on the tape, including the video tracks. However,
at the start of the video overrecording, in the “pre-video
erased area,” the full-track erase head cannot erase the information
physically located between itself and the writing video heads (see
Figure 3). If the recorder has a flying erase head, this underlying
video information will be erased just ahead of the video-recording
process, and the new information will be recorded properly. However,
without a flying erase head, the existing video information normally
will not be completely erased by the video record head in this portion.
When the overrecording ends and is long enough for the video record
heads to have reached the fully erased portion of the VHS tape (at
the end of the pre-video recorded area), then a portion of completely
erased tape will follow the end of the newly written video. At the
end of the completely erased portion, the underlying video information
returns as very short partial-field tracks, which progressively
increase in length until the entire track is present. This portion
of partial tracks is equal to the linear width of the helical-scan
tracks because the full-track erase head produces a vertical, 90-degree
erasure on the videotape across the slanted (about 6 degrees) field
tracks. Therefore, at SP speed, the segment is 96.30 mm in length
(96.67 mm at EP), which is based on solving the sine function using
the 10.07-mm effective vertical height of the tracks and the angle
of 5º58′09.9″ (5º56′48.1″ at EP),
which results in a playback time of 2.89 seconds (8.69 seconds at
EP). If the overrecording is not long enough for the video heads
to reach the fully erased portion, then a segment of the underlying
video will be present between the end of the overrecording and the
completely erased segment.
Audio Changes
When a VHS video recorder is placed in the record mode over an
existing recording, the linear audio record, the linear audio erase,
and the full-track erase heads are activated simultaneously. The
linear audio erase head deletes the underlying audio recording,
except for the short distance between the audio erase and record
heads, where the new signal will mix with the old. The full-track
erase head starts erasing all of the information on the tape, including
the linear audio track(s). At SP speed, the linear audio information
is physically located 175.544 mm (175.923 mm at EP) from the beginning
of its matching video field track because the linear length of the
helical-scan tracks is 96.30 mm (96.67 mm at EP) and the distance
from their end to the linear audio track is 79.244 mm (79.253 mm
at EP), as shown in Figure 3. When an overrecording ends and its
physical length is at least equal to the pre-video erased area,
the linear audio does not return until 175.544 mm (175.923 mm at
EP) after the end of the erased portion. If the physical distance
of the overrecording is less than the pre-video erased area, then
a segment of the underlying audio will be present between the end
of the overrecording and the erased portion.
When the recorder has a flying erase head, the underlying hi-fi
audio information will be erased just ahead of the hi-fi audio-recording
process and the new information will be recorded properly. However,
even without a flying erase head, the FM audio information normally
is completely erased by the hi-fi audio-writing head. When the overrecording
ends and its physical length is at least equal to the distance between
the hi-fi heads and the full-track erase head, then a portion of
completely erased tape will follow the end of the newly written
audio. After the end of the completely erased portion, the underlying
hi-fi audio information is present on the videotape, but as partially
erased tracks. These partial tracks will not play back on most VCRs
because their audio-output circuitry automatically switches to the
linear audio track whenever the hi-fi signal is not present, is
mistracking, or, as in this case, has complete dropouts (Trundle
1999). If the overrecording is not long enough for the audio head
to reach the fully erased portion, then a segment of the underlying
audio will follow the end of the newly written hi-fi audio.
Control-Track Changes
When a VHS recorder is placed in the record mode over an existing
recording, the underlying control-track information is erased by
both the full-track erase head and the control-track record head.
If the overrecording is not long enough for the control-track record
head to have reached the fully erased portion, then a segment of
the underlying control-track information will follow the end of
the newly written control track.
Test Overrecordings
For this experiment, a series of test recordings was prepared
on four VCRs, ranging from consumer-level to professional-quality
units, with the following parameters:
- The video test signals were interlaced, composite, and at a
1.0-volt peak-to-peak amplitude. The test audio signals were sine
waves adjusted to peak at 0 dB on the hi-fi input meter of
each unit. On the VCRs that did not have an audio meter, the test
signals were supplied at an amplitude of -10 dB volt peak.
- For the underlying recordings, continuous audio/video recordings
were prepared at both SP and EP (SLP) speeds, with an all-green
raster and a 400-Hz audio signal.
- Overrecordings consisting of a white crosshatch grid over a
black raster video signal and a 1-kHz audio signal were then produced.
Overrecordings were made with lengths ranging from 2 to 30 seconds,
at both SP and EP (SLP) speeds.
- Additional test recordings were made with a variety of other
audio and video samples to reflect more real-world scenarios.
Examinations of Test Recordings
The test recordings were examined as follows:
- Professional-quality laboratory equipment was used, and all
connections were made using either S-video (separated video) or
BNC (Bayonet Neil-Concelman) cabling, as appropriate.
- The recordings were played back on a VCR, viewed on a high-resolution
monitor (usually in the “underscan” mode), and listened
to with high-fidelity headphones in both linear audio and hi-fi
audio modes. (Note: the VCR automatically switched to the linear
mode when either no hi-fi audio was present or the track information
was partially erased.) Optimized manual tracking, instead of automatic
tracking, was used for the underlying and overrecording segments,
as appropriate, for improved playback.
- The video was cabled through a time-base corrector (TBC) with
digital storage capability, which allowed the capture and review
of both individual frames and fields.
- The recordings were also played back on the same VCR with the
linear audio output selected, cabled through the TBC, and connected
to an external computer video-capture/audio-capture device. The
capture device was connected to a laboratory computer via a high-speed
cable and set to the Moving Picture Experts Group 2 (MPEG-2) format
at 15 million bits per second (Mbps), 720- x 480-pixel resolution,
and 48-kHz stereo linear pulse-code modulation (LPCM). The recordings
were then saved as MPEG (“.mpg”) files.
- An additional set of MPEG files was produced, using the same
procedures as number 4 above, except that the hi-fi audio output
was selected on the VCR.
- The MPEG computer files were then reviewed using software that
simultaneously displayed the video (with frame numbers added)
and the audio information.
- The MPEG computer files and the original recording for each
test were then reviewed to determine the following
- The general visual characteristics of the overrecorded and
underlying video information.
- The timing of the changes in the video information, including
the beginning and end of the overrecording, the fully erased
segment, and the partial-track portion.
- The timing of the changes in the linear audio information,
including the beginning and end of the overrecording, the fully
erased segment, and the partial-track portion.
- The timing of the changes in the hi-fi audio information,
including the beginning and end of the overrecording, the fully
erased segment, and the partial-track portion.
- The visual and time differences for overrecordings and underlying
recordings produced at different record speeds.
- The visual and time differences for overrecordings of different
lengths.
Test-Recording Results
A review of the test recordings revealed two general classes of
results, broken down by the length of the overrecordings. The longer
test overrecordings were of sufficient length to completely erase
the portion of the videotape between the full-track erase head and
the beginning of the helical-scan heads (the pre-video erased area)
(see Figure 3); whereas the short overrecordings did not completely
erase that segment.
Longer Overrecordings—Video Characteristics
When the longer class of overrecordings was played back, it produced
a sequential series of separate video segments, as follows (see
Figures 5a, 5b, 6a, and 6b):
U → O or (OU + OP + O) → E →
UNP → UN → U
where
U = |
Underlying video recording (green angled lines) |
O = |
Video overrecording (red angled lines) |
OU = |
Video overrecording with remnants of underlying
video recording—only present on overrecording VCRs without
a flying erase head—(red angled lines with green shading) |
OP = |
Video overrecording with a pull-down effect—only
present on overrecording VCRs without a flying erase head—(red
angled lines with blue shading) |
E = |
Completely erased portion (gray area) |
UNP = |
Underlying video recording with a pull-down
effect and no control-track sync (blue angled lines) |
UN = |
Underlying video recording with no control-track
sync (light-green angled lines) |
Figure
5a: Color-coded representation of a longer overrecording on
a VCR at SP speed, with the vertical dimension enlarged 35 times.
This VCR has a flying erase head. This figure is in Adobe Portable
Document Format.
Figure
5b: Color-coded representation of a longer overrecording on
a VCR at SP speed, with the vertical dimension enlarged 35 times.
This VCR does not have a flying erase head. This figure is in Adobe
Portable Document Format.
Figure
6a: Example test video clip of a longer overrecording made on
a VCR at SP speed. This VCR has a flying erase head. This figure
is an XviD-encoded MPEG-4 video file with an .avi file suffix. To
view it, you will need an XviD/MPEG-4 video codec installed on your
computer. The XviD/MPEG-4 video codec can be downloaded at no cost
from http://www.xvidmovies.com/codec/.
Figure
6b: Example test video clip of a longer overrecording made on
a VCR at SP speed. This VCR does not have a flying erase head. This
figure is an XviD-encoded MPEG-4 video file.
This sequence is illustrated in Figures 5a and 5b as recorded
on the videotapes and in Figures 6a and 6b as video clips from test
overrecordings. Figures 5a and 5b have a vertical dimension that
has been enlarged 35 times compared to the horizontal dimension.
For example, a 10-mm length in the horizontal direction would be
represented as 350 mm in the vertical direction. This nonlinear
representation is necessary to allow the complete video sequence
to be displayed over a relatively short length. These figures show
the narrow linear audio track at the top, the wide helical-scan
video and hi-fi audio tracks in the middle, and the narrow control
track at the bottom (only the positive sync pulses are displayed).
The white spaces between the tracks are the guard bands. The angled,
vertical lines in the video track represent individual fields, which
are recorded and played back at an angle from the bottom right to
the top left (because of scaling, the field tracks are at an angle
of about 75 degrees in the figures, instead of the standard of about
6 degrees, as shown in Table 1). The video line widths are not to
scale. The color coding in Figures 5a and 5b for the video, linear
audio, and control information is consistent throughout both drawings.
For example, the red vertical lines in the linear audio track are
the physical location of the linear audio for the red angled tracks
in the O video. This color coding also reflects a lack of corresponding
linear audio or control information, for example, in the blue UNP
portion, which has neither control-track sync nor linear audio.
Figures 5a and 5b represent the recorded information at SP speed.
For EP and other linear speeds, some of the values will be slightly
different (see Table 1).
When the overrecording VCR had a flying erase head, the beginning
of the overrecorded portion, O, usually produced an uncorrupted
frame transition from the underlying recorded portion, U, if both
were recorded at the same speed (segments OU and OP
are not present on units with a flying erase head). When the overrecording
VCR did not contain a flying erase head, remnants of the underlying
recording combined with the new video information, producing a pull-down
effect as illustrated in the OU and OP portions
of Figure 5b and in the Figure 6b video clip. During the OU
segment, complete field tracks played back with a mix of video from
the underlying recording, U, and the new overrecorded video. These
complete OU tracks continued until the physical distance
between the full-track erase head and the beginning of the video
record heads, which is located at the beginning of OP
on Figure 5b, was reached. The OP segment started with
the playback of very short track segments of only the overrecorded
video information, followed by the mixed information in the OU
portion. As playback continued, the percentage of the video track
containing only the overrecorded video information progressively
increased, while the mixed portion proportionally decreased, until
a full field of only the overrecorded video was present at the beginning
of the O segment. This progression produced the so-called pull-down
effect, which displayed the pure overrecorded video as a horizontal
band starting at the top of the frame (beginning of the field tracks)
and progressing downward to completely replace the mixed video at
the bottom. This pull-down effect lasted a distance of 96.30 mm
on the videotape at SP speed (see Figure 3) and 96.67 mm at EP.
Therefore, on playback this segment lasted 2.89 seconds (about 87
frames) at SP (96.30 ÷ 33.35) and 8.69 seconds (about 260
frames) at EP (96.67 ÷ 11.12). During the visual review of
OP, because only about 485 of the 525 video lines were
visible because of vertical blanking, the pull-down effect was visible
for no more than 2.67 seconds at SP and 8.00 seconds at EP, even
using the underscan feature on professional monitors. When the overrecording
was at a different recording speed than the underlying information,
the beginning of O (for the units with a flying erase head) or OU
(no flying erase head) usually had a short series of distorted frames
containing information from both the underlying video and the overrecording.
The completely erased video portion, E, followed the end of the
images in the overrecording, O. The length of this erased area was
exactly equal to the physical distance between the full-track erase-head
gap and the beginning of the video record heads on the drum of the
overrecording VCR (see Figures 1a, 1b, and 3). This erased portion
was displayed only as noise, usually in a random, herringbone-like
pattern, as seen in the Figure 6a and 6b video clips.
The next segment on the test recordings was UNP, which
is the return of the underlying video but with a pull-down effect
and no control-track sync. As reflected in Figures 5a and 5b, at
the beginning of UNP, the playback of very short track
segments of video information was followed by the erased information
in the E portion. As playback continued, the video track lengths
progressively increased, while the erased portion decreased, until
a full field of video information was present at the beginning of
the UN portion. Usually, at the beginning of the UNP
portion, the underlying information was not in color and was very
distorted, but it improved in quality as the sequence continued.
This progression produced the visual pull-down effect, which displayed
the underlying video as a horizontal band starting at the top of
the frame (beginning of the field tracks) and progressed downward
to fill the frame with the underlying video. This sequence lasted
a distance of 96.30 mm at SP speed (96.67 mm at EP) on the videotape
(see Figure 3); therefore, on playback this segment lasted 2.89
seconds (about 87 frames) at SP and 8.69 seconds (about 260 frames)
at EP. During the visual review of UNP, because only
about 485 of the 525 video lines were visible because of vertical
blanking, the pull-down effect was visible for no more than 2.67
seconds at SP and 8.00 seconds at EP, even using the underscan feature
on professional monitors. There was no control-track sync in this
portion because the control-track information for the beginning
of each field track in UNP would be located 175.544 mm
“earlier” at SP speed (175.923 at EP) on the videotape,
which had already been erased by the full-track head, as noted in
Figures 5a and 5b. At SP speed, the 175.544 mm was based on the
standardized 79.244-mm distance from the end of each field track
plus the 96.30-mm linear distance of each track (see Figure 3).
At EP speed, the 175.923 mm was based on the standardized 79.253-mm
distance from the end of each field track plus the 96.67-mm linear
distance of each track (see Figure 3). The most obvious effect of
this loss of sync was that the information for individual frames
was often combined with data from either previous or following frames,
producing images that were not a true reflection of the originally
recorded information. An additional effect of the loss of sync was
that the underlying video in this segment played back at the same
speed as the overrecording; that is, SP overrecordings with EP underlying
recordings played back the underlying recording at SP instead of
EP speed.
The last segment of the overrecordings, UN, had all
of the video information from the underlying recording but had no
control-track sync because it had been erased by the full-track
erase head. This portion was exactly 79.244 mm in length at SP speed
(79.253 mm at EP), based on the standardized distance between the
end of the video heads and the linear audio record head (see Figure
3). This segment lasted 2.38 seconds (about 71 frames) at SP
playback and 7.13 seconds (about 214 frames) at EP. As the UP
portion had, this portion played back at the overrecording speed,
regardless of the underlying recording speed, and had sync problems.
Longer Overrecordings—Audio Characteristics
The hi-fi audio information played back only in portions where
complete video fields were present on the videotape and not, for
instance, in the UP pull-down segments. Therefore, hi-fi
audio was present throughout the overrecording (the O, OU,
and OP segments) but then ended and did not return until
the beginning of UN. However, when the overrecording
was at a different speed than the underlying information, the hi-fi
audio did not play back until after UN. Unlike the video
information recorded by VCRs without a flying erase head, there
was no observed mix of the underlying and overrecorded hi-fi audio
information in the OU segment. The start and stop times
in these segments were very close to those of the video information,
except when the overrecording and underlying recordings were recorded
at different speeds. In those cases, a delay usually occurred at
the beginning because of the speed change.
The linear audio was present during the overrecorded video segments
O, OU, and OP, except when the underlying
information and the overrecording were recorded at different speeds.
In those instances, a delay usually occurred at the onset of the
recorded audio. After the end of the overrecording, there was no
high-level recorded linear audio information until the end of UN;
however, low-level audio (30 to 45 dB below the original amplitude)
from the underlying recording was often present in this portion.
Longer Overrecordings—Control-Track Characteristics
The control-track information matched the high-level information
on the linear audio track. Therefore, whenever the control-track
and linear audio track recordings were not present, the video information
lacked control-track synchronization and linear audio, as noted
in Figures 5a and 5b.
Short Overrecordings—Video Characteristics
When the short overrecordings were played back, they produced
a sequential series of separate video segments that were mostly
different from the longer overrecordings, as follows (see Figures
7a, 7b, 8a, and 8b):
U → O or OU → U → UP →
E → UP → U → UN → U
where
UP = |
Underlying video recording with an erased area
pull-down (light-blue angled lines) |
This sequence is illustrated in Figures 7a and 7b as recorded
on the test videotapes and in Figures 8a and 8b as video clips from
test overrecordings. Figures 7 and 8 have the same scaling and other
characteristics as Figures 5 and 6, respectively.
Figure
7a: Color-coded representation of a short overrecording on a
VCR at SP speed with the vertical dimension enlarged 35 times. This
VCR has a flying erase head. This figure is in Adobe Portable Document
Format.
Figure
7b: Color-coded representation of a short overrecording on a
VCR at SP speed with the vertical dimension enlarged 35 times. This
VCR does not have a flying erase head. This figure is in Adobe Portable
Document Format.
Figure
8a:: Example test video clip of a short overrecording made on
a VCR at SP speed. This VCR has a flying erase head. This figure
is an XviD-encoded MPEG-4 video file.
Figure
8b:: Example test video clip of a short overrecording made on
a VCR at SP speed. This VCR does not have a flying erase head. This
figure is an XviD-encoded MPEG-4 video file.
The O and OU segments had the same general characteristics
as the longer overrecordings, except they were shorter in length
and usually had no pull-down area, OP, because the short
overrecordings were shorter than the pre-video erased area.
Between the end of the overrecorded portion and the erased area,
there was a short segment of the original underlying recording,
U, that was not present on the longer overrecordings.
The next segment contained an erased portion, E, in the middle
of the underlying recording. The erased portion physically appeared
on the videotape as a vertically erased area across the angled video
tracks of the underlying information, as seen in Figures 7a and
7b. When played back, the erased, herringbone-like portion started
appearing at the top of the frame, progressively replacing the partial
tracks of the first underlying recording segment, UP.
At the end of the erased portion, E, the partial tracks of the second
UP video started appearing at the top of the frame, with
the erased information appearing as a band below it and the first
UP information beneath it, as seen in Figures 8a and
8b. These three bands progressed downward in the frame, with the
first UP portion disappearing off the bottom first, followed
by the erased portion, and finally, only the second UP
segment is visible at the beginning of the next U segment. The length
and time of the erased area were exactly equal to the length of
the short overrecording, either O or OU. One method of
determining this distance and its timing is to count the total number
of video lines in the erased band and then use the following formulas:
Length (in mm) = |
(linear length of video track) x (scan lines
in erased band) ÷ (total lines in frame) |
Time (in seconds) = length ÷ playback speed
As an example, at SP speed with 350 scan lines in the erased band,
the length and time would be:
Length = 96.30 mm x 350 ÷ 525 = 64.20 mm
Time = 64.20 mm ÷ 33.35 mm/sec = 1.92 seconds
The second UP portion occurred across the full linear
width of the scanning video heads, a distance of 96.30 mm at SP
speed (96.67 mm at EP). On playback this segment lasted 2.89 seconds
(about 87 frames) in SP and 8.69 seconds (about 260 frames) in EP.
During the visual review of the second UP portion, because
only about 485 of the 525 video lines were visible because of blanking,
the pull-down effects were visible for no more than 2.67 seconds
at SP and 8.00 seconds at EP, even using the underscan feature on
professional monitors. This area played back at the correct speed,
even if the underlying recordings and overrecordings were at different
speeds, because the control-track information was present. If the
overrecording measured more than 79.244 mm in length at SP speed
(79.523 mm at EP), then the end of the second UP portion
lacked synchronization.
The second underlying recording segment, U, appeared after the
end of the second UP portion and contained synchronization
and full tracks. However, if the overrecording measured more than
79.244 mm in length at SP speed (79.523 mm at EP), then this U segment
was not present.
The last segment of this overrecording sequence was UN,
which had all of the video information from the underlying recording
but no control-track sync. The loss of sync was caused by the erasure
of the control-track information in segment E, which was located
175.544 mm earlier at SP speed (175.923 mm at EP). The combined
length of UN and the preceding U segment was exactly
79.244 mm at SP speed (79.253 mm at EP), based on the standardized
distance between the end of the video heads and the linear audio
record and control-track heads. The length of UN was
the same as the erased/overrecorded portion whenever UN
did not exceed 79.244 mm in length at SP speed (79.523 mm at EP).
Short Overrecordings—Audio Characteristics
The hi-fi audio information was present throughout the overrecording
and the first U segment (after O or OU) and then returned
at the beginning of the second U segment before UN. These
results reflect that the hi-fi audio was present only in the portions
that contained complete video fields. If the overrecording and underlying
recordings were recorded at different speeds, then a delay usually
occurred at the beginning because of the speed change.
The linear audio normally was present throughout the entire overrecording
sequence, except for the UN portion; however, if the
overrecording was more than 79.244 mm in length at SP speed (79.523
mm at EP), then the linear audio was not present at the end of the
second UP portion (the second U segment was not present).
Because of the erasure of the linear audio track in the E segment,
the audio was not present 175.544 mm ahead of the entire erased
portion at SP speed (175.923 mm at EP); therefore, the entire UN
segment lacked linear audio. If the overrecording and underlying
recordings were recorded at different speeds, then a delay usually
occurred at the beginning because of the speed change.
Short Overrecordings—Control-Track Characteristics
The control-track information matched the high-level information
on the linear audio track.
Examples
The following three examples of overrecording configurations are
commonly encountered in forensic applications and are based on the
previously described video overrecording theory and test results.
All three examples use 80.00 mm for the pre-video erased area, and
the frame numbers have been rounded to the nearest whole number.
Both the underlying recording and overrecording VCRs have hi-fi
audio, and some of the Video Sequence totals in Tables 2 through
4 have slight variances because of rounding errors.
The first example is a 10.00-second overrecording with the following
characteristics: (1) the underlying recording is at SP speed, (2)
the overrecording is at SP speed, and (3) the underlying recording
and overrecording VCRs have flying erase and hi-fi audio heads.
As listed in Table 2 and generically illustrated in Figure 5a, this
overrecording configuration is divided into four segments:
- Segment O is the overrecording, lasting 10.00 seconds, which
is recorded on the videotape over a total of 333.50 mm (10.00
x 33.35) and consists of 300 frames (10.00 x 29.97). The control
track is present during this portion, and so are the linear audio
and hi-fi audio.
- The 80.00-mm erased segment, E, which is the pre-video erased
area, has no video, hi-fi audio, linear audio, or control-track
information because they were erased by the full-track erase head.
This portion lasts 2.40 seconds (80.00 ÷ 33.35).
- Segment UNP is a pull-down portion of partial tracks,
without synchronization, of the underlying recording. It is 96.30
mm in length, lasts 2.89 seconds (96.30 ÷ 33.35), and consists
of 87 frames (2.89 x 29.97). The 96.30-mm distance represents
the linear distance of the full helical-scan video-head track
at SP speed. The pull-down consists of the underlying recorded
information progressively replacing the erased segment. The hi-fi
audio does not play back in this area, even though it is present
on the videotape, because only partial tracks are present. The
linear audio and control-track information are not present because
they have been erased by the full-track erase head. If the audio
and control data had been present, they would have been located
175.544 mm earlier on the tape (see Figure 5a).
The last portion of the overrecording sequence is segment UN,
which contains the underlying recording without synchronization.
It is 79.244 mm in length, lasts 2.38 seconds (79.244 ÷
33.35), and consists of 71 frames (2.38 x 29.97). The 79.244-mm
length represents the distance between the end of the video tracks
and the recording control-track and linear audio heads. The hi-fi
audio information is present in this portion, but the linear audio
and control-track information have been erased by the full-track
erase head. Following this UN segment, the underlying
recording, U, returns with the video, audio, and control-track
information intact.
Table 2: An example of the representative video, audio, and control-track
times, lengths, and frames for a 10.00-second overrecording with
the following characteristics: (1) the underlying recording is at
SP speed, (2) the overrecording is at SP speed, and (3) the underlying
recording and overrecording VCRs have flying erase and hi-fi audio
heads. The distance from the full-track erase head to the video-recording
head has been designated as 80.00 mm, the frame numbers have
been rounded to the nearest whole number, all lengths are in mm,
and all times are in seconds.
The second example is a 20.00-second overrecording with the following
characteristics: (1) the underlying recording is at SP speed, (2)
the overrecording is at EP speed, and (3) the underlying recording
and overrecording VCRs have hi-fi audio heads, but the overrecording
unit does not have a flying erase head. As listed in Table 3 and
generically illustrated in Figure 5b, this overrecording configuration
is divided into six segments:
- Segment OU is the first part of the total overrecording
and contains a mixture of both the underlying recording and overrecording
because there is no flying erase head. This section is 80.00 mm
in length, based on the pre-video erased area between the full-track
erase head and the beginning of the video-head recording. This
segment lasts 7.19 seconds (80.00 ÷ 11.12) and consists
of 216 frames (7.19 x 29.97). The control track is present during
this portion, and so are the linear audio and hi-fi audio.
- Segment OP is the pull-down segment and the second
part of the total overrecording. It starts at the beginning of
the full-track erasure and ends 96.67 mm later. The 96.67-mm distance
represents the linear distance of the full helical-scan video-head
track at EP speed. The pull-down consists of the pure overrecording
information progressively replacing the mixture of the underlying
recording and the overrecording. This segment lasts 8.69 seconds
(96.67 ÷ 11.12) and consists of 260 frames (8.69 x 29.97).
The linear audio, hi-fi audio, and control-track information are
present during this segment.
- Segment O is the pure overrecording segment and the third part
of the total overrecording. It lasts 4.12 seconds (20.00 - 7.19
- 8.69), covers a length of 45.81 mm, and consists of 123 frames
(4.12 x 29.97). The linear audio, hi-fi audio, and control-track
information are present during this segment.
- The 80.00-mm erased segment, E, which is the pre-video erased
area, has no video, hi-fi audio, linear audio, or control-track
information because they were all erased by the full-track erase
head. This portion lasts 7.19 seconds (80.00 ÷ 11.12) and
has an equivalent of 216 frames (although no video information
is actually present).
- Segment UNP is a pull-down portion of partial tracks,
without synchronization, of the underlying recording. It is 96.67
mm in length, lasts 8.69 seconds (96.67 ÷ 11.12), and consists
of 260 frames (8.69 x 29.97). The hi-fi audio does not play back
in this area, even though it is present on the videotape, because
only partial tracks are present and the playback speed is incorrect
(EP versus SP). The linear audio and control-track information
are not present because they have been erased by the full-track
erase head. If the audio and control data had been present, they
would have been located 175.923 mm (96.67 + 79.253) earlier on
the tape.
- The last portion of the overrecording sequence is segment UN,
which contains the underlying recording without synchronization.
It is 79.253 mm in length, lasts 7.13 seconds (79.253 ÷
11.12), and consists of 214 frames (7.13 x 29.97). The hi-fi audio
information is not present in this portion because the playback
speed is incorrect (EP versus SP). The linear audio and control-track
information are also not present because they have been erased
by the full-track erase head. Following this UN segment,
the underlying recording, U, returns with the video, audio, and
control-track information intact.
Table
3: An example of the representative video, audio, and control-track
times, lengths, and frames for a 20.00-second overrecording with
the following characteristics: (1) the underlying recording is at
SP speed (2) the overrecording is at EP speed, and (3) the underlying
recording and overrecording VCRs have hi-fi audio heads, but the
overrecording unit does not have a flying erase head. The distance
from the full-track erase head to the video-recording head has been
designated as 80.00 mm, the frame numbers have been rounded to the
nearest whole number, all lengths are in mm, and all times are in
seconds.
The third example is a 2.00-second overrecording with the following
characteristics: (1) the underlying recording is at SP speed, (2)
the overrecording is at SP speed, and (3) the underlying recording
and overrecording VCRs have flying erase and hi-fi audio heads.
As listed in Table 4 and generically illustrated in Figure 7a, this
overrecording configuration is divided into six segments:
- Segment O is the overrecording, lasting 2.00 seconds, which
is recorded on the videotape over a total of 66.70 mm (2.00 x
33.35) and consists of 60 frames (2.00 x 29.97). The control track
is present during this portion, and so are the linear audio and
hi-fi audio.
- The first U segment is the return of the underlying recording
for 13.30 mm, or 0.40 second (13.30 ÷ 33.35), with the
control track, the linear audio, and the hi-fi audio present.
On short overrecordings, the total of the O and first U segments
equals the pre-video erased area length.
- The E–UP pull-down segment has the same length
and timing as the overrecording, or 66.70 mm, 2.00 seconds, and
60 frames. This pull-down consists of the underlying recorded
information being progressively replaced with the erased segment,
E, until the erased portion is at the top and the underlying recording
is at the bottom of the frame. The linear audio and control-track
information are present; however, the hi-fi audio is not present
because there are only partial video tracks.
- The UP–E pull-down segment is 96.30 mm in
length, lasts 2.89 seconds (96.30 ÷ 33.35), and consists
of 87 frames (2.89 x 29.97). The 96.30-mm distance represents
the linear distance of the full helical-scan video-head track
at SP speed. The partial tracks of the second UP video
start appearing at the top of the frame, with the erased information
appearing as a band below it and the first UP information
beneath it. These three bands progress downward in the frame,
with the first UP portion disappearing off the bottom
first, followed by the erased portion, and finally, only the second
UP segment is visible at the beginning of the second
U segment. The hi-fi audio does not play back in this area, even
though it is present on the videotape, because only partial tracks
are present. The linear audio and control-track information are
present.
- The second U segment is the return of the underlying recording
for 12.54 mm, 0.38 second (12.54 ÷ 33.35), and 11 frames.
The control track, the linear audio, and the hi-fi audio are present.
On short overrecordings, the total length of the second U and
the UN segments equals the 79.244-mm distance between
the end of the video tracks and the recording control track and
linear audio heads.
- The last portion of the overrecording sequence is segment UN,
which contains the underlying recording without synchronization.
It is the same length and time as the overrecording, or 66.70
mm, 2.00 seconds, and 60 frames. The linear audio and control-track
information are not present because they were erased by the full-track
erase head; however, the hi-fi audio is present. Following this
UN segment, the underlying recording, U, returns with
the video, audio, and control-track information intact.
Table
4: An example of the representative video, audio, and control-track
times, lengths, and frames for a 2.00-second overrecording with
the following characteristics: (1) the underlying recording is at
SP speed, (2) the overrecording is at SP speed, and (3) the underlying
and overrecording VCRs have flying erase and hi-fi audio heads.
The distance from the full-track erase head to the video-recording
head has been designated as 80.00 mm, the frame numbers have
been rounded to the nearest whole number, all lengths are in mm,
and all times are in seconds.
Forensic Applications
The analyses of the test overrecordings and the underlying video-recording
principles reflect a number of audio and video parameters that can
be measured and considered by the authenticity examiner. The following
are recommended examination procedures for analyzing VHS cassettes
for suspected overrecordings and a list of observations that can
be important to the forensic examiner.
Examination Procedures
The following are the generally recommended examination procedures
for reviewing an evidential VHS videocassette in the laboratory.
However, other analysis steps or modification of some of the recommendations
may be necessary for a specific recording.
- The VHS videocassette should be played back on a professional-quality
VCR and visually reviewed on a professional monitor with underscan
capability and/or digitized and reviewed using an appropriate
high-resolution computer monitor (with appropriate hardware and
software that enables the user to view individual fields). The
separate optimized manual-tracking setting, instead of automatic
tracking, should be used for the underlying and overrecording
segments because this often produces better playback, especially
when there is a speed change. If audio and/or video problems are
encountered on one VCR, then playback on other brand/model VCRs
is advisable.
- The video signal from the professional-quality VCR should be
routed through a TBC, which will add stability in the areas lacking
a control track.
- The linear audio and hi-fi audio should be listened to separately
on the original videocassette. Those portions containing no hi-fi
audio (when the VCR automatically switches to linear audio) should
be identified.
- The audio and video information in question should be digitized
with a high-quality video-capture/audio-capture device, using
an appropriate format that exceeds the quality of the VHS recording.
- A software program should be used that can simultaneously display
both the audio and the individual video frames and/or fields,
with accurate time registration and frame/field numbering.
- An initial determination should be made of whether the suspected
event is consistent with the longer or shorter overrecording configuration.
- Based upon the specific characteristics of the overrecording
category, the pertinent audio and video times should be measured.
If no audio was recorded on an evidential tape, then record- and
erase-head signals, changes in the noise floor, and other indications
of segment boundaries should be identified.
- The parameters identified with the specific type of overrecording
should be compared with the information on the videocassette in
question.
Test-Recording Observations
- The test recordings clearly showed the importance of the linear
audio information, which also reflects the presence or absence
of the control track. This was useful even with videotapes containing
no high-level linear audio because system artifacts were often
recorded on the linear audio track at segment boundaries.
- It was important to identify the portions that lacked synchronization
because those areas often had video images with a loss of color,
added noise, and vertical instability.
- Pull-down portions usually did not show accurate video information
and had color loss, added noise, and synchronization problems.
- In some cases, the linear audio was not completely erased by
the full-track erase head; the remaining audio signal was about
30 to 45 dB below its original amplitude. This was probably an
effect of the full-track erase head, which is designed to delete
the helical-scan tracks and not the linear track information.
- The linear audio and the control tracks are in the same head
stack, and thus, when linear audio was present, the control track
was also present. Similarly, when the linear audio was erased,
the control track was also erased.
- A thorough examination of the separate video, linear audio,
hi-fi audio, and control-track information provided the best representation
of the overrecording sequence.
- The updating/stationary counter on most real-time counters
is a direct indication of the presence or absence of control-track
pulses.
- Because super VHS (S-VHS) and compact VHS (VHS-C) use pertinent
standards that are identical to VHS, the findings of this paper
apply to those formats as well (IEC 1991; IEC 1993).
Acknowledgments
The authors thank the following individuals who reviewed this
paper and provided important technical and grammatical improvements:
Holly Breault and George Skaluba (FBI, Quantico, Virginia); Marla
E. Carroll (Unilux, Ltd., Fort Lauderdale, Florida); Dean Catoggio,
Bryson Shearwood, and Jason Ferridge (Victoria Police, Macleod,
Victoria, Australia); Steven A. Killion (BEK TEK LLC, Clifton, Virginia);
Graeme Kinraid (Australian Federal Police, Canberra, Australia);
and Wayne R. Runion (U.S. Army Crime Laboratory, Fort Gillem, Forest
Park, Georgia).
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Bruce E. Koenig is the senior examiner and owner of BEK TEK LLC. He has been involved in the Forensic Analysis of Audio and Video Recordings on a full-time basis since 1974. Mr. Koenig & his associates conduct forensic examinations of audio and video media (analog and digital) to authenticate recordings, improve intelligibility, identify/classify voice and non-voice signals, and compare voice samples.
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