Expert Article Library
Verifying The Integrity Of Audio And Videotapes
by Steve Cain
By Steve Cain Email: info@tapeexpert.com
An ever increasing reliance on tape evidence in criminal prosecutions,
especially in organized crime and drug cases, underscores the importance of tape
integrity and the methods used to qualify or disqualify tape evidence.
This article will discuss some of the procedures
utilized in analog and digital editing of tapes and assess their potential
threat vis-a-vis tape tampering issues; the "legal admissibility"
issue surrounding tape recorded evidence to include defining strategies for the
defense to require the government to release the 'best evidence' for analysis
purposes; and an overview of the accepted techniques for the scientific analysis
of recorded tape evidence.
Tape Editing Technology,
The forensic examination of "tampered tapes"
should include an inspection of the original tape(s) and the recorder(s) used to
produce the tape(s). In the simple case, the existence of an electronic edit
and/or evidence of physical splicing will produce acoustic irregularities which
can be viewed with instruments and documented.
Modern day technology was apparently used in the
electronic editing performed on the disputed Gennifer Flowers/Gov. Bill Clinton
tape recordings. The Cable News Network (CNN) asked that I provide an expert
opinion on Mr. Clinton's voice and also asked that I examine the tape submitted
by the STAR News Magazine for any evidence of possible tampering. The later
examination disclosed a number of suspicious acoustic events (anomalies)
including: a total loss of signal (dropouts) ;a change in the speakers'
frequency response during different telephone conversations; and
"spikes" (audible sounds of short duration which are often
attributable to normal stop/start and pause functions of the recorder).
In order to provide any definitive conclusion, I
requested the original recorder and tape to determine if these electronic edits
were intentional edits or possible malfunction/anomalies of the
recorder/microphone equipment. CNN has never received the requested tape or
recorder from the Star News Magazine.
Digital editing of both audio and video tapes, however,
greatly complicates the issue and increases the likelihood that altered tapes
can escape detection.
The Federal Bureau of Investigation (FBI) Signal
Analysis Branch has already acknowledged, "It is difficult to detect some
alterations when a recording is digitized into a computer system, physically or
electronically edited and recopied on to another tape." *1*
The days of utilizing a razor blade and splicing tape to
effectively alter or "doctor" a recorded conversation are all but
gone. Right now there are at least twenty manufacturers of desktop computer
editing work stations or digital recorders which can be used as "turn
key" editing systems. Software and add on computer cards can transform an
IBM personal computer or a Macintosh computer into a sophisticated digital audio
editing machine. Some of the systems require a digital audio recorder for
initial conversion of the analog format before accessing the computer hardware.
These editing work stations were developed to save the motion picture and
recording industries money by precluding the necessity of recording sessions or
to correct subtle errors in multi track releases.
Some computer boards and software cost less than a
$1,000,and provide both recording and editing of sound in an IBM compatible or
Mac personal computer format. Editing options are practically inexhaustible thus
giving the operator the ability to alter the tape in a word processor type of
mode (i.e. cut and paste, copy, delete, etc.) while selected playback files
utilize subdue cross fading effects that can "shape" the sound. The
typical telltale signs of traditional analog recorder editing including
"clicks, pops" and other short duration sounds, can now all be
effectively removed without any detectable, audible clue.
Traditional Editing Techniques
Present tape editing practices include either physical
splices or electronic editing on one or more analog tape recordings whenever
portions of selected conversations are over recorded (i.e. erased) or the
original recorder was stopped and restarted inappropriately. While listening to
the tape, the attorney may first suspect an alteration by noting either
unexplained transients, equipment sounds, extraneous voices, or inconsistencies
with provided written information.
The major categories of tape alterations include; (1)
Deletion; (2) Obscuration; (3) Transformation; and (4) Synthesis *2*
Deletion of unwanted material can readily be done through splicing or by using
one or more recorders to erase, rerecord, or stop/pause the recorder at
strategic points within the conversation. Obscuration involves the distortion of
a recorded signal with the purpose of rendering selective portions
unintelligible. This method, for example, was used during the editing of the
infamous 18 minute gap in the Watergate tapes. This technique is also used to
.mask splices, clicks, or suspicious transients and is more difficult to detect
than deletion methods. By judicious use of two tape recorders, one may add
"noise" to the copy and thereby mask the original recording and render
it less intelligible. One can also reduce the volume of the slave recorder and
thus weaken the amplitude of target conversations on the original tape.
Transformation involves the alteration of portions of a
recording so as to change the meaning of what is said. The technique is similar
to deletion practices but greater skill and care must be applied as a knowledge
of acoustic phonetics is required to avert a suspicious edit.
Lastly, synthesis is the generation of artificial text
by adding background sounds or conversation to the tape copy which were not
present on the original recording. The addition of selective phrases can be
accomplished if a sufficient data base library of recorded conversations is
available. It must be emphasized that all of the traditional analog methods of
altering audiotapes can be more efficiently and surreptitiously accomplished
through the use of digital editing work stations.
Tape Authentication And Detection Of Edits
With the threat of digital editing looming larger, it is
more inoperative than ever that both the official tapes and recorders be made
available for inspection.
The FBI's Signal Analysis Branch has developed a set of
well defined procedures for the acceptance of authentication requests which
provides an excellent overview of what the government considers to be essential
for a scientifically valid tape analysis:
1. Sworn testimony or written allegations by
defense, plaintiff, or government witnesses of tampering or other illegal acts.
The description of the problem should be as complete as possible, including
exact location in recording, type of alleged alteration, scientific test
performed, and so on;
2. The original tape must be provided. Copies of a
duplicate tape cannot be authenticated and are normally not accepted for
examination by the FBI;
3. The tape recorders and related components used
to produce the recording must be provided; and,
4. Written records of any damage or maintenance
done to the recorders, accessories, and other submitted equipment must be
provided.
In addition, there must be a detailed statement
from the person or persons who made the recording describing exactly how it was
produced and the conditions that existed at the time, including:
A. Power source, such as alternating current, dry
cell batteries, automobile electrical system, portable generator.
B. Input, such a telephone, radio, frequencies
(Rf) transmitter/receiver, miniature microphone, etc.
C. Environment, such as telephone transmission
line, small apartment, etc.
D. Background noise, such as television, radio,
unrelated conversations, computer games, etc.
E. Foreground information, such as number of
individuals involved in the conversation, general topics of discussion,
closeness to microphone, etc.
F. Magnetic tape, such as brand, format, when
purchased and whether previously used.
G. Recorder operation, such as number of times
turned on and off in the record mode, type of keyboard or remote operation for
all known record events, use of voice activated features, etc.
Also recommended is a typed transcript of the
recording, to include both English and foreign language versions *3*
It is essential in all tape authentication exams
to obtain the original recorder and tape, as copies cannot normally be
authenticated. If the defense is encountering difficulties in obtaining the
necessary "originals" they may wish to cite Koenig's article'*4*as
an authoritative resource which specifies the reasons why the original evidence
is essential in any tape tampering request.
If the original tape and recorder are not available for
inspection, the forensic expert can still conduct a preliminary examination of
the submitted "copy" for any evidence of discontinuous recorder
operation, although all conclusions must necessarily be qualified regarding
possible editing effects. The examination process normally includes both an
aural, physical, and instrumental analysis of the evidential tape. Phase
continuity, speed determination, azimuth determination, waveform analysis,
spectrographic and narrow band spectrographic analysis are among the techniques
employed to evaluate the tape.
The techniques and tests are usually adequate in the
detection of altered analog recordings. Fortunately, the vast majority of
altered tapes today are still analog tapes.
Defense counsel should have a working knowledge of how tapes
are analyzed.
First, there is a physical inspection of the submitted
tape, the tape housing, the tape recorder and all ancillary equipment used to
make the original recording:
microphones, telephone couplers, transceivers, etc. A magnetic development test
involves the application of a special fluid which under proper magnification
will make visible the head track configuration, off-azimuth recordings,
start/stop functions, damage to recording heads, etc. The forensic expert can
subsequently determine whether the submitted tape is a copy, has been
over-recorded, or was made on a different recorder than the one submitted. The
original recorder can be detected by slight speed fluctuations and deformities
in the rotating parts which provide a unique "wow and flutter"
signature which can be measured. Also, spectrum analysis can be used to measure
slightly different signals transmitted through the microphone or telephone
equipment. All of the signal analysis equipment can be useful in answering
questions related to bandwidth, distortion effects, or unique tones generated
during the original recording process.
Forensic Video Examinations
The forensic video examiner is concerned with the authenticity and integrity of
the signal. Questions relating and whether the tape is a copy, a compilation of
other tapes or an edited version are of important consideration. Forensic
examinations of videotapes usually consist of both a visual and aural
examination. One of the more important pieces of equipment used in forensic
video examinations is a waveform monitor which is a specialized oscilloscope. It
displays the voltage versus time modes and has specialized circuits to process
the signal. If any editing occurs, then its possible to display the signal
aberration on the display screen of the instrument.*5*
Additional tests include measurements of the
chrominance, hue and burst of the color videotape by using a vector scope. The
vector scope measures the chrominance information and allows for the examination
of matching bursts of multiple signals. It also permits the investigation of
edit points.
Vertical, interval and horizontal information known as
video synchronizing information can be observed on a cross pulse monitor. This
"cross pulse" information can be viewed on a cross pulse monitor and
with proper application, one can often determine if the videotape is a copy or
an original. In cases where the helical heads are out of alignment, a set of
marks could exist for each succeeding generation or copy.*6*
Lastly, if one suspects videotape editing, the examination will require a
frame-by-frame inspection, with the use of waveform monitors, vector scopes, and
a cross pulse monitor together with other forensic equipment as deemed
appropriate. It must be noted that there are sophisticated production studios
that can edit videotapes in such fashion that traditional methods of detection
are no longer adequate. Studios capable of producing such tapes are, for now,
generally limited to larger metropolitan areas.
Legal Issues/Admissibility
In their article, "Attacking The Weight Of The
Prosecution's Science Evidence,"*7*
authors Edward J. Imwinkelried and Robert Scofield explore the thesis that the
accused has a constitutional right to introduce expert testimony which can
generate a reasonable doubt. The authors warn, however, that this right to
relevant criminal evidence is in fact very limited in scope, namely; (1)
important or "crucial evidence" and; (2) the defense must show that
the evidence is "trustworthy."
Likewise, authors Nancy Hollander and Lauren Baldwin
point out that the admissibility of an expert's testimony is often dependent on
whether the expert is testifying for the defense or for the prosecution ."*8*
In the field of forensic tape analysis, there exists few
competently trained and certified experts available to the defense to challenge
the accuracy of government tapes and/or the conclusions of the government
experts. Even though I have over twenty years experience in federal law
enforcement and as a Treasury Department crime laboratory supervisor, I am
routinely subjected to concerted efforts by the prosecution to attack my
credibility and the accuracy of my conclusions. As you would expect, as a
government expert, I never received any criticisms from the prosecutor
concerning my credentials or accuracy of my findings.
Access To Evidence
More and more courts are being forced to address the
question of whether the government has the privilege to withhold technical data
from a defendant challenging the integrity of electronic surveillance evidence.
A few courts have recognized "qualified privilege" for the government
to such data (by drawing an analogy to an "informer's privilege"), but
have not been very sensitive to the unique nature of electronic surveillance
evidence nor defined the showing required to overcome the government's
"qualified privilege." Under the due process clause, criminal
defendants should be afforded a meaningful opportunity to present a complete
defense.*9* To safeguard this right the
court has recognized the principal of "constitutionally guaranteed access
to evidence ....*10* This access to
evidence however, is not absolute as indicated in Roviaro v. United States,*11*"
wherein the court recognized the government's limited privilege to withhold the
identity of informers. Two circuit courts of appeal have extended the limited
privilege recognized in Roviaro to the nature and location of electronic
surveillance equipment."*12*
In Angiulo and Cintolo, the appellants
asserted that the district court had mistakenly barred questions concerning
providing them the precise location of microphones hidden in an apartment. Trial
motions for the information had not been made nor had the defendants offered any
technical basis for the value of the information. The government successfully
objected to the questions concerning the microphones location on the grounds
that it would reveal sensitive surveillance techniques and jeopardize future
criminal investigations.
In upholding the district court, the First
Circuit, citing Van Horn *13* and United
States v. Harley,*14* and making an
analogy to the informers privilege in Roviaro held that a qualified
privilege against compelled government disclosure of sensitive investigative
techniques exists."*15* The
privilege can be overcome, however, by a sufficient showing of need. The
defendant must show that, "he needs the evidence to conduct his defense and
that there are no adequate independent means of getting at the same point."*16*
The Cintolo court stressed that the extent to which adequate alternative
means could have substituted for the proper testimony is "a key to
evaluating this claim of necessity.*17*
As technological advances have occurred in digital
editing, there likewise has been a tremendous increase in the number of body
worn FM transmitters and other recording devices used by law enforcement to
collect evidence against defendants. It should be emphasized, however, that some
of this evidence may not be admissible in court if the agencies do not comply
with several Federal Communication Commission (FCC) regulations. First, all
nonfederal agencies must use only transmitters that are approved by the FCC and
without this approval the transmitter is not considered a legal transmitting
device and therefore cannot be legally used to gather evidence. Secondly, state
and local agencies must be licensed in the FCC's Police Radio Service and thus
far most departments reportedly have not met this requirement. These
observations are part of the information contained in "Equipment
Performance Report: Body Worn FM Transmitters," a report of the Technology
Assessment Program (TAP). This program tested nine Body-Worn FM transmitters in
accordance with National Institutes of Justice (NIJ) Standard 0214.01. These
standards require transmitters passing the test to provide intelligible audio
signals that result in acceptable quality voice recordings.*18*
As noted in the Cintolo and Angiulo decisions,
the defense failed to provide a sufficient showing of necessity, thus, it is
imperative that defense experts vouch for the necessity of access to the
government evidence as soon as possible.
The Need For Original Recording Equipment And How To Get If
There are a number of valid scientific reasons for
accessing original tapes, recorders, and related equipment to conduct a proper
analysis.
In practically every creditable forensic
publication dealing with forensic tape analysis procedures, the authors
emphasize the necessity of examining the original evidence or a direct patch
cord copy. In many cases, however, experience has shown
an unwillingness of the government prosecutor and agents to provide such
materials to the defense for examination purposes. The government may object
that the defense never requested the original or direct copy recordings and
therefore, their motions for access at the eleventh hour are basically
"delay strategies." This argument can be effectively countered if the
defense obtains an appropriate court order requesting the defense expert be
provided access to the required "best evidence recordings."
Secondly, the government may contend that it has a
qualified (if not absolute) privilege of withholding technical data from the
defense counsel citing "National Security" or indicating that such
release may jeopardize future criminal investigations. The Anguilo and Cintolo
decisions provide the defense counsel relief from such government actions.
Counsel must show the need for the evidence to conduct the defense and that
there "is no adequate independent means of getting at the same
points."
The importance of the defense obtaining the original or
at least a direct patch cord copy of all evidential recordings cannot be over
emphasized. In practically every case I have seen, the copy initially provided
by the government was not adequate for the best voice identification, tape
enhancement or tape authentication examination. Subsequent motions filed by the
defense citing the aforementioned requisite need for the original evidence often
results in its release by the court. As reflected in the newly approved
International Association for Identification standards for analysis of
questioned voice recordings, the "unknown and known voice samples must be
original recordings, unless listed as a specific exception ...."*19*
Notes:
1. Bruce E. Koenig, Authentication of Forensic Audio Recordings,
JOURNAL OF AUDIO ENGINEERING, 38 No. 1/2, 1990, Jan/Feb, page 4.
2. National Commission For The Review of Federal and State Wiretapping
Laws, pp 223225,1972.
3. Steve Cain, Voiceprint Identification, NARCOTICS, FORFEITURE,
AND MONEY LAUNDERING UPDATE NEWSLETTER, U.S. Department of Justice, Criminal
Division, (Winter 1988).
4. Bruce E. Koenig, Authentication of Forensic Audio Recordings,
JOURNAL OF AUDIO ENGINEERING SOCIETY, 38 No. 1/2, 1990, Jan/Feb. page 4.
5. Tom Owen, Forensic Audio and Video Theory And Applications,
JOURNAL OF AUDIO ENGINEERING SOCIETY, Vol. 36, No. 1/2. 1988, Jan/Feb, page 39.
6. Ibid page 40.
7. Edward J. Imwinkelried, and Robert G.Scofield, Attacking The Weight
Of Prosecution ~Scientific Evidence, THE CHAMPION, PDN, April 1992.
8. Nancy Hollander and Lauren M. Baldwin, Testimony In Criminal
Trials: Creative Uses,Creative Attacks, THE CHAMPION, December 199 1.
9. California v. Trombetta, 467 U.S. 479, 485 (1984).
10. United States v. Valenzuela Bemal, 458 U.S. 858, 867 (1982).
11. 353 U.S. 53 ().
12. See United States v. Angiulo, 847 F.2d. 956,98182 (lst Cir.
1988); and United States v. Cinto1o, 818 F.2d. 980, 100103 (lst Cir. 1987);
United States v. Van Horn, 789 F.2d. 1492, 150708 (llth Cir. 1986).
13. 798, F.2d. 1492 ( ).
14. 682 F.2d. 1018, 1020 (D.C. Cir 1982).
15. Cintolo, 818 F.2d. 1002.
16. See Harley, supra.
17. Cintolo, 818 F.2d. 1003.
18. Copies are available at no charge from the Technology Assessment
Program Information Center (TAPIC), tollfree number 800-248-2742 or (301)
251-5060.
19. IAI Voice Comparison Standards, JOURNAL OF FORENSIC
IDENTIFICATION, January/February, 1992