VERIFYING THE INTEGRITY OF AUDIO AND VIDEOTAPES
By Steve Cain
Champion - July 1993 by Steve Cain
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
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
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
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,
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
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
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
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.
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
In upholding the district court, the First Circuit, citing Van Horn *13*
and United States v. Harley,*14* and making an analogy to the informer’s
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
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,
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,