VERIFYING THE INTEGRITY OF AUDIO
By Steve Cain
Champion - July 1993 by Steve
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 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
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
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
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
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
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.
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
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
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
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,
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,