More articles by Steve Cain
|
Get notified about new articles - join the ExpertPages Mailing List now
|
You're a prosecutor preparing a major drug or conspiracy case
and one of your principal pieces of evidence is the tape-recorded conversations
of the alleged conspirators.
The problem is that the quality of the tape
— as the result of background noises and other factors
— is so marginal that
you run the risk of the jury discounting it.
What do
you-do?
According to Steve Cain, president of Applied Forensic
Technologies, Intl., and one of the nation's leading experts on voiceprint (or
spectrograph) technology, you have tape quality enhanced and authenticated by
qualified specialists.
"An integral part of any voice identification
task," says Cain, "is to attempt to ensure that the most intelligible speech
samples are available for comparison purposes.
All too often, however, the limitations of
surveillance, recorder, microphone, and adverse room and reverberation effects
severely degrade the audio signal. Through the proper selection of a variety of
analog and digital tape filtering devices, unwanted sounds often can be
attenuated."
The output signals of tape recorders can be damaged by three
general factors — noise, interference and distortion — each of which is caused
by a specific condition. In addition, there are what Cain calls "adverse
forensic influences" that include the bandwidth equalization of telephone lines
that limit voice frequencies to between 300 to 3500 hertz.
To reduce or
eliminate various noise and distortion sounds from an audio tape, forensic audio
specialists use a variety of filters. For example, when a tape hiss occurs
within the speech frequency range, a so-called low-pass filter is used to
eliminate it. When what is called a "low-end rumble" occurs, an appropriate
high-pass filter is used. "Comb filters" are used to reduce harmonically related
noise such as a power supply hum.
"Because of inadequate equipment and
poor operator technique," Cain says, "a host of...factors besides noise can
reduce speech intelligibility. Improper recorder speed or transient mechanical
problems, along with poor quality tape or unstable AC power, all can contribute
to a poor quality recording."
The tape enhancement process, therefore,
must start with examination of the equipment used and the recording tape speed.
Once a tape is "cleansed" of interfering noises to make the recorded voices or
other pertinent sounds as clear as possible, it usually must be authenticated
before it can be introduced as evidence in court, to avoid any charges of
illegal tampering.
Probably the most famous tape authentication
examination was the one conducted in 1974 by a group of forensic experts
appointed by then U.S. District Judge John Siica in the Watergate case to
examine the disputed 18-minute gap in a White House recording.
With the
increasing number of drug-related and money laundering cases being prosecuted by
federal, state and local prosecutors, the use of tape-recorded conversations and
related sounds is increasing correspondingly, calling— Cain says — for
professional examination of tapes requiring enhancement and
authentication.
As might be expected, the tape authentication procedures
developed and suggested by the FBI Technical Services Division's Signal Analysis
Branch are detailed and voluminous.
The requirements include sworn
testimony on the circumstances of the recording and equipment used, the original
tape and recording device, written records of any damage, maintenance and
repairs; detailed statements by the operator on the technical conditions
existing at the time of the recording, including such factors as the power
source, background environment, condition of the tape, etc.
The FBI
suggests that forensic experts carefully examine the recorder right down to the
marks left by ferrofluids that adhere to magnetic poles. Each recorder leaves a
distinctive "fingerprint" in the form of electronic imprints along the tape
surface. These imprints — unless physically altered — are identifiable among
different records.
Finally, the FBI suggests a physical as well as
spectrographic (wave form) examination of the tape to make a subjective
determination. The physical examination involves a trained examiner listening to
perceived pitch, quality, rate of speech, mannerisms, amplitude, breathing
patterns, syllable couplings, background sound variation, hum and other acoustic
effects, such as room reverberation. That done, you and your tape are ready for
the trial.
By: Steve Cain Email: info@tapeexpert.com
