By Steve Cain Email: firstname.lastname@example.org
An ever-increasing troubling phenomenon is occurring
throughout the U.S., especially in metropolitan areas.
The Department of Justice (DOJ) and its law enforcement agents are
providing defense attorneys with CD copies of Title III, RICO, and other serious
felony cases involving consensual telephone intercepts or face to face
undercover encounters. Recent
contacts with several of my colleagues who are board certified forensic tape
experts have revealed that what may have started a couple of years ago as a cost
effective means by DOJ of providing digital tape copies to defense attorneys is
turning, however, into a growing number of corrupted CDs which contain
compressed and missing audio data which is not suitable for voice
identification, tape enhancement, or tape authenticity examinations.
In a recent Chicago case, I was
provided with supposedly uncompressed WAV audio files which should have been
easily readable but when finally opened, the different files were found to
contain over 5,000 truncated telephone conversations which had not been labeled
nor were they continuous (linear recordings) of the entire original telephone
call. It was further noted that
even the speech content itself had been corrupted and many of the conversations
were discontinuous and failed to record the entire conversation.
The defense attorney in this case requested additional relevant technical
data such as make and model of recorder, analog or digital original
formats, and any information concerning special encoding algorithms, compression
information, and any other information which the FBI may have included as
proprietary methodology in constructing their digital WAV files from the
original analog cassette tapes. The
U.S. Attorney declined to provide any of the requested data.
The presiding federal judge voiced concern regarding this missing data
and the reluctance of the DOJ to provide it to the defense.
Eventually the prosecuting U.S. attorney declined to provide any of the
requested CD manufacturing data and the tape issue was later made mute as
the original recorded tapes were never introduced into evidence.
The typical CD can store normally up to 74 minutes of music which
involves over 783 millions bytes of encoded digital information.
The evidence CD is normally an audio CD produced in WAV format, which is
sampled at 44.1 KHz, with 16 bits depth. The
material comprising the CD disc is plastic, about 1.2 millimeters thick and
consists of an injection molded piece of clear polycarbonate.
During manufacturing, this plastic is impressed with microscopic bumps
arranged as a single continuous long spiral track of data.
Subsequently a reflective aluminum layer is sputtered onto the disc
covering the bumps follow by a thin acrylic layer which is sprayed over the
aluminum for protection. The
incredibly small size of the bumps (known as bits) make the spiral track of a CD
extremely long and in fact, if stretched out into a straight line it would be
almost 3.5 miles long. To read
something this small you need a very precise
Quality Control Issues Regarding CD Integrity
Some discs develop cracks near the center at high speeds while in others
the protective layers delaminate over time.
High quality polycarbonate substrates should not exhibit these
Improper handling and storage techniques by manufacturers, distributors,
and users are producing an increasing number of discs with scratches, dirt,
smears and other surface contamination. This
material will during readout partially block the laser beam in the playback
process and could result in a permanent defect in the dye layer.
One should carefully examine any CD-R discs under low power magnification
and reject any contaminated surfaces.
Defective CD ROM Discs:
Many discs fail low radial tracking or contain high jitter
characteristics. Poor quality
control at the mastering or molding is normally the cause.
Such discs will read on some drives but fail in others.
CD Radial Acceleration:
Sudden radial jumps in track location are causing unpredictable field
failures. This is normally caused
by either mastering or molding flaws.
Brand of CDs:
Both interchange and longevity can be compromised with poor media
selection. One in depth study by
Media Science of 41 discs from 13 manufacturers showed that 17 passed, 13 were
marginal, and 11 discs failed.
Jewel Case Damage:
In many cases, forensic experts have received CD-R discs which have been
damaged in their jewel cases and also the discs have been scratched or otherwise
physically damaged during shipment. Other
discs contained uncorrectable errors.
Severe defects are often caused by recording systems such as absent or
short post gaps, high jitter, and severe error correction algorithms which
result from flaws in the recordings software or in the CD-R drives themselves.
Regarding Corrupted or Contaminated CD Copies
All data on an audio disc is
organized into frames to ensure a constant read rate.
In addition, each frame consists of 24 bytes of user data, plus
synchronization, error correction, and control and display bits.
The CD-Audio contains data which is not arranged in distinct physical
units but instead one frame is interleaved with many other frames so that a
scratch or defect in the disc will not destroy a single frame beyond correction.
CD Audio and WAV Files
Unlike ordinary data files, a sound file does not contain
error-correcting codes (ECC) within it to handle data lost in transfer.
One of the ways in which CD players can vary in quality is that they are
more or less effective in using ECC to repair errors in reading the audio.
Reading WAV Files
preliminary methods for creating files on a CD ROM include an audio CD track or
a WAV file. Only the former will
play in a CD player although CD-R software can convert a WAV file into the
necessary format which will read the audio file from a CD onto your hard disc as
a WAV file.
What is a WAV File?
A WAV file consists of three elements including a header, audio data and
a footer. The mandatory header
contains specifications for the file including information on interpreting the
audio data. Fortunately not all CD
players or software programs follow all of the same rules.
Consequently, the file format data may not be quite what it should be to
make a valid WAV file. When that
difference occurs, it can cause the CD player to not open the WAV file or at a
minimum it has to be told the format of the audio data for playback purposes.
Digital to Audio Conversion
As all recorded data on a CD is stored in digital format for one to
listen to it, it must be converted back to analog information.
The quality of the signal going into your sound card depends on the
quality of the CD reader/player you are using.
Within the sound card, an analog signal is then converted into digital
data which produces its own set of problems.
Analog to digital conversion is a tougher engineering problem than
converting from digital to analog. In
essence, most sound cards are not that effective in converting all of the analog
and digital data in a continuous fashion without signal losses and potential
corruption of originally recorded data on the CD.
Another key factor in determining quality of encoded audio data on a CD
is the quality of reading. Some
errors derive from the way the track is formed on the disc and are related to
the term jitter. In addition,
the faster the data flies past the laser pickup, the greater the chance that a
bit will be misinterpreted. If the
attorneys CD player does not support digital audio extraction (DAE) the CD
may not playback at accelerated rates and a greater number of ticks and pops
(i.e. bit errors) will be encountered.
Buffer underrun means that the buffer supplying data to the writer
has emptied before the data was completely written.
Consequently often times the CD reader does not perform at the speed it
was expected to deliver. One reason
for this is consistently high error rates typical of the CD-R which is scratched
or a dirty pressed disc. A second
factor which can slow the CD-ROM reader is a severe scratch or similar localized
damage which causes it to read most of the time but to have trouble in one area.
The buffer will be seen to fall rapidly as the reader re-reads to try to
get valid data. You often can see a
light flash as you watch the buffer drain.
Can I Make a Copy of an
Some CD-ROM and CD-R drives have problems extracting digital audio at
high speed so if you are experiencing many clicks and pops, try extracting the
data at a slower speed. You can
also run into problems if you attempt to extract data faster than your hard
drive can write especially if the hard drive requires defragmentation.
How Come the CD Doesnt
All CD writers need a continuous supply of data, and if anything
interrupts that flow the buffers can empty and underrun.
Often times it is better to copy a CD not on the fly (i.e. with another
CD burner) but to extract the files to your hard drive as WAVs and check them
there during playback for signal discontinuities or other corrupted sound
Can One Compress or Encrypt
Data on a CD-ROM?
Yes, there are many encryption and compression algorithms which are
employed in the original duplication and mastering process.
Unfortunately, the end user such as an attorney, may not be able to read
the data unless that information is supplied by the government.
Some of these encryption programs are proprietary while others remain
open to the general public.
What are the Most Common CD
A short list of common causes for tracking an audio readout difficulties
Dirty optics, drawer loading
belts which malfunction, sticky mechanisms, broken parts within CD player, need
for electronic servo adjustments, bad connections such as solder joints or
cracked flex cable traces, defective motors, laser-dead or weak laser diode or
drive problems, photo diode array which produces weak or shortened segments and
loss of power, bad/heat sensitive electronic components, and bad or missing
optical pickup shield ground.
What is Jitter and
Usual meaning jitter refers to a time-based error when digital
samples are converted back to an analog signal.
Jitter correction is a process of compensating for jitter and restoring
the audio to its intended form. Most
modern digital audio extraction programs will perform jitter correction.
What is Firmware?
Firmware is a type of software that lives within the computer.
It is a firmware of your CD recorder that controls the operation of the
device and handles everything from decoding the CD-ROM sectors to writing the
disc table of contents. Sometimes
there are bugs or missing features which are upgradeable.
Firmware upgrades have been used to add features like disc at once
recording and fix bugs like reversed left and right audio channels.
Sometimes however, the upgrade will inadvertently add bugs causing the
recorder to work improperly.
Observations/Recommendations From Forensic Audio Experts Regarding Corrupted CDs
One Philadelphia-based sound engineer whose responsibility it is to write
the algorithms for speech/sound CD recording commented that most mass production
of CDs are accomplished through third party mastering/duplication houses which
may or may not protect the integrity or do other quality control inspections to
make sure that the produced CD copies consist of a continuous stream of recorded
data with no distortion, clipping, or other degrading factors.
He agreed that the only sure way of ensuring that the original tape
recorded content had been accurately transferred to the CD copy was to compare
all of the contents of the original tape versus CD recordings.
He further indicated that most duplication houses will add equalization
which will affect the original tonal quality of the taped conversation but also
will compress some of the data depending upon the needs of the client and the
length of information to be placed on the CD.
A former Supervisory FBI
agent whose job responsibilities years earlier involved forensic audio
examinations, indicated that many of the present day undercover recorders used
by the FBI or other federal law enforcement agencies have changed from an analog
format to a digital one. Though he
did not identify the exact make and model of the digital IC (integrated circuit)
recorders now being used in the field, he indicated that most of them compressed
the audio data by as much as 8 to 1 during the recording process.
In essence this means that as much as 80 percent of the original analog
information is being discarded because of the compression process thus
potentially leaving out important acoustic and forensic information which may be
beneficial in tape authentication cases.
He warned that many of the recorders nowadays do not permit a linear or
continuous uncompressed recording of the evidential conversation but instead
digitizes the information into compressed files.
He agreed that the best evidence for now will continue to be an analog
tape recording which will be the only format which should preserve all of the
originally available speech information.
More than any other factor, it is the compression by digital extraction
methods from the original analog cassette tape that presents the greatest threat
to tape integrity for the defense attorney.
Most experts would agree that it is essential that the defense attorney
request all of the original recording equipment and tapes rather than rely on CD
copies alone for evidentiary purposes. In
addition, there is a loss of fidelity between the original tape and the CD copy.
One CD manufacturer, (Philips), reported that even a personal copy made
from an original CD disc will contain decreased amplitude linearity; increased
signal to noise ratios; and an increase in total harmonic distortion and overall
increased noise levels.
Thus far it has been my experience that DOJ personnel are extremely
reluctant to provide any specific data on the digital encoding of analog
cassette tapes onto a CD-ROM. Therefore,
Ive also supplied a list of some of the more recent and informative internet
sites that address many of the problems discussed above regarding the many
problems involved in properly mastering a CD and later making reliable copies
for dissemination to attorneys.