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CD Copies of Taped Recordings: A Poor Alternative for Defense

by Steve Cain

By Steve Cain  Email:

An ever-increasing troubling phenomenon is occurring throughout the U.S., especially in metropolitan areas.  The Department of Justice (DOJ) and it’s 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 CD’s 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.

CD Manufacturing Information

  •   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 reading mechanism.

    Critical Quality Control Issues Regarding CD Integrity

    Fragile CD’s:  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 characteristics.

    CD Contamination:  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 CD’s:  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.

    CD-R Interchange:  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.

    FAQ 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

    Two 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. 

    Reading Errors

  •   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 attorney’s 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 Underruns

  •   “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 Audio CD?

  •   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 Doesn’t Sound Good?

  •   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 information.

    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 Player Problems?

  •   A short list of common causes for tracking an audio readout difficulties include:

    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 “Jitter Correction”?

  •   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. 

    Other 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, I’ve 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.