In recent criminal cases, the prosecution has given the defense compact discs (CDs), supposed copies of audio recordings which the prosecution intends to use in evidence. In the instances when the original recordings were made on analog cassettes, an important legal question arises: Are the digital CDs true copies of original evidence?
In addition to criminal actions, CDs have appeared in civil actions purporting to be exact duplicates of telephone conversations and transactions. The very same issue of authenticity arises.
I believe an argument can be made that these digital CDs are, in fact, not exact duplicates of original evidence. The primary reason in support of this position is that digital recording is fundamentally different than analog recording.
Digital v. Analog
An analog cassette tape recording is created by reforming the position of magnetic ferric oxide molecules on a moving strip of plastic. The digital CD, on the other hand, is the result of laying down a track of numbers, 0 and 1, in various sequences onto a plastic disk.
Analyzing the cassette soundtrack for tampering is a physical procedure in which interruptions in the recording can easily be detected and visualized via computer waveform. This is not true of a digital recording, in which the 1s and 0s follow each other in a seamless stream that shows no interruption where an edit has occurred.
By the same token, a digital stream, once placed into a computer, can be extracted in any sequence required by the operator. This ease of manipulation, coupled with the difficulty of detecting such manipulation, creates a real potential to tamper with important audio evidence. For example, a crucial statement on an original recording can easily be omitted by the person making digital copies from the original analog cassette.
Most surveillance is made with analog recording instruments, either by strapping a small tape recorder onto the body of a Confidential Informer (CI), or by placing a transmitter onto the CI's person and tape recording the signal from a nearby vehicle. Thus, when the prosecution introduces digital copies of analog surveillance into evidence, an exact duplicate has not been supplied.
Deliberate Sound Editing
In both criminal and civil cases, the issue of deliberate sound editing has arisen.
In a currently-ongoing case (which cannot be mentioned), the State Police picked up a suspect's questioning directly onto an installation in which four optically made disks recorded the interview simultaneously. Three of the disks were immediately impounded while the fourth one was used for making CD copies for the defense.
After listening to the evidentiary CDs furnished by the prosecution, the defendant claimed that an important part of the interview had been omitted. While this could be a defendant's ploy, the possibility does exist that whoever extracted the copies could have been selective in omitting portions of the soundtrack.
In United States v. Edward Whipple, a federal case in Philadelphia, the same soundtrack section appeared twice on a single tape. In many other cases, deliberate stops and starts made by the technical recording transfer operator have been detected. While these analog edits ere easily detected, digital edits are highly difficult and frequently impossible to discern. If, however, a great deal of ambient noise is present, a digital edit can be actually seen by the sudden difference in background shadings on a voice print.
To keep everyone honest and to avoid questions of authenticity of evidence at trial, I believe audio recordings introduced into evidence should continue to be made via the tamper-resistant analog audio cassette.