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The average DC offset values of recordings produced on nine small digital audio recorders were evaluated to determine if they demonstrated characteristics meaningful for forensic audio authenticity examinations when submitted audio files are compared with test recordings made using exemplar digital recorders. Five audio formats (PCM WAV, ADPCM WAV, MP3, WMA, and DSS) were tested with no input, using the internal microphone(s), and with two Olympus external microphones. The average DC offset values were calculated using sound editing, digital analysis, and scientific computing programs. Based on the standard deviations of the DC offsets in these recordings, their wide variation and inconsistency probably preclude meaningful comparisons between one-minute submitted digital audio recordings and exemplars in authenticity examinations. [ABSTRACT FROM AUTHOR]

Detailed data analyses were conducted of the header sections of eleven (l 1) Windows Media Audio-formatted files produced on three small Olympus digital audio recorders and then re-encoded with four common audio editing programs. The purpose of this research was to identify changes made as a result of the re-encoding processes as they relate to forensic audio authenticity examinations. The header format structure, the procedures followed, the numerous changes identified in the headers of the re-encoded files, and a discussion of the authenticity implications are listed. [ABSTRACT FROM AUTHOR]

A detailed methodology is described to authenticate investigative and other forensic audio recordings collected via digital recording systems. These examinations are conducted to determine whether the recorded information is original, contains alterations, has discontinuities due to recorder stops and starts, and matches a submitted recording system, if available. Sections are included on examination requests, laboratory space, contributor-supplied materials, equipment and software, an 11-step authenticity examination protocol, authenticity examples, evidence handling, and expert testimony. The 11-step protocol consists of evidence marking, physical inspection, digital data imaging and playback optimization, critical listening, high-resolution waveform analysis, narrow-band spectrum analysis, spectrographic analysis, digital data analysis, miscellaneous techniques, work notes, and reporting. A formal procedure has been developed to optimally authenticate digital recordings submitted to a forensic laboratory, based on proper education, training, specialized schooling, and experience; appropriate laboratory space; specialized software; scientific protocols; accepted evidence-handling practices; and effective expert testimony. The paper complements an earlier publication (1990) except that it does not present specific procedures of a particular government laboratory. [ABSTRACT FROM AUTHOR]