AES San Francisco 2008
Paper Session P25
P25 - Forensic Analysis
Sunday, October 5, 2:30 pm — 4:30 pm
Chair: Eddy Bogh Brixen, EBB-Consult - Smørum, Denmark
P25-1 Magnetic Field Mapping of Analog Audio and Video Tapes (Invited Paper)—David P. Pappas, Fabio C. S. da Silva, National Institute of Standards and Technology
Forensic analysis of magnetic tape evidence can be critical in many cases. In particular, it has been shown that imaging the magnetic patterns on tapes can give important information related to authenticity and identifying the type of recorder(s) used. We present here an imaging system that allows examiners to view the magnetic patterns while they are playing, copying, or listening to cassette audio and VHS video tapes. With the added benefits of high resolution and polarity sensitivity, this system significantly improves on the accuracy and speed of the examination. Finally, the images, which constitute a true magnetic field map of the tape, can be saved to a computer file. We demonstrate that analog audio data can be recovered directly from these maps with bandwidths only limited by the sampling rate of the electronics. For helical video signals on VHS tapes, we can see the signature of the magnetic recording. Higher sampling rates and transverse spatial resolution would be needed to reconstruct video data from the images. However, for cases where VHS video tape has been cut into small pieces, we have built a custom fixture that allows the tape to be held up to it. It can display the low frequency synchronization track, allowing examiners to quickly identify the side of the tape that the data is on and the orientation. The system is based on magnetoresistive imaging heads capable of scanning 256 channels simultaneously along linear ranges of either 4 mm or 13 mm. High speed electronics read the channels and transfer the data to a computer that builds and displays the images.
P25-2 Magnetic Development: Magneto-Optical Indicator Film Imaging vs. Ferrofluids—Jonathan C. Broyles, Image and Sound Forensics - Parker, CO, USA
Techniques, advantages, and disadvantages of ferrofluids and magneto-optical indicator film imaging methods of magnetic development are discussed. Presentation and discussion of test results with supporting test images and figures. A number of MOIF imaging examples are presented for discussion. General overview on how magneto-optical magnetic development works. Magnetic development examples processed on magneto-optical imaging system developed and built by the author.
Convention Paper 7642 (Purchase now)
P25-3 Extraction of Electric Network Frequency Signals from Recordings Made in a Controlled Magnetic Field—Richard Sanders, University of Colorado Denver - Denver, CO, USA; Pete Popolo, National Center for Voice & Speech - Denver, CO, USA
An Electric Network Frequency (ENF) signal is the 60 Hz component of an AC power signal that varies over time due to fluctuations in power production and consumption, across the entire grid of a power distribution network. When present in audio recordings, these signals (or their harmonics) can be used to authenticate a recording, time stamp the original, or determine if a recording was copied or edited. This paper will present the results of an experiment to determine if ENF signals in a controlled magnetic field can be detected and extracted from audio recordings made with battery operated audio recording devices.
Convention Paper 7643 (Purchase now)
P25-4 Forensic Voice Identification Utilizing Digitally Extracted Speech Characteristics—Jeff M. Smith, Richard Sanders, University of Colorado Denver - Denver, CO, USA
By combining modern capabilities in the digital domain with more traditional methods of aural comparison and spectrographic inspection, the acquisition of identity from recorded evidence can be executed with greater confidence. To aid the Audio Forensic Examiner’s efforts in this, an effective approach to manual voice identification is presented here. To this end, this paper relates the research into and application of unique vocal characteristics utilized by the SIDNI (Speaker Identification by Numerical Imprint) automated system of voice identification to manual forensic investigation. Some characteristics include: fundamental speaking frequency, rate of vowels, proportional relationships in spectral distribution, amplitude of speech, and perturbation measurements.
Convention Paper 7644 (Purchase now)