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Electronic Imaging & Signal Processing

Multichannel real-time magnetic imaging system

A sensing device that ‘sees’ magnetic fields moves forensic testing a step forward.
17 October 2007, SPIE Newsroom. DOI: 10.1117/2.1200710.0879

Imaging of fields generated by magnetic media, for example, magnetic audio or video tapes, is critical to validating evidence in criminal investigations. The most popular of conventional approaches, called the Bitter technique,1 is an imperfect, messy, time-consuming process that involves coating the tape with a solution containing solvent and magnetic particles. We recently designed and fabricated a novel alternative to this technique that is noninvasive, sensitive to field polarity, shows high spatial resolution, and has the potential to make images as the tape is played.2 This new type of magnetic imaging system comprises a linear array of sensors on a die (see Figure 1) that connects the signals from 256 sensor channels to custom, low-noise electronics for imaging and readout. The tape to be analyzed is brought to within a few microns of the sensor array by a transport system and is continuously run past the sensor stripe, also known as the read head, in the same fashion as a traditional tape in a cassette player.

The imager operates at high speed and in real time. It is capable of measuring 256 channels per scan with either 1600 dots per inch (dpi) or 500dpi resolution at an acquisition rate of up to 250K samples/s (see Figure 1). The array, or stripe, is located only a few microns from the bottom edge of the die. Figure 2 shows the layout of the sensor bridges. Each sensor resembles a barber pole and measures 2×)16(μ)m. The die is etched to keep the cassette tape within 10(μ)m of the sensor array to maximize resolution. The system electronics and tape transport are rack-mounted and can sit on a desktop. All movement of the tape and read heads is servo-motor-controlled for repeatability and accuracy.

Figure 1. Die with 256 elements. The pink area is a copper flood that brings power in to the sensor array. The yellow triangle within the pink area indicates the leads coming from the individual sensors. The yellow squares are the signal and power connections with additional pads used for on-chip testing. The sensor stripe runs along the bottom edge.

Figure 2. Close-up of the sensor bridge elements on the die.

Figure 3. Forensic image of audio tape with detail.

Figure 4. (a) Mark left by the erase head is evident in region III. Comparison of results from the traditional technique (c) and from the new imaging system (b).

Figure 5. Image of VHS tape with detail.

Figure 3 shows a picture of a cassette tape obtained by the 1600dpi head. The image focuses on a previously recorded audio signal. The audio was partially erased by the same recording head, which left a characteristic mark. The stop mark revealed through forensic analysis indicates tampering, which helps determine the validity of the tape as evidence in a court of law.

What is especially important about this image is that it was taken while the tape was being played. This capability constitutes a powerful new addition to the tape forensics toolbox. Finally, it is possible to digitally image the entire tape. Up to now, one could digitize the audio, but not the image of the magnetic fields above a tape. And in forensics, it is the magnetic fields rather than the content that count. Given a storage requirement of 8MB of data per second of audio, it would require only 43GB to capture a typical 90min tape.

Figure 6. Color picture of an Intel chip being imaged for current shorts. –Jx and –Jy are current directions. The image demonstrates the system's ability to resolve polarity.

Figure 4(a) shows a 1s scan of a cassette tape taken with the 256-channel sensor array. The bottom half of the image shows three regions. Region I contains a signature of a stereo audio pattern. The audio was erased in region II, where a sharp interface marks the start of an erase head event, which is highlighted in region III. Figure 4(b) shows the interruption measured with the sensor array. The same section is imaged using traditional techniques in Figure 4(c).

The magnetic pattern of the erase stop event is unique for each type of tape recorder (brand and model). The 256-channel array makes it possible to see the pattern in greater detail. Figure 5 shows a .5in-wide videotape taken with the 500dpi head. Note the stereo audio traces at the bottom, the broad video signal dominating the middle, and the synchronization marks at the top. The gray scale follows the polarity of the magnetic field, with black and white representing opposite extremes.

The system is currently being used by the FBI for forensic evaluation but is not limited to criminal investigations.3 It can also be applied in nondestructive testing of integrated circuits (see Figure 6) or in biomagnetic sensing, for example, as a magnetic bead counter in a microfluidic device.

Sean Halloran, David Pappas
Advanced Magnetic and Quantum Materials
National Institute of Standards and Technology (NIST)
Boulder, CO
Fabio da Silva
University of Colorado at Denver and Health Sciences Center
Denver, CO
Anthony Kos
Boulder, CO