We have constructed an imaging device that is capable of showing the spatio-temporal distribution of magnetic flux density in real-time. The device employs a set of AMR (anisotropic magneto-resistance) three-axis magnetometers, which are arranged into a two-dimensional sensor array. All of the magnetic field values measured by the array are collected by a microcontroller, which pre-processes and sends the data to a PDU (processing and display unit) implemented on a smartphone/tablet or a computer. The interpolation algorithm and display software in the PDU present the field as a high-resolution video; thus, the device works as a magnetic field camera. In the experiments, we employ the camera to map the field distribution of the distorted ambient magnetic field caused by a hidden object. The obtained image of field shows both the position and shape of the object. We also demonstrate the capability of the device to image a loaded power-line cable carrying a 50 Hz alternating current.

We propose a novel data hiding method in an audio host with a compressive sampling technique. An over-complete dictionary represents a group of watermarks. Each row of the dictionary is a Hadamard sequence representing multiple bits of the watermark. Then, the singular values of the segment-based host audio in a diagonal matrix are multiplied by the over-complete dictionary, producing a lower size matrix. At the same time, we embed the watermark into the compressed audio. In the detector, we detect the watermark and reconstruct the audio. This proposed method offers not only hiding the information, but also compressing the audio host. The application of the proposed method is broadcast monitoring and biomedical signal recording. We can mark and secure the signal content by hiding the watermark inside the signal while we compress the signal for memory efficiency. We evaluate the performance in terms of payload, compression ratio, audio quality, and watermark quality. The proposed method can hide the data imperceptibly, in the range of 729–5292 bps, with a compression ratio 1.47–4.84, and a perfectly detected watermark.