Daniel R. Fuhrmann

Experimental evaluation of psychophysical distortion metrics for JPEG-encoded images

Two experiments for evaluating psychophysical distortion metrics for JPEG-encoded images are described. The first is a threshold experiment, in which subjects determined the bit rate or level of distortion at which distortion was just noticeable. The second is a suprathreshold experiment in which subjects ranked image blocks according to perceived distortion. The results of these experiments were used to determine the predictive value of a number of computed image distortion metrics. It was found that mean-square-error is not a good predictor of distortion thresholds or suprathreshold perceived distortion. Some simple point- wise measures were in good agreement with psychophysical data; other more computationally intensive metrics involving spatial properties of the human visual system gave mixed results. It was determined that mean intensity, which is not accounted for in the JPEG algorithm, plays a significant role in perceived distortion.

Estimation of overlapping spectral signatures from hyperspectral data

A new method for spectral unmixing is developed. This method accounts for the nonnegativity of the mixing constants. Preliminary experiments to test the method are reported.

Compensation for optical distortion in Fourier transform spectrometers

A calibration method for determining and correcting the optical distortion in the spectral axis of a Fourier Transform spectrometer is proposed. In our method, a narrowband optical calibration source is used, and the resulting interferogram is treated as a sinusoid which is phase-modulated as a result of the optical distortion. A discrete-time quadrature demodulator is used to determine the phase modulation, and the distortion is then characterized using a third-order polynomial fit to this estimated phase. Correction of interferogram data is accomplished by resampling the discrete data onto a uniform grid in the predistortion space. Our method has been applied to calibration data from the Digital Array Scanned Interferometer (DASI), with excellent results.

Sampling theorems for linear time-varying systems with bandlimited inputs

We propose and prove an extended sampling theorem for linear time-varying systems. As a result, we establish a discrete-time equivalent model of the input-output relation of the system for the case of bandlimited inputs and bandlimited system variation. The sampling of the output signal and an equivalent discrete-time model of the system are discussed.

Improving the visual quality of JPEG-encoded images via companding

A companding procedure designed to improve the visual quality of monochrome images encoded with the baseline Joint Photographic Experts Group (JPEG) image compression algorithm is described. The companding procedure consists of two pointwise nonlinearities applied to the image, one before and one after the JPEG encoding. In a series of two-alternative forced-choice experiments designed to measure human visual threshold response to these images, it was determined that the average bitrate at the justnoticeable- difference (JND) point for the companded images is less than that for the uncompanded images. In a suprathreshold experiment, the subjects selected the companded images as being less distorted than the uncompanded images in more than 80% of the trials, for image bitrates ranging from 0.4 to 1.0 bpp.

Considerations in the transmission and storage of JPEG-encoded medical video

High-speed, public network telecommunication services are becoming critical to the health- care sector. We are examining the use of Asynchronous Transfer Mode (ATM) transmission of digital video to support remote real-time diagnostic procedures (e.g. fluoroscopy and ultrasound). Remote visualization may eliminate the need for a physician on-site, thus reducing personnel (physician) costs while providing access to remote physician specialists. A key requirement of cost-effective video transmission and storage is the need for data compression, JPEG compression provides reconstructed images of high quality, inexpensive codecs are now available, and the compressed data streams may be easily transmitted via ATM networks based on DS-3 transport. We will present a telemedicine model, describe a preliminary experimental protocol, and discuss psychovisual assessment of data from fluoroscopy and ultrasound examinations. Early results (monochrome medical video compressed at approximately 1 bit/pixel) provide a basis for examining deployment issues and costs.

An EM Algorithm For Direction-Of-Arrival Estimation For Narrowband Signals In Noiset

We derive a generalized EM algorithm for the maximum-likelihood estimation of the directions-of-arrival of multiple narrowband signals in noise, and prove that all of the limit points of the algorithm are stable and satisfy the neces-sary maximizer conditions. The deterministic signal model is considered here in which estimates of the unknown signal amplitudes are generated.

Interferometric imaging of a rotating sphere using synthetic aperture techniques

Interferometry has been used for decades to image distant electromagnetic sources at long wavelengths. However, the long data collection intervals have prevented imaging rotating objects such as planets. We show that one can craft an interferometric imaging algorithm using synthetic aperture beamforming techniques by linear beamforming in the correlation domain data rather than the signal-domain data. We then use this analogy to show how it is possible to create previously unrealized interferometric images of a rotating spherical blackbody.