Aliasing

Color flow mapping systems have become widely used in the short time since their development. These systems overlay a pseudo-color velocity map upon the gray-scale two-dimensional image. Between 4 and 16 pulses are directed to each line-of-sight, and this requirement reduces the frame rate in comparison with the gray-scale image. Other limitations of color flow mapping include its ability to estimate only the velocity toward or away from the transducer and an increase in the variance in comparison with spectral Doppler. Potential artifacts include aliased velocities and the detection of flow in hypoechoic or hyperechoic nonvascular structures. Clinical applications include cardiology, studies of the abdominal and peripheral vasculature, evaluation of organ perfusion and the differentiation of tumors. Most current systems use narrowband estimators that examine a fixed sample volume and detect a change in phase between two pulses. Wideband estimators that can track red blood cells in two or three dimensions are under evaluation. Narrowband estimators, including the autocorrelator, the short Fourier transform and second order autoregressive filters, are compared with wideband estimators including cross-correlation, sum-absolute-difference and the wideband maximum likelihood estimator. Because the intensity of blood echoes is far smaller than echoes from surrounding tissue, high pass filters have been developed that can reject the larger signal from tissue using the return from a small number of pulses. Other areas of research include strategies for flow estimation with contrast agents, three-dimensional color flow mapping and power Doppler flow mapping.

Abbiamo verificato, analizzando gli istogrammi delle tensioni, che il passo di quantizzazione Q dell’ ADC dipende linearmente dalla tensione di fondo scala.

Abbiamo poi verificato come invece Q non dipenda dall’ampiezza del segnale generato, stimandolo nel caso di tensione di fondo scala VFS = 40 V. L’andamento trovato può essere considerato costante ed è Q = (0.16 ± 0.03) V, compatibile entro una σ_Q con il valore atteso E[Q] = 0.156 V.

Siamo poi passati alla verifica del teorema del campionamento: abbiamo mostrato come due segnali a frequenze ν_N/2 ± ∆ν appaiono uguali se campionati ad una frequenza ν0 < 2ν_N/2, a causa dell’aliasing.

Infine, abbiamo ricavato sperimentalmente la frequenza di Nyquist (mezzi), estrapolandola dal fit eseguito sull’andamento delle frequenze osservate dall’oscilloscopio in funzione delle reali frequenze del segnale in ingresso. La ν_N/2 al di sopra della quale l’oscilloscopio ha visto frequenze ν_osc = 2ν_N/2 − ν_G è stata stimata come ν_N/2 = (48.83 ± 0.04) kHz, compatibile entro una sigma con il valore atteso E[ν_N/2] = 48.825 kHz.

This paper concerns the continuous-time stable alpha symmetric processes which are inivitable in the modeling of certain signals with indefinitely increasing variance. Particularly the case where the spectral measurement is mixed: sum of a continuous measurement and a discrete measurement. Our goal is to estimate the spectral density of the continuous part by observing the signal in a discrete way. For that, we propose a method which consists in sampling the signal at periodic instants. We use Jackson's polynomial kernel to build a periodogram which we then smooth by two spectral windows taking into account the width of the interval where the spectral density is non-zero. Thus, we bypass the phenomenon of aliasing often encountered in the case of estimation from discrete observations of a continuous time process.

This paper presents an architecture for an all digital modulator and upconverter which uses aliasing for coarse frequency positioning. This allows a very efficient, flexible all digital architecture which is capable of supporting an entire groupband of signals while its design primarily uses low speed components.

This paper first addresses the Direction of Arrival (DOA) ambiguity problem in Uniform Linear Arrays (ULAs) and then proposes a wideband implementation through narrowband operation for an aliased array. It is shown that by an appropriate design e.g., frequency band selection and separation, signal spectrum support, and multiples access, one can resolve the aliasing, while maintaining larger array element spacing. A frequency invariant beamformer is used to control a contact bandwidth among the frequency subbands. The algorithm takes an all-angle approach for DOA and transmitted data signal estimation. Finally, the stability of the methodology is justified and the collaborated by simulation results.

I introduce a unified approach for denoising and interpolation of seismic data in the frequency-wavenumber ([Formula: see text]) domain. First, an angular search in the [Formula: see text] domain is carried out to identify a sparse number of dominant dips, not only using low frequencies but over the whole frequency range. Then, an angular mask function is designed based on the identified dominant dips. The mask function is utilized with the least-squares fitting principle for optimal denoising or interpolation of data. The least-squares fit is directly applied in the time-space domain. The proposed method can be used to interpolate regularly sampled data as well as randomly sampled data on a regular grid. Synthetic and real data examples are provided to examine the performance of the proposed method.