24-07-2012, 03:52 PM
Advanced noise models used after 2007
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The principal source of noise in digital images arises during acquisition and/or transmission. The performance of imaging sensors is affected by a variety of factors, such as environmental condition during image acquisition, and by the quality of sensing element themselves. For instance, in acquiring images with a CCD camera, light level and sensor temperature are the major factors affecting the amount of noise in the resulting image. Images are corrupted during transmission principally due to interference in the channel used for transmission.
Main Noise Models Some of the important noise models are:-
Gaussian Noise It is easy to use mathematically therefore it is used commonly in practice even if the model is not perfect. Rayleigh Noise It occurs in range images and can be used to model skewed histograms. Gamma and Exponential Noise Generally occurs in laser imaging and can be used for approximating skewed histograms.
SOFT OUTPUT DEMODULATION — FRONT-END OF THE IDN
The IDN closely cooperates with the SODEM (Soft output demodulator) providing fundamental hypothesis about captured signal Rd. This functional block presents a gateway between the domain of real realizations (realizations of certain random variables), where are operated with scalars (factual realizations), and the probability (metric) domain of the IDN, where are operated with whole densities. The SODEM includes likelihood function (or transfer function) of the IECS-ML channel emulating all noises incident in the image capturing system chain.
LOW-FREQUENCY NOISE HBT MODEL
Possible Low-Frequency Noise Sources and Their Localization Note that, in order to extract the two main low-frequency noise sources, a T model of an HBT is used. However, the T and Pi models are exactly equivalent by taking into account the appropriate frequency dependence of the collector current source.
Microscopic physical noise sources lead to random fluctuations of conductive currents. It then follows that, in a macroscopic electrical representation, every equivalent Norton noise current source must be associated to the conductive element where it originates.