среда, 29 декабря 2010 г.

SA update to version v : OFDM Module.

SA update to version v

Long enough time ago SA users SA had paid attention that the copy of the selected on the sonogram fragment visually looks shorter than the original. This interesting phenomenon, entirely linked with property of uncertainty of FFT transformation.

The matter is that transition from the time area into the frequency area, which provides FFT transformation, excludes possibility of exact definition of position of concrete frequency component by the time. FFT only "says" that the component is entering into the block, but where is the component located in the block is unknown.

It is the very old and standard problem. It has no exact decision, but we counted possible to lower a sharpness of this phenomenon. As the basic problems arise within the small blocks, when the resolution on time on
the sonograms is rather good.

This is realized in the current version.

We just make replacement of one problem by another. If in the old versions, copied by the sonogram fragment, physically had such duration which had been displayed at the moment of copying. That in the current version, the physical duration is longer than it is specified at copying.

We have considered possible to go on such exchange, for two reasons.

In the first, users confuses that the copied fragment visually looks shorter, and this situation demanded actions to regulate it.

In the second, the work on the sonogram, assumes visual perception, and it is better to see the copied fragment the way it has been selected, neglecting real accuracy. Even more so further it is necessary to work with the fragments in the area of the sonograms displaying.

The mode of a signal review has been added by the users offer.

The review is provided by FFT window with the size in 1024 counts. The primary goal of the mode it is to pass part of the signal with preamble or another problem segment (by having placed the start point in the necessary position), before the correlation triangle search will begin.

In the review mode, FFT window moves on the signal loaded in OFDM module, by the slider which is shown on the picture.

After key parameters of the signal are gotten, it is possible to define the detailed parameters of the preamble or other fragmen, by moving symbol-by-symbol backwards on the part passed earlier.

The detailed example on the real signal is described in the article OFDM analysis in SA version

Also the problems concerned users bug reports are removed.