TV Explorer HD ATSC manual - Promax

USER’S MANUAL. TV EXPLORER HD ATSCAnalogue and digital carriers are very different in terms of signal contents andpower distribution over the channel. They, therefore, need to be measured differently.The modulation error ratio (MER), used in digital systems is similar to the Signal/Noise(S/N) ratio in analogue systems.MER represents the relation between the average power of DVB signal and theaverage power of noise present in the constellation of the signals.When measuring MER, it also shows the noise margin in ATSC, C, S and the Linkmargin in DVB-S2, which indicates the distance from the QEF point at the current signal.By example, QAM 64 demodulators require a MER greater than 23 dB to work.Though it is preferable to have at least a 3 or 4 dB margin to compensate for anypossible degradation of the system. While QAM 256 demodulators require a MERgreater than 28 dB with margins of al least 3 dB. Normally, the maximum MER valueseen in portable analysers is of approximately 34 dB.EnglishFinally it is shown a status line, which displays information about the detectedsignal.5.15 Constellation DiagramThe constellation diagram is a graphic representation, of the digital symbolsrecived over a period of time.There are different types of constellation diagrams for the different modulationmodes. With the TV EXPLORER HD ATSC it is possible to display constellations forATSC, DVB-C, DVB-S and DVB-S2 signals.In the case of an ideal transmission channel, free of noise and interferences, allsymbols are recognised by the demodulator without mistakes. In this case, they arerepresented in the constellation diagram as well defined points hitting in the same areaforming a clear dot.Noise and impairments cause the demodulator to not always read the symbolscorrectly. In this case the hits disperse and create different shapes that at the end willallow to determine at a glance the type of noise in the signalEach type of modulation is represented in a different way. A 16-QAM signal isrepresented on screen by a total of 16 different zones and a 64-QAM is represented by adiagram of 64 different areas and so on.The constellation shows in different colours the density of hits and includeszooming, scrolling and clearing functions for a better graph representation on screen.03/2012 Page 65