UNDERSTANDING DIGITAL TV - Ro.Ve.R. Laboratories S.p.A.

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RADIOFREQUENCY MEASUREMENTS DVB-T MODULATION The carriers are modulated: in Amplitude – vector length in Phase – vector angle Until the decoder detects that the vector falls within the right square, there is no error and reception is perfect. Each carrier is modulated independently from the others. Each carrier carries a piece of the total information. The modulation is both for phase and amplitude, for example “64QAM” (the most used) or “QPSK” or “16QAM” (or “256QAM” for DVB–T2) 10
The modulation is the same type for each carrier, but carries different pieces of binary information and hence the amplitude and phase of the various carriers are different from each other. This gives a confused representation of the spectrum, that appears to have a beard like the noise. In fact it is very similar, because the information is random and randomly variable, so that the British have coined the phrase “noise like signal”, which gives the idea of a completely unrecognizable signal within the spectrum. The noise picked up by the antenna, or interference, makes the carrier’s vector moving randomly within its square; if the noise increases the amplitude, it can throw it out and then there will be an error and the video image will become unrecognizable suddenly and without notice. Instead, in an analog signal, the noise, or interference, has a progressive action and is immediately visible on the signal. In the case of a digital signal, by observing the spectrum and the received power, it is not clear when the various carriers are received correctly, because you do not know how much noise disturbance there is, and how it affects the reception. Later you will see what to do when working on headends and antenna pointing, in order to obtain the best signal and work out how to measure the signal quality. 11
Page 1 and 2: UNDERSTANDING DIGITAL TV • Radio
Page 3 and 4: SUMMARY 6 Radio frequency spectrum
Page 5 and 6: The difference between the two sign
Page 7 and 8: The function that allows you to fin
Page 9: The power of the digital signal is
Page 13 and 14: We know that signals can be “poll
Page 15 and 16: The DVB-T signal has this behavious
Page 17 and 18: Here you can see a schematic block
Page 19 and 20: When it is said that a DVB-T system
Page 21 and 22: Let us assume that the electromagne
Page 23 and 24: We will illustrate a laborious, but
Page 25 and 26: This chart is useful, even if it do
Page 27 and 28: Another situation where the antenna
Page 29 and 30: The back to front antenna ratio is
Page 31 and 32: The reflections caused by a mismatc
Page 33 and 34: Since the system is immune to refle
Page 35 and 36: Operation is based on being able to
Page 37 and 38: Nearly every instrument has an SFN
Page 39 and 40: Here are a number of rules to impro
Page 41 and 42: This picture shows an example of an
Page 43 and 44: As an example, we have superimposed
Page 45 and 46: The diagram clearly shows the trans
Page 47 and 48: To obtain minimum reception stabili
Page 49 and 50: These predictions are verified when
Page 51 and 52: The electromagnetic field is measur
Page 53 and 54: Even if the MER is a good indicator
Page 55 and 56: The diagram shows once again the th
Page 57 and 58: This table is calculated for system
Page 59 and 60: Micro-interruptions are not always
Page 61 and 62:
So ripple (waves) in the spectrum p
Page 63 and 64:
In Figure 3 and 4 you can see the s
Page 65 and 66:
Figure 5 shows DVB-T signal ch.8 th
Page 67 and 68:
Using this case study you can see h
Page 69 and 70:
Directive 2004/108/EC requires that
Page 71 and 72:
The diagram shows the real case of
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MPEG4 REAL TIME SPECTRUM 6-HOUR BA
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mod. HD COMPACT STC THE MOST ADVANC
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UNDERSTANDING DIGITAL TV - Ro.Ve.R. Laboratories S.p.A.

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