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

More documents

Recommendations

Info

SFN (Single Frequency Network) ECHOES AND GUARD INTERVAL direct path + echo = signal received symbol N symbol N intersymbolic interference symbol N+1 symbol N+1 intrasymbolic interference The echo is an attenuated and delayed copy of the desired signal A downtime is inserted called the GUARD INTERVAL • If the reflections fall in the guard interval, they do not generate errors (the decoder simply throws away all the wrong information in the guard interval) • As soon as they fall outside the guard interval, the decoder see them as interference and ... this is pain! 34
Operation is based on being able to delete the wrong part of the information from the delayed reflection. It’s as if, trying to talk in the presence of an echo, you could eliminate the first part of the pronounced syllables. An echo, if not too delayed, only ruins the first part of the syllable next to the one just pronounced. Of course you would need a whole synchronized system to be able to guess where to cut off. To function properly, the so-called SFN Single Frequency Network, needs transmitters with power that is calibrated and not too high, so as not to get too far away, where it would have a delay, against to local transmitters, outside the maximum distance allowed. The maximum delay is called the GI, Guard Interval, and is 224 microseconds for SFN mode (8k carriers). Often transmitters have intentional delays, inserted to adjust the arrival times of signals in the area, but it is not always possible to meet all the requirements in difficult territories. In these cases, an installer should know how to search for and measure the various signals arriving at the antenna and to point it with accuracy in order to eliminate the signals that arrive with too many delays. 35
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 and 10: The power of the digital signal is
Page 11 and 12: The modulation is the same type for
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: Since the system is immune to refle
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
Page 73 and 74: MPEG4 REAL TIME SPECTRUM 6-HOUR BA
Page 75 and 76: mod. HD COMPACT STC THE MOST ADVANC

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

Create successful ePaper yourself

Delete template?

Save as template?