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

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RADIOFREQUENCY MEASUREMENTS BEFORE & AFTER VITERBI ERROR CORRECTION - NOISE MARGIN DEMODULATOR 1st ERROR CORRECTION VITERBI 2nd ERROR CORRECTION REED SOLOMON MPEG DECODER bBER aBER • Absolutely minimum bBER and MER: bBER=2x10 –2 and a MER of approx. 20 dB (You must remain above these limit values, 6 dB noise margin (with 64QAM) • Viterbi reduces errors until 2 x 10 –4 (after Viterbi aBER) • Reed Salomon corrects what is left up to BER 1 x 10 –11 • This is QEF (quasi error free) – an error event statistically visible every 30 minutes 16
Here you can see a schematic block of the structure of a DVB-T set top box, where error correction is done in two stages, as is carried out in <strong>TV</strong> and SAT set top boxes. The Viterbi circuit is common to all types of digital broadcasting and clears up most of the errors and adapts the system for the Reed Solomon error correction. What happens is that the number of errors at the decoder input is highly variable, and after Viterbi error correction it is much lower and more consistent. In the picture on page 18 you can see the error limits tolerated by the system, but as we already said, the limits must be much higher and if you want an acceptable and stable quality you must have some Margin (usually the Noise Margin in DVB-T must be 6 dB). IMPORTANT: The real innovation in the work of installation and development of an antenna system for digital signals (unlike analog), lies in the noise margin concept, to be respected for all digital systems, including cable and satellite distribution. Since the minimum values of various parameters to be analyzed differs between the various systems satellite, terrestrial and cable (see table on page 2), some meters automatically provide the signal quality FAIL–MARG–PASS, and this greatly simplifies the job. 17
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: The DVB-T signal has this behavious
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
Page 73 and 74:
MPEG4 REAL TIME SPECTRUM 6-HOUR BA
Page 75 and 76:
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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