The Analysis of DVB-C signal in the Digital Television Cable Networks

Transcription

1 The Analysis of DVB-C signal in the Digital Television Cable Networks Iulian UDROIU (1, Nicoleta ANGELESCU (1,Ioan TACHE (2, Ion CACIULA (1 1- "VALAHIA" University of Targoviste, Romania 2- POLITECHNICA University of Bucharest, Romania Abstract: - In this paper the quality parameters of the DVB-C signal transmission in a digital TV network by cable are studied. The measurements in order to analyze the parameters of DVB C signal were made in Romania, more exactly on the TV network by cable of Targoviste. These measurements refer to the following parameters: (Bit Error Rate), MER( Error Ratio), C/N(carrier-to-noise), margins noise, power signal DVB C, constellation diagram and frequency spectrum. To evaluate these parameters, an equipment of TV EXPLORER II+ type made by PROMAX Spain was used. Key-Words: - MER,, C/N, DVB C, noise margin 1 Introduction The DVB C (Digital Video Broadcasting Cable) standard known as ETSI EN which regularizes the digital TV transmissions by cable was issued in Since 1994 the digital TV transmissions by cable DVB - C were implemented. Since 2007 the digital TV transmissions by cable were introduced in Romania, at national level with SD resolutions, and since the 1 -st of October 2009 the digital TV transmissions of high definition (HDTV) were implemented. The DVB C system supposes the transmission of fluxes MPEG 2 and MPEG 4 using the modulation with digitizing of channel. For the digital TV transmissions by coaxial cable, it uses the 64 modulation and also, for the fiber optic transmissions of DVB-C signal, the 256 modulation is frequently used. Since 2008, the second generation of the digital TV transmissions by cable known as DVB C2 was implemented. The main characteristics of the DVB C system in regard to the new generation DVB C2 are presented bellow in Table 1: 2 Details of the experiment To evaluate the parameters of transmission of DVB C signal, a digital TV network by cable was realized, in the different four points of this network. used in this network for transmission of the TV signal is 256 type. Table 1. The main characteristics of the DVB C system DVB C DVB C2 Input Interface Single Transport Stream (TS) Multiple Transport Stream and Generic Stream Encapsulation Modes FEC Pilots Guard Interval Schemes Constant Coding & Reed Solomon (RS) Bit- Single Carrier Not Applicable Not Applicable 16- to 256- (GSE) Variable Coding & and Adaptive Coding & LDPC + BCH Bit- Time- and Frequency- COFDM Scattered and Continual Pilots 1/64 or 1/ to In this experiment are measured the following parameters: the carrier to noise (C/N ratio), the bit error rate (), the modulation error ratio(mer ratio), noise margin, constellation diagram and frequency spectrum. ISSN: ISBN:

2 C/N ratio is defined for a signal having same bandwidth between the ratio of the modulated signal power and the equivalent noise power (equation 1) CHANNEL BW SYMBOL RATE MODULATION 7,93MHz 6.900KSymb 256 C [ ] P db = 10 lg carrier (1) N P noise It is better if the values of the C/N ratio are higher than the quality of the received DVB C signal. Measurement of the error bit rate () is realized after demodulation of the signal and before the error correction (FEC). The bit error rate () is defined is[4]: = bit errors / transmitted bits (2) The digital receiving system of the TV signal by after the demodulation process occurs a correction of the errors using the Reed Solomon method in Fig 1 presented bellow: TUNER Figure 1. Digital reception system via cable Another parameter which is evaluated in this experiment concerning the performance of the transmission channel is the modulation error ratio (MER) which has the following equation (3)[4]: MER A/D P 10 (3) [ ] error db = lg Psignal before FEC DEMOD. 3 Experimental results Measurements on the DVB C signal were made in Romania by means of the digital TV network by cable which there is to Targoviste. For analysis of the parameters above mentioned, many tests on five TV channels in four different points of the network were made. In order to realize these measurements an equipment of TV Explorer II+ type made by Promax Spain was used. This equipment can measure the DVB C signals. The transmission parameters of DVB C signal are presented bellow in Table 2: Table 2. Parameters of the DVB C signal SIGNAL DVB C SYSTEM PAL FRAME RATE 50Hz VIDEO FORMAT 4/3 or 16/9 REED-SOLOMON The quality measurements of transmission of DVB C signal were made on five TV programs having the following characteristics which are presented in Table 3. Table 3. Characteristics of channels Channel Network UPC UPC UPC UPC UPC VIDEO MPEG2 MPEG2 MPEG2 MPEG2 MPEG2 AUDIO MPEG2 MPEG2 MPEG2 MPEG2 MPEG2 FRAME 50Hz 50Hz 50Hz 50Hz 50Hz Program Rezolutia 720x x x x x576 The measurements tried to establish the reception conditions in the measurements points, the evaluation being made by measuring the mean parameters of the signal, which are referring to the signal level at the receiver input, to the signal/noise ratio, to qualitative digital signal reception parameters DVB C (MER,, C/N, noise margin). Measurements made on digital TV channels by cable,,,, are presented in Table 4. From the present data, the values of the P in, C/N,, MER and noise margin were represented by curves in five points of DVB C network (Fig. 2, Fig. 3, Fig. 4, Fig. 5 and Fig. 6). Power[dBuV] 63, , , , ,5 Figure 2. Input power of receiver versus points 1-5 ISSN: ISBN:

3 Table 4. The measurements values for channels,,,, MER C/N POWER Noise Ch P [db] [db] [dbuv] 36,2 7,30E-08 >39,8 61,2 5,2 1 35,6 1,50E-07 >40,6 61,9 4,6 1 36,5 4,30E-08 >39,4 60,6 5,5 1 35,7 3,90E-08 >38,7 60 4,7 1 36,4 3,00E-08 >38,5 61,9 5,4 1 35,5 8,30E-08 >35,6 62,8 4,5 2 36,1 4,90E-08 >35,9 62,6 5,1 2 Figure 4 of DVB C signal versus points ,1 3,30E-08 >33,2 61,7 5, ,00E-08 >35,1 61, ,7 4,90E-08 >33,7 63,2 5,7 2 35,7 4,20E-08 >35,4 62,6 4,7 3 35,3 4,80E-08 >36,1 63 4,3 3 36,4 5,10E-08 >32,9 61,8 5,4 3 35,2 5,30E-08 >34,9 61,3 4,2 3 36,7 3,30E-08 >34,2 63,2 5,7 3 36,1 7,90E-08 >35,5 62,9 5,1 4 35,4 4,20E-08 >35,9 62,8 4,4 4 36,7 3,30E-08 >33,2 62 5,7 4 35,5 7,40E-08 >34,5 61 4,5 4 C/N[dB] Figure 5. C/N ratio of DVB C signal versus points 1-5 6,5 6 37,1 2,90E-08 >33,6 63,3 6,1 4 37,5 Noise 5, ,5 MER[dB] 36, Figure 6.Noise margin of DVB C signal versus points ,5 35 Figure 3. MER of DVB C signal versus points 1-5 In Fig. 7 and Fig. 8 are presented the variations bit error. Rate () depending on the signal power at receiving in other four points of the network, respectively, the variation of bit-error-rate depending on the carrier to noise(c/n). ISSN: ISBN:

4 59, , , , ,5 Power[dBuV] Figure. 7 versus Input Power(P in ) of DVB C signal Figure. 9 Constellation diagram for the DVB C transmission in channel C/N[dB] Figure. 8 versus C/N of DVB C signal From Fig. 7, it is observed a low variation of (Bit error rate ) depending on the signal power in case of measurements made on the channel [f=330mhz], respectively a high enough variation in case of the [322MHz] channel. These vary from a value of 4E-08 to 1, 5E-07 having a signal power of 61,9dBµV. Other measurements have small variations of depending on the DVB C signal power which is received. In Fig. 8 is shown a high enough variation of in regard to C/N in case of the measurements made for [322MHz] channel, these being from a 4E-08 at a C/N 36dB and 1,5E-07 to 40,6dB. All measurements made for others channels,,, present low variations of in regard to C/N. For analysis of the quality transmission of DVB C, among the most important parameters, there is the constellation diagram and the frequency spectrum. To simulate the constellation diagram for channels [314MHz] and [338MHz] which are analyzed, we will not obtain only a discrete point, but we will obtain many points constituting a cloud representing each state as in Fig. 9 and Fig. 10. Figure. 10 Constellation diagram for the DVB C transmission in channel In case of noises, the cloud increases, and the receiver can take the wrong decisions resulting the bit errors. As it is observed from the analysis of other two figures, the constellation diagram of other two channels DVB C are not affected of noise and therefore there are not some problems concerning the reception of image and sound. As it was above mentioned, one of the most important parameter of the transmission is the frequency spectrum which is constant on the whole frequency range, but in fact this is different. Analysis of the frequency spectra or evaluating the frequency spectrum on the channel [314MHz] and for other on the channel [338MHz] are shown in Fig. 11 and Fig 12. ISSN: ISBN:

5 Figure. 11 Frequency spectrum for the DVB C transmissions in channel Figure. 12 Frequency spectrum for the DVB C transmissions in channel As it is presented in Fig. 11 and Fig. 12, the frequency spectra are not constant on the frequency range, this thing being produced by linear distortions on the whole transmission channel. This thing does not affect the image quality and the sound. References: [1] ETSI EN , Digital Video Broadcasting (DVB): Framing Structure, Channel Coding and for Cable Systems, April [2]. ETSI ES , Digital Video Broadcasting (DVB): DVB Interaction Channel for Cable TV Distribution Systems (CATV), October [3]. Bit Error Ratio in DVB as a Function of S/N, Application Note,Rhode & SCHWARZ Brodcasting Division 4. [4] W. Fisher, Digital Television- A practical Guide for Engineers, Springer-Verlag Berlin Heidelberg New York, 2003 [5] Lars-Ingemar Lundstrom, Understanding Digital Television: An Introduction to DVB Systems with Satellite, Cable, Broadband and Terrestrial TV Distribution [6] Bo Shen; Junhua Tian; Zheng Li; Jianing Su; Qianling Zhang, A high performance receiver for digital cable TV with integrated A/D and FEC decoder, Design Automation Conference, Proceedings of the ASP-DAC Asia and South Pacific, Volume 2, Jan. 2005,pp [7] Alekseev, S.S.; Krivosheikin, A.V., Digital implementation of DVB-C reverse channel receiver, ISCE '06. IEEE Tenth International Symposium on Consumer Electronics, pp.1-3 [8] Jing Lei; Wen Gao,. A Backward-Compatible Solution for Next Generation DVB-C System, ICC '08. IEEE International Conference on Communications, 2008,pp Conclusions In conclusion, modulation used for the digital TV transmission by cable, more exactly in this case for 256 allows the reception in good conditions of the TV programs for this support. By reason of the error correction, the demodulation process makes as the reception of TV programs to be made in the best conditions. The sent data fluxes can be used for the video transmissions and due to the utilization of the MPEG compression can be sent many TV programs on one channel for an efficiency of the frequency spectrum. From analysis of the parameters presented in this paper a good working of this DBV-C system has resulted. ISSN: ISBN: