The contribution deals with the simulation of the digital video signal transmission through the baseband transmission channel model. The simulation model that covers selected phenomena of DVB (Digital Video Broadcasting) system signal processing is presented. The digital video signal is represented with the digital data of one non-compressed video frame that is channel encoded and protected against errors with the forward error correction (FEC) codes. The transmission channel model has influence on transmitted digital data and its distortion and the pertubative signals affect on the data decoding. The developed interactive simulation software (Matlab application) features are outlined too and the conclusion presents efficiency of the used FEC codes.Doprinos rada je u simulaciji prijenosa digitalnog videosignala putem modela kanala za prijenos u osnovnom pojasu. Prikazan je simulacijski model koji uključuje odabrane postupke obrade signala u sustavu za radiodifuziju digitalnog videosignala (DVB, Digital Video Broadcasting). Digitalni videosignal je predstavljen u obliku digitalnih podataka nekomprimirane slike, koja je kanalno kodirana i zaštićena od pogrešaka uporabom kodova za unaprijedno ispravljanje pogreške (FEC, Forward Error Correction). Model prijenosnog kanala ima utjecaj na prijenos digitalnih podataka, tako da izobličenje i smetajući signali u kanalu djeluju na dekodiranje podataka. Također su opisane značajke razvijene interaktivne programske podrške za simulaciju (Matlab aplikacija) i zaključci koji prikazuju djelotvornost korištenih FEC kodova

Tomáš Kratochvíl

English

HRČAK - Portal of Croatian Scientific and Professional Journals

ISSN 0005–1144ATKAAF 45(1–2), 41–46 (2004)Tomá{ KratochvílDigital Image Transmission Simulation Using the DVBForward Error Correction CodesUDK 621.397.13:004.932IFAC IA 5.8.4Original scientific paperThe contribution deals with the simulation of the digital video signal transmission through the basebandtransmission channel model. The simulation model that covers selected phenomena of DVB (Digital VideoBroadcasting) system signal processing is presented. The digital video signal is represented with the digitaldata of one non-compressed video frame that is channel encoded and protected against errors with the for-ward error correction (FEC) codes. The transmission channel model has influence on transmitted digital dataand its distortion and the pertubative signals affect on the data decoding. The developed interactive simula-tion software (Matlab application) features are outlined too and the conclusion presents efficiency of theused FEC codes.Key words: digital image, source and channel encoding, transmission channel model1 INTRODUCTIONDigital television (DTV) or Digital VideoBroadcasting (DVB) refers to the sourcecoding of audio, data and video signals, thechannel coding and the methods for thetransport of the DVB signals via all kindsof transmission media. The DVB standardsare determined for satellite DVB-S, cableDVB-C and terrestrial DVB-T broadcas-ting. There are common principles andcharacteristics of source and channel enco-ding in baseband for mentioned transmis-sion media (source encoding MPEG-2, chan-nel encoding FECs by block Reed-Solo-mon code and convolution code with inter-leaving). The systems are different by usedmodulation methods (QPSK for DVB-S,16-256 QAM for DVB-C and COFDM forDVB-T) caused by different bandwidth, li-mits of radiated power, levels of distortionand perturbation in transmission mediaetc. [1].The contribution presents partial solu-tion of the grant project »Simulation andanalysis of the digital signal transmission andtransmission distortions in DTV and DVBarea« no. B2813302 of the Grant Agencyof the Academy of Sciences of the CzechRepublic that was solved in 2003–2004 atInstitute of Radio Electronics, Brno Uni-versity of Technology and the author issupervisor of the project.AUTOMATIKA 45(2004) 1–2, 41–46 41Fig. 1 The simulation and analysis transmission simulation model2 TRANSMISSION SIMULATION MODELThe transmission simulation model deals with the se-lected baseband DVB processing phenomena (see Figure1). The input signal corresponds to analog RGB image(one non-interlaced and non-compressed frame of thestandard television signal) that is converted into digitalYCBCR image samples and multiplexed into serial data.The conversion and multiplexing is according to CCIRITU R-601 recommendation. This way created one-dimen-sional digital signal is optionally channel encodedwith FEC (Forward Error Correction) codes andsymbols are interleaved. The protected digital sig-nal is then transmitted with the model of the digi-tal transmission channel. The model has variableparameters and can affect signal with the lineardistortions and pertubative and reflected signals.After the transmission the signal is received, FECdecoded, deinterleaved, demultiplexed and the RGBimage samples are counted and RGB image is visu-alized. Then the objective (bit-error rate BER eva-luation) or subjective (image comparison) checkof the transmission quality can be evaluated (das-hed lines in Figure 1) in according to set upparameters of each block. The simulation modelwas applied in Matlab and the developed simu-lation software is used for research and educa-tion of DVB signal baseband processing in labo-ratory education. The next paragraphs specify par-ticular blocks and the way of the model signalprocessing. The conclusion presents efficiency ofDVB protection codes.3 DIGITAL IMAGE AND VIDEO SIGNALOne frame of the PAL CCIR 625/50 televisionsignal is the standard analog image with 576 vi-sible lines and according 720 pixels per line. Thisimage consists of three components RGB. TheDVB non-compressed image corresponds to theCCIR ITU R-601 recommendation [1] and thetransmission components are the luminance signalY and chrominance signals CB and CR. Each sig-nal component is sampled by the standardizedsampling frequency and quantized by 8 bits persample. Table I presents possible sampling formatsand subsequent frame resolution. The 4 : 4 : 4 sam-pling format is used for master record, the 4 : 2 : 2and 4 : 2 : 0 sampling formats are used for stan-dard television broadcasting (quality accords toPAL CCIR 625/50).for the sampling formats in baseband including themultiplexed serial data (MUX). The data rates are0.768 times multiplied when only active part ofthe image is in-process [2].4 ERROR PROTECTION CODES IN DVBTwo relevant methods of error protection in thetransmission of digital television are forward errorcorrections by block Reed-Solomon code (FEC1)and convolution code (FEC2) with interleaving(see Figure 2). For the transmission of digital TVover satellite and via terrestrial transmission net-works the RS (n, m) code − n = m + k where m isthe number of information symbols and k is thenumber of correction symbols – is concatenatedwith the convolution code by means of an inter-leaver. The RS code is based on the Galois fieldGF (28), and therefore has a symbol size of 8 bits.The RS (255, 239) was chosen which processes adata block of 239 symbols and can correct up to8 symbol errors by calculating 16 redundant cor-rection symbols. As an MPEG-2 packet is 188 by-tes long, the code was shortened, i.e. the first 51information bytes were set to zero and not trans-mitted at all. In this way the RS (204, 188) is ge-nerated [1].42 AUTOMATIKA 45(2004) 1–2, 41–46Digital Image Transmission Simulation Using the DVB... T.šKratochvílTable I Sampling formats and image resolutionFormatResolution Y Resolution CBCRhorizontal vertical horizontal vertical[pixels] [pixels] [pixels] [pixels]4 : 4 : 4 720 576 720 5764 : 2 : 2 720 576 360 5764 : 2 : 0 720 288 360 288The data rate of the digital video signal com-ponents and its serial data multiplex depends onthe sampling format. Table II shows the possiblesampling frequencies fs and according data rates HTable II Sampling frequencies and data rates4 : 4 : 4 4 : 2 : 2 4 : 2 : 0Signal fs H fs H fs H[MHz] [Mbps] [MHz] [Mbps] [MHz] [Mbps]Y 13.5 108 13.5 108 13.5 108CB 13.5 108 6.75 54 6.75 27CR 13.5 108 6.75 54 6.75 27MUX 40.5 324 27 216 27 162Fig. 2 Forward error correction codes in the transmission of theDVBAfter the outer code a convolutional interleaverwith depth I = 12 is used. From the frame lengthof the outer code with n = 204 the base delay re-sults as M = n/I = 17. Finally a convolution code isapplied to the interleaved symbols. Its rate R == m/n – where m is the number of input bits and nis the number of output bits – is equal to 1/2, theconstraint length is K = 7. Optionally the 2/3, 3/4,5/6 and 7/8 rates are possible. Coding for errorcorrection by transmission over cable is similar,only the convolution code is not required as thesignal-to-noise performance in the cable channel isvery much better than in the satellite channel.5 TRANSMISSION CHANNEL MODELThe philosophy of the digital transmission chan-nel simulation appears from the similarity with thecharacteristics and the process of design and rea-lization of the non-recursive digital filters FIR.These filters have finite impulse response, numeri-cal stable algorithms of design and relatively easyhardware implementation on Digital Signal Proces-sor (DSP) platform. Another advantage of thesefilters is the linear phase characteristic in mentio-ned frequency band that is important especiallyfor digital components signal transmission in digi-tal television. The conventional model [3] for digital transmis-sion channel simulation for baseband digital videosignal transmission is the FIR filter with low-passcharacter and variable parameters and method ofdesign. Design of the proposed digital transmis-sion channel model deals with the input parame-ters of the channel in according to chosen methodof design. The acceptable design methods for simu-lation in Matlab are the weighting of the impulseresponse method, sampling of the frequency cha-racteristic method and design by approximation offrequency characteristic with LS algorithm. Thesemethods give stable results and ensure successfulimplementation [4].The developed model [5] can change the charac-ter of the filter (LP, HP, BP, BS, multiband), pos-sibility of design method selection and it has vari-able parameters (filter order, cut-off frequencies,attenuations and allowed ripples of transmissionmodule in passband and stopband, eventually inter-active design by using the tolerant field of thedigital transmission channel model). The pertuba-tive signals influence on the transmitted digitalsignal is substituted with one source of pertuba-tion signal. This source is optionally added to use-ful signal – additive pertubation and may be mo-deled for e.g. as the white noise with normal dis-tribution. The next possibility of pertubative sig-nal is the reflected signal and its influence on thetransmitted digital signal.6 SIMULATION SOFTWARE FEATURESThe developed Matlab GUI application [6] forthe simulation and modeling uses functions of theSignal Processing, Image Processing and Commu-nication Toolboxes of Matlab and allows:⇒ input image selection from file – BMP, PCX,JPG, PNG, TIF, FIG; ⇒ digital video signal sampling format selectioncorresponds to ITU R-601 recommendation –4 : 4 : 4, 4 : 2 : 2, 4 : 2 : 0;AUTOMATIKA 45(2004) 1–2, 41–46 43T.šKratochvíl Digital Image Transmission Simulation Using the DVB...Fig. 3 The transmission channel model parameters editor – interactive design(example of low-pass transmission channel FIR design using tolerant field)⇒ transmission quality selection corresponds toITU R-601 recommendation – 8 or 10 bitsper component samples;⇒ transmission multiplex type selection corres-ponds to DVB standard – serial or parallelmultiplex;⇒FEC1 parameters setting of RS code, n and mparameters in according to DVB standard –RS (255, 239), RS (255, 235), RS (255, 223),RS (255, 205);⇒ intereleaver depth I settings – up to 20;⇒FEC2 parameters setting of convolution code,rater R = m/n selection in according to DVBstandard – 1/2, 2/3, 3/4, 5/6, 7/8;⇒ design and visualization of the baseband trans-mission channel model parameters, including in-teractive graphics methods of design and sa-ving the model parameters – low-pass, high--pass, pass-band, stop-band and multiband fil-ter type;⇒ additive noise pertubation model and reflectedtransmission signal model that are used in thetransmission channel model;⇒ decision level setting in according to used sour-ce coding – NRZ unipolar/bipolar, RZ unipo-lar/bipolar;⇒ channel and source decoding, restoration of theimage from transmitted component signals andobjective and subjective evaluation of the trans-mission influence on transmitted image;⇒ objective evaluation of transmission quality –block and bit error rates of protected/non-pro-tected digital signals;⇒ subjective evaluation of transmission quality –the input/output image in spatial domain com-parison including Zoom function for detail dis-play;⇒ interactive help – HTML text (in czech).7 RESULTSIndependently of whether the encoding and de-coding is in frequency or time domain, the effi-ciency of the RS code is the same. The residualbit-error rate (BER) of various RS codes basedon GF (28) is shown in Figure 5. Although theRS codes are symbol-oriented codes, the analysisof the efficiency takes bit errors into account.The efficiency of the code increases with an in-crease in the number of test symbols. At an inputbit-error rate of 2 ⋅ 10−3 the residual bit error rateof the RS (255, 205) code is approx 1 ⋅ 10−10 –the coding gain is thus more than 10 to the powerof 7 – whereas in the case of the RS (255, 239)code at the same input bit-error rate the outputbit-error rate is 9 ⋅ 10−4 – the coding gain is onlyslightly greater than 0.5. For all DVB transmis-44 AUTOMATIKA 45(2004) 1–2, 41–46Digital Image Transmission Simulation Using the DVB... T.šKratochvílFig. 4 Subjective check of transmission quality – comparison of input RGB and B&Wimages with transmitted images that are overlaid by the transmission errors (only examp-ple without specification)sion standards a modified (shortened) RS (255,239) code is used which makes it possible residu-al bit-error rate of approx 1 ⋅ 10−11 at an input bit--error rate of 2 ⋅ 10−4 while correcting up to 8symbol errors per block.The residual bit-error rate of convolutional co-des of rate R is a function of Eb/N0 (energy trans-mitted per bit divided by the noise-power densi-ty of the white Gaussian noise) and the parame-ter K describes the length of the code. The per-formance of the error correction increases withincreased K. For the DVB standard a convoluti-onal code of rate R = 1/2 with a constraint lengthK = 7 is used. With Eb/N0 = 3.2 dB it is possibleto achieve a bit-error rate of less than 2 ⋅ 10−4at the output of the decoder, this ratio corres-ponds to the maximum of the bit-error rate atthe input of the RS decoder, so finally a bit-er-ror rate at the output of the RS decoder of lessthan 1 ⋅ 10−11 is obtained.8 CONCLUSIONDescribed philosophy, model for simulation andanalysis and software implementation is the par-tial solution of the mentioned grant project andgives some general results for baseband digitaltransmission channel simulation and analysis ofdigital television transmission. The developed Mat-lab GUI application for the simulation and analy-sis uses functions of the Signal Processing, ImageProcessing and Communication Toolboxes of Mat-lab. It can be used for the illustrative modelingand experimental education of the digital televi-sion technique area. This work presents the openmodular application that can be ensemble with theother transmission phenomena of the DVB stan-dard signal processing (digital modulation techni-ques, transmission in RF band etc.)ACKNOWLEDGEMENTThe contribution was supported by the grant projectof Grant Agency of the Academy of Sciences of theCzech Republic, no. B2813302, Simulation and analysisof the digital signal transmission and transmission distor-tions in DTV and DVB area and Research programmeof Brno University of Technology, Research of electro-nic communication systems and technologies, CEZ MSM262200011.REFERENCES[1] U. Riemers, Digital Video Broadcasting. The InternationalStandard for Digital Television. Springer-Verlag Berlin Heil-deberger New York, 2001.[2] G. W. Collins, Fundamentals of Digital Television Trans-mission. A Wiley-Interscience Publication, John Wiley &Sons, Inc., 2001.[3] T. Kratochvíl, V. Øí~ný, Characteristics of an Ideal Low--Pass Discrete Digital Filter Used for Transmission of theVideo Signal in TV Technique. In proceedings ERK 2000,pp. 25–28, Portorose, (Slovenia), September 2000.[4] T. Kratochvíl, Simulation of the Digital Real TransmissionChannel Characteristics. Research report of developmentproject of the Ministry of Education of the Czech Repub-lic no. 156/2001, Brno (Czech Republic), January 2001. [5] T. Kratochvíl, V. Øí~ný, The Digital Image TransmissionSimulation. In proceedings VIPromCom 2002, pp. 401–406,Zadar (Croatia), June 2002.[6] T. Kratochvíl, J. Hrdina, Utilization of Matlab for Educa-tion of the Digital Image Transmission. In ProceedingsMatlab 2002, Vol. 1, pp. 261–264, Prague (Czech Repub-lic), November 2002.AUTOMATIKA 45(2004) 1–2, 41–46 45T.šKratochvíl Digital Image Transmission Simulation Using the DVB...Fig. 5 Residual bit-error rate value of various RS (255, 255−2t) codes46 AUTOMATIKA 45(2004) 1–2, 41–46Digital Image Transmission Simulation Using the DVB... T.šKratochvílSimulacija prijenosa digitalne slike uporabom DVB kodova za unaprijedno ispravljanje pogre{ke. Doprinosrada je u simulaciji prijenosa digitalnog videosignala putem modela kanala za prijenos u osnovnom pojasu. Pri-kazan je simulacijski model koji uklju~uje odabrane postupke obrade signala u sustavu za radiodifuziju digital-nog videosignala (DVB, Digital Video Broadcasting). Digitalni videosignal je predstavljen u obliku digitalnihpodataka nekomprimirane slike, koja je kanalno kodirana i za{ti}ena od pogre{aka uporabom kodova za unapri-jedno ispravljanje pogre{ke (FEC, Forward Error Correction). Model prijenosnog kanala ima utjecaj na pri-jenos digitalnih podataka, tako da izobli~enje i smetaju}i signali u kanalu djeluju na dekodiranje podataka.Tako|er su opisane zna~ajke razvijene interaktivne programske podr{ke za simulaciju (Matlab aplikacija) i za-klju~ci koji prikazuju djelotvornost kori{tenih FEC kodova.Klju~ne rije~i: digitalna slika, izvorno i kanalno kodiranje, model prijenosnog kanalaAUTHOR’S ADDRESSTomá{šKratochvíl, M.Sc.Institute of Radio Electronics, FEEC BUTPurkyòova 118, Brno 612 00, Czech RepublicE-mail: kratot@feec.vutbr.cz,Received:  2004–10–23

Simulacija prijenosa digitalne slike uporabom DVB kodova za unaprijedno ispravljanje pogreške

Abstract

Similar works

Full text

Available Versions

HRČAK - Portal of Croatian Scientific and Professional Journals