HDTV işaretlerinin uydu üzerinden iletiminde özel problemler

Abstract

Günümüzde kullanılmakta olan TV sistemlerinin yerine kullanılması planlanan HDTV sistemleri 1000'den fazla tarama satırı ve 16:9 görünüş oram kullanarak 35 mm. sinema filmi ile aynı görüntü kalitesi sağlarlar.Bu sistemlerde birden fazla kompakt disk kalitesinde ses kanalı vardır. Bu tip sistemler bölüm 2'de anlatılmıştır. Bölüm 3'de iletim ortamları incelenmiştir.HDTVnin band genişliği büyük olduğundan mevcut frekans planlaması bozulmadan bu ortamlardan iletilemez.HDTV işaretlerini iletebilmek için iki yol vardır. Birincisi işaret bandım ikiye bölerek iki kanaldan iletmektir. Bunun avantajı kanallardan biri uygun şekilde düzenlenerek klasik alıcılarla ve/veya spektrumla uyumluluk sağlamasıdır. Sakıncası ise iki kanal işgal etmesidir. Bu tip iletim sistemleri bölüm 3'de anlatılmıştır. Diğer yol band genişliğini sıkıştırmaktır. Bu şekilde tek kanal kullanılarak HDTV işaretleri iletilebilir. Bunun sakıncası klasik sistemlerle uyumlu olmamasıdır. Band sıkıştırma yöntemleri bölüm 4,5 ve 6'da anlatılmıştır. örnekleme yöntemleri ile bandı sıkıştırılan işaret mevcut frekans planlaması bozulmadan bugün kullanılmakta olan uydu ve yer istasyonları ile iletilebilir.

Since 1970, in order to improve the quality of picture and sound of the existing TV systems, more extensive studies have been made. In these studies, the characteristics of human visual system have been considered. Number of scanning lines, aspect ratio and viewing distance are fixed for these characteristics. After these studies, varios systems have been offered. These systems are called ATV (Advanced TV). ATV systems consist of IDTV (Improved Definition TV ), EDTV (Extended Definition TV ) and HDTV (High Definition TV ). The term IDTV refers to improvements to NTSC television which remain within general parameters of NTSC emission standarts. The term EDTV refers to a number of different improvements that modify NTSC emissions. HDTV is a new signal format and has twice the horizantal and vertical resolution of the conventional systems. HDTV will offer the some image quality as 35 mm. movie film, the improved resolution being achieved by employing more than 1000 lines instead of current standarts, a larger cinemascope-like aspect ratio, suppression of spurious picture artifacts and noise. Wide RF bandwidth is needed to transmit such signals. In HDTV systems there is multichannel sound of compact disc quality. Aspect ratio is 16/9 (width of screen is 1 6 and height of screen is 9. This ratio is 4/3 in the conventional system.) In the above explained system. There are some propesed HDTV systems, due to the fact that various studies have been made in many countries. In each system the number of scanning lines, field rate and signal format are different In chapter 2 various HDTV systems have been explained. Transmission systems have been explained in chapter 3. HDTV signals can be transmitted by means of terrestrial systems, cable systems and satellite systems. Terrestrial HDTV systems must be compatible with the existing receiver and/or spectrum. The biggest problem in the terrestrial systems is to find unoccupied channel. For this reason taboo channels in the VHF and UHF bands can be used with various modulation schemes. SC-HDTV is this type of a system. In this system 28.9 MHz HDTV source signals is coded in band of 6 MHz. So this system is transmitted by means of one channel. So compatibility of the existing spectrum is provided. Because the modulation scheme is different from the conventional type this system is transmitted by means of taboo channels. The second example is HDNTSC system. In this system HDTV source signal is split into two parts by means of electronic processing. One part is prepared so that it will be compatible with the existing receiver and spectrum. Another part which is called augmentation channel carries additional scanning lines, wide aspect ratio, digital sound channels and etc. This augmentation channel is transmitted in the taboo channels. Another alternative method in the terrestrial transmission is microwave transmission. But it is useless method. Because, there are a lot of looses in microwave band due to the rain, ice, atmosphere and etc. So more repetion station must be used to transmit HDTV signals. HDTV signals can be transmitted through the cable distribution systems, but the disadvantage of the system is that, it is not widespread over the country. B-ISDN (Broadband -Integrated Service Digital Network ) has been developed for this purpose. In this project, digital fiber optic telephony network will be used to transmit high definition signals. The most appropriate way is to get the use of satellite to transmit HDTV signaLBut HDTV signal has a video bandwidth about 30 MHz. In order to transmit this signal by means of broadcast satellite which has an RF bandwidth of 27 MHz ( or 24 MHz ), one method is to split the bandwidth of HDTV signals by means of electronic processing into two parts. One part is compatible with the existing systems which is transmitted on one DBS channel and can be VII received by ordinary TV sets equipped with suitable DBS receivers. The second part of source signal which is transmitted on a second channel carries additional information but with a 16:9 aspect ratio. Two channels are motrixed by a home HDTV receiver to reconstitute the original HDTV signals. CBS system is this type of a system. CBS system has been explained in chapter 3. Various experiments have been done for transmission by satellite. First of all these trues is Y-C seperate transmission system. Within this system luminance information and chrominance information are transmitted by seperate channels. No electronic processing is applied to luminance and chrominance signals. This system uses two channels and wide RF bandwidth to transmit HDTV signals so it is a useless system. Another experiment is TCI transmission system. This system uses a channel to transmit HDTV signals. Within the this system, line-sequential chrominance signal is compressed in the time and multiplexed in the horizantal blanking interval time division. In this system RF bandwidth is 60 MHz so this syMcm is a useless system because of 27 MHz ( 24MHz ) RF bandwidth is used in the current salellites.ln the tranmission experiment of this system, Carson rule have not been considered. Another experiment is interlaced scanning conversion. In this system scanning lines are less than other systems. So narrower bandwidth is provided. But quality of picture and sound is same as other systems. But bandwidth of this system is still bigger than anticipated bandwidth. So this system is not useful. The most important way to transmit HDTV signals is bandwidth reduction. In this method, the bandwidth of HDTV signals is reduced by means of sampling according to the content of image. Method of sampling changes whether image is moving or stationary. The most impartant bandwidth reduction system is MUSE (Multiple Sub-Nyguist Encoding). In this system 1125 scanning lines, 60 Hz field rate, 2:1 intarface ratio and 16:9 aspect ratio are used. The bandwidth of the signal is reduced to 8.1 MHz (-6 dBm) using sub-Nyquist sampling system. Using MUSE, a satellite channel of width 24 or 27 MHz in the 12 GHz band can carry an HDTV picture, digitally coded four channel sound and mdependent digital sound at about 100 kb/s. In MUSE system HDTV signals are sampled according to the motion of the picture. For moving parts of the picture line -offset subsampling is used and frame and field offset subsampling are used for stationary parts of the picture. Result sampling patterns must be same hnpotant techical basis in MUSE system as follows: 1) A bandwidth reduction technique using multiple sub-sampling and motion compensation 2) Anolog sampled value transmission technique consist of automatic equalization 3) Syne system wtich provides correct re-sample phase. This system uses positive syne pulse 4) A Non-linear emphasis system with 9,5 dB gain 5) This system uses quasi-constant luminance system 6) Baseband multiplexing system is used for sound and mdependent digital data. Signal format is same as video MUSE system uses TCI (Tine Compressed Integration) format Chrominance signals are transmitted as line sequantial R-Y is multiplexed in odd lines. B-Y is multiplexed in even lines. There are 480 samples in a line. 12 of them are for HD signal (horizantal syne signal of MUSE). 94 samples are for C signal and 974 samples are Y signal. First and second lines are frame pulses. Signals which are inserted vertical blanking interval are as fallows a) Frame syne signal b) Control signals c) Clipping level signal d) VITS (Vertical Interval Test Signal) e) Sound and mdependent information signal. Received sampled signal is spatially and temporally interpolated pixelwise, according to the motion of the picture, in a decoder. A complete MUSE system has been developped and domestic and international emission experiments have been carried out. MUSE system which has 8.1 MHz bandwidth is not compatible with the existing systems and can not be transmitted by means of the single channel of terrestrial distrubition systems have 6MHz channel bandwidth. MUSE systems which are compatible with existing receiver and/or spectrum have been developped for the above missing conditions. Norrow-MUSE of these systems has 6MHz bandwidth and is compatible with spectrum. It can be used for simulcasting. In this system source signal is 1 125 lines. 1 125 lines are converted to VIII 750 lines, it is transmitted as 750 lines. Base bandwidth of this system is 4.86 MHz. Since the transmission format is different from the conventional systems, Narrow-MUSE can not be received the conventional receiver. Sampling method is same as MUSE. Inter-field and inter-frame interpolation are used for stationary parts of picture intra-field interpolation is used for moving parts of picture Two mode are used for digital sound. A mode has four channels, B mode has high quality two channels. MUSE-6 system has 6MHz RF bandwidth and is compatible with NTSC. In this system source signal is 1 125 lines. These 1 125 lines are converted to 525 lines. MUSE-6 system has 16/9 aspect ratio. This wide aspect ratio is provided by top and bottom masking method. MUSE-6 is transmitted as 525 lines. It has 4.2 MHz base bandwidth. So MUSE-6 can be received by the conventional receiver. MUSE-9 has two channels. A channel has 6MHz RF bandwiolih (4.2 MHz base bandwidth) and is compatible with NTSC. The other channel has 3MHz RF bandwidth (2,1 MHz video bandwidth) and carries additional information which is wide aspect ratio, additional lines and etc. MUSE-9 has four channels sound. The quality of systems which are compatible with NTSC are less than Narrow-MUSE due to the NTSC restrictions, MUSE family systems have been explained m chapter 4. Transmission experiments by satellite of MUSE system have been carried out during the Seoul Olympiad Games. In these experiments frequancy modulation, 27MHz RF bandwidth, 9.8 MHz frequency deviation, non-linear emphasis and keyed AFC have been used. These experiments have been carried out with communication and broadcasting satellites. Within the these experiments different bandwidth power and C (6/4 GHz), Ku (14/12 GHz) and Ka(20/30 GHz) bands are used. Transmission characteristics have been changed for every experiments. European countries have been developping the EUREKA-95 prject about HDTV system. The developped system is based on MAC principle and is colled HD MAC. In this system 1250 scanning lines, 50 Hz field rate, 2:1 interlace ratio and 16:9 aspect ratio are used. The bandwidth of this system exceeds 30MHz with these parameters. Because the high definition image contain abaut four times about the information of the basic MAC image, the bandwidth reduction required would suggest that three dimensional filtering and sampling would be necessary at the encoder. This would require the use of a complementary three dimensional interpolating decoder at the receiver. The technique adapted uses a combination of horizantal, vertical and temporal filtering paths that can be selected on an adaptive basis according to the image content There are two type HD-MAC systems. One of them is basic system and the other is extended system. Basic system has 1250 lines (1 152 active lines) 50 Hz field rate and 20 MHz video bandwidth. For the luminance two times compression is necessary and four times compression is neccesary for chrominance. Digital assisted TV(DATV) is inserted vertical blanking interval. 21 lines are used for this data. Second part of vertical blanking interial is reserved to code motion vectors. Extended system provide a lot of features for example motion compensation techniques, global motran prameters, 72MHz sampling frequency and compatibility with MAC. Transmission experiments of HDMAC system are planned to be carried out in 1990's. In chapter 5 wideband MAC system has been explained. Further more, various systems with the basis of NTSC compatibility have been suggested for Northern America. One of them is HDS-NA which has two formats. One format is for cable an dor terrestrial transmission and the other is for stallite transmission. Fallowing three technique can be used for satellite HDTV signal format 1) Same signal format as terrestrial transmission 2) Independent signal format which is not relation with terrestrial transmission format 3) Signal format which is relation with terrestrial transmission format but It is not same signal format. line format of HDS-NA satellite signal is defined as a superline. This superline consists of 8 unit of time division information in 127.1 1 us. There are 131.2,5 superlines in a field. There are 525 superlines in four field. Four field is defined a superframe. This signal has a 9.54 MHz bandwidth. In this system 525 lines, 59.94 Hz field rate, 1:1 progressive scanning. 8.2 \is is reserved for digital data in every superline. Every vertical interval consist of 1 1,25 superlines. In a super frame there are 480 superline for active video and there are 45 sperline for digital data. This system uses 10.125 Mbit/s with duobinary coding and its performation is better than like. FM satellite links. In this system emphasis is verified simple pole and zero circuits as well as NTSC, B-MAC and D2-MAC. Satellite format of HDS-NA system has been explained in chapter 6. IX It is important to supply generalized satellite link model for satellite transmission. This model has been described in chapter 7 C/N ratio is very important in the satellite transmission, Because it is relation with receiving antenna diameter. First, traditional satellite link model has been described, and then generalized satellite link model has been described. There are two important different between traditional model and generalized model. 1) The gain of transponder can not be defined as a single. Noise and gain are appraised every carrier 2) There is intermodulation between carriers in the transponder and output link noise If there is no intermodulation between output link noise and carriers and if constant gain is let for transponder generalized model degenerotes traditional model. In chapter 8 interference and spectrum calculations have been studied. FM modulation is ;ised when HDTV is transmitted as anolog format. Hence interference and spectrum calculation of FM-TV signal must be done. In addition to calculate bandlimiting distortion, co-channel Interference and adjacent channel interference must be calculated to make efficient use of the avaible spectrum. Because number of the systems increases very quick. FM video signal is very important in the analysis on the transmitting TV by satellite. Either powers or bandwidths are large in DBS satellites. To model of FM video signal is very difficult. But transmitting spectrum of the signal must be correctly calculated to decide level of interference. Inportant part Of the distortion occuring in FM-TV systems is due to bandlimiting of the signal. Bandlimiting distortion of FM-TV systems is defined ratio of total signal power to out-band of power as fallows total signal power SDR=101og total signalpower- in -band power 1 1 B^+(tfs? B*+(frfsJ The spectrum of FM signal is provided with its autocorrelation conversion. Baseband spectrum can be defined as luminance components and chrominance compenents of video signal. These components have double-sided power spectral density. Baseband power spectral density of luminance component of signal as fallows. pıBı 1 ît Bj+r Chrominance power spectral density as follows PB Sc(f)=-£_f 2tc where Pl(f) = total power of luminance signal Pc(f) = total power of chroma signal Bi(f) = bandwidth of luminance signal 3c(f) = bandwidth of chroma signal fs = color subcarrier frequency Total spectrum of video signal as fallows St(f) = Si(f) + Sc(f) If the video signal is applied to the frequency modulator, output FM signal of modulator has Rvj(j( x ) correlation function as below Rvid (t ) = e x p (-Kvid (t )) Cos Wcx In general K^T^/jS^OSiriVxyf2. df Where Sx is double-sided baseband spectrum. Using above these formulas, spectrum of luminance companent is find as below S{f)= V2tcP, This equation gives approximate estimation of spectrum vicinity carrier. So this equation can not be used in calculation of adjacent channel interference. In similiar power spectrum of chrominance is find as fallows nvB, Sc(t)=e-^ -4-5 Be *l c, c ?M») (f-lf+B* (f+ff+B* Total spectrum of video signal is find as fallows Svid(f> -3^1 fexp(-^p | +T^ [expf -f^M^expf+f^ numerical methods must be used for calculation on tails of spectrum. Energy dispersion signal must be used calculations of co-channel and adjacent channel interference This signal is added to base modulation signal to decrase high power components. Various waveforms and frequency have been offered. £1 general the signal which has triangular waveform and same frequancy with frame is used. Auto correlation of energy dispersion signal must be calculated for calculations of adjacent and co-channel mterf erence. Adjacent channel interference is find as fallows. /. fc+B/2 St(f) df (dB) fc-B/2 where fc channel spacing St(f) spectrum of transmitted signal which consist of energy dispersion signal. Co-channel interference is defined as fallows CCI= f St (f) df Co-channel interference is due to high power density in transmitting spectrum. Additional subcarriers decrease CCI. Analytic and numerical methods can be used real estimation of FM spectrum in satellite communication system. Analytic method is valid vicinity of carrier i.e. co-channel interference Numerial methods are used to calculate adjacent channel interference i.e tails an spectrum. But numerical methods have more computer time than analytic methods. As a result HDTV is a new signal format and it has very large bandwidth. But this signal can be transmitted by means of satellite. For this two ways are used. One of them is band splitting and the other is bandwidth reduction. Compatibility with receiver and/or spectrum is provided when the band of HDTV signal is split. Bandwitdth reduced system is not compatible with spectrum and receiver. In transmitting by satellite - Anolog and digital transmission can be done - Bandwidth of all systems must be reduced - Narrow RF bandwidtih systems are characterized with anolog modulation and high degree bandwidth reduction. - 50-120 MHz bandwidth must be used in large RF bandwidth -12 GHz band can not be used wideband HDTV with anolog or digital modulation. - All bands from 12 GHz to 23 GHz are suitable view of propagation - More large bandwidth is provided when the communication satellites are used. So larger fregvency deviation is provided. It means that smaller S/N or C/N. So smaller diametes dishes can be used or earth stations with lower EIRP can be used. The quality of compatible systems are lower than non compatible systems. For example the quality of Narrow-MUSE which is not compatible with NTSC is better than NTSC compatible MUSE-6 and NTSC compatible MUSE-9. 61 general non -compatible with single channel systems use frame and field memory. This increases price of receiver and quality. As a result decreasing of number of offered HOTV systems will be useful. Standartization is very important. Furthermore Digital transmission techique must be used for the noise and distortion in the tranmission.