Signals and Communication Technology Digital Video and Audio Broadcasting Technology A Practical Engineering Guide Bearbeitet von Walter Fischer 1. Auflage 2010. Buch. xxvii, 800 S. Hardcover ISBN 978 3 642 11611 7 Format (B x L): 15,5 x 23,5 cm Gewicht: 1430 g Weitere Fachgebiete > EDV, Informatik > Professionelle Anwendung > Digitales Video Zu Inhaltsverzeichnis schnell und portofrei erhältlich bei Die Online-Fachbuchhandlung beck-shop.de ist spezialisiert auf Fachbücher, insbesondere Recht, Steuern und Wirtschaft. Im Sortiment finden Sie alle Medien (Bücher, Zeitschriften, CDs, eBooks, etc.) aller Verlage. Ergänzt wird das Programm durch Services wie Neuerscheinungsdienst oder Zusammenstellungen von Büchern zu Sonderpreisen. Der Shop führt mehr als 8 Millionen Produkte. 2 Analog Television Throughout the world, there are only two major analog television stan- dards, the 625-line system with a 50 Hz frame rate and the 525-line system with a 60 Hz frame rate. The composite color video-and-blanking signal (CVBS, CCVS) of these systems is transmitted in the following color transmission standards: • PAL (Phase Alternating Line) • NTSC (National Television System Committee) • SECAM (Séquentiel Couleur a Mémoire) Vertical blanking Visible A B C lines Visible horizontal part Horizontal blanking Fig. 2.1. Dividing a frame into lines PAL, NTSC and SECAM color transmission is possible in 625-line sys- tems and in 525-line systems. However, not all the possible combinations have actually been implemented. The video signal with its composite co- ding is then modulated onto a carrier, the vision carrier, mostly with nega- tive-going amplitude modulation. It is only in Std. L (France) that positive- W. Fischer, Digital Video and Audio Broadcasting Technology, Signals and Communication Technology, 3rd ed., DOI 10.1007/978-3-642-11612-4_2, © Springer-Verlag Berlin Heidelberg 2010 8 2 Analog Television going modulation (sync inside) is used. The first and second sound subcar- rier is usually an FM-modulated subcarrier but an amplitude-modulated sound subcarrier is also used (Standard L, France). In Northern Europe, the second sound subcarrier is a digitally modulated NICAM subcarrier. Al- though the differences between the methods applied in the various coun- tries are only minor, together they result in a multiplicity of standards which are mutually incompatible. The analog television standards are numbered through alphabetically from A to Z and essentially describe the channel frequencies and bandwidths in VHF bands I and III (47 ... 68 MHz, 174 ... 230 MHz) and UHF bands IV and V (470 ... 862MHz); An example is Standard B, G Germany: B =7 MHz VHF, G = 8 MHz UHF. In the television camera, each field is dissected into a line structure of 625 or 525 lines. Because of the finite beam flyback time in the television receiver, however, a vertical and horizontal blanking interval became ne- cessary and as a result, not all lines are visible but form part of the vertical blanking interval. In a line, too, only a certain part is actually visible. In the 625-line system, 50 lines are blanked out and the number of visible li- nes is 575. In the 525-line system, between 38 and 42 lines fall into the a- rea of the vertical blanking interval. To reduce the flickering effect, each frame is divided into two fields combining the even-numbered lines and odd-numbered lines in each case. The fields are transmitted alternately and together they result in a field re- petition rate of twice the frame rate. The beginning of a line is marked by the horizontal sync pulse, a pulse which is below the zero volt level in the video signal and has a magnitude of -300 mV.. All the timing in the video signal is referred to the front edge of the sync pulse and there exactly to the 50% point. 10 s after the sync pulse falling edge, the active image area in the line begins in the 625-line system. The active image area itself has a length of 52 s. In the matrix in the television camera, the luminance (luminous density) signal (Y signal or black/white signal) is first obtained and converted into a signal having a voltage range from 0 Volt (corresponding to black level) to 700 mV (100% white). The matrix in the television camera also produ- ces the color difference signals from the Red, Green and Blue outputs. It was decided to use color difference signals because, on the one hand, the luminance has to be transmitted separately for reasons of compatibility with black/white television and, on the other hand, color transmission had to conserve bandwidth as effectively as possible. Due to the reduced color resolution of the human eye, it was possible to reduce the bandwidth of the color information. In fact, the color bandwidth is reduced quite signifi- cantly compared with the luminance bandwidth: The luminance bandwidth 2 Analog Television 9 is between 4.2 MHz (PAL M), 5 MHz (PAL B/G) and 6 MHz (PAL D/K, L) whereas the chrominance bandwidth is only 1.3 MHz in most cases. Fig. 2.2. Analog composite video signal (PAL) Fig. 2.3. Vector diagram of a composite PAL video signal 10 2 Analog Television In the studio, the color difference signals U=B-Y and V=R-Y are still used directly. For transmission purposes, however, the color difference signals U and V are vector modulated (IQ modulated) onto a color subcar- rier in PAL and NTSC. In SECAM, the color information is transmitted frequency-modulated. The common feature of PAL, SECAM and NTSC is that the color information is modulated onto a color subcarrier of a higher frequency which is placed at the upper end of the video frequency band and is simply added to the luminance signal. The frequency of the color subcarrier was selected such that it causes as little interference to the lumi- nance channel as possible. It is frequently impossible, however, to avoid crosstalk between luminance and chrominance and conversely, e.g. if a newsreader is wearing a pinstriped suit. The colored effects which are then visible on the pinstriped pattern are the result of this crosstalk (cross-color or cross-luminance effects). Vision terminals can have the following video interfaces: • CVBS, CCVS 75 Ohms 1 VPP (video signal with composite • coding) • RGB components (SCART, Peritel) • Y/C (separate luminance and chrominance to avoid cross color or cross luminance effects) In the case of digital television, it is advisable to use an RGB (SCART) connection or a Y/C connection for the cabling between the receiver and the TV monitor in order to achieve optimum picture quality. In digital television only frames are transmitted, no fields. It is only at the very end of the transmission link that fields are regenerated in the set top box or in the decoder of the IDTV receiver. The original source mate- rial, too, is provided in interlaced format which must be taken into account in the compression (field coding). 2.1 Scanning an Original Black/White Picture At the beginning of the age of television, the pictures were only in “black and white”. The circuit technology available in the 1950s consisted of tube circuits which were relatively large and susceptible to faults and consumed a lot of power. The television technician was still a real repairman and, in the case of a fault, visited his customers carrying his box of vacuum tubes. 2.1 Scanning an Original Black/White Picture 11 Let us look at how such a black/white signal, the “luminance signal”, is produced. Using the letter “A” as an example, its image is filmed by a TV camera which scans it line by line (see Fig. 2.4.). In the early days, this was done by a tube camera in which a light-sensitive layer, onto which the image was projected by optics, was scanned line by line by an electron beam deflected by horizontal and vertical magnetic fields. 1 2 3 4 5 6 7 8 9 10 11 12 700mV = white Line 3 0mV = black Fig. 2.4. Scanning an original black/white picture H- blanking 700mV = white 0mV = black -300mV H-sync Visible part Fig. 2.5. Inserting the horizontal sync pulse Today, CCD (charge coupled device) chips are universally used in the cameras and the principle of the deflected electron beam is now only pre- 12 2 Analog Television served in TV receivers; and even there the technology is changing to LCD and plasma screens. The result of scanning the original is the luminance signal where 0 mV corresponds to 100% black and 700 mV is 100% white. The original pictu- re is scanned line by line from top to bottom, resulting in 625 or 525 active lines depending on the TV standard used. However, not all lines are vi- sible. Because of the finite beam flyback time, a vertical blanking interval of up to 50 lines had to be inserted. In the line itself, too, only a certain part represents visible picture content, the reason being the finite flyback time from the right-hand to the left-hand edge of the line which results in the horizontal blanking interval. Fig. 2.4. shows the original to be scanned and Fig. 2.5. shows the associated video signal. 2.5 lines Vertical sync puls 1 line Begin of line 1 of 1st field Center of line 313 of 2nd field Fig. 2.6. Vertical synchronization pulse 2.2 Horizontal and Vertical Synchronization Pulses However, it is also necessary to mark the top edge and the bottom edge of the image in some way, in addition to the left-hand and right-hand edges.