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Herwig A. Benning TV in cinema format: SAW filters drive PALplus espite different systems and stan- Surface acoustic wave ● Downwardly compatible with dards for frequency, color and defin- standard PAL (SAW) filters are indispen- ition, a common trend can be ob- D ● sable to PALplus – the en- served in television technology: the screen Conversion from 4:3 to 16:9 format hanced version of the PAL format of the future will become wider. possible at all times Compared to today’s TV picture, which has ● Improved signal quality thanks to standard for wide-screen a width to height ratio of 4:3, the future for- greater effective luminance and formats. They are the only mat will have a 16:9 aspect ratio. This comes chrominance bandwidth type of component which much closer to the human field of vision, ● Elimination of cross color and cross since the area perceived by the eye has a luminance can meet PALplus specifica- greater horizontal emphasis than a vertical ● Improved sound quality (optional) tions for frequency response one. What’s more, cinema* films with their ● Echo suppression (optional) and group delay in the IF 16:9 format can be projected and broadcast without annoying black bars above and Table 1 PALplus system requirements stages of TV receivers and below the picture. video recorders. SAW filters In 1989, broadcasting corporations and from Siemens Matsushita consumer electronics companies in Europe Components are now used set up a project team to develop a standard plus broadcasts in January 1994, the new by all major TV manu- based on PAL for wide-screen TV. An impor- technique has put Europe in the lead in tant requirement was that the new stan- development of 16:9 wide-screen TV. For facturers. dard, to be known as PALplus, should be 1995, broadcasting corporations and com- 100% downwardly compatible with the panies have planned more than 12,000 existing PAL system. program hours (over 5000 of them in Germany alone) in the new PALplus stan- At the 1993 International dard. Consumer Electronics Ex- hibition in Berlin, the Enhanced signal quality first experimental PAL- plus broadcasts began, The improved signal quality (Table 1) re- and the specification quired for PALplus is achieved in particular was finalized. In No- by greater effective luminance and chromi- vember 1994, Nokia nance bandwidth. In addition, the Color Plus launched the first PAL- color coding technique prevents the cross- plus TV set in Germa- talk between luminance and chrominance ny, which, incidentally, typical of PAL. This technique is designed contained a SAW filter to process high-frequency luminance and from Siemens Matsu- chrominance for two fields at a time. The shita Components, and Color Plus technique has been modified for all major TV manufactur- PALplus to produce a motion-dependent ers now offer PALplus variant, which has proved superior and be- receivers. Since the intro- come an integral part of the PALplus system duction of regular PAL- concept. 1 16:9 and 4:3 screen formats Under the PAL standard, a complete frame comprises 625 lines. 576 of these are used The PALplus receiver presents the 16:9 picture to present the actual picture. The remaining with 576 lines including 49 lines are located in the vertical blanking Transmission channel decoded helper signals interval – invisible to the viewer – and used to fill the screen for various purposes, such as transmitting teletext. Helper Fig. 1 shows the transmission channel and The conventional 4:3 screen presentation of 4:3 and 16:9 screens. receiver presents the 16:9 picture in letterbox format So that the 16:9 picture can fit the frame of with 432 lines the 4:3 screen, it is down-converted in the vertical direction. In this process, 144 of the 576 active lines are used to display black bars at the top and bottom edges of the picture. A total of 432 lines remain for the Fig. 1 Transmission channel and screen presentation on 4:3 and 16:9 screens Camera/film Helper Helper demodulator Inverse Y Y D Low- L letterbox RF + A pass * filter Yh SAW IF CVBS A Tuner IFA Mixer filter demodulator D 432 D U,V PAL U,V + demod- IFA Mixer A ulator 576 IFA: Intra-frame average Status Motion Camera/film Camera/film YL: Low-frequency luminance spectrum decoder detector Yh: High-frequency luminance spectrum Fig. 2 Block diagram of a PALplus TV receiver actual picture. Experts graphically call this A PALplus receiver requires the 144 “lost” er signal is practically ruled out in standard compromise the “letterbox picture”. This is lines as well as the 432 visible lines. The receivers. To ensure further reduction of any how wide-screen cinema films have been high-pass component suppressed by interference in reproduction on convention- broadcast on television in the past. filtering – the signal known as the vertical al PAL sets, the helper is transmitted at a helper – is transmitted separately. After a reduced signal level. However, as soon as TV sets with a 16:9 as- nonlinear amplitude pre-emphasis, it is mod- pect ratio matching the wide screen format ulated by the color carrier. Here the energy What a PALplus filter must do are available, all 576 active lines can be used of the helper signal centers mainly on the with PALplus to produce a picture that region of the color carrier frequency. This Optimum feeding of the helper signal to the completely fills the screen. In view of the color carrier modulation frees the resulting helper demodulator makes exacting de- required compatibility between PALplus spectrum of the helper from low-frequency mands on the IF stage (including the SAW and standard PAL, corresponding tech- signal components. Stability problems are filter that it contains): niques must be implemented at both the thus avoided in the synchronization stages ● maximum possible video bandwidth, transmitter and receiver ends. of older receivers, and visibility of the help- ● very high color carrier, 2 How SAW filters work SAW filters make use of the piezoelectric effect: a change in the applied voltage trig- gers a mechanical wave on a crystal. Con- versely, mechanical pressure leads to changes in potential difference. Lithium niobate is the preferred material for the crystal chip. The lithium niobate substrate has the great advantage of a relatively large physical coupling coefficient. This means that SAW filters can be manufac- tured with a small insertion loss. The electrodes are comb-shaped with interlocking fingers, the spacing between the fingers being in the order of microns. An RF voltage applied to the input transducers gen- electrodes, such as the appropriate size, plain the predominant use of SAW filters erates a mechanical (acoustic) surface number and spacing of the fingers. by manufacturers of color TV sets and vid- wave on the crystal, which runs to the out- Compared with conventional filters using eo recorders around the world. But SAW put transducer and is reconverted there coils and capacitors, SAW filters are cheap- components are also found in satellite re- into an electric voltage. Special frequency er, smaller, have greater long-term stability, ceivers, cordless telephones, cellphones, transmission functions can be implement- do not need tuning and provide significant- keyless entry system for cars, garage door ed by suitable mechanical design of the ly better performance. These reasons ex- openers and many other applications. * ● maximum possible suppression of sound High-performance filters for PALplus lay ripples. The very important group delay carrier (>30 dB) at 33.4 MHz, and in the region of the helper signal should be ● group delay ripple, very important in the Basically, only the video filter or the video as flat as possible after the IF stage. But the region of the helper signal (34.47 MHz channel of a quasi split-sound filter have to group delay ripple of the TV transmitters, ± 500 kHz) meet special requirements for PALplus re- which varies from country to country, as well ception. The maximum possible video band- as the group delay of the SAW filter, also Many years of experience in volume produc- width and reduction of the sound carrier at have an effect, as do the group delay of the tion (see panel) and the use of state-of-the- 33.4 MHz are contradictory demands. An sound traps or the sound traps themselves. art simulation software were instrumental extremely steep slope is the only way of The group delays of the TV transmitters and in development of SAW filters for such implementing a very high color carrier at the sound traps cannot be easily changed, demanding specifications. The excellent 34.47 MHz and signal suppression of more so the SAW filter cancels out the group business relations between Siemens Mat- than 30 dB at only 1 MHz lower. This was delay. Fig. 3 shows the frequency and sushita and its customers opened up a made possible by using a chip of maximum group delay function of the G3264K (video wealth of expertise. This was used for length made from the piezoelectric sub- channel). co-development of the new filters to the strate lithium niobate (LiNbO3). The selec- benefit of components and equipment tion of various SAW filters for PALplus The level of the color carrier in the G3264K manufacturers alike. (Table 2) results from the different group de- is 0.1 dB. Despite the very high color carri- Table 2 Special SAW filters are available for all requirements in PALplus receivers Filter Application Picture carrier Color carrier Sound carrier Group delay Package type frequency suppression suppression G3956M Video filter B/G 38.9 MHz 1.0 dB 39 dB

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