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THE WAY I SEE IT By NEVILLE WILLIAMS High definition TV: lots of problems at the transmission end As a follow-up to his observations in the August issue, we give correspondent Keith Walters an opportunity to list the transmission problems that would arise with the introduction of high definition, wide screen television. He says they would not have occurred to readers unfamiliar with professional TV production technology. Keith Walters (K.W. for short) assumes that our original correspondent W.G. is just such a person: quite familiar with video home movie gear but blissfully ignorant of the complicated equipment and procedures used every day of the week in assembling programs and commercials from a variety of sources. "Through kilometres of high quality 75-ohm coax, countless BNC connectors, Tokyo-by-Night control panels and mixing desks, multiple signal sources have to be synchronised and sub-carrier phases matched to within 2°. Not simple", he says. Most of this equipment, according to K.W., is configured to the TV standard in the particular country ( usually a variant of PAL or NTSC). If another standard was adopted, it would all have to be discarded and production facilities re-equipped and re-cabled at enormous cost. The situation would have little in common with the changeover from monochrome to colour in the 1970s. Most of the old monochrome equipment was nearing the end of its useful life, anyway. It could be replaced progressively with new colour equipment, able to work well in b&w mode until the changeover was actually made. High definition, wide screen television, he says, calls for a "component video" approach to signal processing, already familiar in some applications but having its first public exposure in TV broadcasting in the various MAC formats. The "components" referred to are simply the luminance (Y) signal and two chrominance signals (B-Y and R-Y). Until fairly recently, normal practice has been to encode the three signals into composite video (luminance and a standardised colour sub-carrier) right at the source - the TV camera, special effects generator, &c. All such signal source equipment can be synchronised to a master sync and subcarrier generator. Once this is done, and allowance made for the various lengths of connecting cable, signals can be wiped and splitscreened, and otherwise processed, without further ado. The changing scene TV video production based on composite video was regarded as quite satisfactory up to the late 1970s and still is for most applications. However, the emergence of digital video processing - which bends, folds and otherwise "mucks about" with images - has exposed the limitations of composite video as a production format. At this point, I quote directly from Keith Walters' letter, with some abbreviation to conserve space: "Basically, these gadgets work by breaking up the picture into a half-million or so pixels, and storing them digitally in a high-speed RAM. By varying the manner in which the memory is accessed on playback, the shape of the output image can be manipulated. "You can't do this with composite video because stretching or shrinking would change its sub-carrier frequency. The signal must first be decoded back to its original components - luminance, B-Y and R-Y, commonly referred to as 'YUV'; this, so that the effects machine can handle the components separately and simultaneously. "The problem is that there is no known process whereby composite video can be turned back into YUV components identical to those originally obtained from the camera or whatever. "This statement stands, despite what happens in an ordinary TV set. Usable U and V information is recovered and (with PAL) averaged by the delay lirte over two lines. A 4.43MHz trap extracts most of the colour subcarrier, leaving the high frequency luminance signals reasonably intact. The residual SEPTEMBER1989 93 THE WAY I SEE IT - CTD chroma sidebands are not too intrusive because the subcarrier is specifically chosen for minimum visibility". OK for receivers ... but Overall, says K.W., the components so recovered are OK for direct display on the screen of a TV set, being visually quite pleasing under optimum conditions. But a very real problem arises in a TV production situation if recovered YUV components need to be reencoded on a new subcarrier. As he says: "It is imperative that all traces of the original subcarrier be removed from the Y signal, because even a minute frequency difference between the old and new subcarriers will cause unsightly patterning on the reconstituted chroma signal. " The simplest way to counter the effect is to use a low-pass filter at 3.1MHz, the lower limit of the chroma sidebands. But this also limits the luminance bandwidth. What's more, the roll-off has to be fairly sedate to avoid ringing effects. The upshot is that practical filters are typically flat to 2.5MHz, 3dB down at 3MHz, and rolling quite rapidly above that. "2.5MHz? Where have we heard t?at _before? Yes, anti-patterning filtermg can drag luminance bandwidth back down towards the limits suggested earlier for VHS, Beta or any other helical-scan recorder that uses the 'colour-under ' system". At this point in his letter, K.W. digresses into an explanation of the colour-under recording system and the problems which show up when an incoming composite signal has to be taken apart for recording and playback, then reconstituted to suit the receiver. It is informative but interrupts his main theme. To carry on: "What we really need is something that completely removes the colour subcarrier while leaving the high frequency luminance components intact - something that has not been achieved to date. "Techniques using delay lines or 94 SILICON CHIP digital field stores ('spatial filtering') offer a partial solution. With NTSC, a simple glass delay line can provide good chroma/luminance s_eparation. With PAL, two delay Imes are required, with more complex circuitry, and it doesn't work as well. In both cases it is at the expense of vertical resolution. "The techniques work best on stationary images containing lots of vertical lines, which means that they work better on electronically generated test patterns than they do on real pictures! This is not to say that the process is impractical. By us~ng elaborate digital filtering techmques, quite impressive results can be obtained - at considerable cost. " But the bottom line is that the recovered YUV signals are still never identical to the originals and if they have to go through the sam~ process again, in the course of video production, the deterioration becomes more marked. There's a limit to the number of generations possible". Avoiding compromise Such problems could be avoided s~ys Ke~th Walters, by keeping ali signals m a production facility in component form right up to the end of the process, converting them to composite video at the transmitter. Component signals would be more compatible with digital video effects machines; NTSC " green faces" problems would be elim- Satellite broadcasts: MAC or PAL? "It's not as if satellite links can't handle composite video. You may need a little more power or a slightly larger dish but it's hardly the disaster area some champions of the MAC format would have us believe. "Personally, I reckon that the adoption of the B-MAC system by the ABC had a lot more to do with bureaucratic desire for a fully remote controllable distribution system than any technical consideration". (K.W.) inated; SECAM would become a more practical system; special effects such as chroma-key and electronic captions would benefit in all systems; and NTSC/P AL standards conversion would be facilitated e_ach. becoming virtually indis~ tmghishable fr om the original material. These desirable benefits do not come easily or cheaply. I quote again with some abbreviation: "The preferred approach to operating 'component' is to run three cables everywhere the signal has to go, instead of one as with composite video. However, the length of the three cables has to be closely mat ched and rigidly maintained. " Component vision mixers have only recently become available rend~red pra ctical by the adoption · of highly a ccurate and drift-free signal processing. Slight gain errors which, with composite video would cause negligible contrast change, could produce noticeable colour er rors with component video. " Component video r ecorders have been available for some time notably the Sony Betacam cam~ corders used by news crews. Using two sepa rat e pairs of heads mounted in parallel on the head drum, the luminance and multiplexed chrominance signals are recorded separately on a specially modified Beta cassette run at nine times normal Beta speed". Betacam & Betamax At this point, K.W. digresses to emphasise that Betacam has little else in common with domestic ~etamax VCRs. Similarly, profess10nal C-format video recorders have nothing to do with domestic ':HS-C, despite fr equent implications to the contrary in both instances. C-format uses "one-inch" (25 .4mm) tape and a 152mmdiameter drum rotating at the field rate. It provides for various head options and can cope with a composite signal bandwidth of up to 6MHz. But, to continue with the main theme: " Getting b a ck to component recorders, standard Betacom has a bandwidth of about 4MHz. The new SP (superior performance) using a new metal tape boasts a luminance bandwidth of over 5.5MHz - in my experience a useful response to over 6MHz. On a component input monitor, the picture is incredible". K.W. also mentions the " MII" format, jointly developed in Japan by Matsushita and NHK. Basically similar to SP Beta cam, it offers more features, having been essentially redesigned from scratch as the basis for a full range of TV station equipment, from camcorders to editing facilities. "Finally, many TV stations are looking seriously at S-VHS camcorders [fitted with 3-chip CCD TV cameras) as a low cost format for news gathering, particula rly in hazardous situations. At a round $15,000, they cost about one-sixth of a Betacam set-up. The SIN ratio is not as good but, if they're edited directly onto a higher quality production format, they're adequate for most situations. State of the art "But state of the art in video recording is in the new digital video recorders [D-1 format) made by Sony and BTS [Broadcast Television Systems, a collaboration of Philips and Bosch). "D-1 machines r ec or d components digitally at an industry standard sampling rate known as 4:2:2. They use special large cassette tapes and involve a data rate of 200 megabits per second. "Three principal strategies are used to achieve this seemingly impossible recording rate: • The data stream is multiplexed onto two record hea ds mounted in parallel; • The head drum rotates at three times the field rate; • A special metal particle tape is used, formulated so as to optimise the distinction between ones and zeros. "There is virtually no deterioration between as many generations as one is likely to encounter in a production situation. They guarantee 20; more than enough! "Again, with Quantel ' s new 'Harry' system, about 90 seconds of digital component video can be stored on a combination of hard PAL isn't dead yet! "There's an awful lot of life left in our present TV systems. Over the past 10 years, the SMPTE (Socie'ty of Motion Picture and Television Engineers) has published numerous articles on compatible improvements to current TV systems. Judging by the accompanying photographs, they could otter a substantial improvement over what we have now." (K.W.) discs and RAM - long enough for most commercials. It provides virtually instant access to any frame, which appear on the monitor screen like strips of film, making editing a simple task for nontechnical operators. One Harry and one digital VTR can replace a whole multi-VTR editing suite. To be realistic "Component video has many theoretical advantages but they only add up if a system is component all the way - and that simply isn't practical at the moment. There's too much money tied up in composite equipment, which works well for most present purposes. " Where component systems are introduced, they are usually in the form of component "islands" , essentially isolated from the existing composite chain. Any video that has to be brought in from outside the island has to be decoded from YUV, with a corresponding drop in quality". In an aside, K.W. points to the above as a reason for the lukewarm reception by broadcasters of the various MAC formats. They would be ideal if everything was shot, edited and released in component form. In reality, present program material comes mainly from 1-inch tape which has to be turned back into YUV. As reproduced by a special MAC receiver, the end result may still be pretty good but in most cases, it has to undergo further ·composite encoding/decoding before it appears on domestic receiver screens. The basic point K.W. is seeking to make in all this is that HDTV is not just another extension of present systems. Before addressing the problems of presenting high definition wide screen images in the home, the TV industry must first come up with suitable program material and that, in turn, will involve a n a llcomponent production format. I quote: "Despite all the complexities described above, converting to fully component format might prove to be a mere detail compared to going to full-scale HDTV. As I said right at the start, I don't believe HDTV is anywhere near ready for general release". Summing up K.W. concludes by saying that he was anxious to see HDTV in the Japanese pavilion at Expo '88 , expecting to see real HDTV sets on display, alongside a conventional receiver. What he saw were thr ee HDTV projection screens, shared by slide projectors, viewed from a minimum distance of 3 metres. The cameras on show produced noticeably noisy pictures, even in fairly bright light, which is what one would expect with a 30MHz bandwidth. They're supposed to have solved that problem with a new type of camera tube but the display was scarely indicative of mature technology. As for shooting movies on HDTV instead of film , K.W. sees little advantage in the forseeable future. I quote: "The low-light performance of the best HDTV camera today is laughable compared with relatively cheap and available 400 ASA film. "Admittedly, it's easier to create special effects on video but what is more likely to happen is that footage captured on film will be processed digitally by a special high resolution telecine scanner [straight off the original negative). The effects will be produced by computer and the resulting image transferred in negative form onto film again. Special effects don't have to be done in real time" . There's more but by this time, you've probably had your fill of HDTV. Even the writer admits that the letter is a bit " long-winded" but it certainly seems that the picture he paints is not a HDTV one. ~ SEPTEMBER198 9 95