RF Column #38 - December 1994 Copyright (c) 1994,1995 H. Douglas Lung ALL RIGHTS RESERVED TOPICS: SBE / SMPTE at World Media Expo 94 in Los Angeles Grand Alliance HDTV transmission test results COFDM versus 8-VSB -- Which is better? Transmitter topics at SBE Comark transmitter at the transmission tests Harris UHF transmitter design highlights Thomson tetrodes provide high peak powers for HDTV --------------------------------------------------------------------------- This month I'll cover some of the more interesting sessions at the 1994 SBE Conference. The show was one of several making up "World Media Expo" in Los Angeles. In addition to the SBE (Society of Broadcast Engineers) gathering, RTNDA (Radio Television News Directors' Association), SMPTE (Society of Motion Picture and Television Engineers) and NAB-Radio conventions were going on simultaneously. Unlike the annual NAB convention, I find SBE and SMPTE valuable more for the engineering sessions than the exhibits. Manufacturers usually don't introduce new gear at these shows. The number of exhibitors showing RF gear was about the same as at SBE/RTNDA in Miami last year. There seemed to be a more international flavor to the convention this year. I credit this to SMPTE's participation, which attracted engineers from around the world. RF - related engineering sessions focused on HDTV / ATV, with perhaps the most interesting being the SMPTE session titled "Testing of the Grand Alliance HDTV Transmission Subsystem." The U.S. seems to be heading down a different path than Europe for a digital TV transmission system and this session led to a long discussion about the merits of 8-VSB versus COFDM modulation. I won't take the space here to give you a description of the two types - Merrill Weiss has done an excellent job of that in his Advanced Television columns. The SMPTE session was held in the Concourse Theater and it was too dark to use my non-backlit HP-95 for taking notes, so I'm working on memory for some of this. A preprint of presentation by Jules Cohen and Victor Tawil on the results of the tests in Charlotte, NC is available from SMPTE (ask for preprint number 136-27), 595 W. Hartsdale Avenue, White Plains, NY 10607. At the conference they were charging $3.00 per paper per copy. Buy a copy of the preprint if you want the full statistical information on the tests and diagrams of the test set up. The results of the tests were surprising -- ATV coverage tested better than NTSC coverage! Tests were conducted on VHF channel 6 and UHF channel 53. The channel 6 NTSC transmitter had a peak visual effective radiated power of 10 kW while the UHF NTSC transmitter's ERP was 500 kW. The average ERP for the ATV digital transmissions was 12 dB below the peak NTSC visual power. You'll recall that the 8-VSB digital modulation system has a high peak to average ratio, so the peak ATV power was about five or six dB below that of the NTSC signal. The same transmitter was used for both ATV and NTSC transmission. The tests didn't use anything exotic for the transmitting antennas. For UHF, a Dielectric TFU-24G was used and for VHF a single bay, omnidirectional panel antenna manufactured by Harris was used. For receiving, an all channel antenna similar to conventional home antennas was used. From the data sheet it looks like the gain at channel 6 was about 4 dB and the gain at channel 53 was around 8.5 dB, referenced to a dipole. A D-3 digital tape machine provided the video for the NTSC tests. For the ATV testing, a digital transmission test generator was used to evaluate the bit error rate performance of the system. As you will recall from my columns on satellite video compression, there really isn't a bad picture with a digital transmission system -- its either there and as perfect as it can be or its not there at all. Reception of the NTSC signal was considered adequate if the picture had a CCIR "Grade 3" or better. Grade 3 corresponds to "Slightly annoying" impairments. The ATV signal was considered adequate if the bit error rate was 3 parts per million or less. Refer to my previous columns for a discussion of bit error rate and how it relates to the digital signal. After seeing what some people considered "adequate" analog satellite reception, I question if "CCIR Grade 3" is a low enough threshold for determining actual coverage area. Sometimes people will endure a pretty bad picture to watch a program . Here's the good news. The tests included 199 locations for UHF and the bit error rate was low enough for ATV reception at 182 of the sites. For NTSC, the CCIR Grade 3 or above signal quality was available in only 152 of the sites. During the session it came out that at some sites ATV reception would have been satisfactory when the NTSC signal was completely unviewable. The tests at VHF showed an even greater advantage for ATV with 138 sites out of 169 getting adequate bit error rate for ATV reception while only 67 sites received a CCIR Grade 3 or better NTSC picture. Channel 6 wasn't the best choice for VHF since it was prone to interference from power lines, an FM station and distant co-channel TV stations. The digital modulation seemed to hold up better under these conditions. After Jules Cohen's and Victor Tawil's session there were presentations on the OFDM (Orthogonal frequency division multiplex) and COFDM (Coded OFDM) methods of modulation for ATV digital transmission. Europe and Japan seem to be focusing on this method of modulation instead of the 8-VSB system. While this system has been extensively tested for digital audio transmission there is some question about its performance versus 8-VSB for terrestrial digital video transmission. Most of the OFDM papers presented information that Merrill Weiss has already covered in this magazine, so I won't repeat it here. The discussion between Victor Tawill and engineers from Canada and France (whose names I didn't catch) revolved around three issues. First, was it practical to build an OFDM modulator with enough carriers to handle the 18 Mbs data rate required for ATV? Second, would the OFDM modulation require linearity and peak power performance difficult to achieve in high power transmitters? And finally , which system handles multipath better? In answer to the first question, the OFDM proponents said Phillips and Thomson had developed circuits that could generate COFDM with a data rate high enough for ATV transmission. On the second issue there may be more of a philosophical difference, since Europe tends to use more low power transmitters instead of centrally placed high power transmitters as done in the U.S. and Latin America. Because OFDM is more immune to co-channel interference it works well with many lower power transmitters. Because it has a significantly higher peak to average ratio it will require more head room in the amplifiers. This isn't as difficult to achieve at low power as it is at high power. Because one of the current applications for Coded OFDM (COFDM) is digital audio broadcasting (DAB) it is logical to assume COFDM must handle multipath well. DAB must work in moving automobiles where multipath is a major problem. As the discussion bounced back and forth on whether 8-VSB would work as well or better in this environment the only consensus I could see was that multipath performance really depended on the adaptive equalizer used to correct the incoming signal. Although I didn't hear it mentioned elsewhere, there was a comment that the Grand Alliance 8-VSB system had been tested in a moving vehicle using a simple antenna and it performed well. As my regular readers will remember, I've said that an ATV standard must work well with portable antennas if it is to offer an advantage over satellite or cable delivery methods. If the system does work as claimed, that is good news for broadcasters. It will be interesting to see where the modulation debate stands at the 1995 NAB show in Las Vegas. From what I saw at SMPTE, there is little support for 8-VSB ATV transmission in Europe or Japan. On the other hand, there seems to be growing support for keeping the COFDM option open for a US ATV system. Both systems have their merits. I like 8-VSB because of its simplicity and lower transmitter linearity requirements. On the other hand, if COFDM permits more efficient spectrum use by allowing on channel repeaters, it could benefit Low Power TV broadcasters who otherwise might lose their channels. If the U.S. system is switched to COFDM to match what the others in the world are proposing, it will delay introduction of ATV transmission. Currently, it is my opinion the U.S. is slightly ahead of the rest of the world in efforts to get a terrestrial VHF/UHF ATV system functioning. Transmitter manufacturers are already considering ATV compatibility to be an important transmitter feature. Comark, due to its early adoption of klystrode technology, is the pioneer in high power UHF ATV transmission. It supplied the transmitter used for the field tests discussed above. Harris is testing its transmitters for ATV transmission. Thomson sees its tetrode and Diacrode tube technology as a good solution for ATV. All three manufacturers presented papers at SBE this year. Mark Aitken delivered Comark's paper on "Transmitter Equipment Operation and Performance During the ATV Tests in Charlotte, NC" He covered the same tests Jules Cohen spoke about in the SMPTE session. The UHF transmitter used a 60 KW EEV IM7360 IOT operated at 40 KW, with a 6 dB peak to average ratio. An extra headroom of 6 dB was allowed, so the average power was 12 dB below the peak NTSC visual power. In an effort to see if extra power would extend the coverage the transmitter power was increased 6 dB (giving up the headroom). At the higher power 63 percent of the sites between 50 and 56 miles from the transmitter received a satisfactory ATV picture, versus 56 percent at the original power and less than 30 percent when NTSC was used. During the testing the transmitter linearity was measured at 2.4 percent and 4 degrees, differential gain and phase respectively. Mark commented that if COFDM is used the transmitter must correctly transmit each of the thousand or more QAM carriers tha t make up the signal. This means the linearity, phase and intermodulation performance of the transmitter must be exceptional. Based on the results of tests using QAM, he felt the Comark transmitter could meet these requirements. Jerry Collins, Director of TV Engineering at Harris Corporation, talked about Harris's IOT transmitter. Most of the talk focused on the circuit design for the NTSC / PAL transmitter, with particular attention to the AGC circuitry and their method for monitoring visual and aural carrier powers separately in a common-mode transmitter. The system mixed down the on channel carriers to a 45 MHz. IF, then, using a 50 MHz. oscillator, mixed the result down to 4.25 MHz. (visual) and 8.75 MHz. (aural). At this frequency it was easy to separate out the video carrier and demodulate it to obtain sync and blanking level powers. The 8.75 MHz. aural carrier was filtered and mixed with a 10 MHz. oscillator to yield a 1.25 MHz. signal which could easily be filtered again and detected for aural power level measurements. The system is flexible enough to permit filtering and independent detection of the multiple aural carriers used around the world. To tie the paper back into to the HDTV topic, Jerry sh owed how good the NTSC linearity, ICPM and differential phase tests results were. He cautioned, however, that while these results were very good, specific tests for digital ATV transmission are going to be needed to properly test transmitters for ATV performance. In his presentation, Michel Langlois of Thomson Tubes in France gave us an overview of amplifier options for HDTV RF output amplifiers. He compared transistor, klystron, tetrode, IOT and Diacrode amplifiers. I'll focus on Michel's comments on the Diacrode amplifier, since I've written about the comparative performance of the other devices many times before. If you are interested in the details refer to the 1994 SBE Conference Proceedings. (The 1994 SBE Conference Proceedings are available from SBE, 1994 Proceedings, 8445 Keystone Crossing, Suite 140, Indianapolis, IN 46340, telephone 317-253-1640.) The Diacrode was introduced at NAB in 1994. I had a tough time understanding what was going on in this tube, even after reading the proceedings. I got a better explanation at the booth. Tetrodes work well up to about 30 KW power, but at powers above that it is difficult to get enough power out of enough area on the cathode. If you make the cathode diameter bigger the tube starts show waveg uide resonances. Increasing the length by itself wouldn't help because the RF current will be concentrated in one area. The Diacrode uses a longer cathode but gets around the current problem by shifting the voltage maximum to allow almost twice the area of the cathode to deliver RF current. This is tough to describe without the pictures. Ask Thomson for a data sheet on the Diacrode. The 60 KW Diacrode specifications call for a gain of about 15 dB and an operating voltage of 7.5 KV with an efficiency of 40 percent at black current. Look for Diacrode based transmitters at NAB in 1995. Michel Langlois also discussed tests Thomson had done on tetrode tube performance for HDTV. The results were very interesting. For ODFM, the 30 KW TH563 tube could operate at an average power of 5.5 KW with a bit error rate (BER) of one part in one thousand million (1E-9). Since the peak to average ratio of 8-VSB is lower, a higher average power is possible for that mode. This power level was obtained without using linearity correction. With correction, Michel thought 11 KW average power would be possible. I was surprised to see that the much lower power TH382 5 KW tube was able to deliver a BER of 1E-9 at an average power as high as 2.35 KW. The paper points out these results include the non-linearities in the driver stages. These results should be encouraging to stations that want to put an ATV signal on the air with minimum transmitter cost. If we use a peak to average power ratio of 10 dB for the OFDM signal, that would mean the 5 KW TH382 was able to handle digital TV peak powe rs as high as 23.5 KW without any linearity correction! An air cooled 5 KW TH382 based transmitter using a 6 KV power supply has to be a lot less expensive than a 30 KW peak IOT based transmitter requiring a 30 KV supply and more complicated cooling. That's all the space I have for this month. Next month I'll continue my report on the SBE 1994 technical sessions. The session on new RF hazard standards prompted some interesting discussions on how RF affects animals (including humans). If you thought the hazard was in your head, you may be right, but you probably don't know the whole story! The FCC panel is always interesting and this year's prompted some serious discussion on the increasing crunch on broadcast auxiliary spectrum. I'll also have some more information on transmitter technology, including my observations after a day testing the I.T.S. Model ITS-830 low priced 1 KW solid state transmitter. Results are still coming in on the informal tube reliability survey. I've heard from klystron and MSDC klystron users but so far no results from klystrode or tetrode users. I'll give you a preliminary report along with some observations on how well the various tube technologies are holding up. I'll also have some more info on interesting broadcast stuff on the Internet along with some observations on accessing the Internet using America On Line (AOL). If you haven't sent me your tube life numbers for the reliability survey, please do it soon. Full details were in my October column. If you don't have it, what I'm looking for a report on how long your high power transmitter tubes have lasted, how they failed and any unusual or on-going problems. Please include transmitter type, operating power level and a description of the tube. The description is the type of tube (tetrode, klystron, etc.), cooling method, manufacturer, model number and any other information you feel important. What is the longest life you've gotten from a tube and in what service? Let me know. There are several ways to reach me. The best and fastest is email - my address is dlung@gate.net via the Internet or 70255,460 on Compuserve. Fax works too, the number is 305-884-9661. You can try me at my office phone, 305-884-9664. Please call after 6:30 PM Eastern when I'm not likely to be as busy. Letters and disks for software I've written should be sent to me at 2265 Westwood Blvd., Suite 553, Los Angeles, CA 90064. Please allow six to eight weeks for mail. If you are interested in the spreadsheet I developed for terrain profiles for path plotting contact me for information on how to get it. I plan to add fresnel zone clearance and path distance / angle calculations to it in future columns. The completed version will be posted on Compuserve's Broadcast Professionals Forum sometime in early 1995. Copyright (c) 1994,1995 H. Douglas Lung ALL RIGHTS RESERVED