Implications of Ghost Cancelling to the Cable Industry
Gary Chan Rogers Engineering Toronto, Canada
AJ:)stract The ghost cancelling and the CableLabs' tests in effort is steadily migrating Washington, D.C. from a developmental stage to commercial applications and will eventually target to a Introduction wider application into consumers' television sets and Mul tipath is one of the VCRs. The influx of ghost major impairments cable cancelling into the cable companies encounter in industry is imminent. This receiving over-the-air paper will examine the issues television signals. Tradition that are important to the cable ally, RF cancellation technique industry; it is based on (sometimes baseband technique) earlier experiences on testing is used. However, it requires ghost cancellers done in: trimming long lengths of cable Vancouver - British Columbia to match the appropriate delays (B.C.), Kitchener- Ontario, or adjusting sensitive phase-
Figure 1 Ghost Cancelling Crew Investigating at a System Location.
394 -1992 NCTA Technical Papers shifting networks and matching and fifty test locations were precisely the amplitude of the visited. reflections. In addition, the resultant signal suffers a The cable test followed signal-to-noise degradation immediately after the field during signal recombination test. It consisted of two and, most often, 100% cancel parts: the cable impairment lation cannot be achieved. test and the cable system test. Rogers Cablesystems initiated The impairment test encompassed the ghost cancelling project in a series of cable impairment early January of 1990. A simulations which was carried prototype ghost canceller was out in the CableLabs HDTV tried in Salt Spring Island, testing facility in Alexandria, B.C. on April 6 & 7, 1990 Virginia. The cable system immediately after the NAB test was performed on four convention. A month later, the cable systems in the vicinity first ghost canceller was put of Washington D.c. and into full operation serving comprised of thirty-three subscribers in Vancouver and subscriber locations. its vicinities. Figure 1 shows Measurements were made at a photograph of the Vancouver headends, AML hubs, distri ghost cancelling crew invest bution taps and within igating at a system location in subscribers' homes. The Victoria, B.C. subscriber locations were carefully selected with a balanced mix of AML, trunk and ATSC' s Effort to Establish a optical link. GCR Signal Standard The laboratory test was In 1989, the Advanced carried out at the Communicat Television Systems Committee ions Research Center in ottawa formed a specialist group on last January. The test Ghost Cancelling called T3S5 to consisted of computer examine the various ghost simulations of the different cancelling techniques. There GCR signals being impaired by were five systems - AT&T, BTA, ten different combinations of NA Philips, Sarnoff and Samsung ghosting and noise conditions; - submitted to the ATSC for observations were focused on consideration. It was hoped each proponent's GCR signal and that with a succession of field its software implementations, test, cable test and laboratory such as, its ability in test, a single voluntary characterizing the transmission standard could be established channel, the duration required for the television industry in to obtain a good approximation North America. Six of the channel response, and organizations were involved in the number of iterations this effort. The broadcast necessary to achieve field test was conducted in convergence. The laboratory mid-September of 1991. It took test also used segments of the place in the Washington D.c. CCIR test tape, originally area. Three television prepared to assess video stations, one in the VHF and codecs, to subjectively two in the UHF bands, were used evaluate the picture quality for the test. over a hundred improvement. Ghosts and random
1992 NCTA Technical Papers- 395 noise were introduced at IF and TEK 14S:0 O£MJOULAT0Fl OUTPUT HT p;i'A'SER RF frequencies. The laboratory test was concluded at the end
lfll 1 • I~<~E of January. 1 110 188
The results of the field .;e8 90 I S0 test, the cable test and the 70 laboratory test will be 60 submitted to the ATSC. ATSC will recommend a standard to 40 its membership for a vote in 21:10 30 April of 1992. At the same time, a line in the vertical blanking interval will be determined for the transmission of Ghost Cancel Reference (GCR} signal. The tabulation of the -!.·) vote will be disclosed in June i='l"'~e::oe :~ 'r~:l of 1992. Figure 2 GCR Signal Received at the Fraser Hub.
Vancouver Test processed by the ghost canceller, the ghost canceller Vancouver was the initial interpreted the undershoot as a test site chosen for the ghost ghost and tried to cancel it. canceller test. There were a The net result was that a ghost few important findings in the was introduced by the canceller Vancouver test that prompted itself. further investigations into the impact of ghost canceller to a cable system. For example, AM Kitchener Test and FM microwave systems are widely used for cable Prior to the commencement television distribution. of the Ki tchener test, there However, if they are not well were many uncertainties about maintained, non-linearity ghost cancellers operating in a introduced by AM and FM cable environment: microwave links can adversely affect the ghost cancellation - Group delay caused by duplex performance and convergence filter and mop-up equalizers time. Figure 2 shows the often cause chrominance to received GCR at the Fraser AML luminance delay inequality. receive site. The GCR signal Could these ghost cancellers was inserted before the AML be able to improve these transmitter in Burnaby on distortions and to what channel 32. Notice that there extent? was considerable undershoot but - How would these ghost no overshoot on the integrated cancellers perform in a (sin x) jx signal. Under normal typical cable subscriber's circumstances, the undershoot environment, with the did not create any problems presence of a variety of because it was blacker than cable distortion products, black. However, when the GCR which satisfied the signal was captured and specifications outlined in
396 -1992 NCTA Technical Papers the FCC rules Part 76? cable signal at a distribution Microreflection is one of tap and Figure 4 illustrates the major concerns for in the same signal after it was house wirings that do not processed by the ghost conform to CATV standards. canceller. Notice the small Would these ghost cancellers improvement on the 2T pulse and be as effective as to cancel
ghosts which were distinct GHOST C~I'ICELLEIIl TESTir«i IN 1CITCHE!'£1l
and far apart than to cancel cnonnel A ghosts which were clustered very close together near the ""' f [RE II picture carrier? 1 ree In August of 1991, a test """I ~r~· ~ f
different brands of ghost GHOST CAf"'CE~EP TESTIP«i IN KiiCHEt£R
Channel A canceller react differently to 13-Aug-91 14:4~:35 the signal that was contaminated by ghosting and other distortions, to simplify the analysis, it was decided that only one ghost canceller would be used. There were a few specific = : ::re~ findings as a result of the 2eef f 2e+ Kitchener test. From the j I ,.:
subjective data, the results \-.JJJ0· al------"-!t--:~n~lt::,..;trrd'..J--Jii--lJ~-\i-.J.A..i 1 -tet indicated the process through ; - 1992 NCTA Technical Papers- 397 Contrary to the result of due to the loss of GCR the Fraser hub in Vancouver, signal. Thus, there is a the Kitchener data indicated need for ghost cancellers that AML equipment, if well being able to freeze the maintained, did not impose any last set of filter coeff restriction on the ghost icients during which the GCR cancelling equipment to signal is missing. perform. - If electrical interference (or sometimes called impulse noise) is present on the Suggestions on purchasing Ghost received signal, here is Canceller another feature to ask for: its robustness to electrical The technology to interference. Figure 5 implement ghost cancelling shows the BTA GCR signal system clearly exists. Japan corrupted by electrical introduced the ghost cancelling interference. As a result, system as part of the there may be momentary loss ClearVision implementation in of the picture for a short 1988. North America will have duration. If the impulse a GCR signal standard this noise happens often enough, year. It is believed that the momentary loss of commercial ghost cancellers picture becomes very will be introduced to the annoying. broadcast and cable industries even before the 1992 NCTA iiHOST CANCELLEP AT f'I1T HE!.t"CKEN Convention. Yet, what does a Chonne I 8 cable company need to consider 398 -1992 NCTA Technical Papers baseband devices. If Acknowledgement channel processors are presently used to process The author would like to thank over-the-air signals, one Rogers Cable TV in Vancouver, may want to ask ghost British Columbia and in canceller manufacturers Kitchener, Ontario for their whether there is an IF assistance in conducting the interface option available. field tests. The evaluation of - Lastly, line 19 on the ghost cancellers is sponsored vertical blanking interval by CableLabs. The cooperation is being proposed by the of Tektronix, KOMO, KIRO and ATSC for the transmission of KING stations, Communications GCR signal and, as a Research Center, and the consequence, the usage of support of various ghost line 18 and line 20 may be canceller manufacturers is restricted to time invariant vital to this project. Their test signals only. But, help and support are most line 20 is being used by appreciated. some cable operators for conditional access and the information on this line is References time-varying. Cable operators may want to "Field Tests of Ghost Canceling solicit ghost canceller Systems for NTSC Television manufacturers to incorporate Broadcasting", by National GCR signal deletion Association of Broadcasters and circuitry once the Association for Maximum Service television signal has gone Television, January 31, 1992. through the ghost cancelling process. And, maybe, there "Results of Computer are other alternatives than simulations and Laboratory deleting the GCR signal. Tests of Proposed Ghost Cancelling Systems", by Bernard Caron - Communications Research Conclusions Center, (preliminary) February 1992. The list of suggestions presented on buying a ghost "Shades of Ghost Canceling", by canceller is by no means Wendell Bailey, pp .18, Communi complete. It is the intention cations Engineering and Design, of Rogers Engineering to share December 1991. the experiences of our ghost cancelling system development "Ghost Cancelling and Cable", effort with other cable by Nick Hamilton-Piercy and companies and, via the sharing Gary Chan, pp.113, NCTA of ideas, to stimulate more Technical Paper, March 1991. discussions and generate more ideas on the implementation and deployment of ghost cancellers. 1992 NCTA Technical Papers- 399 Gary Chan is a Staff Engineer at Rogers Engineering, Toronto Canada. He received his B.Sc.E.E. degree in 1980 from the University of Manitoba and Master of Engineering (Electrical) degree in 1986 from the University of Alberta. Since his graduation in 1980, he has engaged in CATV design, operation, maintenance, research and development of advanced cable television technologies. Most recently, he leads the PCN test on using the cable plant as a transport media in Toronto. He is a member of the IEEE and a registered Professional Engineer of Ontario. 400 -1992 NCTA Technical Papers