IEEE TRANSACTIONS ON BROADCASTING, VOL. 36, NO. 4, DECEMBER 1990 255

Field Testing of a Ghost Canceling System for NTSC Television Broadcasting

Victor Tawil Lynn D. Claudy

Washington, D.C. Washington, D.C. Association for Maximum Service Television National Association of Broadcasters

ABSTRACT

One of the leading causes of television picture degradation in over-the-air television reception results from multipath propagation. Transmitting a television signal through a multipath channel results in the creation of multiple images (ghosts) being overlaid along with the desired image on a television receiver. Great strides have been made in the past decade toward developing technologies that reduce the visual effect of multipath propagation and eliminate ghosts. One such technoloa, developed by the Broadcasting Technology Association of Japan (BTA), was recently field-tested using over- the-air television signals in and around the city of Atlanta, Georgia.

The BTA technology was effective in eliminating or reducing ghosts in nearly all the locations surveyed The amount of improvement varied depending on the type and complexity of the ghosting impaiment, but, in general, was on the order of two steps as measured on a five-step CCIR impaitment scale. The technology was most effective in correcting static or non-varying ghosting conditions, while least effective in the case of time- varying or rapidly changing propagation conditions.

I. INTRODUCTION

Ghosts are time-displaced, attenuated, multipath- distorted versions of a transmitted television signal that are inadvertently picked up along with the intended signal by television receivers. These distorted versions of the signal are created as a result of reflection or scatter of the transmitted signal from nearby buildings, towers or hills, etc., traveling over a variety of paths to reach the receiving antenna. These signals may be time-varying, advanced (leading ghost) or delayed (lagging ghost) in time relative to the intended signal and may be stronger or weaker in amplitude than the intended signal.

The visual perception of ghosts varies depending upon how they originate. Ghosts created by specular reflection from large, flat conductive surfaces that .are synchronously detected by the television receiver appear as delayed replicas of the original signal. Ghosts created by non-specular reflection over different path lengths appear as smeared versions of the original signal. Color problems such as improper hue and saturation of the picture may result from the overlap of ghost signals' chroma burst with the main signal's chroma burst.

The perceptibility of ghosts is strongly affected by picture content and quality. Busy or cluttered pictures hide ghosts;

heavy noise or weak signals can mask the visibility of ghosts and reduce the annoyance level in moderate or weak ghosting conditions. Time-varying ghosts are generally perceived as being quite objectionable.

In the past decade, significant progress has been achieved toward reducing the visual effects of multipath propagation and eliminating ghosts. The television industry and receiver manufacturers in the U.S and abroad have been experimenting with advanced digital processing and filtering techniques to achieve some real-time reductions in the visual effects of multipath. Some have experimented with using the vertical synchronizing pulse of the TV signal to detect and eliminate ghosts inside a television receiver (Ref. 2, 3, 4); others have elected to develop a unique ghost canceling reference (GCR) signal for transmission along with the television signal that is subsequently used by a receiver for eliminating ghosts (Ref, 4, 6, 7).

More recently, development work by Nippon Hoso Kyokai (NHK), the national television broadcaster in Japan, and the Broadcasting Technology Association (BTA) in Japan has produced what is believed to be an effective and reliable ghost canceling system. NHK and BTA, in cooperation with several Japanese consumer electronics firms, initiated development and production of consumer-grade equipment.

To assess the performance of this new technology under a variety of propagation conditions, the National Association of Broadcasters (NAB) and the Association for Maximum Service Television (MSTV) designed and implemented a field observation and measurement program in and around the city of Atlanta, Georgia, using over-the-air television signals (Ref. 1). The present paper describes the joint NAE%/MSTV measurement program in detail and presents some findings and observations along with general statistics on the overall performance of the BTA ghost canceling technology for a major urban area in the United States.

11. GENERAL DESCRIPTION OF THE BTA GCR SIGNAL, SEQUENCE

The BTA ghost canceling system requires that a specialized reference training signal (called the Ghost Canceling Reference or GCR signal) is inserted on line 18 in the Vertical Blanking Interval (VBI) of the transmitted television signal. This signal is subsequently analyzed at the receiving end with the result of configuring an adaptive filter to reduce the ghosting impairment created by the transmission medium.

0018-9316/90/12OO-O255$01.00 0 1990 IEEE

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The GCR signal used in the BTA ghost canceling system actually consists of two waveforms: a bar waveform with a sin x/x characteristic applied to the leading edge and a full horizontal line held at a constant 0 IRE amplitude (Figure 1). The GCR pulses and blank lines are alternately inserted on line 18 in the VBI in a predetermined repeating 8-field sequence. In addition, line 17 must be blank or contain only static signals (non-changing from field to field). At the receiving end, the GCR and blank lines in selected fields are subtracted from each other. This operation cancels contamination of the GCR signal that could otherwise be caused by long time duration ghosts. For the subtraction process to yield only the GCR signal, line pairs must be

0 60 220 227.5SC 0 16-76 61.46 6 3 . 5 6 ~ s

GCR SIGNAL

0 63- 56 PS

PEDESTAL SIGNAL

GHOST CANCELING REFERENCE WAVEFORM

FIGURE 1

Previous Previous F i e l d l i n e CCR F i e l d l i n e CCR

t,- is p o l a r i t y of chroma s i g n a l

GCR SIGNAL %FIELD SEQUENCE

FIGURE 2

chosen from fields that have the same color burst phase. The &field sequence is shown in Figure 2. M e r subtraction, the GCR signal can be recovered for analysis by the receiver's processing section.

111. DESCRIPTION OF THE MEASUREMENT PROGRAM

S k local Atlanta television stations -- WSB-TV, Channel 2; WAGA-TV, Channel 5; WGTV, Channel 8; W m - m , Channel 11; WPBA, Channel 30; and WATL, Channel 36 - volunteered to transmit the BTA GCR signal along with their normal televised signal for a 7-day period during the time of the 1990 National Association of Broadcasters Convention in Atlanta, Georgia.

A five-by-five equally spaced rectangular grid was used to select measurement locations within the FCC predicted City Grade service areas of the six television stations. Grids were spaced at approximately eight-mile intervals where one location near the center of each grid was selected, for a total of 25 measurement locations. Twelve additional measurement locations were selected including five private homes and seven locations recommended by local Atlanta television station personnel as being plagued by multipath and ghosting. In all, the over-the-air television signals from the participating stations were observed at 37 different locations. Figure 3 shows the location of the station transmitters, FCC predicted City Grade service coverage contours and an overlay of the measurement grid.

It is important to note that no rigorous attempt was made to accurately assess the overall relative prevalence of multipath conditions throughout the Atlanta area. Consequently, the data from the present study should not be used to estimate the level of ghosting that would be encountered on a statistical basis.

IV. MEASUREMENT EQUIPMENT

Test and measurement equipment was installed in a mobile van for gathering data for the project. The equipment complement included an AC power generator, 30-foot mast, field strength meters, a Radio Shack VU-75 antenna, a two- way RF signal splitter, a Tektronix model 1450 television demodulator, an NEC consumer-type ghost-canceler/television tuner with video outputs, an NEC/NHK baseband video ghost canceler, a Tektronix model 1480-R video waveform monitor, a Sony model BVW-45 Betacam SP video cassette recorder, a Sanyo AVM-222 20" video monitor, a video switcher, a microphone and other miscellaneous equipment. Figure 4 shows a block diagram of the measurement system.

V. METHODOLOGY AND PROCEDURE

At each outdoor measurement site the following data was collected: RF signal level at the input to the television receiver; photographs of the general vicinity of the observation location showing reception conditions; waveform monitor photographs of the GCR signal before and after correction; subjective evaluation of quality level and degree of ghosting impairment before and after correction; and video tapes of the observed signals for off-line analysis. The video tapes were subsequently used to produce still photographs showing the uncorrected, ghost-impaired picture and the corrected picture. Private home observations included written subjective quality and impairment evaluations and photographs of the television

257

0 10

Miles

ATLANTA FIELD TEST MEASUREMENT GRID

FIGURE 3

RSVU75 Anlenna on 30 Fool Mas1

TEK 1450

Ghost

NEC GC

Ghost Canceling

Vldeo Switcher

sony Bvw45 I

Waveform Monitor U

TEK 1480R

MOBILE MEASUREMENT SYSTEM BLOCK DIAGRAM

FIGURE 4

Sanyo

Monllor

screen. Logistical constraints prevented the video recording of the observed pictures or observations of the GCR signal waveform in the private homes.

Two expert observers were used to conduct the subjective evaluation portion of the project. Both observers had extensive experience in critical technical assessment of video picture quality. The visual evaluation of each received picture was based on the subjective quality and impairment scales used by the International Radio Consultative

Coverage Key

Channel 2 . . . . . . . . . . . . Channel 5 - 0 - Channel 8- - - Channel 11----- Channel 30 Channel 36 ------

Site ID

A - Channel 2 (WSB-N) B - Channel 5 (WAGA-TV) C - Channel 8 (WGTV) D - Channel 11 (WXIA-TV)

Channel 30 (WPBA) Channel 36 (WATL)

Committee (CCIR) (Ref. 8). The five-grade CCIR scales are as follows:

Ouality Imaairment

5 Excellent 5 Imperceptible 4 Good 3 Fair 3 Slightly Annoying 2 Poor 2 Annoying 1 Bad 1 Very Annoying

4 Perceptible, but Not Annoying

These subjective rating scales were used to evaluate the overall quality of the signals received at each site and the level of ghost impairment before and after the ghost canceler was activated. To rate the signal quality and impairment level, the two trained observers each rendered individual opinions. When a difference was noted it was discussed, and if no agreement was reached, the average of the two opinions was recorded.

At each site the antenna was raised to a height of 30 feet above ground and oriented toward the appropriate station's transmission tower. As can be seen from Figure 3, all the Atlanta station transmission towers that were included in the project are located in the same general vicinity with the exception of Channel 8 which is to the east of Atlanta, approximately 12 miles from the other stations' transmitter locations.

The combination RF tuner/ghost canceler unit was used to receive channels 8, 11, 30 and 36. (Channels 2 and 5

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were not available on this particular tuner since it was designed for use in Japan where the low VHF channels are not allocated for terrestrial broadcasting). The NEC/NHK baseband unit was principally used (in conjunction with a Tektronix model 1450 television demodulator) to evaluate channels 2 and 5. However, it was occasionally used for all six channels. The private home observations employed only the combination RF tuner/ghost canceler which was connected to the home antenna and, where possible, to the home receiver as well.

When the selected signal was tuned in, the ghost canceler was reset and a period of about 5 to 10 seconds was allowed for the equipment to perform the ghost-reduction process. The VCR was then used to record approximately one minute of the received signal for archival purposes. A broadcast- quality VCR was used to accurately record the full bandwidth of the video signal and preserve the nature of even very subtle ghosting effects. However, as these VCRs are intended for use in a relatively unimpaired signal environment, their design does not tolerate a significant level of sync instability as can be produced by multipath. Thus, some of the recordings exhibited horizontal and vertical sync and color decoding problems induced by multipath that were not seen on the television monitors in the measurement van. Nonetheless, the multipath impairment to the visual image is clearly identifiable in the tape recordings. Since the quality and impairment assessments were all made in real time, the project results were not affected by characteristics of the tape recordings. During the recording the canceler was turned off and on, or periodically bypassed, to more readily show the effect of the ghost canceler.

The recorded video tapes from each site are contained in the files of the National Association of Broadcasters in Washington, D.C.

VI. EXAMPLE OF COLLECTED DATA

Figures 5 through 7 show an example of the type of measurement data and documentation collected at each site. Figure 5 shows a photograph of a typical site location. Figure 6 is a photograph of the received GCR signal on line 18 with and without the ghost canceler as photographed from the waveform monitor in the measurement van. Figure 7 shows the video image with and without the ghost canceler. It was observed in examining the still photographs that the psychovisual coherence of a ghost image is reinforced by the presence of motion. The still photographs did not always recreate the same impression of ghosting level or indicate the same level of annoyance as a moving scene where the ghost is viewed in temporal context.

VII. DATA ANALYSIS

As previously mentioned, a total of 37 sites were visited during the 7-day test period. Data was collected from all six television stations (four VHF and two UHF) for a total of over 200 field strength measurements and subjective assessment observation sequences. An observation sequence included a quality evaluation of the received picture prior to the activation of the ghost canceler and an impairment evaluation of the received picture before and after activation.

Since most of the measurements were located within the FCC predicted City Grade service contours of all six television

EXAMPLE OF TYPICAL OBSERVATION SITE

(Georgia Institute of Technology)

FIGURE 5

BEFORE GHOST FILTER

AFTER GHOST FILTER

EXAMPLE OF WAVEFORM MONITOR PHOTOGRAPHS

(Georgia Institute of Technology - Channel 2)

FIGURE 6

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BEFORE GHOST FILTER AFTER GHOST FILTER

EXAMPLE OF PICTURE MONITOR PHOTOGRAPHS

(Georgia Institute of Technology - Channel 2)

FIGURE 7

stations, most of the observations were made under "strong" or "moderate" signal conditions. Recognizing, however, that predicted signal levels may vary greatly from measured ones, it turned out that 70% of the signals measured exhibited field strength levels equal to or greater than the FCC predicted City Grade contour levels, 86% exhibited levels equal to or greater than Grade A contour levels, and all the signals were within the Grade B contour levels.

To assess the overall quality of the received pictures for the geographic area under investigation prior to the activation of the ghost canceler, subjective evaluation of quality level was performed. Using the CCIR quality assessment scale, approximately 80% of the observations were rated as "good" or "excellent"; the remaining 20% were rated as either "fair", "poor", or "bad". While these assessments were not directly used in the impairment analysis, they were useful during the measurement phase of the project in preparing the observers for the impairment evaluation.

All 37 sites experienced some level of ghosting conditions on one or more channels. Ghosts were visible in 75% of all

the observations made. Before the ghost canceler was activated, 31% of the observations had impairments rated as "perceptible but not annoying", 21 ?h as "slightly annoying", 18% as "annoying", and the remaining 5% as "very annoying". In contrast, after the ghost canceler was activated, ghosts were visible in only 28% of the observations. 8% were rated as "slightly annoying", "annoying" or "very annoying". Table 1 presents the impairment statistics before and after activation of the ghost canceler.

TABLE 1

CCIR ImDairment Scale Before &r (%) (%I

Imperceptible 25 72 Perceptible, but Not Annoying 31 20 Slightly Annoying 21 5 Annoying 18 2 Very Annoying 5 1

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The ghost canceler was effective in reducing or eliminating ghosts in most locations where ghosting conditions were observed. The amount of improvement, however, varied depending on the type and complexity of the ghosting impairment. Almost half of the visible ghost observations measured a two-step improvement on the five-step CCIR impairment scale and approximately one-third measured a one-step improvement. 19% of the observations recorded a three-step improvement. The BTA system, however, was unable to reduce or eliminate ghosts in 7% of the observations. These observations were identified as conditions where the received picture quality was poor, the ghost canceler improperly processed the training signal or a substantial leading ghost was encountered.

Measurements from the channel 8 station exhibited the highest and worst ghosting impairments. Ghosts were visible on channel 8 in 93% of the locations surveyed. This higher- than-usual incidence of ghosting reported may be attributed to the difference in the transmitting location of channel 8 relative to the other stations. Since the channel 8 transmitter was located over 10 miles to the east of all the other transmitters, the channel 8 signal traversed markedly different topographic and environmental configurations -- encompassing different foreground terrain, buildings, vegetation, etc., than signals originating from downtown locations, which may have resulted in increased incidence of ghosting.

Table 2 presents the percentage of observations by channel where ghosting conditions were rated as "imperceptible" before and after the ghost canceler was activated.

TV Channel

2 5 8 11 30 36

TABLE 2

Irnuairment Rating of "IrnDerCeDti ble"

Before After (%I ("/.I

29 85 so 93 7 61 27 73 17 59 26 65

Except for one VHF channel (channel 8), ghosting conditions were somewhat more prevalent on UHF channels than VHF channels. Prior to activation of the ghost canceler, some level of ghosting was visible in 72% of the observations (66% if channel 8 observations are removed from consideration) made at VHF compared to 78% at UHF. After the ghost canceler was activated, ghosts were visible in only 17% of the observations at VHF compared to 37% at UHF. The data suggests that the ghost canceler was generally more effective in eliminating ghosting conditions at VHF than UHF frequencies.

To assess the performance of the ghost canceler under different signal level conditions, the data was sorted according to signal levels measured in the field into three separate categories: (1) a "strong" signal level category which encompassed field strength levels equal to or greater than FCC predicted City Grade contour levels; (2) a "moderate" signal level category that included levels equal to or greater

than FCC predicted Grade A levels but less than City Grade contour levels; and (3) a "weak" signal level category which encompassed levels equal to or greater than Grade B levels but less than Grade A levels. The ghost canceler was effective in totally eliminating ghosts in 70% of the measurements that met the City Grade service levels. This was reduced to 61% for measurements lying between the City Grade and Grade A levels, and was further reduced to 16% for measurements between Grade A and B levels. Table 3 presents the percentage of Observations categorized by different levels of signal strength where ghosting conditions were rated as "imperceptible" before and after the ghost canceler was activated. Note, however, that the ghost canceler was equally effective in reducing (but not necessarily totally eliminating) the visual perception of ghosts in all three grade of service.

TABLE 3

Simal Condition IrnDairment Rating of "ImDerceutible"

Before After

"Strong" 51 70 (at least City Grade)

"Moderate" 12 61 (Grade A < Signal < City Grade)

0 16 I z::i: B < Signal < Grade A)

In the case of indoor observations, the performance of the ghost canceler varied significantly depending on the particular arrangement of the receiving antenna system. Homes with outdoor antennas generally displayed stable (non- varying) ghosting conditions, and were largely corrected by the ghost canceler. Homes with indoor antennas, such as rabbit- ears or monopoles for VHF and bow-ties or loops for UHF, exhibited changing (dynamic) ghosts. These varying ghosting conditions were exacerbated by people moving around in the room, trees swaying, vehicles passing by, etc. The ghost canceler generally was not able to adequately compensate for, or track, these conditions. Annoying artifacts were sometimes produced by the canceler unit attempting to correct for a condition which had changed, or was no longer in existence; the canceler would effectively create pseudo-ghosts under these conditions.

VIII. CONCLUSIONS

A field measurement program was conducted to evaluate and document the performance of the BTA ghost canceling technology under different multipath conditions for a major urban area in the United States. Based on the information collected, the BTA technology was effective in reducing or eliminating ghosts in nearly all the locations where ghosting conditions were observed. The amount of improvement varied depending upon the type and complexity of the ghosting impairment, frequency band and transmitting location.

W i l e this field-test program did not attempt an exhaustive assessment of all aspects of this technology, it is hoped that

the Atlanta data, together with other data collected elsewhere in the U.S. and Japan, can serve as the technical basis for a rigorous evaluation of the application of this technology to improvements in terrestrial broadcasting service.

ACKNOWLEDGMENTS

We wish to thank the following individuals for their assistance with this project: the BTA, especially S . Matsuura, for equipment availability and installation coordination and assistance; H. Miyazawa from "K for advice and participation with the expert observers; E. Williams from the Advanced Television Test Center (A7TC) and J. Hidle from the Carl Jones Corporation for serving as expert observers; Tektronix, A7TC and the Capital Cities/ABC Washington D.C. News Bureau for equipment loans; and local Atlanta television stations WSB-TV, WAGA-TV, WGTV, WXZA-TV, WPBA and WATL for their cooperation in inserting the GCR pulse in their broadcast transmissions.

REFERENCES

NAB/MSTV, "Results of Field Tests of a Ghost Canceling System for Television Broadcasting", June, 1990.

M. Obara, H. Miyazawa, J. Murakami, S. Makmo and K. Ohzehi, 'TV-Ghost Cancel Adapter", Journal of the ITE, IT52-1 (1982).

M. Obara, S. Ohnishi, J. Murakami, S. Makino and K. Sakagami, "A Ghost Canceler Using a CCD Transversal Filter", Journal of the ITE, Vol. 37, No. 2, (1983).

Tzy-Hong S. Chao, "Multipath Equalization for NTSC video using Digital IR Filter", IEEE Transactions on Consumer Electronics, Vol. 34, No. 1, February 1988.

H. Obara, S . Ohnishi, H. Miazawa, M. Sakurai and J. Murakami, "Ghost Cancel Reference Signal", Journal of the ITE, RE81-6, (1981).

S. Matsuura, H. Miyazawa, S. Takayama, M. Usui, R. Kobayashi, H. Iga, "Development of a Ghost Cancel Technology for TV Broadcasting", 1990 NAB Engineering Conference Proceedings.

R.E. Keeler, B.R. Saltzberg and J.D. Wang, "Training Signal and Receiver Design for Multipath Channel Characterization for TV Broadcasting", Advanced Television Systems Committee Document T3S5/1070, August 1, 1990.

CCIR, "Recommendation 500-3 (Mod F) -- Method for the Subjective Assessment of the Quality of Television Pictures".

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Victor Tawil received a B.S. degree in Electrical Engineering from New York University, New York, in 1970, and an M.S. degree in Electrical Engineering from the University of Rochester in Rochester, New York, in 1974.

From 1974 to 1988, Mr. T a d was employed by the Federal Communications Commission. W e at the FCC, he held various positions in the Private Radio Bureau and the Propagation and Analysis Branch of the Office of Science and Technology, specializing in the area of tropospheric propagation and systems engineering.

In 1988, Mr. T a d joined the Association for Maximum Service Television (MSTV), an association of approximately250 local television stations created to protect the quality of over-the-air broadcast signals. He was promoted to his current position of Vice President for MSTV in 1990.

Lynn Claudy received a BSEE degree from Washington University in St. Louis in 1977 and an MSEE degree from the Illinois Institute of Technology in 1979. He also holds a BA degree from Oberlin College.

In 1977, he joined Shure Bros. Inc., a manufacturer of professional audio equipment, as a development engineer. From 1980 to 1988 he was employed at Hoppmann Corporation, a systems integration and consulting fnm for the communications industry, where he held various managerial and systems engineering positions.

In 1988, Mr. Claudy joined the Science and Technology Department of the National Association of Broadcasters. In his current capacity as Director of Advanced Engineering and Technology, he is responsible for NAB technical involvement with advanced television developments, includmg standardization efforts for high definition television and improvements to exkting television service. He is also an adjunct professor of physics at the American University where he has taught Acoustics and other audio technology courses since 1984.

Mr. Claudy is a member of the Audio Engineering Society, the Society of Motion Picture and Television Engineers, the lnstitute of Electrical and Electronic Engineers, and the Tau Beta Pi and Eta Kappa Nu engineering honorary societies.