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Abstract
The monitoring purpose of the broadcast transmission system is to provide a means of determining how the transmission system is performing and where a fault or abnormal condition exists in the transmission chain. This article presents the monitoring system for the digital DBS (Direct Broadcast Satellite). The author describes the monitoring requirements relating to which signals need to be monitored, where to monitor in the transmission chain, and how often each signal needs to be monitored.

 

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A Signal Monitoring Approach for the Korean DBS System

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Jeong-Hyun Park
ETRI

 

Broadcasting television directly from satellite to homes, apartments, or businesses is a very attractive service for areas where over-the-air VHF/UHF reception is either poor or lacking in variety, or cable distribution facilities are absent. It has emerged as the dominant new application of satellite technology. In fact, even today there are thousands of privately owned earth stations receiving television from low-power satellites on an informal basis (i.e., the owner of the satellite does not necessarily know or approve of the reception). Thus, it is not surprising that many government and private entities are planning Direct Broadcast Satellite (DBS) for television [1, 2].
Korea Satellites #1 and #2 with 120 W output and six broadcast transponders with 27 MHz bandwidth were placed 36,000 km above the equator at 116° latitude for commercial broadcast and communication services in 1995 and 1996. Korea has developed DBS to provide direct-to-home broadcast services such as pulse code modulation (PCM) voice and multichannel audio, and TV with full coverage of Korea.
The following are the characteristics of the DBS transmission system developed for Korea. It can accept analog and digital inputs such as component analog video with 44.1 kHz AES/EBU digital audio, composite National Television Standards Committee (NTSC) video with analog audio, SMPTE 125 M parallel component digital video with analog audio, and SMPTE 259 M serial component digital video with 48 kHz AES/EBU digital audio. 1.2 kb/s to 19.2 kb/s asynchronous data, and 2.4 kb/s to 2 Mb/s synchronous data are also supported. The digital transmission system generates an MPEG-2 transport stream using MPEG-2 compression for video and audio. The system is designed with channel error correction using convolution coding (Viterbi decoder) and Reed Solomon coding (204, 188, T = 8), and quadrature phase shift keying (QPSK) modulation for quasi-error-free performance over a satellite channel. The MPEG-2 transport streams are modulated with a 70 MHz IF carrier and transmitted to the satellite using high-power Klystron amplifiers (HPAs) after upconversion to Ku band (14.5 ~ 14.8 GHz). One transponder carries four channels. The transmission system has 1:1 redundancy. The transmission system also has a centralized control system consisting of a server and workstations used to configure and control the DBS system, and download the parameters. There is also a monitoring system. Monitoring performance plays an important role in providing reliable DBS services.

Monitoring Points and Equipment

The determinations of the monitoring frequency, monitoring points, monitoring signals, and monitoring tools for the transmission system are important for fast troubleshooting and adjustment, and quick decisions on monitoring results.

Monitoring Points

The monitoring is performed at the source, loopback, IF/RF chain, and destination to verify that the program is normal during processing in the transmission system, and is received from the satellite without quality degradation.
Source -- To monitor the video signals at source, there are dedicated video monitors for monitoring NTSC video signals 24 hours a day, high-resolution main video monitors for evaluation of picture quality, and a video analyzer (waveform monitor + vector analyzer) for analysis of video parameters. For the audio signals at the source, there are also dedicated volume unit (VU) meters for monitoring of analog audio signals 24 hours a day, and an audio test set (amplifier, speaker, headset) for evaluation of audio quality, and an audio analyzer for analysis of audio parameters. For the data signals at source, there is a data monitor with a logic analyzer for monitoring the data and clock 24 hours a day.
Loopback -- The monitoring is done periodically or nonperiodically for checking of the video/audio/data signals at each loopback point, such as baseband (at the output of the MPEG-2 encoder and multiplexer) and RF (at the output of HPA and HPA RC: HPA redundant controller) in the transmission chain. A test loop translator (TLT) and loopback receive test units (RTUs) are used to reproduce the video/audio/data signals.
IF/RF -- The IF/RF signals are monitored nonperiodically at inputs and outputs of the IF/RF subsystem, such as the modulator (MOD), equalizer (EQL), uplink power controller (UPC), upconverter (UC), UC local oscillator (LO), IF output of the beacon receiver (BR), HPA, low-noise amplifier (LNA), low-noise block box (LNB), and received RF signals from the RTU antenna by the spectrum analyzer (SA) and power meter (PM).
Received -- Also, the monitoring is done for 24 hours for program and data signals at RTUs via a 45 cm antenna.
BER -- For fault isolation and total quality measurement of the transmission system, a quick, reliable, and repeatable indication of channel BER and a packet error count which provides a long-term performance is performed by the data RTU.

Monitoring Equipment

Table 1 shows some tools and their functions for monitoring the system.

Monitoring Scheme

Figure 1 shows the conceptual diagram of the monitoring system in a digital DBS system.

Source Monitoring

At the input to the transmission system, source program signals are fed to the video/audio format converters (FCs) and the redundant encoder/multiplexer (ENC/mux) via the program signal synchronizers (PSSs). The FCs ensure that NTSC video and analog audio signals are output to the monitoring equipment. All the video and audio signals are visually monitored by console-mounted dedicated "source" video monitors and VU audio monitors, respectively. Any one of these same signals can also be analyzed at the console by video and audio analyzers after being routed through video and audio routers, respectively. Also, any one of four programs can be routed to either of two high-quality video monitors for a better representation of the video signal. Any of six audio channels associated with every program service can also be routed to either of two stereo audio amplifiers for monitoring at the console through loudspeakers or headphones. An audio signal generator and video pattern generator are provided and connected to the audio and video routers, respectively. Audio test signals such as sweep signal, multitone, audio frequency, and dynamic gain for initial setup and performance evaluation can be routed to the audio amplifiers or audio analyzer. Video test signals such as color bar, convergence, matrix, pulse and bar, multiburst, and Sinx/x for initial establishment and performance evaluation can be routed to the main video monitors or video analyzers. Also, at the input to the transmission system, source data services are fed to the ENC/mux and data monitor via the data service signal distributor. The status of four data services can also be visually monitored continuously at the console via a PC-based logic analyzer. The data monitor bridges onto the clock and data lines of each data service and displays the signals on the screen of the data monitor in real time, much like a logic analyzer. The data monitor has the capability to select variable signal thresholds via a computer keyboard, thus allowing the monitoring of RS-232 and RS-422 signals without any hardware changes or rewiring. Since RS-422 signals are synchronous and therefore differential, only the "+" output of these signals (clock and data lines) is monitored. Figure 2 shows the source monitoring scheme.

Loopback Monitoring

The video and audio outputs of the redundant ENC are monitored/analyzed by employing a monitor decoder. Output from each monitor decoder is fed to the monitoring equipment. These signals are fed to the video and audio routers, respectively, which can then be routed to either of the two main video monitors and/or video analyzer, and the audio amplifiers/audio analyzer. The ENC/mux outputs are fed to the mux crosspoint switch (MCS). The MCS connects the active ENC/mux to the modulator and also provides an output to the monitoring equipment. This output to the monitoring equipment is monitored by employing a baseband loopback RTU (BBLB RTU), which provides either the video/ audio or data outputs. Any one of the four video programs and the associated audio outputs from the BBLB RTU are fed to the video and audio routers, respectively, which in turn can be routed to either of the two main video monitors and/or video analyzer, and the audio amplifiers/audio analyzer. The two audio outputs from the BBLB RTU (L and R channels) are fed to a dual-channel audio distribution amplifier (ADA) for conversion from unbalanced to balanced signal format, multiple outputs, buffering, and also for level coordination before being fed to the audio router. The data output from the BBLB RTU is fed to the data monitor for continuous visual monitoring. The operator selects, via the BBLB RTU, the program or data service to be monitored. The RF loopback RTU provides either video/audio or data information as sampled from the HPAs or the HPA RC via the TLT. Figure 3 shows the loopback monitoring scheme.

IF/RF Monitoring

The monitoring equipment also provides the capability to monitor and/or analyze the frequency, amplitude, and noise signals from the HPAs and the HPA RC via the TLT/LNB, as received from the satellite via the transmitter antenna, ortho-mode coupler, LNA, RF coupler, downlink coupler, and LNB, and as received from the satellite via the RTU antenna/LNB. The outputs of the redundant modulators are fed to redundant equalizers, which feed redundant UPC units, which in turn feed redundant upconverters. The outputs of the upconverters are fed to redundant HPAs which feed the HPA RC, and the ortho-mode coupler, which in turn feeds the transmitter antenna. The modulator, equalizer, and UPC permit monitoring of such parameters as power level, signal spectrum, and carrier frequency either at the units themselves or via an IF patch panel located in the IF rack with either a spectrum analyzer or power meter. Each upconverter, HPA, and HPA RC provides a monitor output for similar monitoring at the equipment only. Also, from these same monitor ports a TLT can be connected to perform RF loopback tests without the need of an actual satellite. (The TLT is normally connected to the HPA RC. The operator is required to manually connect the TLT input cable from one port to the other.) The output of TLT/LNB feeds a 75- four-way splitter with insertion loss of about 6 dB. One port (dc blocked) of the splitter permits monitoring of the RF signals with a spectrum analyzer (this port is terminated in 75 when not used). A second port (dc passive) feeds the RF loopback RTU, while a third port (dc blocked) feeds the five receive RTUs via the RF switch and further splitters, and a fourth port (dc coupled) is extra and is terminated in 75 . The splitter allows dc current to pass through one output port only. The signal received from the satellite via the transmitter antenna is fed via an LNA and the RF coupler to a beacon receiver (BR), which provides a power control signal to the UPC. The BR provides a monitor port for monitoring its IF output with either a spectrum analyzer or power meter. Figure 4 shows the IF/RF monitoring scheme.

Received Baseband Monitoring

The output from the HPA RC via TLT/LNB, as received from the satellite via the transmitter antenna/LNA and as received from the satellite via the RTU antenna/LNB, is fed to RF switch. The output from the RF switch is fed to a dc block (to prevent the receive RTUs from powering the RTU antenna/LNB) and then to a two-way active splitter having a gain of about 3 dB at 1 GHz. This is done to partially compensate for losses through the various splitters and to ensure that the input to the RTUs is within the proper range. One of the outputs of the active splitter feeds the data RTU via a dc block to prevent the RTU from powering the active splitter. The input level to the data RTU is then approximately 2 to 4 dBm below the LNB output, depending on which LNB is providing the signal. The second port of the active splitter is fed to a four-way splitter with an insertion loss of 5.7 dB which permits dc current to pass through one port only. The dc passive port is connected to a dedicated RTU, and it is the RTU which provides power to the active splitter. The other three outputs of the splitter are dc blocked, and each output feeds one of the other three programs' RTUs. The input level to the program RTUs is then about –19 to –40 dBm. Similar to the "source" signals, the video and audio information from the outputs of the four programs' RTUs can be visually monitored by dedicated "receive" video monitors and VU audio monitors. These same signals can also be analyzed by video and audio analyzers after being routed through video and audio routers, respectively.

BER Monitoring

From an operational perspective, BER monitoring is required to provide a quick indication of how the system is operating, an "all well" indicator. This requires a relative but reliable, fast, and repeatable measure of the link performance. BER monitoring can be by the direct comparison scheme or the forward error correction (FEC) estimation scheme. The direct comparison scheme provides the most accurate BER measure; however, it requires considerable development and is also only practical at the transmission system. The FEC estimation method is simpler to implement and is usable for all RTUs in the field or at the transmission system. The estimate error is insignificant compared to the tolerances for BER measurements. When the Eb/N0 is 3 dB better than threshold, the estimated BER accuracy is 99.9983 percent. This scheme uses the features available with the large-scale integrated (LSI) L64709 [3] channel-decoding chip. The LSI L64709 includes a convolution coder to re-encode the output from the Viterbi decoder (Fig. 5). The input signal to the Viterbi decoder is delayed and compared bit for bit with the output of the convolution coder; this includes punctured code operations as well. There is also an error detection flag, which is an output bit asserted at the beginning of a frame, which contains an uncorrectable error and is deserted at the end of the frame. The RTU calculates BER and automatically updates the BER display every 32 readings. This estimated BER is displayed every 0.3 ~ 1.5 seconds in the range 1 x 10–2 to 5 x 10–5 using the measurement interval of 2.0 Mb (50 ms) or in the range 2 x 10–3 to 2 x 10–7 using the measurement interval of 67.1 Mb (1.5 s) at data RTU.

Implementation Issues

To implement an effective, fast monitoring scheme, there are implementation considerations such as converting, routing, switching, and interfacing.

Converting

A program signal synchronizer (PSS) in the digital DBS system is connected at the source stage to support both analog and digital input signals at the same time. The PSS outputs to the MPEG-2 encoders are put in a single common output format from a number of different video/audio input signal formats. The PSSs are used to synchronize video frame rates and audio sample rates to rates derived from the system timing reference signals. The frame synchronizer in the PSS is also used to delay the audio signal to compensate for the delay introduced to the video signal. Video/audio format converters are used to convert the PSS output signal, SMPTE-259 M serial digital component video signals, and serial digital audio signals to NTSC composite video and baseband analog audio signals for monitoring at the source. Data inputs with asynchronous unidirectional RS-232-C from 1.2–19.2 kb/s data and synchronous unidirectional RS-449-C (RS-422) from 2.4 kb/s–2 Mb/s data are distributed to multiple data service formatter units via data signal distributors. Each of these data formatter units formats the data service signals into transport stream packets. Data signal distributors are used to provide data service signals to primary and redundant data service formatter units and the monitoring equipment as well.

Routing

For effective monitoring at the source, loopback, and as received from the satellite, audio/video routers are used to redirect signals to monitoring equipment capable of monitoring and analyzing essential signals. There are one video and three audio routers with a minimum of 14 inputs and three outputs for video and 14 six-channel inputs and three six-channel outputs for audio. Three levels of audio routers with dual-channel capability are used to accommodate the six channels of audio. There are also remote controllers for controlling simultaneously (1 video + 6 audio) the video and audio routers from the console.

Switching

An RF switch with DC-1450 MHz frequency range and three inputs/one output are used to provide monitoring of the RF signals sampled from the HPA using the TLT, the received signals from the transmitter antenna, and the received signals from the RTU antenna. A RF switch controller with four-position single-position triple-throw (SPTT) is used to select the LNB output signal from RF loopback, transmitter antenna, and RTU antenna.

Cabling and Interfacing

All video signals in the monitoring system are composite NTSC format as specified in International Consultative Committee for Radiocommunication (CCIR) 624-4 [7]. All NTSC composite video signals are at a nominal level of 1 Vp-p (140 IRE). All video connections between monitoring equipment use RCA or BNC connectors with highquality 75- coaxial cables. All audio signals are at a 1 kHz test tone level of + 0 dBu with a headroom of + 18 dB (dBu is a signal level referenced to 0.775 V rms irrespective of impedance), and the frequency range is 15 Hz–20 kHz. All audio connections between monitoring equipment use screw terminals or balanced XLR connectors with 110 shielded twisted pair cables. All speaker cables are 16 additive white Gaussian (AWG) two-conductor parallel cable. All data connections between monitoring equipment use DB-9, DB-25, or DB-37 connectors with shielded multipair cables or flat ribbon cables. The RTU data interfaces conform electrically to RS-232-C specifications. All IF cables from/to LNBs, splitters, RF switch, and RTUs are 75 RG-6U or equivalent. Interface cables between IF/RF subsystems such as the modulator, equalizer, UPC, and upconverter use RG-59 with BNC connectors. The upconverter to HPA uses rigid coaxial cable with SMA connectors. WR-75 waveguide connects the HPA to the HPA RC. The MCS to modulator uses 100 twisted pair data cable with DB-9 to DB-50 connectors.

Concluding Remarks

Television pictures become contaminated by different kinds of noise arising during the signal's transmission. At present, cable TV operators find out about picture degradation only from subscriber complaints or measurements of system parameters. Some of these measurements require stopping a program's transmission.
This article describes the monitoring requirements relating to which signals need to be monitored, where to monitor in the transmission chain, and how often each signal needs to be monitored. Measurements of video signal parameters (such as color bar, multiburst, convergence, Sinx/x, differential gain, and differential phase), audio signal parameters (such as frequency spectrum, noise, dynamic gain, and signal level), IF/RF signals, and estimated BER provide direct diagnostic clues, service quality measures, and performance evaluation of the transmission system. The 24-hour monitoring of the video/audio/data at the source and as received, periodical or nonperiodical loopback monitoring at the MPEG-2 encoder, multiplexer, and HPA output, and BER monitoring allow efficient troubleshooting of the transmitter chain. Main video monitors assess color accuracy and picture quality, and a video/audio analyzer assesses key video and audio parameters such as the amount of distortion of picture and audio signal. The power meter and spectrum analyzer assess key IF/RF signal parameters to ensure reliable and economic operation of the transmitter station. Potentially, the power meter, spectrum analyzer, and video/audio analyzer could also serve as a low cost maintenance tool to keep linearity and group delay adjustments optimal. A TLT provides a test loopback at RF and allows the creation of worst-case situations.
This article also presents monitoring schemes for the DBS system and implementation issues such as routing, switching, converting, and interfacing. The proposed monitoring system makes troubleshooting in the transmission chain easy, and provides the important idea of deciding whether the transmission system operation and picture quality are good or not.

References
[1] D. Scherer, "Measurement Tools for Digital Video Transmission," IEEE Trans. Broadcasting, vol. 39, no. 4, Dec. 1993.
[2] T. S. Rzeszewski, "Television Technology Today: Direct Broadcast Television from Satellites," IEEE Press, pt. II, 1985, p. 77.
[3] LSI Logic, "L64709 FEC concatenated Decoder Technical Specifications Engineering Version," Aug. 2, 1994.
[4] M. Connel, "Digital Signal Distribution in a Combined Digital/Analog Environment," Proc. SMPTE Adv. Television and Elect. Imaging Conf., New York, NY, Feb. 5, 1993, pp. 93–94.
[5] K. Feher et al., Telecommunications Measurements, Analysis, and Instrumentation, Englewood Cliffs, NJ: Prentice Hall, 1987.
[6] Tektronix, "Television Measurements: NTSC System," 1990.
[7] CCIR Rec. 624-4, "Characteristics of Television Systems," 1992.

Biography
Jeong-Hyun Park received B.S. and M.S. degrees in electronics engineering from Soongsil University, Seoul, Korea, in 1982 and 1985, and a Ph.D in computer science from Chungbuk National University, Korea, in 1997. He joined the Electronics and Telecommunication Research Institute (ETRI) in March 1982 as a junior member of research staff in the Communications Systems Research Division. From February 1994 to September 1995 he was a senior member of the Transmitter Team for the DBS Joint Project in SATCOM DIVISION at MPR Teltech, Canada. He is currently a senior member of research staff at the Mobile Communications Technology Research Division of ETRI. His current interests include security for mobile and satellite communications, communication protocols, monitoring schemes, and authentication.