Technical Tutorial - Moving forward in the digital world Interoperability for satellite news gathering equipment
Misko Popovic
12/30/98
Satellite Communications
Copyright 1998 by PRIMEDIA Intertec, a PRIMEDIA Company. All rights reserved.
Digital technology provides the television broadcasting industry with tremendous benefits. The obvious benefits bring improved quality and potentially lower costs to an industry continually searching for ways to boost the bottom line. Improved compression techniques and processor technologies have helped spur the use of digital video techniques.
Until recently, a number of factors have prevented the widespread use of digital video . Perhaps the single most significant issue facing broadcasters is the failure of different compression hardware to communicate, or interoperate. While this issue may be less frequent within closed networks (where equipment purchases are controlled), it has been a significant issue among satellite news gathering (SNG) operators and news pool feeds, in which equipment types vary. In certain instances, all the equipment may comply with MPEG-2/DVB specifications, but in the end these specs turn out to be too broad to guarantee true interoperability.
Understanding the advantages of digital compression from both the space segment and ground segment is paramount to ensuring interoperability among manufacturers.
Space segment factors Analog video occupies a bandwidth of 20 or 30 MHz, depending on the desired video signal-to-noise ratio. When an analog video signal is converted to an equivalent quality digital signal, the bit rate of the non compressed digital signal is typically well over 100 megabits per second (Mbps). In terms of bandwidth capacity, this bit rate would require an entire 72 MHz transponder at C-band and a 150 MHz transponder at Ku-band. This bit rate is too high for most networks and processors to handle. Therefore, to lessen these bandwidth requirements, compression techniques were developed to reduce the amount of digital video information transmitted. These techniques have continually evolved and improved over the last two decades and digital transmission is in widespread use today.
The digital revolution is advancing as digital techniques, combined with advanced modulation, allow very efficient utilization of bandwidth for broadcasting. In fact, four to five digital channels of analog-comparable quality can be broadcast within the same amount of bandwidth as one analog channel. This is extremely important as the radio spectrum becomes more congested.
In addition to efficient use of bandwidth, digital transmission offers several other advantages, including: * excellent quality; * less likelihood of interference than an analog carrier; * operation at lower transmit and receive power levels, which translate into lower costs of operation; * use of smaller Earth stations and antennas for either the transmit or receive side of the transmission link; * multiple quality levels, and the integration of operational features, such as encryption.
Table 1 represents the capacity comparison between analog and digital TV transmissions for a 72 MHz transponder with the available power resources of 33 dBW at the bit error rate (BER) threshold of 10-7 and 3 dB system margin. As indicated in Table 1, two analog 30 MHz bandwidth TV signals can fit within a 72 MHz transponder when transmitted to a Standard A antenna both for contribution and distribution (although 2 dB higher signal-to-noise ratio [S/N] quality is required for contribution). When two 20 MHz analog carriers are used, it is possible to design the link based on the Intelsat Standard F-2 size antenna for the cable head-ends.The digital side of the table explains how many digital carriers of the same size can fit within a given resource (in this case a 72 MHz transponder - 33 dBW worth of power), according to the desired information rate and receive antenna size. It is interesting to note that either up to 2 x 45 Mbps carriers (for contribution or distribution) or up to 11x 6 Mbps carriers can be transmitted to Standard A or B antennas. Similarly, a single carrier as large as 8 Mbps could be transmitted through the same transponder to a 2.4-meter antenna.
Compression results in approximately a 50 percent reduction in bandwidth requirements for distribution-quality video. Similarly, digital video for cable head-end applications translates to significant reduction in capacity requirements. Applications for DTH TV quality call for small antennas on the receive side, which demand more satellite power. Although using large receive antennas (such as Standard A or B) would result in about a 12 time reduction in bandwidth, such antennas will not be acceptable at the user premises. Higher power satellite transponders must be utilized to serve small DTH antennas. This decreases the satellite capacity available.
It is this financial tradeoff that led analog TV operators to remove operational FM baseband and RF ground equipment before expiration of the equipment's economic life. However, a reduction in space segment cost and the ability to provide service in situations where transponder capacity would have been previously unavailable has stimulated demand. It also gives incentives to operators to transition from analog to purely digital video operation.
Transmission modes The broadcast of compressed TV signals can be achieved by means of two different transmission modes: multichannel per carrier (MCPC) and single channel per carrier (SCPC). The MPEG-2 standard allows both modes. In the MCPC mode, several TV channels (and their associated audio channels) are first multiplexed and then the Forward Error Correction (FEC) and QPSK modulation are added before the composite signal is uplinked to the satellite.
On the receive side, the demodulator and channel demultiplexer are used to recover the selected channel. Independent channel selection enables one MCPC carrier to support a combination of sports, movies and news.
In the SCPC mode, one TV channel uses one digital carrier. Therefore, the transponder's bandwidth is shared among different carriers. The multicarrier operation in the transponder requires that an output power back-off be applied to reduce the effect of intermodulation products. Power back-off refers to how far from saturation the transponder should operate so all that carriers in that transponder operate in a linear region. The smaller the transponder back-off, the closer each carrier is toward the saturation or non-linear region, and the higher the chances are that the transponder tube will eventually be damaged. There are a few significant drawbacks of using SCPC operation that are worth noting: * the transponder cannot be used at saturation; * each channel/carrier requires an individual data and control channel; * each channel requires its own multiplexer. The advantage of MCPC over SCPC is that a single carrier per transponder improves the network efficiency by: * avoiding the waste of power caused by the transponder back-off required in the case of multicarrier operation; * achieving savings through the use of smaller dishes at the receive site, due to more power being able to be generated by the satellite; * accommodating more TV channels per transponder through the more efficient use of satellite capacity.
DTH and satellite distribution to cable head-ends normally utilize the MCPC mode to take advantage of the power of the entire transponder. Usually in these cases, the service is power limited, so the main objective is not spectral efficiency, but rather to minimize the level of interference. Therefore, in the case of MCPC, QPSK with Reed Solomon Coding and FEC=1/2.
Ground segment factors and interoperability issues Interoperability among manufacturers was never an issue in the analog world since the equipment dealt with unprocessed signals. Digital coding and compression techniques that have been developed by the different manufacturers create interoperability issues and challenges. As equipment at the distant end of a satellite link tries to decode and reprocess the signal, a certain amount of sputtering occurs.
As part of the ongoing efforts by Intelsat - the international satellite operator with a constellation of 19 communications satellites - to promote the growth of compressed digital video transmission services, the organization has facilitated three rounds of interoperability testing over the course of the last two years. Intelsat has recently undertaken a new round of testing, which will begin early next year.
The focus of these initial tests was on single-channel video compression equipment used for SNG, occasional-use and low data rate contribution applications. The testing was intended to demonstrate how different manufacturers' equipment, operating within the MPEG-2 and DVB framework, would work together when set to specific parameters. The ultimate goal of these tests was to achieve plug-and-play operation for both NTSC and PAL standards which are the main video standards used around the world. Plug-and-play is defined by equipment that needs no further adjustments once an initial configuration is set up.
To provide an equal opportunity for all manufacturers, Intelsat announced an open invitation to all manufacturers to participate in the trials. As a result, the following 10 equipment manufacturers participated: * California Microwave/STS * DiviCom * NDS * General Instruments * Scientific-Atlanta, Inc. * Tadiran Scopus DVC * Tandberg Television AS * Thomson Broadcast Systems * Tiernan Communications, Inc. * Wegener Communications, Inc.
Based on the manufacturers' comments on the test plan, the following performance parameters were chosen for interoperability testing among codecs: * presence of acceptable video; * presence of acceptable audio; * automatic retrieval of MPEG-PSI (Program Specific Information) provides information necessary to enable the automatic configuration of the receiver to demultiplex and decode the various streams of programs within the multiplex; * subjective evaluation of lip-sync; * objective measurement of lip-sync.
Note that Conditional Access and Service Information (SI) were not addressed in this round of testing.
The results obtained in these tests were, in general, very encouraging and demonstrated which encoders and IRDs would plug and play. In some cases, the interoperability between an encoder and decoder pair was not demonstrated due to the limitations in hardware and software available at the time of testing.
In response to the broadcasting community's requirement for a news pool feed at predetermined data and coding rates that would enable communication between any given encoder and decoder pair, test organizers decided to continue interoperability testing in a modified way: * Testing of the equipment for interoperability should be performed by using only a single set of transmission parameters preferred for news pool transmissions (8Mbps/FEC=3/4). * Signatories and service providers would assess their own inventories of encoders and decoders and submit such a list to Intelsat, including the equipment model, as well as the software version. * Intelsat would provide free space segment so that the equipment could be tested using a live satellite feed (in lieu of the facilities in the Technical Labs). * This phase of testing would be performed by Intelsat signatories and broadcasters, via their teleports, using the equipment from their existing inventory, located at their respective teleport locations. Therefore, equipment manufacturers' representatives were not allowed during this phase of the test, which would help ensure true results regarding "plug and play" capabilities.
These tests are still in progress, but some interesting preliminary results include the following: This phase of tests revealed different understandings within the industry of a standard 8 Mbps transmission. Some manufacturers and users assume that Reed-Solomon Coding is embedded into 8.448 Mbps rate, which makes a symbol rate of 5.632 Mega-Symbol-Per-Second (MSPS), while other manufacturers consider an 8.448 Mbps rate as the pure information rate prior to applying any overhead, inner and outer coding.
When the match of this news pool rate on both the transmit and receive sides was achieved, the test results indicated that some manufacturers' decoders proved to be true plug and play. Others had to be manually retuned to be able to lock onto the signal, and other IRDs could not lock onto the signal at all.
To attempt to finalize this live testing, Intelsat released a Statement of Work (SOW), which outlines the parameters for the next round of interoperability testing. The SOW was issued to those manufacturers of MPEG-2/DVB equipment which participated in any of the previous three rounds of testing.
The objective of the next round will be to focus strictly on ensuring the interoperability of news pool feeds. Manufacturers who successfully complete this round of testing will be certified as compliant, and will be included in a comprehensive catalog published by Intelsat for broadcasters.
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