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The Evolution of Broadcast TV Technology

An excerpt from the Ninth Edition of
The World of Satellite TV for the Americas

All of today’s pay TV networks use some form of encryption to secure the video and audio components of their program services to maintain control over the distribution of their signals. Subscribers to these networks must use some form of decoder to reconstruct the original picture and/or sound information. Before we can contemplate the many different aspects of video encryption, however, we must first understand some of the basic characteristics of the standard analog TV signal. The following tutorial provides an overview of the evolution of the broadcast TV signal, from the inception of broadcast television itself, to the more recent introduction of encrypted pay TV services and the long anticipated arrival of a new digital TV (DTV) standard for the United States.

The World's First TV Transmission Systems

The early “vidicon” TV cameras developed by RCA’s Vladimir Zworykin converted moving images into hundreds of separate horizontal lines, with the more lines per complete image or “frame”, the greater the resolution or clarity of the TV picture. These early experimental TV transmissions delivered monochrome (black and white) images that used either 343 or 441 lines to produce each full frame of video. The low-resolution images that these systems produced, however, clearly were not suitable for commercial broadcast applications.

On July 31, 1940, the U.S. Federal Communications Commission (FCC) convened a group of 168 electronic specialists and charged them with the task of setting a common video transmission standard for TV broadcasting in America. The National Television Standards Committee (NTSC) responded by submitting a series of recommendations that delineated a broadcast TV signal consisting of 525 scanning lines per video frame transmitted at a rate of 30 frames per second. Moreover, each frame would consist of two alternating “fields” consisting of 262.5 lines each. The NTSC concluded that the speed at which the two “interlaced” fields would alternate--60 times per second--was so fast that the human eye would perceive the two alternating fields as a single image. A 4:3 picture “aspect ratio”, the ratio of screen width to screen height, was also selected. On March 8, 1941, the FCC approved these recommendations, thereby establishing what is now known as the NTSC video standard.

The Move to Color TV

The FCC decided to reconvene the National Television Standards Committee to recommended the technical parameters of a color TV transmission system that would be backwards compatible with the existing monochrome TV sets then in use. The solution was to use a secondary “subcarrier” frequency embedded within the monochrome TV signal to carry the color or “chrominance” component of the TV signal. Older monochrome TV sets would therefore be able to tune in to color TV broadcasts and receive a black-and-white picture.

The NTSC also decided to allow the amplitude of the chroma-color subcarrier determine the image’s color saturation level, while the color’s “tint” would be determined by comparing the chroma-color subcarrier's “phase” to a reference signal generated within each color TV set. In late-1953, the Commission subsequently ratified the NTSC's recommendations, establishing a new U.S. color TV transmission standard. Most of the other countries in the Western Hemisphere have also adopted the NTSC system, as well as Burma, Japan, the Philippines and Taiwan.

It took twelve years for the U.S. household penetration of color TV set to reach 4.9 percent. In 1965, however, the three major TV broadcast networks began offering their affiliate stations a substantial amount of color TV programs during the so-called “prime time” viewing hours. This fueled the demand for color TV sets so that by 1970, color set penetration had jumped from 4.9 percent to 35.4 percent, and six years later it reached 73.6 percent.

Never Twice the Same Color

NTSC also suffers from unwanted interference byproducts called “picture artifacts” that reduce the quality of the TV image. These artifacts are caused by the interaction between the luminance and chrominance components of the NTSC video signal.

During the 1950s, European engineers also complained about the low resolution of the NTSC standard’s 525-line images, which is why Europe adopted two alternate higher-resolution standards dubbed PAL (Phase Alteration by Line) and SECAM (Sequence with Memory). Each of these systems features 625 lines per video frame with a frame rate of 25 frames per second. Alternate techniques for transmitting the color component of the video signal also were adopted. All of Europe and the Middle East--as well as most countries in Africa and Asia--have officially adopted either the PAL or SECAM video standard.

Nevertheless, the PAL and SECAM standards have their own unique problems. This gave the Americans the face-saving opportunity to comment that PAL actually stood for “pretty awful looking” and SECAM was just the normal French way of producing a “system essentially contrary to the American method”.

The introduction of three different analog video standards has given broadcasters major technical difficulties over the years. These difficulties first became a major concern when satellites began to make it possible for TV signals to be transmitted live from one part of the world to any other. The various national broadcast networks worldwide were impelled to develop special standards converters and “transcoders” to convert live or taped foreign TV broadcasts to the local video standard.

The Move Toward Encryption

On January 15, 1986, Home Box Office became the first U.S. pay TV operator to use encryption when it began scrambling its HBO and Cinemax feeds to cable TV affiliates nationwide. Other programmers, such as CNN Headline News, Showtime and the Movie Channel, also followed suit. Due to legislation passed by the U.S. Congress in 1984, however, all pay TV programmers using encryption technology were obligated to offer their services to any home dish owner who was willing to pay for authorized access.

The Technology of Encryption

Total control is achieved by assigning a unique address code to each subscriber’s integrated receiver/decoder or IRD. The authorization center uses this code to turn on, or turn off, any individual IRD, or even to control a selected group of set-top boxes. The authorization center, for example, can “black out” specific regions of the country when a particular programmer is transmitting a sporting event for which it does not own the national broadcast rights.

Whenever I refer to a set-top box as an analog IRD, I am actually referring to the type of satellite signal that the IRD is capable of receiving. As we shall see below, the so-called analog IRD must be capable of processing a variety of digital instructions before it can deliver any analog-based satellite TV program to the viewer’s TV set.

Videocipher RS

VCRS also features an electronic “smart card” that provides the essential conditional access (CA) data that the IRD needs before it can decode the encrypted signals. This smart card, which plugs into a CA card reader slot located on the front panel of the IRD, contains a single Large Scale Integrated Circuit (LSIC) chip that holds the mathematical algorithms or “keys” that unlock each IRD in the system.

Encrypting the Video Signal. All TV sets, whether NTSC, PAL or SECAM, contain a cathode ray tube (CRT) that uses an electron gun to shoot a stream of electrons at the interior wall of the TV screen. This beam of energy excites the screen's phosphor coating, which produces the images that we see.

Encryption Technology Overview

The VCRS encoder removes both of these sync pulses, with the result that the TV set has no way of determining precisely when to end one video line or field and begin the next. The encoder also inverts the TV signal’s luminance component to a negative state and also shifts the location of the chrominance signal component, further confusing the TV set.

To make amends, the encoder digitally delivers a set of instructions that tells the IRD exactly when to insert the missing sync pulses, where to find the missing color information and just how to return the luminance component of the video to the normal “upright” state. However, this digital set of instructions is encrypted so that the signal can only be resolved by those set-top boxes that have the electronic keys required to correctly read these instructions.

The General Instrument factory electronically programs a unique twelve-digit unit address into each IRD that it manufactures. The TV screen will display each IRD’s address code whenever the viewer uses the remote control to enter the “SETUP 1” command. Each programmer requires this address number whenever a new subscriber wants to order an encrypted TV service.

The programmer provides the IRD’s address code to General Instrument’s DBS Authorization Center in La Jolla, California. The Authorization Center in turn will send an authorization message over the satellite that provides the IRD with the set of instructions that it needs to begin decoding the service. Alternatively the Authorization Center can remove the authorization message from the data stream to shut off any individual IRD. Either process can be completed within a matter of seconds.

VCRS also includes a “SETUP 0” command that will display diagnostic data on the TV screen. In the event that subscribers are unable to receive a TV program service for which they have already been authorized, the Authorization Center may ask the subscriber to provide some of these codes to assist in determining the cause of the service interruption.

Encrypting the TV Audio. The VCRS encoder is not merely satisfied with transforming the video component of the TV signal into a jumbled mess. It also digitizes the audio and then inserts the resulting digital bit stream into the encrypted TV signal’s horizontal blanking interval (HBI), which has enough space to accommodate one stereo audio channel plus a utility data channel.

The encoder also encrypts the bit stream by adding each digitized audio sample to a sequence of binary numbers that have been generated by the encoder’s pseudo-random binary sequence generator, or PRBS. This random binary sequence is the “electronic key” that the IRD needs to restore the audio to its original state. In the case of all U.S. encryption systems, this is the 56-bit DES (for Digital Encryption Standard) algorithm mandated by the U.S. National Bureau of Standards.

To generate this 56-bit key, the IRD must also contain a PRBS that can be synchronized with the PRBS at the encoder so that both units are producing identical strings of random numbers. The encoder transmits information over the satellite, called the “seed”, that provides each IRD’s PRBS with the required synchronization information.

The encoder also encrypts the seed so that the IRD must also have an electronic key for unlocking this synchronization information. The smart card is the system component that supplies this function.

Similar to the digital set-top boxes discussed in the previous chapter, VCRS also transmits forward error correction (FEC) data to assist the IRD in detecting and correcting errors caused by link noise. The FEC data is transmitted within the HBI along with the audio’s digital bit stream. VCRS-compatible, set-top boxes can detect and correct single-bit errors as well as conceal or “mask” double-bit errors.

OTHER ANALOG ENCRYPTION SYSTEMS
There are several other encryption systems that satellite dish owners in the Americas may encounter as they travel along the satellite superhighways. Let’s take a brief look at a few of the more commonly encountered ones.

VideoCipher I. The “VCI” system is exclusively used to encrypt satellite TV signals that are not intended for viewing by the general public, such as news and sports “back-hauls” from the site of a live event to a TV network’s home studios. There is no such thing as a consumer-grade IRD for VCI.

VCI is an improvement over VCRS in that it digitally encrypts both the video and audio components of any satellite TV broadcast. VCI uses what is known as the “cut and rotate” method for encrypting the video signal. The encoder samples each video line and converts each line segment to an equivalent digital message. The encoder also determines the cut points for each digitized line and then rotates each line segment so that it can be inserted between alternate cut points. The location of the cut points vary from line to line and all vertically oriented picture information is stepped back and forth across the screen in a sequence that changes from one field to the next. The audio component is encrypted much in the same way that VCRS encrypts the audio for the services that it handles.

Like VCRS as well as most other modern encryption system, VCI uses synchronized pseudo-random binary sequence generators at the uplink and downlink locations to generate the algorithms required for encrypting the video and audio signals. With VCI, the IRD receives the set of instructions needed to perform complementary cut-and-rotate operations that reassemble the image into its original state. The VCI encoder uses the vertical blanking interval (VBI) to send instructions to the IRD concerning the location of the cut points.

B-MAC. Developed by Great Britain’s NTL and exclusively licensed to Scientific Atlanta, B-MAC (for Multiplexed Analogue Component, Type B) is used by satellite service providers worldwide to encrypt their TV program services. At one time or another, B-MAC has been used for DTH operations in several countries, including Australia, Indonesia, South Africa and the United States. Within North America, however, B-MAC hasn’t been used for DTH applications since Primestar switched to an all-digital delivery system in the mid-1990s.

B-MAC uses an encryption technique known as the "line translation" method, where each video line is delayed in a random fashion by several microseconds. This creates the familiar cross-hatched, diamond-shaped patterns that satellite dish owners the world over encounter occasionally.

The B-MAC encoder uses the VBI to send addressable packets to its IRD universe, while the HBI is used to transmit as many as six digitally-encrypted audio channels plus one utility data channel.

In addition to its role as an encryption system, B-MAC also eliminates some of the inherent flaws with NTSC that were mentioned earlier in this chapter. For example, B-MAC totally eliminates the picture artifacts caused by the “cross-talk” between the luminance and chrominance components of the NTSC signal. With B-MAC, the chrominance information is transmitted within one-third of the active scanning time, with the luminance information contained in the remaining two-thirds.

Viewguard. This is yet another encryption system that is not used for DTH applications. Broadcast networks around the world use this system to encrypt their news and sports back-haul feeds. Like VCI, the Viewguard system digitally encrypts video through the use of the cut and rotate method. The audio component of the satellite TV signal also is digitally encrypted.

DIGITAL DTH ENCRYPTION
The available encryption techniques increase dramatically whenever an analog TV signal is transported in to the digital domain. As I pointed out in the last chapter, DVB-compliant digital DTH systems can be rendered unavailable by restricting access to compatible set-top boxes as well as any information concerning the symbol and FEC rates in use. Digital DTH signals also are transmitted in addressable packets that can be subjected to algorithms in a greater number of ways than what is possible when working in an analog domain. For example, the conditional access data for a digital DTH transmission does not have the bandwidth constraints imposed on analog encryption systems, which must insert the CA data into the analog TV signal’s HBI and/or VBI.

With analog encryption, the satellite TV viewer will at the very least see a jumble of squiggles running through the TV screen. Digital TV signals, however, are all but invisible unless you have a spectrum analyzer at your disposal. Digital signals appear to the uninitiated IRD as virtually indistinguishable from random noise.

Now having said the above, it is also fair to point out that digital DTH encryption systems share many of the key aspects that have already been described with regard to analog encryption systems. Pseudo-random binary sequence generators are used to generate the required electronic keys and the precise synchronization of the encoder and decoder is also an essential requirement. Smart cards and their corresponding conditional access (CA) readers, which are built into all digital DTH IRDs, are also an integral part of the digital encryption equation.

In fact, some digital DTH services use a specialized version of a conditional access system that is already in use elsewhere to encrypt analog satellite TV broadcasts. For example, DIRECTV uses a version of News Datacom’s conditional access system that also is employed in Europe for analog TV encryption purposes under the name VideoCrypt. The VideoCipher RS (analog) and DigiCipher II (digital) systems developed by General Instruments also share many common elements.

ADVANCED DEFINITION TV SYSTEMS
In 1983, an organization known as the Advanced Television Systems Committee (ATSC) was created to coordinate the technical details for a replacement standard for the then 41-year-old NTSC standard. In 1987, the FCC appointed an Advisory Committee for Advanced Television Systems (ACATS) to inform the Commission about the technological advances required to produce an advanced television system (ATV) for the United States. ACATS initially evaluated a total of twenty-three different analog-based systems. In 1990, General Instrument submitted the first digital advanced TV system for consideration. ACATS immediately recognized the superiority of digital delivery for any new ATV standard and abandoned the search for an analog alternative that would be backward compatible with the existing NTSC video standard.

In 1990, the FCC responded to the ACATS recommendation to pursue a digital solution by proposing a “simulcast” plan for the introduction of ATV. During the initial phase-in period, existing TV stations would continue to broadcast in NTSC while new ATV stations would broadcast on TV channels that are currently unused within each local TV market.

With the NTSC system, adjacent TV channels are left unoccupied to prevent interference between TV stations operating within the same broadcast area. Digital ATV services, however, can occupy these unused channels without causing interference to analog TV stations. The FCC therefore would not need to assign new ATV channel frequencies to introduce ATV services. What’s more, there would be total use of the VHF and UHF frequency spectrums assigned for terrestrial TV broadcasting for the very first time.

THE DIGITAL GRAND ALLIANCE
Several organizations introduced digital ATV systems to ACATS for consideration, including two digital systems jointly developed by General Instrument and the Massachusetts Institute of Technology, a system jointly developed by Zenith and AT&T, a system jointly developed by Thomson and David Sarnoff Laboratories, and a system developed by Philips. After all systems were fully evaluated, ACATS was unable to designate any one system as inherently superior to the others.

In February of 1993, ACATS encourage all of the digital system providers to combine the best features from all the systems to create a unified ATV standard for recommendation to the FCC. On May 24 1993, the seven companies announced the formation of a consortium called the “Grand Alliance” to jointly develop a single, “best-of-the-best” ATV system.

Field trials of the new system were held at various locations around the country, including Washington, D.C. At the conclusion of the testing period in early 1996, the final proposal for a new ATV system was submitted to the Commission.

FROM ATV to DTV
On December 24, 1996, the FCC adopted a technical standard for what the Commission now calls digital television, or DTV, that is a modification of the system recommended by the ATSC. The new DTV standard offers a multiplicity of digital TV, audio and data formats. These include the broadcast of one or two High Definition Television programs; five or more Standard Definition Television programs at a visual quality superior to an analog NTSC signal; numerous CD-quality audio signals; and the delivery of large amounts of data. The DTV standard does not require DTV broadcasters to use specific scanning formats, aspect ratios and lines of resolution. Instead, the DTV standard offers each broadcaster a variety of options from which to choose (see chart below).

DTV promises to enhance the quality of both the sound and the images to be displayed on new DTV-compatible TV sets. DTV images will present more than twice the number of lines (1080) over what conventional TV pictures provide today. The video images even have the potential to deliver pictures with a sharpness that approaches the clarity of 35-millimeter film.

The flaws inherent in the old NTSC standard will be eliminated. The DTV standard will accurately portray all the colors of the original image without viewers scrambling to adjust the tint controls on their TV sets. The DTV standard takes advantage of sophisticated digital filtering and forward error correction techniques, so that the DTV set can detect and “mask” out noise, “ghosting”, as well as electrical interference from automobiles and electronic appliances. Video “crawl” and other NTSC picture artifacts will also be a thing of the past.

THE DTV SPECIFICATIONS
DTV offers broadcasters the option of delivering their signals in either the standard 4:3 aspect ratio used by today’s NTSC TV sets or in a wide-screen, 16:9 aspect ratio that more faithfully reproduces the dimensions of film-based materials. DTV also will use digital “Dolby” audio transmission techniques to broadcast programs in stereo with surround sound.

Pixels and Lines. The DTV standard supports four fundamental arrays of vertical lines and horizontal picture elements or “pixels” that can be displayed on the TV screen: 480 x 640, 480 x 704, 720 x 1280, and 1080 x 1920. Although the NTSC standard is a 525-line system, only 483 of these lines are “active” lines, with the remaining “inactive” lines contained in the vertical blanking internal. Moreover, NTSC generates 756 pixels per line. Therefore the 480 x 704 and 480 x 640 DTV formats, which are roughly equivalent to NTSC in terms of vertical resolution, are also referred to as “STV” (for standard TV) formats. The 720 x 1280 array has been dubbed the “ADTV” (for advanced definition TV) format because its vertical and horizontal resolution exceed the performance characteristics of NTSC, PAL and SECAM, while the 1080 x 1920 array is an “HDTV” (high definition television) format.

Field and Frame Rates. Field rates of 60, 30 and 24 fields per second are available. The 60 and 30 fields per second best accommodate video source material using interlace scanning, while the rate of 24 frames per second is advantageous for the transmission of all film-based source materials using progressive scanning. Media resources that use progressive scanning, such as film-based materials, differ from video resources using interlace scanning in that each line of an image is presented in sequence.

Video Compression System. DTV uses the MPEG-2 specification as the basis of its own compression system. DTV takes advantage of the layered structure of MPEG-2. One layer can transport an STV signal to less-expensive DTV sets, while at the same time additional layers can transport signal enhancements that will allow more expensive ADTV or HDTV sets to display higher-resolution images from the same digital TV broadcast. MPEG-2 data packets also provide for the transmission of virtually any combination of video, audio and data information. One major difference between an MPEG-2 DVB-compliant signal and a DTV signals is that the former uses a modified version of MUSICAM for the creation of CD-quality digital audio while DTV will use the 5.1 channel Dolby AC-3 surround sound system.

THE ROAD AHEAD
On April 3 1997, the FCC formally established a schedule for a phased transition from NTSC to DTV beginning in 1999. Each TV broadcaster must provide a free digital video programming service that is at least comparable in resolution to their existing NTSC service and also aired during the same time period. The FCC's transition time table requires that the affiliates of the major four networks in the country’s top ten TV markets to be on the air with a digital signal by May 1, 1999. Affiliates of the major four networks in markets eleven through thirty must be on the air by November 1, 1999. All other commercial and non-commercial stations must complete their transition to DTV by May 1, 2002 and May 1, 2003, respectively. (The top ten markets include 30 percent of all American TV households, while the top 30 markets include 53 percent of all U.S. TV households.)

Released on April 21 1997, the FCC's “DTV Table of Allotments” calls for the eventual location of all DTV channels in a core spectrum of VHF (2 to 13) and UHF (14 to 51) channels that have been deemed to be technically most suited to DTV broadcasting. At the end of the transition period each broadcaster must return the frequency spectrum that it currently uses to transmit analog TV signals. UHF TV channels 60 through 69 (764 to 806 MHz) will be returned prior to the end of the transition period so that the spectrum can be assigned for other purposes, such as fixed and mobile services for public safety use.

The Commission has set a target date of 2006 for the total phase out of the NTSC system. The FCC, however, will periodically review this date in light of its evaluation of the transition to DTV.

In the near future, DTV manufacturers will begin to market digital set-top boxes that will permit a terrestrially transmitted DTV signal to be displayed by any NTSC TV set. A variety of DTV sets will also soon appear in the marketplace, offering the satellite service provider with an opportunity to sell and install DTV sets along with satellite receiving hardware. The new DTV sets featuring a 16:9 aspect ratio will also compel the digital DTH service providers to begin offering ADTV and HDTV broadcasts. The advanced capabilities of DTV will give terrestrial TV broadcasters and cable TV systems the opportunity to more effectively compete with digital DTH service providers. The availability of the new DTV sets in the marketplace will also allow DBS operators such as DIRECTV to begin broadcasting movies in an high definition, wide-screen format as early as 1999.

The old Chinese curse “May you live in interesting times” in this case actually should be regarded as a blessing in disguise. The transition to DTV will provide all of us with new opportunities and greater freedom of choice as we enter the first decade of the new millennium.

This tutorial is an excerpt from the new Ninth Edition of The World of Satellite TV for the Americas. Copyright 1998. All Rights Reserved.

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