Television Transmission and Interface Issues

The world of television has become vastly more complex with technological and service options multiplying the choices. The demand for displays and the number of modes of utilization are huge. This article explores part of the complexity of this exciting situation.

by Walter S. Ciciora

A GOOD FRIEND OF MINE likes to say, "I'm so old, I can remember when telephone calls came on wires and television programs came over the air." This witticism dramatizes how much and how quickly things change in television and in display technology.

The National Television Systems Committee (NTSC) television standard set in 1941 for black and white and modified in 1953 to include color was capable of supporting image quality that far exceeded the capabilities of the then-existing displays. This lasted for decades. Committee discussions during the setting of the High-Definition-Television (HDTV) standard expressed the hope that it too would last for decades and stimulate further advances in display technology. That hope has been frustrated by the accelerated progress in displays. HDTV displays quickly reached the full potential of the HDTV standard and even progressed beyond it.

What is Television?

"Television" once meant the "electronic fireplace" in the living room, a 19-in. black-and-white picture in a wooden box around which the entire family gathered to watch one of three network programs. It now has a much more expansive meaning.

As shown in Fig. 1, a viewer might begin watching a game transmitted in HDTV on a large display. The viewer might then continue on a laptop computer in HDTV or Standard- Definition Television (SDTV). The end of the game might be watched on a cell phone or a personal digital assistant (PDA). Some of the challenges of switching between these displays include adjusting the resolution and, as seen in Fig. 1, serving more than one aspect ratio. Note that while the cell phone or PDA does not require HDTV, the laptop probably does since it is viewed from inches away and might have a fairly large screen. The cell phone or PDA might have a mini-projector that would require more resolution than a 2-in. screen on the device itself.



Fig. 1: It's a new age for television, as viewers can watch programming on devices ranging from an HDTV unit, a laptop computer, and a cell phone.


The Advanced Television Systems Committee (ATSC) defined a set of standards encompassing three vertical line numbers (480, 720, 1080), four numbers of pixels per line (1920,1280, 704, 640), two aspect ratios (4:3, 16:9), and three different frame rates (60, 30, 24) most in both interlaced or progressive scanning in its A/53 document.

"Transmission" is the means to transport the signal to the user's equipment. Broadly speaking, it can be via electrical, optical, or physical means. The signal may be encoded in analog form or digital. The electrical or optical signal can be multiplexed by a variety of methods, including frequency-division multiplexing (FDM), which uses a variety of types of modulation to place different signals in various parts of the transmitted spectrum; time-division multiplexing (TDM), which breaks the signal into time segments that are interleaved for transmission to the user; or space-division multiplexing (SDM), which transmits different signals to locations according to the needs and interests of those at the locations. Physical transmission is the mechanical transport of a medium that contains the signal in recorded form.

The method of transmission is dictated by the nature of the transmission medium. Optical fiber requires a modulated light beam. A metallic medium uses either a baseband or modulated signal. Transmission through space usually uses a modulated radio-frequency signal. The types of modulation fit the media. Broadcasters in the U.S. have selected 8VSB (8-Level Vestigial Side Band) modulation for their transmission, while cable has selected Quadrature Amplitude Modulation (QAM) to best fit the technical capabilities of their Hybrid Fiber Coax (HFC) infra-structure. Direct Broadcast Satellite (DBS) uses yet another modulation to best fit its needs.

What Is an Interface?

An interface is the connection between two entities. There are two interfaces to be considered: (1) a signal interface between one piece of equipment and either another piece of equipment or the transmission medium and (2) the human interface, which facilitates the use and enjoyment of the service. Both interfaces have become more complex as technology advances and more service options arise.

Time: The Fourth Dimension

The VCR and the personal video recorder (PVR) are interesting case studies of how both the human interface and the hardware interface have become more complex. The addition of the Internet provides another transmission mechanism to the broadcast, cable, and DBS mix. HDTV challenges the storage capacity of these devices.

Harking back to the good old days, time was defined by the broadcaster. The program was viewed when transmitted or not at all. This has been termed "linear television" because it is tied to "real time" and time flows linearly in just one direction. The signal interface consisted of an antenna connection that was either two wires separated by about a half-inch, called twin lead, connected directly to screw terminals on the back of the receiver or a coaxial cable attached to an appropriate connector. If the receiver had the opposite connector, an interface device called a "balun" (balanced-to-unbalanced signal converter) would be used. The human interface was a tuner knob with detents requiring the viewer to physically go to the receiver to change the channel. As broadcasters began transmitting over the higher frequencies of the UHF band, existing receivers were unable to tune the new channels. These receivers had to be interfaced with a converter that received the new channels and converted them to a channel the existing receiver could tune.

Today, everything has changed. Thanks to advanced signal processing and massive, inexpensive data storage, "real time" is not the limitation it once was. With "time shifting," the human interface is more complex. While the VCR initially served this purpose, the PVR is much more capable. The PVR is a set-top box (STB) with a large hard drive for storing digital video. In the case of DBS, the signal is transmitted in digitized form and can get to the hard drive with little processing. This makes it easy to include multiple tuners. More than one program can be recorded at the same time while any previously recorded program is viewed. The PVR that works with analog off-air and cable transmissions must digitize the signal. The cost of a second analog-to-digital converter was prohibitive in early models, so this feature was unavailable. As analog signals disappear, multiple tuners will become common in PVRs because the signal is already digitized.

Both the VCR and the PVR introduce additional interface issues for the consumer, particularly when an external cable or DBS converter is required. Not only is there a "rat's nest" of wires interconnecting everything in a confusing manner, but multiple remote controls have to be skillfully operated in a carefully choreographed manner for the time-shifted recording to appear later as expected. The issue is particularly daunting when a consumer wants to watch one program while recording another.

HDTV is a major challenge for both the VCR and the PVR. The additional bandwidth of the signal requires substantially more storage. The rapid and continual expansion of hard-drive capacity gives the PVR the advan-tage in home recording of HDTV transmissions.

The combination of the PVR, the Internet, and a television signal source leads to some fascinating capabilities. The latest PVR features allow the user to schedule recording via the Internet from any location. Some PVRs allow the downloading of programs via the Internet, bypassing the broadcaster, the cable operator, and the DBS provider. Other systems feature multiple PVRs in the home with the ability to share their stored programming with each other. When a home network is installed, the PVR can access music, videos, and slide shows stored on personal computers. Once again, the interface becomes more complicated. Now, the PVR must interface to a high-speed Internet connection for these features to work. Fortunately, this can be accomplished with an in-home radio-frequency system, called Wi-Fi (Wireless Fidelity), which avoids the need to directly wire the PVR to the home Internet network.

Yet one more television transmission approach is found in a service called "MovieBeam." A 1.5-Mbps data stream is quadrature modulated onto the video carrier of an analog television signal. The auxiliary spectrum is pre-distorted in a manner that prevents it from interfering with the analog television signal in an ordinary television receiver. The STB is delivered with a hundred movies on a hard drive, most in SDTV and some in HDTV. MPEG-4 compression is employed and is critical to accommodating HDTV on reasonably sized hard drives at this time. Each week, about 10 of the movies are replaced with the data from the quadrature signal. This bypasses cable and DBS and gives the broadcaster a service to sell. The addition of one more STB further burdens the interface and operation of the entertainment system. Possibly, all of the inputs on the display device have already been filled.

Video on Demand (VOD) gives the viewer total control of the time aspect of viewing including pause, play, rewind, and fast forward. Sufficient spectrum must be available to allow an individual data stream to be allocated to each VOD viewer during a session. A two-way communications interface is also necessary to be able to send control signals back to the server. This is no problem in a fiber-rich cable system.

Bandwidth Horse Race

There seems to be no end to the appetite for bandwidth. An almost endless supply of narrow interest channels is becoming available. They need bandwidth. Everyone wants an even higher-speed Internet. Telephony requires bandwidth. HDTV and VOD are huge consumers of bandwidth. And the remaining old analog signals are horribly spectrum-inefficient.

Competition is another driver of demand for bandwidth. Conventional cable companies are franchised locally and tend not to compete with each other; the same is not true of the relationship between the cable operators and the telephone companies. Although cable franchises are almost never exclusive, the economics of competing cable systems has failed almost without exception. Investors simply will not make money available for this purpose.

The telcos have a different situation. The cell-phone business and cable companies are severely eroding their base telephone business. Adding video services is almost a survival issue resulting in intense competition. More programming choice is an important competitive position. Verizon is rolling out "FiOS" Fiber to the Home (FTTH) by using a system similar to the cable HFC infrastructure. The main difference is that FiOS deploys fiber from the neighborhood node directly to each home while cable uses shared coax. Also, FiOS is all-digital, while cable still carries some analog channels. AT&T's "U-verse" Fiber to the Node (FTTN) retains the existing twisted pair to the home. DBS is also adding satellites for more HDTV programming, further increasing the competitive pressure.

Conventional cable operators must respond to this competition by reclaiming analog bandwidth. The low-hanging fruit is the harvesting of bandwidth devoted to analog television. The first step is to convert all premium services to digital. Premium customers pay more and so can support the cost of a STB. There are fewer premium customers, so fewer STBs are needed. Then, the progression continues in inverse order of channel popularity. Each displaced analog channel can support 8–10 or even 14 digital streams, depending on the sophistication of the compression.

Government rules require all analog broadcast television be terminated by February 17, 2009. Because the cable industry is not monolithic and there is no legal requirement in this regard, we can expect differences in the way cable operators respond. Some have announced plans to go "all-digital" with the clarification that a small number of channels will remain analog. A few have even proposed that they will have no analog signals at all. Clearly, if all channels are digital, then every television, VCR, and other receiving device must either be capable of receiving the cable operator's digital signal, have a converter, or be abandoned. Since the majority of receiving devices in subscriber's homes are analog-only receivers, a massive capital investment is needed to switch to all-digital.

Switching to a more-efficient compression method such as MPEG-4 and/or a higher modulation method, such as 1024 QAM, is more difficult. "Legacy" used to mean "the old man's money." But in the television industry, it means the technology one is stuck with. In this case, it's MPEG-2 and 256 QAM in cable and 8VSB in broadcast. Some cable operators are now specifying STBs that do both MPEG-2 and MPEG-4 in anticipation of a switch. The switch can begin such as the transition from analog to digital; i.e., switch the premium services with the lowest penetration first, requiring the fewest MPEG-4 capable boxes. Of course, this is a headache for consumers hoping to connect their new digital television sets directly to the cable. Most of these receivers do not do MPEG-4.

In the race for more bandwidth, one option is to simply add more. The bandwidth of television coaxial cable does not have a "hard limit." Bandwidths in excess of 1 GHz are practical, particularly as the length of the cable run in the system is shortened as the fiber-optics portion reaches closer to the home. However, the interface issue again looms large. If the television receiver does not tune the additional frequencies, a STB is required.

Switched Digital Video

A new transmission technique, Switched Digital Video (SDV), is rapidly being adopted and is dramatically improving the utilization of cable-system bandwidth. It is based on the realization that the subscribers in a cable-system node serving a neighborhood are watching only a fraction of the continuously offered linear television programs. While these programs may occupy a few hundred digital program streams, only a few dozen are in use at any one time in each node. Much of the bandwidth to the node that could otherwise be used to offer more linear program channels, especially HDTV channels, or to serve more VOD customers, is wasted.

In an SDV system, only the most popular programs are continuously present. The total of continuously present programs may be a few dozen rather than the few hundred continuously available on an ordinary digital cable system. When a subscriber uses the remote control to "tune" to a channel not already present in an SDV system, a message goes to the headend requesting that program. The program is placed in an available multiplex in an available 6-MHz band and that location is communicated to the STB. The STB displays the requested channel number even though it has no relationship to the actual channel used to deliver the program.

If the viewer changes channels, this information is conveyed to the headend servers. If a second viewer requests the same channel, he is simply provided with the correct tuning information. SDV does not include pause, fast-forward, and rewind. There are problems for any receiving device that cannot send requesting signals to the headend. A STB is required.

The Interface Problem

The bandwidth of digital video and especially HDTV create technical issues that require new interface connectors. The latest is the High Definition Multimedia Interface (HDMI), which was first released in 2002. It has several versions, at least one of which is still under development and supports 1080p video at 60 Hz. The Digital Video Interface (DVI) is an earlier effort. Digital audio arrived before digital video and it also required a new interface. The Sony/Philips Digital Audio Interface Format (S/P DIF) is commonly used to connect HDTV receivers to home-theater audio systems. Sony and Apple developed an early high-speed data interface standardized as IEEE 1394. Sony calls it "i.Link" and Apple terms it "Firewire." It is used extensively to connect consumer digital video products, particularly digital camcorders and personal computers that are used for editing. Each of these standards has versions which generate headaches. Compatibility problems arise when a component is missing a connector-type expected by another component, or worse yet, the connector is present but the expected application is not supported. An example is a version of 1394 called DTVLink, intended for interconnecting HDTV receivers to cable set-top boxes, DVRs, and other consumer devices, that was standardized by the Consumer Electronics Association (CEA) and the Society of Cable Telecommunications Engineers (SCTE), but is not supported by the 1394 interface on most consumer devices. Table 1 lists the interface connectors that may be found on the back of current HDTV receivers. A major issue for consumers is the availability and expense of the cables.

The rapid advance of technology provides options and opportunities and also confusion and frustration. Troubles multiply when high-technology devices and services have to work together. The cable and consumer electronics industries have struggled with the interface between cable service and consumer-electronics products for decades. While the technical issues are not trivial, the more-difficult matters involve economics, business, and the ability to migrate technology and services to new levels. If neither technological changes nor service innovations took place, the interface problem could be solved with only modest difficulty. But technological change and service innovation are critical to both businesses. Cable's continuing technological advance and new services keep making the existing hardware obsolete. A critical issue centers on who owns the hardware. If the cable operator owns it, a business decision can be made comparing the cost of replacing the STBs, other hardware, and software against the expected rate of return. The problem becomes much more difficult when the subscriber owns the hardware.

The most recent effort is the "CableCard" device that contains the conditional access capability so that subscribers with "cable ready" television receivers can receive digital programming. Some new consumer-electronics equipment comes with one or more CableCard sockets. The CableCard is leased from the cable operator and provides access to one-way cable services. This last part is critical and is almost certainly not understood by the majority of consumers. The cable-ready receiver with the CableCard installed cannot participate in two-way services, including VOD and SDV and interactive television. Yet these are important reasons for subscribing to cable from the consumers' perspective. Work on a two-way solution for consumer-electronics continues.

As of July 1, 2007, cable operators are no longer allowed to add STBs to their inventories that have built-in conditional access. Conditional access must be implemented via a CableCard. The theory is that the significant increase in the number of CableCards in use will force the solution of any remaining problems and the higher volumes will lower the costs.

The old practice of "surfing" the channels to see what's on is no longer a reasonable approach. It would take longer to surf through all of the channels than the time required for most programs. The human interface is greatly helped by a comprehensive Electronic Program Guide (EPG). The PVR's EPG, especially the TiVo guide, is easy to use and intuitive. The guide's descriptions of the programs include the date of production, whether the show is a repeat, a listing of the actors, and a description of the content. This allows a keyword search of the program listings to find programs of interest.

A further issue with the EPG involves the "look and feel" of the cable service. This is an important part of the way the subscriber views the cable operator. Unfortunately from the consumer-electronics manufacturer's perspective, this gets in the way of the television receiver's "look and feel." Some television receivers have their own EPG, displaying information gleaned from the broadcast television signal. But the first thing the cable subscriber sees when turning on the television receiver is the cable operator's EPG. The design of the EPG is more art than science with several players trying their best. Consequently, there are several EPGs in various cable systems representing differing opinions as to what works best for the subscriber.


Table 1: Interface connectors that may be present on HDTV receivers.
Interface Notes
75-Ω antenna connector Frequently there are two, one for an antenna and one for cable TV input
NTSC analog composite
S Video
Component analog HDTV Complies with EIA/CEA Standard 770.3
L/R audio Analog stereo pair
S/P DIF Digital audio; coax, and/or optical
IEEE 1394 Digital video, digital audio, and control
DVI Digital video
HDMI Digital video, digital audio, and control
Cable Card Must comply with SCTE Standard 28
RS-232 Telephone line and/or control
VGA Usually accompanied by subminiature computer sound connector


Efforts continue on improving the interface. CableLabs is working on the Open Cable Applications Platform (OCAP) (see http:// intending to make it possible for consumer electronics to directly implement services offered on cable. The work is complex and difficult and progress isn't as fast as most would wish.


Technology and service innovations have spawned a huge variety of ways of transmitting television, including HDTV. This comes at a cost of myriads of interface issues. Despite this, HDTV is rapidly gaining acceptance. •


Walter S. Ciciora is a consultant and can be reached at 45 Hulls Farm Rd., Southport, CT 06890-3000; telephone 203/259-5183, fax 203/259-0556, e-mail: