Quick, answer this question: After your signal leaves the antenna, what device receives it at the other end? If you answered "a television," you are both right and wrong. A television may be used to view the signal, but in about 80 percent of U.S. households, a demodulator at a cable television headend receives the signal. The signal may be downconverted to an IF frequency, and then upconverted and multiplexed with a number of other signals and fed to the consumer's television. Frequently, the device that is used to tune to different signals is a set-top box (STB). The STB may also be required to access special services that are not available through the TV.
STB basics In the very early days of cable, all cable channels were in the normal VHF television spectrum. It was not long, however, before cable systems ran out of capacity. In a bid to increase channel capacity, cable systems put additional television channels in the superband - the frequencies between VHF and UHF stations. Of course, TV tuners at the time were unable to tune to these frequencies. To resolve the issue, cable systems provided the customer with a STB. The box contained a downconverter that would tune all conventional VHF television channels and the additional superband. The output of the STB was VHF channel 3 or 4.
As time went on, the cable market became more sophisticated. Cable companies began providing tiers of subscription service. STBs began to employ descrambling technology. At first these were simple schemes involving sync stripping, but later they included complex digital scrambling algorithms. In recent years, STBs have taken another turn. The WebTV STB allows the viewer to interact with television programming using Web-based technology. Replay and TiVo both allow viewers to selectively record and playback programming. PC tuner cards turn PCs running special software into advanced viewing devices.
I never thought much about STB technology until becoming involved in a project that allowed viewers to select different subtitle languages for display on their TVs. While subtitling technology has been around for some time, at the time of this project, subtitling in Asian languages using computers had not been implemented, due in part to the extremely large number of characters in some Asian languages.
As part of this project, we learned a great deal about set-top box technology: Cost is everything. In addition to cost, other important factors in STB design are low shipping weight, tariff costs, few if any moving parts and security (both electronic and physical).
Cost is king In our industry we frequently find that a manufacturer is reluctant to make changes to their product unless a key customer demands it. This is because the profit from the sale of a custom system rarely pays for the custom development. Sometimes, manufacturers do customize their product to suit a customer's need - automation systems for example. But in the set-top box world, cutting costs is the key to making products successful. For this reason, customization is out of the question. Unless you are buying hundreds of thousands, or millions of units, expect to get a standard product. This is not bad, as it helps manufacturers take advantage of volume production and it helps you as the customer save money.
During our subtitling project, we had extended discussions with a number of STB manufacturers. The technical solution for handling the over 40,000 characters of one variant of the written Chinese language seemed simple - add memory. The STB manufacturers agreed that, technically, there was no problem adding the memory required. However, they pointed out that the additional memory would make the box financially impossible. This is hard to fathom until one looks at the number of potential set-top box sales there could be in the Chinese market. There are more televisions in China than in any other country in the world. If boxes manufactured for China are on the ragged edge of profitability, and if the volume of boxes sold into this region absorbs a majority of the manufacturing capacity of a set-top box company, that company may suffer significantly from incorporating a costly technology into the box.
At the risk of this becoming Economics 101, you might wonder why the manufacturers cannot simply increase the price of the box to compensate for the cost of additional components. The reason is the market is price sensitive. Most STB purchasers negotiate for thousands if not hundreds of thousands of boxes per contract. These negotiations put incredible pressure on manufacturers to keep prices as low as possible. If a manufacturer adds a $1 part, it is unlikely that they will be able to get an additional $1 in revenue because of the extremely competitive nature of the business.
Important STB considerations Another factor to consider is shipping weight. STB manufacturers and STB purchasers are very conscious of shipping weight. Why? STBs are shipped more often than you would think. They are shipped from manufacturing centers offshore. They are warehoused by the vendor and then shipped to the purchaser. The purchaser then has them shipped to various cable outlets and perhaps retail centers. The boxes may then be shipped from the cable company to the user and, finally, the user may ship a defective unit back to the cable company for repair. That is a lot of shipping. When the boxes have a low per-unit cost, shipping costs become a part of the total cost equation, especially when purchases can be in the hundreds of thousands of units. The design mandate is, "make it light." This is simple to understand once you see the problem, but it is not something we would normally consider.
International tariffs are also a significant consideration in STB design. STBs are used all over the world, not just in the U.S. Different governments have different tariff structures, some of which are very strict. It is not unusual for some companies to impose a tariff on a STB that exceeds the value of the STB itself. It can be difficult for manufacturers to address this problem because tariff regulations are different the world over.
Cable companies have incorporated security features into their STB designs for years. STBs have traditionally incorporated hardware security, software security or both. Hardware security can consist of special screw heads on boxes, special labels that if broken void the warranty on the boxes and, in extreme cases, systems that detect whether a box has been opened. When the box is opened, vital components in the box are rendered inoperable. Software security is a topic unto its own. Security has moved from simple sync scrambling and carrier trapping to digital audio scrambling to complete digital signal scrambling and beyond.
In all cases, cable system operators and STB designers are between a rock and a hard place. On one hand, security features should be strong enough to keep the cable systems from losing revenue. On the other hand, remembering that cost is king, security features must be inexpensive to implement. Cable operators do not mind spending substantial amounts for system security at the head-end because that is typically a one-time charge. Expensive security features implemented in the STB are paid for every time the operator purchases a box. Some ingenious security schemes have been developed even though cost constraints are severe.
On another note, perhaps the label should say, "no serviceable components inside." In the world of consumer products, the cost to repair a device may frequently approach or even exceed the purchase price. Have you priced the repair of a consumer VCR lately? The same situation exists for STBs. STBs are not designed for in-depth troubleshooting. Usually they consist of a single PC board. If the box does not work and the problem is not a switch, replace the PC board. Some large STB purchase contracts specify that the manufacturer is responsible for warranty replacement of the STB for the life of the unit. In most cases, even though the STB may be sent back to the manufacturer, it is discarded. Why throw out an STB which may only suffer from a broken switch? Because the manufacturers have had years of experience repairing STBs. On average, they know exactly how much time it will take for a technician to locate a problem and fix it, and they know exactly how much the replacement parts will cost. If the average cost to repair the product exceeds the cost of a new unit, the old unit is discarded.
Set-top vs. computer You might wonder, what is the difference between an STB and a computer? The short answer is not much. In the early days, STBs included a tuner and perhaps some descrambling circuitry. It was not long before STBs started to incorporate microprocessors. These processors provided enhanced security and billing technology. Today, set-top boxes can have multiple processors, hard disks, memory and modem connections. Operating systems include embedded systems programming languages, Windows CE and the many variants of UNIX.
What about the cost issues raised earlier in the article? Manufacturers are constantly pressed between incorporating more functionality in their boxes and holding down costs. As the price for major components such as hard disks and memory fall, manufacturers are able to incorporate these into their designs without driving the cost of the STB through the roof.
Programming STBs is an art in itself. In the past, memory prices were so high that programs were optimized to be as small and efficient as possible. Most programs were written in machine code, or perhaps assembler. As memory prices fall, the requirement to make software as compact and efficient as possible is decreasing. In any case, the conventional STB software program is much more tightly coded than the typical desktop application.
Most of this article focuses on issues surrounding traditional STBs, but new STBs such as WebTV, Replay and TiVo are bound to change the rules. These boxes cost substantially more than conventional STBs. The design rules for these boxes are different. Costs of critical components such as memory and hard disk storage continue to fall at incredible rates. Several years ago, there was much talk in the industry of the "killer app," the application that was going to drive a huge spike in computer/television/STB sales. While the search for the killer app goes on, one thing is clear: The economics of STBs are changing and the complexity of STBs is increasing dramatically.