Today’s cable systems are increasingly digital. Armed with a rank of receivers, multiplexers and remultiplexers, a headend can produce hundreds of channels for delivery to viewers. Photo courtesy Scientific-Atlanta.
Demands on distribution
If a facility is not actively producing HD content locally, any HD signals coming into the facility are invariably in compressed form. Until recently, technological limitations forced local stations to first convert the incoming compressed HD signals back to baseband SMPTE 292 before performing network distribution functions such as switching between local and network, branding, and EAS insertion. But the onslaught of HD and the continued proliferation of cable networks are pressuring the cable industry to employ new ways of handling an ever-increasing traffic load.
The cable industry grew up as an increasingly complex RF reception and distribution system. Coax was the backbone of the system. That technology, plus the television receiver's limitations, restricted the program load these systems could carry. Today, anyone able to obtain satellite transponder space can aspire to become a national network and come calling on the cable companies for distribution. It takes staggering amounts of capital and lobbying to gain access to today's cable distribution chain, but the “wanabes” keep coming. That demand, along with the growing pressure that HD puts on bandwidth, has forced cable infrastructure to evolve.
Compression technology has made it possible to pass more programming through the same bandwidth. Fiber technology can increase the available bandwidth in a given path. Receiver technology, in the form of set-top boxes, has allowed cable operators to offer increased programming to viewers. Most cable systems are evolving from mere distribution systems to something resembling WANs. Just as computer technology has pervaded most areas of daily life, telecom technology is pervading the entire television food chain, from acquisition to viewer. There's a good chance that broadcasters will eventually handle their program streams much as the cable folks now do.
Many of today’s rack spaces more closely resemble dense LAN installations than traditional analog distribution centers.
The bit rate bugaboo
To handle HD from the terrestrial or satellite broadcaster, cable systems have added what is commonly referred to as a “digital tier.” This tier is mainly virtual in nature because it still occupies bandwidth (i.e., channels) just as have traditional analog channels. The difference between the two is essentially the same as the difference between terrestrial analog and DTV transmission. Like terrestrial broadcasters, cable operators digitally modulate channels to carry digital data streams. Also like terrestrial broadcasters, these channels can carry more than one program stream. On many cable channels that carry local HD streams, two HD streams occupy each channel. That means that the local affiliates' ABC and CBS HD offerings can travel to the STB on the same channel. The modulation scheme that often facilitates this throughput is 256 QAM. With a constellation this dense, the operators must monitor the system's carrier-to-noise (C/N) levels closely, meaning cable engineers will have to live with the same concern about how close their digital signals are to the error cliff as do broadcasters.
Cable operators, like terrestrial broadcasters, digitally modulate channels to carry digital data streams.
Enter the remultiplexer
Most broadcasters today have been confronted with MPEG encoders that take SD and/or HD and produce program elementary streams that feed a multiplexer. The multiplexer takes the various streams and interleaves them into a transport stream. The stream travels to a transmit site where it modulates an ATSC carrier using 8-VSB. Early on, most multiplexers commanded encoders to build program streams for insertion into the transport stream using a constant bit rate (CBR). The program stream used a fixed bit rate whether or not the encoder needed that bit rate for good MPEG encoding. Now, many multiplexers command their associated encoders to use a variable bit rate (VBR) based on the complexity of each individual program.
For a local affiliate, this is the typical entry point into a cable headend. A fiber feed from the local station with SD and HD programs enters at the right end of the Maxcom unit. The center coax is the local station’s HD feed wrapped in ASI. The left coax is the baseband NTSC feed from the local station for entry into a traditional analog cable plant.
The encoder/multiplexer combination has allowed broadcasters to assemble transport streams. But the next entity in the food chain often doesn't want the total package. As satellite transponder usage increasingly has become digitally modulated, cable operators have discovered they need a way to extract the programming and services they want. Rather than decoding and basebanding the entire stream to cull out the desired programming, it is more efficient to keep the programming in the compressed domain. Enter the remultiplexer. This device inventories program services by looking at the program allocation table (PAT) and the program map table (PMT) and filtering out only the desired program streams. It also allows cable operators to add new program services and then generate a new transport stream. Remultiplexers will soon allow cable systems to insert local spots, along with EAS, into digital streams. These boxes are now sophisticated enough to perform local branding, EAS insertion and grooming. Grooming allows the operator to use CBR or VBR techniques to decide how much bandwidth a selected program stream can have, no matter what bandwidth the individual program stream has when it arrives.
These devices can have few or many I/O ports and can be fairly large, allowing cable operators to receive multiple transport streams from a variety of sources for distribution on the necessary channels. From a technical standpoint, there is the increasing realization that, unless there is a compelling reason why a terrestrial transport stream can't be taken apart, it probably will be.
Here are two generations of cable remultiplexers. The two units on the bottom were state-of-the-art five years ago. The top unit is today’s more powerful version.
Thus, when it comes to handling HD, cable systems and terrestrial broadcasters will use processing techniques that are similar in many ways. Many broadcasters, and many cable operators, lean toward non-modulated HD pass-through, riding on ASI through the plant. Cable operators have used remultiplexing to route programs from one transport stream to the next, and broadcasters might find they can add this technique to their bag of tricks. If broadcasters keep migrating toward a distribution role, with limited local contribution (in most cases, news), television infrastructure might increasingly resemble cable headend topology.
In the bag
Jim Boston is an industry consultant based on the West Coast.