Now that standardization work has begun in the Video Services Forum for high bit-rate video transport service interoperability, it’s a good time to review why uncompressed video transport makes sense not only for SD signals, but also for HD signals in the near future. With carriers starting to offer local uncompressed HD transport (such as Verizon’s Hi-Def DVTS, which launched last summer) and rumors about long-haul uncompressed HD circuits becoming available late this year or early next, it appears that the United States might finally be moving to catch up with Japan, where uncompressed HD services have been available for several years.
(click thumbnail)While uncompressed video occupies more bandwidth, advances in carrier technologies are making its use more practical.Let’s begin with a look at the reasons why broadcasters prefer uncompressed transport, and follow this with a look at the reasons why carriers might grow to love uncompressed as well.
There are so many benefits of uncompressed transport for broadcasters that it might be hard to cover all of them, but here are the most important ones:
Signal Integrity—With an uncompressed network, barring any bit errors along the path or signal interruptions, the signal that comes out of a link is exactly identical to the one put in. This removes a whole host of potential problems for a broadcaster, such as compression artifacts, loss of color depth, audio/video synchronization issues and incorrect handling of audio or other embedded ancillary signals.
Flexibility—For a compression system to work properly, it normally needs to be tuned to each application. When different styles of video are presented, say 1080i live sports versus 720p from film, the compression encoders are typically adjusted through the use of software controls to get the best results from each type of content. The configuration of the embedded audio signals also needs to be configured—bad things can happen when a codec tries to compress a digital audio signal that is already compressed, such as Dolby E. With uncompressed transport links, these considerations vanish, because SMPTE was wise enough to specify that all different types of HD video would use either 1.485 or 1.4835 Gbps serial links regardless of format. This consistency gives broadcasters a great deal of flexibility to move all styles of content with many different configurations of embedded signals over a common medium.
Low Delay—Virtually all uncompressed video networks have low delay, simply because there are so many megabits of data flowing that it is too expensive to provide enough memory at any point in the network to store a large amount of data (and thereby create delay). In addition, avoiding compression eliminates multiple frames of delay that can occur in both the compression encoder and decoder. With low-delay circuits, broadcasters can do live interviews directly over these links, and the need for adding audio delay to circuits to compensate for video delay is almost completely eliminated. Low-delay circuits also make service restoration faster in the event of a disruption—problems can be recognized more quickly, and the time needed for a corrected signal to propagate through the network is also reduced.
Of course, the costs for uncompressed transport need to be reasonable. Broadcasters simply aren’t willing to pay six times as much for uncompressed HD transport as they currently pay for uncompressed SD transport simply because HD’s bit-rate of 1.5 Gbps is six times SD’s bit-rate of 270 Mbps. Customers know that carrier costs don’t increase linearly with bit-rate—it’s often the case that quadrupling the capacity of a local telephone circuit only increases the price by a factor of two or less. So, any price premium charged for HD needs to be reasonable for the service to be accepted by broadcasters.
As one of the highest bandwidth signal types on many networks, video transport technologies are subject to a lot of scrutiny. In general, carriers tend to focus on the cost of bandwidth more than on the other costs involved with providing a video transport service. However, more carriers might be willing to transport uncompressed HD video if some of the key benefits are taken into consideration:
Reduced Equipment Complexity—Uncompressed signals require simpler, less expensive devices to adapt them to a network. This is due to the less intensive processing that is required simply to adapt a signal to a specific type of network interface (IP, SDH/SONET, or dark fiber), which is much less complex than the processing that is needed to both compress and adapt the signal to a network. Also, in both cases, care must be taken to ensure that the video timing is accurately reproduced at the far end, which can actually be more complicated with variable bit-rate compressed signals than with constant rate uncompressed signals.
Uniform Service—Uncompressed links allow a carrier to offer a “one size fits all” service to video customers, greatly simplifying the order fulfillment process. Carriers can avoid the headaches of having to reconfigure equipment to adapt to different signal types when all customers use the same interface and signal format.
Simplified Management—The need for problem solving and troubleshooting in an uncompressed network is greatly reduced as compared to a compressed network. By eliminating the complexity of configuring every circuit for each type of customer video, a carrier can greatly streamline the circuit turn up and provisioning process. This process is one of the largest cost centers for many carriers, and is also the source of many of the problems that occur on a daily basis in a modern network.
A general rule of thumb in the telecom industry is that equipment purchase costs account for roughly one-third of the total life cycle costs of providing a service, with the majority of the other costs consisting of system management and maintenance. The large savings possible in these areas with uncompressed transport could conceivably cover all or most of the cost of the additional bandwidth.
A PERFECT STORM?
Transporting video always involves tradeoffs among cost, bandwidth and performance. Fortunately, as technology improves, cost trends are all moving in favor of uncompressed transport. As network capacities grow, and as the cost of bandwidth continues to decline, it will become easier for carriers to offer uncompressed HD service.
A clear precedent has been set in the world of SD video transport, where a large number of both local and long distance carriers have adopted uncompressed 270 Mbps signals as a major part of their service offering. In the local loop, uncompressed 270 service has emerged as the dominant technology for new SD connections. In fact, a significant number of these links are being used to transport compressed HD signals. As more carriers take a hard look at their total costs, including not only bandwidth, but also the costs of endpoint equipment and the life cycle management costs, it is inevitable that uncompressed HD services will proliferate someday. Here’s hoping that that day is coming soon!
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