The variety of digital video formats being deployed today is in direct response to consumer demand for more program choices across multiple platforms such as Over-the-Top and Mobile TV applications. Content providers are turning to these advanced formats in an effort to reduce bandwidth, increase HD content, and add additional services while maintaining operational expenses. Digital television transmission, whether it is terrestrial, IP, or via satellite, depends highly on efficient use of compression codecs and transmission schemes. Bandwidth is another important factor that must be considered since an increase in bandwidth comes at an additional and fairly significant cost. Making the best and most efficient use of available bandwidth is critical to providing the additional HD programming and services consumers demand.
Compression Scheme Varieties
Looking at the current video delivery markets, Western Europe and North America have a functional legacy infrastructure that consists largely of MPEG-2 encoders, multiplexers, modulators, and set-top boxes (STBs). While predominant, MPEG-2 also has its limitations. This is because this compression standard was designed about 15 years ago to match the processing power that was economical for video encoders at the time.
In contrast, todays processors are nearly a thousand times more powerful. However, because there is a limit beyond which codec losses from lower bites can be recovered with acceptable quality, dramatically faster processing does not translate into commensurately lower bit-rates for MPEG-2.
To gain desirable video and audio quality at lower bit-rates, many content providers have moved to the MPEG-4 Part 10 (AVC) or H.264 standard. This standard yields better quality as the technology supports the use of multiple reference pictures, up to 16 frames or 32 fields, and multiple motion vectors that improve the predictability of GOP sequences. H.264 also takes advantage of improvements in entropy coding that enable better probability analysis for data and picture detail.
Due to the high cost of retrofitting entire networks and replacing STBs, MPEG-2 will remain the most common format for quite some time, in spite of inherent shortcomings. However, to gain some of H.264s bandwidth and cost-saving benefits, cable operators and broadcasters who currently rely on MPEG-2 can adopt a system that mixes the two compression schemes. As an example of the potential savings involved, a satellite operator offering 100 channels of MPEG-2 SD and 12 channels of MPEG4/H.264 HD using an early generation of multiplexers and encoders would require 11 satellite transponders for the standard definition (SD) content and three for the HD. If that same operator took advantage of new processors and H.264 compression in the head-end, the number of transponders needed for HD content drops to two, and for SD, this number falls to nine; ultimately freeing up three transponders for additional services or programs.
From DVB-S To DVB-S2
In addition to employing H.264 compression, satellite providers are also transitioning from the more commonly used DVB-S format to DVB-S2, which offers nearly a 30 percent increase in bandwidth efficiency. Implementing DVB-S2 also allows an increase in the bits per hertz that are transmitted, which translates into lower operational expenditures without sacrificing video quality or consistency. In addition to DVB-S2, advanced formats such as 16-APSK and 32-APSK have the potential to further increase capabilities.
Satellite APSK modulation schemes exist in many variations, each having a different signal constellation, including 4+12 APSK or 5 + 11 APSK for 16-APSK and 4+12+16 for 32-APSK. A content provider using 16-APSK can improve bandwidth efficiency because 16-APSK offers more bits per symbol. Specifically, an 8-PSK (2^3) system offers eight positions in the constellation, while the extra bits in 16-APSK (2^4) adds another eight additional positions for data packets to be included in the transmission of the signal. Given the nonlinear characteristics of the amplifier in the transponder, the constellation or modulation schemes that achieve the best performance are 4+12 APSK and 4+12+16, which have been adopted in DVB-S2.
To better understand what the satellite constellation diagram is revealing, a quick review is in order. A satellite constellation is a plot of symbols on a rectangular space. We can create a constellation for PSK modulation by drawing a circle of radius =√Es. For this example, give the I and Q channels an amplitude of 1, so that the radius of the circle becomes 1.414.
The 16-APSK points, or symbols, on the constellation are 22.5 degrees apart. The 32-APSK modulation constellation is developed from three concentric rings of uniformly spaced 4, 12, and 16 points. The inner ring has radius R1, the intermediate ring has the radius of R2, and the outer ring has radius R3, which is represented as Y1= R2/R1 and Y2 = R3/R1. These ratios are also adapted to the FEC channel coding method. The Y-ratios for DVB-S2 32 APSK are specified as follows:
The data presented above shows how newer, advanced advance modulations schemes such as 16-APSK and 32-APSK allow content providers to further compress the video data and thereby reduce the necessary transponder space that was once needed with older modulation schemes.
Until recently, DVB-S2 (16-APSK or 32-PSK) was thought to be useful in point-to-point applications, but not in multipoint instances. The format was considered effective for ENG or DSNG, but not for Direct-To-Home (DTH) or contribution. However, SENCORE and its partners have undertaken multipoint DVB-S2 testing with impressive, industry-changing results, and we will begin to see more trials and implementation of these advanced formats in a wider variety of applications.
Downsides Of Advanced Formats
We have considered some of the efficiency benefits that can be derived from advanced modulation formats. One unfortunate tradeoff is that these formats require higher transmit power and higher gain-receive antennas to accommodate the link budget that resolves the carrier-to-noise separation at the receiver. Additionally, 16-APSK and 32-ASPK are much less forgiving of non-linearities associated with high-power amplification in satellite transponders.
When transmission deviations cause interference in a digitally compressed signal, the loss of data or bits can result in a variety of reception issues that disrupt the viewer experience including complete loss of signal. Other potential data issues include dropped packets, metadata errors, and inconsistencies like PCR jitter and overloaded buffering any of which can cause tiling or lack of A/V synchronization.
As every device in the delivery network that touches the MPEG stream has the potential to cause a problem, one invaluable support for video consistency is strategic monitoring and analysis at multiple points throughout the delivery network. Advanced signal monitoring devices assess compressed audio, video, and data services for missing packets, clock pulse, overall jitter, and compliance with ETR-101-290 standards. These types of devices also offer a variety of inputs including fiber/copper GigE, ASI and RF, which enable the operator to monitor any portion of the network for quality of service issues on a 24/7 basis.
Relying On Flexible Operational Gear
Beyond monitoring, manufacturers have developed operational devices to help customers take advantage of the gains being offered through DVB-S2 with 8-PSK, 16-ASPK, or 32-APSK modulation, as well as H.264 compression.
These include compact, high-density transcoders with flexible I/O, supporting H.264 to MPEG-2, H.264 to H.264 transcoding and H.264 transrating, as well as audio pass-through. The latter enables providers to take MPEG-2 content and output H.264 for satellite transmission, or, to take H.264 content on the receive side and output MPEG-2 content for cable distribution.
Modular platforms for satellite modulation are capable of either single or multi-stream modulation of MPEG. Some can support either one or two independent streams while variable coding and modulation (VCM) capabilities allow each stream in a single RF carrier to have its own modulation parameters.
Advanced integrated receiver decoders (IRD) can also be a cost-effective means of supporting SD applications that offer satellite or MPEoIP input and either a composite or SDI output. IRD units should also offer an upgrade path from SD to HD, so operators can leverage existing SD infrastructures while putting a solution in place to enable SD to HD migration when appropriate. SENCORE offers a variety of modular receiver decoders that combine dual-channel processing for MPEG-2, H.264 (4:22 and 4:2:0) SD, and HD video decoding with a wide range of interfaces that make the device adaptable to contribution, distribution, or backhaul environments.
The International View
The international market is fragmented and reliant on satellite-carried video content, which creates a unique opportunity for manufacturers of video distribution equipment as they look to help content providers and distributors to leverage existing infrastructure. DVB-S2s multi-stream capability will be very useful in implementing single frequency networks for DVB-T terrestrial distribution.
As mentioned, Western Europe and North America have a functional legacy infrastructure based primarily on MPEG-2. Meanwhile, the countries in Eastern Europe and elsewhere in the world are in the initial stages of building digital television infrastructure from scratch and have the advantage of learning from the digital launches in the U.S. and Western Europe. These countries can seamlessly bypass the limitations of yesterdays technology and take full advantage of todays advances.
The worldwide equipment demand for DVB-S2/H.264 is expected to grow nearly 40 percent over each of the next two years. India is one of the worlds largest and most powerful emerging markets, and the Telecom Regulator Authority of India has recommended the use of DVB-S2 and H.264, even for broadcasting SD video, due to the technical advantages it offers. The Indian DTH market is one the fastest growing satellite markets in the world and is expected to quadruple from 5 million today to 20 million in 2012.
Recognizing the global need, companies such as SENCORE are providing versatile and advanced operational equipment that in many cases is also interoperable with legacy systems. Together with multipoint monitoring, these products will allow content providers to create hybrid systems that leverage MPEG-2 infrastructure even as they take advantage of the benefits of more advanced encoding and formatting.
To meet viewer demand for higher image resolution, 3D, and video that is available on any screen at any time, video distribution models will need to be even more flexible. Likewise, continued growth in the number of channels means a shortage of available bandwidth and may prove to be a limiting factor technology that takes advantage of existing, available bandwidth will be crucial in order to meet consumer expectations and remain competitive.
SENCORE is among the manufacturers supporting new and innovative video delivery networks with products designed to meet the global demand for more video content. The Company recently partnered with a leading U.S. content provider to test our equipments ability to transmit and receive more than 100 Mbps of video, audio, and data formatted as 16-APSK using a live C-band satellite link and a 4.5m receive dish. Following this successful trial, we went a step further and transmitted 32-APSK modulation using a 9m receive dish. While these modulation techniques are still in their infancy, the results of our testing is good news for content providers hoping to leverage technology to expand service offerings while maintaining reasonable operational expenses and improve bandwidth efficiency.
About the author
Jack Vickers is the Senior Product Manager at SENCORE, where he focuses on designing and implementing technology to help content providers maximize bandwidth and existing infrastructure. SENCORE is a leader in the development of high-quality signal transmission solutions for the broadcast, cable, satellite, IPTV, telecommunications, and professional audio/video markets. Learn more at www.sencore.com.