SDV/ADVANCED COMPRESSION - Reclaim Bandwidth and Maintain HD Video Quality

Sun, 02/28/2010 - 7:45pm
Gil Katz, senior director of cable solutions at Harmonic

A hybrid switched video, statistically multiplexed environment does the trick

HDTV is now a fact of life for broadcasters, cable operators and content providers, and most are focused on how to effectively deliver the widest variety of HD content to subscribers (witness the fabled “100 channels” offerings that are now appearing in the lineups of some cable MSOs).

By its very nature, HDTV demands the highest video quality, yet operators are often forced to balance quality against price/performance and finite bandwidth.

The challenge, therefore, is to recover and preserve enough bandwidth to meet growing consumer demand for bandwidth-hungry services such as HDTV and video-on-demand – as cost-effectively as possible and without compromising quality.

Simple digital turnaround (DTA), rate shaping, 3:1 or 4:1 encoding for HD transport, and switched digital video all provide options, each with advantages as well as tradeoffs in terms of complexity, cost and video quality.

One option is a hybrid solution that incorporates technologies, including SDV and advanced compression, in a linear broadcasting operation to allow operators to maximize and reclaim bandwidth, while at the same time delivering the highest-possible HD video quality.

The ability to maximize bandwidth in a cable operation rests on the complementary aspects of SDV and advanced compression. Switched video can play a powerful role in improving an operation’s overall compression ratio by tying up only the QAM bandwidth that is needed as channel requests come in.

However, SDV is unsuitable for channels in constant demand. In fact, popular channels will consume more bandwidth in a switched architecture compared with standard delivery of linear channels.

As service providers ponder HD and SD deployment options for both the near- and medium-term, they must first evaluate whether their existing hybrid fiber/coax spectrum can accommodate full DTA transmission for a given group of channels. If the operator is willing to compromise to some extent on video quality, HD rate shaping is an acceptable solution.

The strong caveat here is that most rate shaping technologies draw the line at 3:1 compression in terms of being able to squeeze multiple channels into a single QAM without seriously affecting quality. Moreover, rate shaping performance will vary depending on the three-channel mix, and occasionally pre-processing by an encoding platform will be required.

Beyond rate shaping, the operator is left with two choices: SDV transmission or advanced compression using MPEG-2 encoding for 3:1 or 4:1 compression.

SDV has become an important fixture in digital video distribution over traditional cable TV systems using QAM channels, saving bandwidth over the last mile for channels distributed to viewers’ homes over coax cable. The role of SDV comes into sharper focus, particularly given the “long tail economics” at work in today’s cable industry, which dictates that operators must provide larger numbers of channels and niche programming in order to stay competitive and serve a more diverse subscriber base.

Switched video can really pay off in bandwidth savings for niche channels further out on the long tail. By putting lesser-watched channels up on a switched platform, which only allocates QAM resources when the channels are being watched, operators are able to free some QAM bandwidth for other applications and services and expand programming by adding new channels to the lineup.

However, the higher cost of SDV makes it impractical for high-viewership channels at the lower end of the channel count, such as major broadcasters’ newscasts. Since the highly watched programs are always “on” and are constantly being switched, they are high-bandwidth consumers in switched mode.

Figure 1: Advanced Compression - High-Demand Channels

Also, since SDV delivery requires constant bit rate (CBR) for QAM resource allocation, in some scenarios the fixed configured rate value may not be adequate to ensure that outgoing video quality will match that of the source – another good reason not to switch the more popular channels.

Figure 1 shows a 100-channel scenario in which SDV becomes the more efficient option in situations in which a single QAM can be utilized by five or more channels.

For the more popular channels from 70 on down, statistical multiplexing makes advanced compression a more feasible and cost-effective option for maintaining video quality, while at the same time preserving bandwidth (with up to four channels that can be allocated to one QAM).

In addition, statistical multiplexing offers high service resiliency for these channels through central operation at the headend. Since QAMs are kept at a minimum – together with rack space, power consumption and cabling – capital and operating expenses are reduced, and the system is easier to operate and integrate. Further, statistical multiplexing is easy to scale as the operation grows, and new channels are added simply by adding additional multiplexers.

As opposed to SDV, advanced compression using statistical multiplexing is the best choice for the most frequently watched channels in the lineup, particularly for high-demand HD channels. However, encoding becomes crucial to the operator’s ability to deliver the maximum number of channels with every QAM and still maintain the highest quality levels.

For HD, an industrial-strength MPEG-2/MPEG-4 encoding solution – such as Harmonic’s Electra 8000 – becomes a critical success factor. This system offers encoding and transcoding to support the MPEG-4 AVC (H.264) codec, as well as MPEG-2, up to full frame-rate 1080p 50/60.

With the ability to decode and encode both MPEG-2 and AVC, operators can accommodate both existing and future encoding requirements with a single hardware platform. For instance, the encoder can be used to receive AVC content and re-encode in MPEG-2, supporting the growing trend toward primary distribution of content in AVC and HD formats.

Also, the dense form factor and multifunctional capabilities of these new-generation compression solutions deliver savings both in power and rack space, making it easier for providers looking to keep operating and capital expenses under control.

In order to ensure maximum bandwidth out of their facilities and keep a lid on capital and operating expenses, progressive cable providers are strategically deploying a hybrid SDV/advanced compression environment that maximizes the best qualities of both. Advanced compression technologies are playing a key role by enabling operators to add new and differentiated services while providing business continuity. The real winners are the home viewers, whose choice of HD channels have never been higher and continue to expand, along with superior video quality they’ve come to expect from HDTV.




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