April 20, 2009 – Advances in MPEG-based compression technology will soon set important new bandwidth-efficiency benchmarks for cable and telco networks, offering network operators the option to increase the number of video streams over a given amount of spectrum with no diminution in quality or to significantly increase the quality at current stream counts.
What this means for cable operators, say Harmonic, Inc. and Imagine Communications, the two suppliers who have announced the new performance benchmarks, is the ability to pack four HDTV channels into a single 6 MHz RF channel in conjunction with dynamic selection of content to optimize the four-channel performance. "What was the quality level for 3:1 HD ratios can now be achieved at 4:1, and you can get 2:1 quality going to 3:1," says Jamie Howard, CEO of Imagine.
Similar gains are available to telcos using MPEG-4 AVC compression, which could have an important impact on strategic decisions regarding the extent to which they need to upgrade DSL-based access networks to accommodate delivery of greater volumes of HD streams to the home. Where the current capacity allocated to HD channels has been pegged around 6-8 megabits per second, telcos will now have the option to operate HD at around 5 mbps, says Nimrod Ben-Natan, vice president of product marketing, solutions and strategy at Harmonic.
"These are CBR (constant bit rate) data rates," Ben-Natan notes. "Using VBR (variable bit rate) the average MPEG-4 data rate per HD channel would be even lower." VBR encoding is commonly used in situations like the cable RF channelization scheme where several individual content streams are multiplexed together in a given RF segment.
While both companies have their roots in cable, Harmonic has long been a major supplier in the telco IPTV realm as well. Imagine, which has designed its high-performance video processing tools for IPTV as well as cable, is primarily focused in cable and the TV content source market with some six million cable households now served by operators who employ its ICE (Interchangeable Compressed Elements) Broadcast System. Both companies displayed their new compression capabilities, which rely on various combinations of hardware and software advances, at the recent National Cable Show in Washington, D.C.
The practice of squeezing three HD channels into a single 6 MHz 256 QAM (quadrature amplitude modulation) output has become more commonplace over the past year as operators invest in advanced processing systems from these and other vendors that allow them to accomplish such ratios at acceptable quality levels. But most cable systems still deliver just two HD streams per RF channel, and no one is doing four.
Now, however, with the volume of linear HD channels growing rapidly as cable operators push to catch up with satellite competitors and to keep pace with telcos, the ability to double the number of HD channels offered within a given bandwidth allocation could prompt many who have been reluctant to spend on the 3:1 advancements to opt for 4:1, even if they're using other means such as switched digital video to increase capacity. With HD content permeating everything from the least watched VOD movies and TV programs in the long tail to live broadcast, operators have the opportunity to leverage varying quality expectations for these different classes of content to create an overall service experience that easily meets consumer requirements at these higher ratios, Ben-Natan says.
"As you get into a broad mix of popular and long-tale programming to choose from, some of it formatted at 1080p, some at 1080i, some at 720p, some full resolution, some three quarters resolution, you can afford to combine them at a four-to-one ratio while maintaining good quality," Ben-Natan says. "As cable operators struggle with how to accommodate more than 100 channels of HD, this technology offers a good tradeoff between quality and bandwidth. The feedback on this from our customers has really been positive."
At a 4:1 ratio over a 256 QAM channel, the average bit rate per MPEG-2 HD channel works out to just over 9 mbps, or about half the bit rate that was once considered essential for acceptable HD quality. To achieve this capability the system must be able to dynamically assign HD streams into specific QAM channels so that high-motion, relatively high-bandwidth-consuming content is mixed with less motion filled content and the varying bandwidth requirements of the different HD formats are exploited as well.
"Your average may be 9 mbps, but at any given moment you might have one channel stream operating at 4.5 mbps and another at 18 mbps and others in between," Ben-Natan says. "You want to balance things out with combinations of high-value and lower value content multiplexed together so that, if push comes to shove and you have to sacrifice quality to maintain the ratio, you can cut the quality on the lower-value content. The flexibility to dynamically mix streams and manage bit rates is really important."
Harmonic has packaged the new platform as the Electra 8000, the successor to the widely deployed Electra 7000, but Ben-Natan makes clear customers operating on the Electra 7000 can upgrade to 8000 capabilities on the existing platform. "The Electra has been in the market for some time," he says. "The ability to make improvements over time is part of our original promise."
Ben-Natan stresses there is a lot of innovation beyond applying new generations of hardware that keep Harmonic at the cutting edge of compression technology, which has seen different vendors surging ahead of the pack at different points in time over the past many years. "The secret sauce is in the video architecture and video compression core," he says. "There are a lot of components working together. We use the best of breed in ASICs (application-specific integrated circuits), FPGAs (field-programmable gate arrays) and DSPs (digital signal processors) that are available at any given time. But it's not just about capturing the benefits of the latest generation of ASIC, FPGA or DSP."
The capability to decode and encode both MPEG-2 and AVC enables operators to accommodate both existing and future architectures using a single hardware platform, Ben-Natan notes. For instance, the Electra 8000 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.
"The MPEG-4 element is very compelling to operators," he says. "They're starting to look at MPEG-4 as a delivery format whether for getting content to new set-tops or for experimentation in IPTV. The Electra platform gives you the insurance policy to make adjustments as you need."
Imagine, too, has its own secret sauce, which includes a means by which operators can maintain assurance that they are maintaining acceptable quality levels at whatever ratio of HD channels to 6 MHz they employ. This capability, which Imagine calls ICE-Q, depends on video quality measurement algorithms that accurately emulate the human visual perception system in response to the quality of each frame element.
The ICE encoding process recompresses each macroblock – a fixed set of pixels within each MPEG-2 or MPEG-4 frame – in the video stream to the minimum bit rate for a given level of quality. The ICE software compares the compressed video with the source video using the ICE-Q video quality measurement algorithms. Source video sequences are replaced by recompressed sequences when the Imagine ICE-Q algorithms determine that an expert eye could not discern the difference, thereby ensuring the highest video quality at any given bit rate.
By breaking down the video performance analysis to the macroblock level the system is able to determine the impact of an artifact on perception depending on its position in the frame. An artifact on the periphery typically has less impact than one in the center and is scored accordingly, Howard notes.
In the ICE-Q scoring system, which uses a scale of 1 to 100, a score of 96 is deemed to be the level where no defects are perceivable. "When an operator sets the target at 96 and wants to transmit four HDs per QAM, the system dynamically chooses the four streams that can be fed through that QAM to meet that quality target," Howard says. "Our system is monitoring, measuring and reporting the actual delivered video quality so operators know they've delivered video to the set-top that meets their quality requirements."
This monitoring and measuring is taking place on the streams at a rate of once every 300 milliseconds, Howard adds. "We're collecting the data and rolling it up into a suite of reports the operator can customize to suit their needs," he explains. "If a video stream goes below the targeted threshold, the operator is alerted and can take action to remedy the situation."
In side-by-side demos of each of four channels that were running through the 4:1 processing on one side and 2:1 processing on the other with the quality threshold set at 96, Imagine challenged visitors to its booth at the cable show to determine which was which. Observers consistently found no discernable differences between the two feeds, Howard notes.
Imagine also demonstrated another use of its technology where no discernable differences were registered with regard to content that was highly compressed in comparison to content that wasn't. Here the idea was to show how source video delivered at 50 mbps could be compressed to 15 mbps at no loss in discernable quality. "The technology has important applications across the entire delivery chain," Howard says, noting that along with supporting four HD streams per QAM the new platform is capable of processing eight HD streams in a single RU.
The new ICE Video Processor, to be made commercially available later this year, is based on the latest advances in Intel processors, Howard notes. "We tune our algorithms to make maximum use of next-generation processors," he adds.
Howard revealed that customer reaction to the benefits of the quality measurement system has prompted Imagine to work on ways to expand the process to where probes can be inserted downstream from the ICE platform to provide the same level of frame-by-frame, macroblock-by-macroblock quality assessment. "We're looking at how we can add this capability in a way that is seamless to the existing network monitoring processes," he says.
The expanded capabilities will also focus more on quality assurance in the Internet content domain in response to providers' growing interest in delivering a TV-quality experience to viewers of such content. "We're looking at a variety of innovative tools to package along with the existing video quality management into a suite of applications and services that gives operators the ability to manage and control quality of experience whatever the type of video might be," Howard says.
This means the Imagine platform will be able to judge performance on progressive downloads as well as streamed content, Howard notes. "Progressive download is going to be impacted by congestion," he says. "If you're trying to watch an HD movie and the bit rate suddenly throttles down from 5 or 6 mbps to 3 or 1.5 mbps, the buffer is going to kick in, resulting in a 30-second or so interruption. Providers need to know this is happening in order to ensure that customers who have paid for content are getting a consistently good experience."