The cry of more HD, more VOD has grown to a roar.

While there isn’t a one solution fits all answer, trends include switched digital video (SDV) and analog reclamation. No matter which direction an operator heads, however, one thing is clear: More content means a need for more quadrature amplitude modulation (QAM) channels.

The same 6 Mhz that held one analog channel supports 10-15 SD channels or two HD channels. Yet, if it’s more HD content an operator wants, then it’s more QAM channels the operator needs, even considering technology that can squeeze up to four HD streams into a QAM channel.

"The efficiency on the edge QAM network is less (for HD) than standard definition," said Rick Swiderski, CTO, RCN, which is migrating to an all-digital platform.

More HD means more QAM channels for VOD as well, but take rates are also increasing as subscribers become more comfortable with the concept.

"A few years ago when (we were) selling QAMs for VOD, it was pretty much four QAM channels per service group…Now we are seeing, two or three years later, that number (has) grown from four to somewhere between six and eight channels," Ramin Farassat, vice president, marketing, RGB, said.

This growing need for QAM channels will continue so long as operators are still using coax. "In an HFC world, QAM modulation will be the predominant vehicle for a while to propagate signals over that type of network," Jeff Winn, RCN, director of video engineering said.

Customer premise equipment is a factor too. "Until you get rid of QAM demodulation in the set-top and cable modem, you still need a modulator…If you have an Ethernet receiver or Ethernet modem or set-top you wouldn’t need (it)," Tony Pierson, vice president business development, LiquidxStream, said.

Switching tactics   

Analog reclamation may have superseded the buzz about SDV, with a few larger operators making a decided push to go all digital, but SDV is still being implemented. With SDV, programs are transmitted only upon request for switched channels. Bandwidth is not congested with rarely watched video.

"(Analog reclamation) does not preclude (using) a switched architecture to increase bandwidth…Even when you reclaim channels, when you pump in more content in the digital domain (you) might want to switch that. Content is infinite these days," Charles Cheevers, ARRIS VP, product line management, multimedia, and CTO, Europe, said.

For competitive reasons, operators may want to use SDV to provide a more personalized and unicast service. "That is the experience you get when you do over the top," Nimrod Ben-Natan, vice president, product marketing, solutions and strategy, Harmonic Inc. said.

In any case, SDV requires a high channel count. "(For) all companies running SDV, channel counts go to eight to 16 per F-connector (output)," Cheevers said.

For SDV, an edge QAM device also needs to be able to copy programming in real time for multiple viewers without dropping packets or losing information.  "It needs to do it at the speed of a channel change," Doug Jones, BigBand Networks chief architect, cable, said.

More for less

Buckeye CableSystem is in the deployment stage of an SDV rollout. The plan is to switch 110 channels, which should gain back approximately seven QAM channels of spectrum. The company has purchased 70 BigBand BEQ Chassis for SDV.

Buckeye takes into account the cost per QAM channel. "We are not going to pay double the cost for something that is not double the value," Jim Brown, director of engineering, said, noting density also is important.

There are so-called ultra high-density modulators on the market. LiquidxStream ‘s LxS 3616, for example, offers 36 QAM channels per port, but says it is the density that drives functionality.

"If you lit up certain services (like) SDV, and want to add DOCSIS 3.0 or more VOD capacity, you don’t have to roll a truck. You don’t have to do anything. You can sit at a local terminal and turn them on," Pierson said.

Alternatively, using a stack of four-channel per port QAM modulators need combining networks, which take up rack space and are harder to manage, Pierson said. "This is all alleviated if all the QAMs you need forever are coming from the same coax port."

Ben-Natan talked about generating up to 100 QAM channels on a single RF port to a specific network region. The HectoQAM would combine voice, video and data services in an IP rather than an RF network.

"We are shaping octal-based QAMs, but we still have customers that buy quad-based QAMs," Ben-Natan said when asked about the timing of the HectoQAM. "Our goal is to have the right density that is required at the right time for the market."

As for Harmonic’s octal technology, the company announced octal-based 48 MHz field-upgradable modules to the NSG 9000 that support up to 144 QAM channels per two-RU chassis, in June 2008, at Cable-Tec Expo.

The market for higher channel counts depends in part on digital conversion, Jones, said. For example, with a 750MHz cable plant, there would be approximately 116 channel slots, but thirty of those might still be analog for local off-air channels.  "That leaves 80 odd slots that could be converted to QAM."

"It really depends on operators’ strategy for reclaiming analog channels," Jones added, noting channel counts would be higher if a plant is 850 MHz or 1GHz.


Many of the edge QAM devices on the market today can support broadcast television, VOD, SDV, DOCSIS 3.0 and a modular-CMTS architecture (M-CMTS). QAM modulation can be removed from the CMTS with the edge QAM modulator acting as a downstream device.

"If you strip down the functionality (of the CMTS), put it in the edge QAM, and run IPTV through the edge QAM…the economics might be available to let that work," Pierson said.

The market hasn’t adapted the concept of the M-CMTS as fast as expected, Farassat said. "The architecture is complex. It also does require competitors to work with each other—the vendor building the QAM modulator and the one selling the CMTS."

When Atlantic Broadband purchased edge QAM modulators recently, it went with Harmonic. "We wanted to ensure some ability to upgrade (and) support both SDV and or M-CMTSs even though we don’t have those at this time," Al Kuolas, CTO, said. However, he isn’t sure that integrating the CMTS functionality into the modulator makes sense for smaller operators where all equipment is in a single headend facility.

Universal universe

While current models of edge QAM devices can support various applications, it’s not done concurrently. That is changing, Farassat explained. "All call (their products) universal modulators, but operators buy multiple (devices) for different applications," he said, noting that before the end of the year, RGB will announce a new architecture to address this.

BigBand’s Jones said his company also is planning to have this functionality in a QAM modulator by the end of the year. "Operators (will) not longer (be) tied to buying one QAM (modulator) for each service…There are operational efficiencies to moving toward a universal edge QAM. That will be the trend over the next three years."

"One edge QAM serving more applications at once. I would love for that to be the case," Brown said, adding, however that Buckeye probably would not be an early adopter. "That would be putting a lot of eggs in one basket."

—Monta Hernon (An earlier version of this article appeared in CT’s QAM Tech Guide, August 2009)

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