In-Band, Out-of-Band, Whatever It Takes

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In case you missed it, the cable-TV industry reached 10 million digital video subscribers at the end of March.

This week, another 110,000 or so digital boxes will settle into U.S. cable homes. Next week, too. And so on, or at least until second-quarter statistics reshape weekly run rates.

Physicists call this "inertia." Things in motion tend to stay in motion. Things that aren't don't.

Apply this doctrine to the digital set-top box itself. Units like Motorola Broadband Communications Sector's DCT-2000 and Scientific-Atlanta Inc.'s Explorer 2010/2100 (and all clones) are in motion. They'll probably stay in motion.

The hot-rod boxes from both suppliers are not yet in motion. From the sound of things, they probably won't launch in significant numbers this year.

That means MSOs are motivated to extract more dollars from those 10 million (and growing) digital customers. Revenues, after all, accumulate faster when $1 per sub, per month spurts from 10 million somethings, than they do when $20 per month trickles from a negligible number of somethings.

This, in turn, affects interactive-TV applications. Few advanced boxes means few "combo" units that include a cable modem, and, by extension, a Data Over Cable Service Interface Specification (DOCSIS) signal path. Applications dependent on DOCSIS and Internet protocol (IP) suddenly need a different route in and out of existing boxes. (This is largely why Liberate Technologies bought Morecom, Microsoft Corp. bought Peach Networks and, marginally, why OpenTV Corp. bought Spyglass.)

Finding the right mix of interactive services — so more dollars spill out of existing set-tops — means venturing into the world of sessions, in which the set-top works in tandem with a headend server.

Today's set-tops are called "thin" for a reason: They can't eat a whole lot. They're too lean to multitask very heartily. Doing much more than adding channels and a guide requires server help.

Pockets of the 10 million digital- cable subscribers do
get more than a guide and a digital tier — Insight Communications Co.'s work in the Midwest comes to mind.

But asking these boxes to do much more without a remote server to help is like asking a nail to pound itself into a board without a hammer.

Setting up a work session between a set-top and a server requires two things: Two-way plant and a way to use it. (The track, after all, is fairly useless without the trains.)

Enter this week's translation: There are two ways to harness today's digital set-tops to servers, for session-based interactivity.

One way, now about 20 years old, is the out-of-band
signal path. Loosely speaking, what's "out" about it is its spectral location (the frequency band), which is unrelated to the frequencies that carry video. Nor is the out-of-band path correlated to any specific channel.

The out of-band passageway dates back to the first addressable analog converters, when it was concocted as a control channel. Mostly, the out-of-band path shuttles the secrets of scrambled channels, delivers software updates, and couriers electronic program guide data to the box.

While specifics vary between the out-of-band signal paths on Motorola and Scientific-Atlanta boxes, it's reasonably safe to count on at least 1.5 mbps of shared throughput (more down than up, usually) in the existing digital boxes from both suppliers.

The second type of signal path is known as "in-band." In-band means within a channel, or within a digital video multiplex. In analog television, in-band information becomes imprinted on an audio subcarrier, or in the vertical blanking interval (VBI). In digital television, information gets slotted into the MPEG-2 transportation stream.

Either way, you pretty much need to be tuned to the channel to see any embedded ITV elements — which raises interesting questions about what happens when someone is interacting, then changes the channel.

Interactive information can ride the in-band path two ways. If IP-based, it can be encapsulated into an MPEG-2 stream. Or, the interactive element — such as a Web page or a commerce screen — can be captured as a sort of still frame (in tech-speak, the MPEG-2 "initialization frame") and delivered to the TV.

Again, specifics from the industry's two largest suppliers vary. Scientific-Atlanta includes an in-band path in its Explorer 2010/2100 boxes. It moves over QAM (quadrature amplitude modulation), which equates to between 27 and 38 mbps of downstream throughput, shared among interactive-capable homes. Motorola's DCT-2000 can be tweaked to behave similarly.

Meanwhile, companies like Big Band Networks and SkyStream, among others, are cooking up ways to address the same challenge of getting real-time, IP-based applications to existing set-tops. (Just to, not from.) Some technologists call these devices "IP gateways," because they make IP data talk MPEG-2, which is the language the boxes understand.

Other outfits, like Navic Networks, address both the "to" and the "from," and how to harvest more useable ITV bandwidth in both directions.

Which ITV applications work best with which signal path? That depends, predictably, on the type of application. A good way to start sifting through what works — and what doesn't — is to circle back to your ITV suppliers and ask them which of their apps (a) makes money and (b) works with the signal path you're using.

Choking on acronym soup? Send translatables to, with "Translation Please" in the header.