About Switched Broadcast Video: Don’t Look Now, It’s Back!


Funny how things come back around again. It’s no secret that cable providers are examining ways to make better use of the bandwidth surging through their $70 billion pipes — without ever having to revisit a backhoe.

At least six techniques are underway: There’s “all digital/less analog,” which Charter Communications Inc. put on the map with its digital simulcast effort in Long Beach, Calif.

(Aside: It came through loud and clear at last week’s Cable & Telecommunications Association for Marketing Summit that “all-digital” almost certainly won’t happen for decades, which probably makes “all-digital” a misnomer. “Not no analog” was suggested, as in “all-digital is not equal to no analog,” but that’s too grammatically clumsy for sober conversation. Marketers, have at it.)


Then, there are bandwidth extensions, at least to 860-MHz. And there’s optical resegmenting of plant, to lower the number of people sharing bandwidth legs. Advanced forms of modulation and codecs often wind up on the “bandwidth options” list. That’s four.

Fifth are the better methods to “groom” digital traffic onto carriers, known in the lingo as “statistical multiplexing,” or “stat-muxing,” for short. The vendor community often calls it “rate-shaping,” too.

The sixth is the subject of this week’s translation: Switched broadcast video. It appears under several labels, like “switched digital video,” or just “switched video,” or “switched digital.”

If you’ve been around since the last time the telcos attempted a video attack, this one may jangle a distant discord. Switched digital video: Wasn’t that the pitiable technical option for people with “bandwidth issues”? (As in, not enough?)

One of the payoffs for taking verbatim notes over the past 15 years is the merriment in reminding people of what they said in prior industrial chapters. The response to switched broadcast from cable’s most distinguished technologists, back in the early to mid 1990s, was amusingly gruff. “Switching video is frivolously expensive!” they harrumphed. “Gold-plated!”


At the time, the essence of those exclamations was true. It was expensive.

Now, thanks to the flourish of Gigabit Ethernet gear to pump more bits faster, it’s not so improbable to picture a switch plunked in near the pump. Instead of sending all channels to the back of the TV set (including the stuff nobody watches), put the lightly-viewed channels into a switching pool. Make passage only when people tune to them.

The space they formerly inhabited, and the bandwidth it represents, suddenly becomes available for other purposes — more VOD, HDTV, or broadband offerings, for example.

By my count, two of the top five MSOs are testing the technologies associated with switched broadcast.

That means a switch, for the headend, to mind things, made by companies like BigBand Networks. There’s also a minor piece of “client” software, for the set-top box.

Neither MSO is ready to go on record with the who, where and what yet. Both agreed to background-elaborate on the “how and why.”


Understanding how video delivery changes with switches starts with knowing how it works without switches.

Say that’s you, there on the couch with the remote. You tune to a lesser-watched program. (Let’s pick on my own esoteric viewing habits, and say it’s Victory Garden, before PBS ruined it.)


Your digital-cable box consults its internal channel map. Say you asked for channel 222. The channel map correlates what frequency and multiplex holds channel 222, “points” to it, converts it from digital to analog (your TV likely needs analog), and sends it to the TV, for display.

In a switched world, perhaps Victory Garden is deemed low enough in viewership to be put on a switch. Still, you tune in. The box consults its channel map, and sees a placeholder: This is a switched channel. Proceed to the headend.

There, the switch assesses the situation: Somebody in service group X wants channel 222. Is anyone else in service group X already watching it? If so, you join that session. If not, the switch readies your stream from its channel pool. It opens up a session, not unlike establishing a VOD stream.

So far, engineers say they like it (big time like it) for two reasons. One, it’s the least expensive of the six options. Two, channel changing happens way faster than with “regular” digital channels. (The latter strikes me as glorious news, as a digital cable customer.)

They don’t like it because it’s the same technology being used by anyone contemplating “IP video,” which means — you guessed it — the telcos.

And, switched broadcast isn’t without a helping of politics. It is, by its nature, dependent upon a two-way network. (How else will that channel map placeholder get up to the switch for handling?)


Anything that isn’t two-way won’t work on a switched broadcast architecture, which includes those digital TVs with the CableCARD slots.


Nonetheless, switched broadcast will likely emerge as an active technology, especially as bandwidth-hungry services, like HDTV, continue to fuel bandwidth efficiency techniques.

Plus, switched video carries attributes that please several constituencies. It doesn’t touch the plant, which makes Wall Street happy. It barely touches the set-top and is comparatively easy to implement, which makes engineers happy. It opens the way for more services, and lets digital customers change channels appreciably faster, which makes marketers happy.

And it’s the cheapest of the alternatives, which makes everybody happy.

Stumped by gibberish? Visit www.translation-please.com.