The Bandwidth Math of Dual Must-Carry

If your company’s growth depends on the cable industry’s available bandwidth, then you’re probably wondering what all this crazy double-talk about the “digital-TV transition” means. Especially the part about “dual must-carry.”

You’re not alone. The “DTV transition” is a dense and trippy subject. For that reason, this week’s translation shows how to do the bandwidth math of a cable system’s carrying capacity.

Let’s put a finer point on it. Say you’re a program network. You’re pitching three new HD channels to the person at the cable company who decides what goes on and what doesn’t.


For the past few months, you’ve been hearing variations of “love the idea, Bob, but for each of your new HD channels, I need to remove four standard-definition (SD) channels.”

You cross your arms. Nod. And silently wonder: How’s that?

The math goes like this: Cable systems built to 750 Megahertz have about 33 digital “channels,” each of which is 6 MHz wide and has a total carrying capacity of 38.8 Mbps. (Cable systems built to 860 MHz have 51 digital channels.)

One digital, SD stream uses 3.75 Mbps of bandwidth. About 10 can fit comfortably into a 6-MHz channel. The math: 38.8 divided by 3.75.

Likewise, one HD stream, using conventional compression, uses 15-ish Mbps of bandwidth. About two can fit into a 6-MHz channel, with some wiggle room. The math: 38.8 divided by 15.

So, for an operator whose shelves are full, adding one HD stream (15 Mbps) could well mean removing four SD streams (3.75 times 4 equals 15 Mbps).

In both cases, SD and HD, operators often apply a method interchangeably known as “rate shaping,” “grooming” and “statistical multiplexing” to squeeze, say, two more SD streams, or one more HD stream, into that 6-MHz “container.”

Refresher on statmuxing: It’s like driving at rush hour when you’re in a hurry. You seek the blank spaces between the cars in the other lanes as your way to dart ahead. Same with rate shaping — it’s a way of organizing the bits more efficiently for the ride.

But all of this is before the Federal Communications Commission decided that cable must carry broadcasters in digital and in analog until 2012. Take a market like Los Angeles or New York, each of which supports a couple dozen over-the-air channels. What’s the worst that can happen?

In a dual must-carry environment, for a market with, say, 20 broadcasters seeking that treatment, and they’re transmitting in HD, cable operators would be forced to clear off 10 6-MHz channels (assuming those stations aren’t already carried).

The good news is, the FCC didn’t mandate must-carry of “all content bits.” Had that passed, operators would’ve been stymied to statmux, recompress or otherwise handle the incoming broadcast signal, for the sake of transmission efficiency.

But back to you. At this point, you’re probably wondering: If dual must-carry is such a squeeze on bandwidth that’s already pinched, then why, oh why, would the National Cable Television Association be glad that its constituents are now “allowed” to dual-carry broadcasters for three years after the transition? Isn’t the point to take back that analog bandwidth (to make more room for your HD stuff)?

It’s not that operators planned to yank the broadcast networks as soon as their analog signal went dark. Most realize the power of being the only guy in town who can serve all the TVs in the house — even that junky one in the back bedroom — with the wire that comes out of the wall (meaning, without having to put a box on every set).

Moreover, the good of the 9/11 FCC decision is that its dual-carry obligation lasts three years, and not “perpetually,” as had been proposed. Plus, it is any trade association’s preference that matters like this be handled by the businesses involved, not by the government.

And, you wonder: What happens, then, in 2012, after the sunset? A bandwidth glut? Answer: Probably not.

Probably, operators will continue to reclaim analog channels (including yours, as negotiated), at a measured and steady rate — which is, hopefully, fast enough to accommodate all the “more HD” that’s coming.

Stumped by gibberish? Visit Leslie Ellis