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One More Time: The ABCs of Q-A-M

3/21/2008 8:00 PM Eastern

Making sure people in the cable-television industry know QAM is an odd fervor of mine. It would be good if everyone in the business of digital TV transmission just knew it, cold. It is foundational. What the alphabet is to words, QAM is to shelf space on cable-TV plant.

Before we even unpack the acronym, know that QAM is tri-multaneously a technique, a thing, and a unit of measure. That’s why it borrows so many beats from the speed of thought when you hear it: It’s an unintended riddle.

Few other things have such varied characteristics as QAM. “What’s black and white and read all over” becomes “what’s a pizza box that shimmies that’s five gallons.” No wonder people glaze over.

QAM stands for quadrature amplitude modulation. Modulation is a technique, used to turn spectrum into bandwidth. (“Spectrum comes from God, bandwidth comes from cash,” as Knology chief technical officer Ricky Luke should get credit for saying.) People say it “kwahm,” like Mom, and “kwam”, like Sam.

QAM is exclusively digital. By contrast, think of the AM/FM radio in your car. It’s analog. (You can tell by the volume of radio ads encouraging you to upgrade to digital HD radio.)

The “M” in AM/FM is for modulation. The “M” in QAM is for modulation. I’m (vastly) over-simplifying, but, QAM does for digital TV what AM/FM does for radio: It imprints what you want to see onto the plant, to get it to you.

So that’s the technique part.

The box part is the flourishing marketplace for the metal boxes that are required for every digital service currently riding on cable systems. Lots of companies make them; pricing is expected to commoditize.

The wow factor in QAM hardware tends to be the density — how many digital channels can be packed into one box. Average is 24. Good is 48. Some vendors are talking about “hecto-QAMs,” where “hecto” means 100.

The unit of measurement part goes like this: One QAM equals one analog (basic) channel, equals 6 MHz of bandwidth, equals ten to 12 standard-definition video streams, equals two to three high definition video streams, equals 38.8 Mbps of broadband data, equals thousands of voice conversations. (For purists, that’s one 256-QAM, and MPEG-2 compression on the video.)

It’s good to know the carrying capacity cold. It’s the fundamental math of digital. It’s how operators reckon how much shelf space exists for new HD channels, or any other digital service.

Know this and thrive!

Stumped by gibberish? Visit Leslie Ellis atwww.translation-please.com.

 

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