Memorize This About QAM (Please)
by Leslie Ellis // March 17 2008
Making sure people in the cable television industry know QAM is an odd hobby 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.
Yes this is a “spinach column.” Eat it. It’s good for you.
It is nearly impossible to have even a mildly technical discussion about a new cable product without bumping into QAM. The term is rife. Whether you’re talking HDTV channels, HDTV on demand, anything else on demand, DOCSIS 3, voice, switched digital video, or bandwidth in general, QAM is there.
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 CTO Ricky Luke should get credit for saying.) People say it “kwahm,” like Mom, and “kwam,” like Spam.
QAM is exclusively digital. By contrast, think of the AM/FM radio in your. 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’ the technique part.
The box part is the flourishing marketplace for the metal boxes called QAMs, which 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 (with H names) 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 spectrum, 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.)
This is the part it’d be good for you to know, cold. Memorize it and keep it handy. It is the fundamental math of digital, and it tends to come up a LOT. It’s how operators reckon how much shelf space exists for new HD channels, or any other digital service.
Know this and thrive!
This column originally ran in the Platforms section of Multichannel News.
What’s a DSG, and Why Does it Matter?
by Leslie Ellis // March 10 2008
If you spend much time around the people responsible for pushing the software behind tru2way into more and more cable systems, chances are high that you’ve heard or will hear this one: “DSG.”
The last time this column examined DSG was in March of 2003. Back then, it was “the new new stuff.” CableLabs had just released the specification for it, one month prior.
DSG is a nested acronym. It stands for “DOCSIS set-top gateway.” (Marketers, fear not. This is a behind-the-scenes term.)
Boiled way down, DSG is the shuttle bus for command and control information, moving in and out of cable-connected digital devices. “Command and control” is the dull but mission-critical work of the back office: Is this a paying customer? Does this box have the right guide data? Does it need new code — like for tru2way?
“Cable connected” implies a device that contains a cable modem. A set-top with an embedded cable modem, for instance, is equipped to signal with DSG. So are some two-way HDTVs, and so will many of the cross-platform services on the product roadmap.
Cable engineers consider DSG a far superior method than what they used to use to move data (and still do, in some cases), which, in some cases (read: Motorola boxes) was called “slotted aloha.”
DSG is a headend thing. A pizza box, in a rack. It is widely viewed as the replacement for the 16 mil. or so boxes out there that only know slotted aloha.
The “go-zinta” (goes-into) of the DSG is the traffic cop for all cable modems: The CMTS, tech-talk for Cable Modem Termination System.
The “go-zouttas” (outputs) of the DSG are the back-end servers or systems needing the information it shuttles. Maybe you began a VOD session. The request, sent upstream from your house to the headend, moves over the cable modem (and therefore over the Internet Protocol, or IP path), to the CMTS. The CMTS hands it to the DSG: “This is for you.” The DSG takes it to the VOD controller, to begin setting up your session.
As a category, this type of data shuttling is called “out of band” signaling. It’s out of band because it doesn’t move inside any particular TV program or network.
For that reason, the engineer inventors who created DSG, five or so years ago, wanted to call it the “DOCSIS Out-of-band Gateway.”
In shorthand, that’d be the “DOG.”
It just never gets old. 🙂
This column originally appeared in the Platforms section of Multichannel News.