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Letter Inspires Look Into MPEG Transport Picture

A telephone engineer named Tim wrote in the other week, wanting to impart his interpretation of the recent exchange between Microsoft Corp. chairman Bill Gates and Comcast Corp. CEO Brian Roberts.

(That's the one where Roberts publicly queried Gates on what he meant by going "all-IP," that is, Internet protocol. )

There was some back and forth with Tim, during which he calmly declared that "there is no such thing as MPEG-2 transport."

Having spent many befuddled moments listening to cable engineers refer to "MPEG [Moving Pictures Expert Group] transport," the word from Telco Tim that it doesn't exist seemed a perfect opportunity to take a closer look.

25M boxes use it

This week's translation thus centers on this thing called "MPEG transport," and the 25 million reasons why it matters to this industry — especially as talk continues about "all-digital" and "all-IP."

MPEG transport slinks around the edges of technical conversations, usually in a tangle of techno-terms.

Here's a random example from a recent batch of notes: "You'd still need a set-top with MPEG-4 decompression on an MPEG 2/4 chip, and then it's a matter of putting a new PID in the MPEG transport stream."

If you're like me, your brain bunched up over a chip that does MPEG-2 and MPEG-4. Then the "PID" hit you in the frontal lobe — and the transport tiptoed away.

Yet cable technologists familiar with the origins of MPEG-2 describe the standard as essential for compressing and moving digitized video from one place to another. Like from a headend or satellite to a set-top.

It's confusing, in part, because the term "transport" implies motion, yet "MPEG transport" is a protocol, not a modulation technique. Protocols are rules that describe how two or more machines talk to one another. Modulation, like QAM (quadrature amplitude modulation), dictates how many digital things can be transmitted within the width of a 6-MHz channel.

Monk-ish organizer

It turns out that MPEG transport is more of a bit organizer than a conveyor belt. It groups packets of many flavors for the ride. ("Many flavors" means that cable modems and voice-over-IP devices also use MPEG transport.)

Put another way, if MPEG transport didn't exist, the bit streams carrying the guts of all the different shows airing on digital cable would look like the inside of my brother's car. (Okay, my car too.) Total disarray.

An analogy: Say you run a factory. You make gadgets that ship in three disassembled parts. You put the three parts in a box, with the "some assembly required" directions. That box, and others, moves down a conveyor belt to a truck. The driver gets a detailed bill of lading, including a precise inventory of what's in which box. The merchandise starts moving toward its destination.

The three parts, relative to MPEG-2, are the streams of squished data that make up a digital TV show – the audio, video, and any corollary data, such as timing information and conditional access.

The instructions are the identifiers that assure proper re-assembly of the show, once the packets get into the set-top. All MPEG-2 packets, for example, contain a "PID," or "packet identifier." A PID might say the packet holds a portion of video from the TV show Monk, airing on USA Network. Another PID could hold the audio for another show, on another channel.

The conveyor belt and delivery truck are the QAM modulators, necessary to shuttle the packets over the cable plant to recipient set-tops.

The detailed bill of lading is the MPEG transport stream. It contains a table (known in tech-speak as a "Program Map Table," or "PMT"), which links the identifiers (the "PIDs") of everything inside the boxes. It knows how to synchronize timing, so that shows air without glitches.

That's a gentle overview of the MPEG transport protocol. All of the 25 million fielded digital set-tops in the U.S. use it. Almost none of those boxes use IP transport. That's because almost none of those boxes contain an embedded cable modem, with its DOCSIS (Data Over Cable Services Interface Specification) signaling mechanisms.

That's not to say MPEG transport is a hobbler to advanced-service launches. It's pervasive, true. But it's fairly flexible. For example, MPEG transport can organize and handle the bits (PIDs) of multiple services, not just digital video. That includes cable modems and voice-over-IP devices.

The main thing to remember is this: MPEG-2 isn't just about squeezing data. It's also about how those bits — because there are zillions of them — are organized for transit.

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