‘Grooming’ Means Customizing Digital Video Lineups

by Leslie Ellis // January 07 2002

A letter showed up in the e-mailbox a few weeks ago, seeking translation on a term that snuck into digital video lingo about five years ago: “Grooming.” It resurged just before the Western Show, when a supplier company, BigBand Networks, said it would outfit “grooming” gear to two Time Warner Cable systems.

“Grooming,” in the digital video sense, has nothing to do with standing in front of mirror with a comb, a toothbrush and a bar of soap. Instead, grooming has to do with customizing digital video channel lineups. Tactically, grooming usually happens after a clump of digital video channels enter the headend, but before the customized channel clump moves over the plant to subscribing homes.

Technically, “grooming” is also known as “add-drop remultiplexing.” It is performed by a piece of headend equipment, made by a (very) short list of vendors. In a signal flow sense, grooming gear generally sits after the satellite receiver and before the encryption and modulation gear.

Most people who use grooming equipment nowadays use it to add or remove programs from AT&T Broadband’s “Headend In the Sky (HITS)” feeds. Maybe a HITS affiliate wants some of the channels in the feed, but not others. (The obvious example is the removal of risque programming in deeply religious areas.) The process of breaking the feed open, plucking out the desired channels, and repackaging it all together, is what necessitates the “grooming” gear.

Most businesses have some form of “grooming.” They just don’t call it that. Consider a gourmet food store, in Vermont, that does a brisk business in gift baskets. Perhaps one of its bigger basket suppliers always seems to include piña colada mix, when a tin of hot cocoa would probably have more appeal (given the season.) When the proprietor removes the colada mix and substitutes the cocoa, the basket has thus been repackaged – “groomed” – for sale.

The mother of “grooming” gear is the statistical multiplexer. Statistical multiplexers and talk about “grooming” always seem to go together, mostly because one (statistical multiplexing) begat the other (grooming).

Technologists like statistical multiplexers, or “statmuxers,” as they’re sometimes called, because they extend to the passageway what MPEG-2 video compression aimed to do in the first place: Eliminate wasteful, redundant bits. Nip and tuck. Be efficient.

Sometimes, redundant bits in a video image are hard to find. Smoky scenes, for example, as well as high-action sports scenes and explosions, are complicated to compress, and require lots of bits for a given amount of time. At the other extreme, a newscaster’s head and shoulders don’t change much, frame to frame, yielding lots of duplication that could be removed.

Those are two compression extremes. In between is a suitable-enough distribution of bit rates and complexities to apply statistical analysis (the “stat” in “statmux”), which yields averages and means and all sorts of intricate, useful ways to create a package of squeezed, digital channels (the “mux” in “statmux”) more efficiently.

Think of the need for statistical multiplexing in terms of any congested highway, at rush hour. If that highway is a 6 MHz channel, the statistical multiplexer is what assures that the fast lane is going uniformly fast. Ditto for the slow lane.

In video terms, that statmuxer is finding the most efficient way for digital video channels of differing bit rates to ride efficiently within the same 6 MHz channel. The amalgam of the compressed programs is the “multiplex,” or “mux,” as engineers say. Grooming is the method and equipment used to customize that mux, before it goes to set-top boxes.

Statmuxing and grooming are in the lexicon right now because digital video is reality. It won’t be long, in fact, before other digitized material – high-speed Internet, telephony – will be candidates for statmuxing and grooming, too. That’s certainly the intent of the supplier companies active in the field.

Cable technologists have long mulled the day when they don’t have to deal with anything analog, and can then treat the entire 750 MHz channel-load as one, gigantic channel – instead of a series of 6 MHz channels – where content is muxed in and out at peak efficiency. In a phone call where there’s silence – “Hang on, somebody’s at the door” – that bandwidth gets reclaimed for use elsewhere, until the conversation resumes.

That sort of thing will require even more sophisticated “grooming” devices. Right now, it’s mostly just MPEG-2 digital video that gets groomed. Over time, watch for it to expand to Internet Protocol (IP) and other digital signal types. 

This column originally appeared in the Broadband Week section of Multichannel News.