Squeezing something down, usually so that more can fit. In broadband, compression happens after video or audio material has first been encoded (digitized), or transformed into a series of digital bits. Any video, voice or data signal can be compressed. In fact, just about anything that can be digitized can be compressed.
In digital video, the prevalent compression method is MPEG-2, where the “MPEG” stands for Moving Picture Experts Group. With MPEG-2, video can be compressed in such a manner that digitized material that would otherwise occupy 10 or more traditional television channels can comfortably be delivered in a single channel.
The MPEG-2 specification works by looking at each frame of video (there are 30 frames each second in a television signal), one at a time, and comparing it to both the preceding and following frames. Any bits that represent sameness from one picture to the next — the background behind the talking head, for instance — are removed.
The wrinkle in excising duplicative information from one frame to the next is the high-motion show. Hockey games, for example, are particularly challenging to compress, because of the swiftness of the game — and especially the puck. Because of that, high-motion shows demand more bits per second, and, consequently, a lower compression ratio, so as to do justice to the action.
More recently, digital video providers began examining the use of advanced video compression (see “advanced codec”) as a way to further squeeze digitized material. At this writing, advanced codecs include the acronym soup that is MPEG-4, H.264, MPEG-4 part 10; plus VC-1, Windows Media 9, and Real 10.
The logic in advanced compression goes like this: A piece of video encoded with the new stuff, at a rate of 1 Mbps and dropping, looks essentially the same as a piece of video encoded with the existing MPEG-2 stuff, at 3.5 Mbps. Translation: A thinner stream works just as well as a thicker stream, to do the same thing.
The rub with advanced compression is that the fielded base of devices using MPEG-2 can’t do it without a box swap out. (A truth for cable and satellite boxes.) That makes it great for “green field” providers (think telco video here), but not so great for companies (cable and satellite) with vast and growing numbers of planted boxes.
Still, though, if advanced compression means an operator can cram, say, three HDTV channels into one digital channel, instead of two, swap outs may still make sense. The existing HD boxes could be reapplied as “standard definition” boxes, avoiding stranded capital. At this writing (autumn 2005), HDTV subscriber numbers are still low enough for an advanced compression swap-out to be an option.
Usage: Using advanced compression and 1024 quadrature amplitude modulation, technologists estimate six to nine HDTV channels could fit into a standard 6 MHz width.