What it Takes to Offer HDTV Over Cable
by Leslie Ellis // August 05 2002
Last time, we toured the route of the high definition television signal, pausing to peek at the rough spots along its way. This time, a more thorough look at what it takes, technically and operationally, to offer HDTV over cable.
Despite the techno-politics and daunting economics that have long occluded the migration to high definition television, the plumbing of it over cable isn’t all that difficult, technologists assure. Get the signal, manipulate it, stuff it into the transmission mechanism. At the house, a new box.
An HDTV signal arrives at the cable headend from one of two places: Satellite, or broadcast. Broadcasters increasingly send their television payload (both analog and digital) directly over a fiber link, although most still transmit their stuff over-the-air, for the sake of consumer TVs equipped to receive digital signals from the antenna on the roof.
Step two – signal manipulation – is fairly similar to what gets done to “standard definition” digital TV signals. “Standard definition” means a digitized and compressed version of regular old analog TV. SD, then, is what “digital TV” is today: Multiple channels, usually 10, of digitized and compressed TV, slotted into one, 6 MHz channel.
HD is also a digitized signal; it also uses the MPEG-2 compression mechanism. The difference is, what’s being compressed contains a lot more information – more than 6x that of a “regular” digital video picture.
In that sense, then, and before we go any further, it’s useful to note that what’s “digital” about “digital TV” is the journey, not the destination. It’s the transmission mechanisms, not the set itself. TVs themselves are not “digital,” really. The vast majority, for example, don’t yet have a digital input connector.
A TV sold as “digital,” then, is a TV that contains the receiver circuitry to pluck a digitally-transmitted signal out of the air, or off of a wire, and display it.
So, when people talk about “digital HDTV” sets, they’re usually referring to high-end sets that can display the extra information that comes with a digitally-conveyed HDTV signal. HD sets are built to display a different type of “pixel” (short hand for “picture element”) that’s square instead of rectangular, and they’re capable of rendering those pixels in a widescreen format similar to movie screens.
Before any HD channels can be squirted into the plant to travel to connected homes, a few bandwidth decisions must be made. Six times more picture information has a predictable effect on bandwidth: HDTV needs more.
In raw numbers, a compressed HDTV signal needs 19.2 Megabits per second (Mbps). By contrast, most SD signals currently take up about 3.5 Mbps.
As discussed many times before in this column, cable uses a modulation type called “QAM,” for “quadrature amplitude modulation,” to move digital signals from headend to home. The earliest form of QAM was 64-QAM, which affords about 27 Mbps of useable bandwidth. Today’s QAM implementations run at 256-QAM, which boost the rate to just under 39 Mbps.
Simple math (19.2 Mbps x 2) shows that two HDTV signals can fit into one 6 MHz channel modulated with 256-QAM, and that’s exactly what some MSOs are doing. In some cases, three HD channels can slip into a 256-QAM channel, depending on the source material. Talk also persists about further manipulating the incoming HD signal to pack even more bits into the transmission pipe.
(In practice, re-squeezing HDTV pictures will probably elicit skirmishes. Remember the early days of video compression, when 24 channels of video were going to fit snugly into one 6 MHz channel? That much snug affected picture quality, which made content creators grimace. Now, most operators don’t push more than 10 or so SD channels into one 6 MHz channel.)
At the house, two things can happen, depending on which digital set-tops are in use for “regular” digital TV. Some suppliers, like Scientific-Atlanta, offer an “integrated” HD set-top. That means that the stuff that knows how to recognize and deal with HD signals is built-in. Others, like Motorola, offer a “sidecar” HD device. When HD signals enter the existing digital box, it sends them off to the sidecar for processing.
The output of the HD box, the last few feet of wiring that moves the HD signal from the box to the TV, and the input to the TV is perhaps the most contentious of HDTV’s techno-political conundrums. It’s about piracy. Digital pictures don’t degrade, and are thus prey for perfect copies. The contentious details of this discussion will fill a future column.
In short, cable technologists involved with HDTV launches are almost ho-hum when discussing the to-do list for launch. It’s no more remarkable, or unremarkable, most say, than any other new service launch.
So, if the devils of the HDTV transmission are indeed in the details, they’re not in the tactical particulars of launching the service.
This column originally appeared in the Broadband Week section of Multichannel News.