April 04, 2008
| by Ben Hardy
Since the advent of HDTV, few things have aroused more anger in HD viewers than the purported use of “HD-Lite” by television service providers. Before declaring oneself a victim, the consumer needs to understand just what HD-Lite is, when it might be used, and what else could be responsible for that alleged less-than-HD image up on the screen.
HD-Lite or HD Compression?
The phrase HD-Lite is thrown around by many as a blanket explanation for any perceived degradation in an HD picture. The finger of blame has been pointed at the service provider (most notably the satellite companies, although recently Comcast has come under fire for it), and though it’s a major consumer talking point, few service providers are talking. One definition of HD-Lite that surfaces most often is “any alteration of original HD source content.” Well, if compression counts as alteration, then guess what? All your HD is HD-Lite.
In order to send multiple channels over a limited bandwidth, satellite companies, cable companies, OTA broadcasts and Telco’s all have to compress. The satellite companies are using MPEG-4, cable companies are using a mix of MPEG-2 and MPEG-4, U.S. broadcasters exclusively use MPEG-2, and some Telcos have been known to use VC-1 (Microsoft’s Windows Media compression system) – regardless of compression method, they ALL do it. (With the possible exception of Verizon FiOS, whose spokesperson claimed they do NOT compress their HD. While it is true that their fiber-to-the-premises roll-out provides an exceedingly large bandwidth, this writer believes that the spokesperson may have confused over-compression – i.e., downsampling or excessive bit-rate shaping—with just good-old-fashioned compression.)
Service providers receive HD content from ABC, ESPN, CNN, etc., in either 720p or 1080i. To send that content to the end-user, the content gets compressed. “When HD content is compressed, three things are done to it,” says Peter Symes, Director of Standards and Engineering for the Society of Motion Picture and Television Engineers (SMPTE). “First, redundant info is thrown away, then info is thrown away that will produce an infinitesimal difference in picture not visible to the human eye. Finally, even more info is thrown away that will only have a minimal effect on what is seen on the receiver.” In short, that HD channel you watch has been chopped up and squeezed as much as possible without having a huge impact on the final image…or so the provider hopes. “It’s a big juggling act,” insists Symes, who also believes that the large number of variables in the HD equation make it improbable that one regulated standard – for the way HD is sent from network to provider, for the way it is sent from provider to consumer, or even how HD televisions are built – will ever emerge.
X, Y, and Z – The HD Variables
Before you go switching your digital television service provider because your current one compresses its HD content, know this: that HD content has been altered even before it reached your service provider. In preparation for the final delivery stage, HD content (like a live basketball game, for example), might be routed across the country via satellite to the service provider. When both HD and SD content are sent to service providers on the same channel, the network sending the content will use statistical multiplexing to maximize image quality on a limited bandwidth. Statistical multiplexing allocates bits to the program that needs it most, but compromises are made for all the programs sent on any given channel. This includes the HD content. So even before the cable company or satellite company gets to send it to your set-top box, the HD image may have already been compressed, and/or had its bit rate shaved to save bandwidth.
We’re not all watching HDTV on the same television sets, through the same set-top box, from the same service providers, in the same room. HDTVs vary drastically, and many consumers are purchasing the wrong set for their specific viewing situation, i.e. too big a screen for too small a room. This will impair one’s viewing experience. Set-top boxes play their role as well. Set-top boxes have the responsibility to upscale or downscale content for viewing on a television. They are not all created equal; some do a good job, some do not.
The way that any given service provider elects to “shape” HD content varies, too. This is dependent on the bandwidth available to that service provider and the number of HD channels being provided. “They are all trying to do a good job,” says Symes, “and they are dealing with the bandwidth available, playing with a balance of number of channels and compression.” Subscribers want more channels, and more channels mean more advertising. It’s easy to see how the service provider would sacrifice some quality for those reasons.
Between watching re-runs of the The Jetsons and convincing his Insteon and Z-Wave controls to get along, Ben Hardy is immersed in the world of home automation, home control, and home networking.