Pulling Digital Television Out of Thin Air

December 27, 2006 | Leisure Bytes | Author

CN Tower
Photo By Tim Brown

A fundamental requirement of any home multimedia environment should be the capability to access off-air high definition television (HDTV) signals.

Experts remain convinced that HD program sources are overly compressed regardless of whether they’re delivered through digital cable or satellite. With this in mind, many viewers have noted significant image quality improvements when viewing the same program source directly off-air. In addition to better image quality, off-air sources are free once you’ve covered the meagre cost for installation of the antenna!

The concept of high definition television isn’t new. In fact, as far back as the 1970s the industry was trying to carve out high definition standards. It’s a good thing nothing was ratified because the ensuing quarter century of innovation in digital encoding and modulation has taken us a long way in some key areas.

Today’s HDTV is based on standards for digital television. People often incorrectly use HDTV and digital television interchangeably. Digital television as set out by the Advanced Television Systems Committee (ATSC) is a delivery mechanism for HDTV, but the payload for digital television isn’t limited to HDTV.

As with analogue TV, the bandwidth of a digital channel is 6 MHz. The beauty of using the 6 MHz digitally is broadcasters can ‘slice up’ the bandwidth as they please. For example, a digital TV station can create a multiplex of four or five standard definition streams (or sub-channels) on a weekday morning. When prime time arrives in the evening, the station can reconfigure its multiplex into a single HDTV stream.

We don’t necessarily have digital television today because of consumer demand. We have it because it frees up radio frequency spectrum currently hogged by inherently inefficient NTSC (National Television System Committee) television channels.

The Advanced Television Systems Committee (ATSC) sets standards for digital television transmission in North America. The modulation scheme selected by the ATSC is known as 8-VSB (8-level vestigial sideband modulation).

There were many engineers who wished the ATSC had adopted COFDM (Coded Orthogonal Frequency Division Multiplexing) instead. CODFM held the promise of greater multipath immunity — something urbanites would definitely benefit from. CODFM has been demonstrated convincingly as a viable modulation scheme in the United Kingdom and parts of Europe where DAB (Digital Audio Broadcast) listeners enjoy very reliable service both in fixed and mobile environments. While it is argued CODFM holds the advantage in multipath immunity, 8-VSB boasts greater coverage per unit power. In fact, 8-VSB claims a ten fold improvement in link budget efficiency over NTSC.

Regardless of the favoured modulation scheme, it’s safe to say that most, if not all, digital transmission technologies are more spectrally efficient than NTSC. Spectrum is an expensive commodity and governments have learned that auctioning portions of this precious commodity can yield huge dividends. In addition to new corporate applications for radio frequencies, the US military has an insatiable appetite for spectrum. The VHF frequencies (NTSC channels 2-13) have excellent propagation characteristics and are likely being eyed by the US military. As of this writing few, if any, North American VHF analogue TV stations will hang on to their VHF frequency once the transition to digital is complete.

So where are those VHF TV channels moving to when they go digital? Up to join their cousins in the UHF band…

Laws of physics dictate that both VHF and UHF signals propagate in a line-of-site fashion. In addition, however, both VHF and UHF signals exhibit a tendency to bend slightly beyond the horizon. How much ‘bend’ is determined by the signal’s wavelength (lambda). With VHF’s longer wavelength, that bending effect can result in a significant coverage advantage over UHF. By extension, VHF signals also tend to spread beyond hilly terrain and other obstructions where UHF signals get blocked.

While on the surface one would assume UHF’s inherent coverage constraints would be a disadvantage for digital television, there are some benefits. The greater propagation predictability inherent in UHF means ATSC channels can be reused with greater regularity, much in the same way PCS mobile telephone frequencies are reused. Also, the 8-VSB modulation scheme allows co-channel and adjacent channels to be located geographically closer together. The 8-VSB modulation scheme lends itself to a more favorable C/(N+I), or put another way, carrier to noise plus interference ratio. The denser the reuse pattern for a given channel assignment, the greater the spectral efficiency.

While high quality, technically superior HDTV programming will likely come from other sources, even in the relatively near future, it’s still worthwhile to install an antenna now and enjoy the superb image quality coming from your local HDTV stations.