Look at this. IB
The Clicker: Single Frequency Networks and OTA HDTV
Posted May 5, 2005, 4:20 PM ET by Peter Rojas
Related entries: Displays, HDTV, Home Entertainment
Every Thursday Stephen Speicher contributes The Clicker, a weekly column on television and technology:
One look at the eye-popping picture and you were hooked. The window-like effect dazzled you. You all but drooled over the rock-solid picture. There's no snow. There's no ghosting. The resolution is to die for. You saw the light and its name was High-Definition. There's only one problem: you can't seem to pull in a reliable OTA (over-the-air) signal.
Oh sure, cable is an alternative for many. However, over the years, you've been spoiled; you're hooked on your PVR. And, as a connoisseur of PVRs, you understand that the cable companies offer anemic light-weight versions. You demand more. You're not happy unless you've got your TiVo or your MCE (Microsoft Media Center Edition). You've even been known to utter the phrase, They'll have to rip my TiVo out of my cold dead hands. Worse yet, you meant it - LITERALLY.
Fear not - all hope is not lost. Despite previous attempts to scale your house with antenna in hand only to be denied a clean signal, there are two reasons to still have hope: a) Single-Frequency Networks and b) 5th generation receivers.
Since the beginning of broadcasting nearly all stations have followed the same steps when it comes to transmitting their signals: a) do your best to locate the tower in the middle b) build the tower high and c) add power. The problem is that, in some regions, this method just doesn't cut it. Natural terrain presents issues as do man-made buildings. Signals bounce around or, worse yet, fly right over your head. If State College, Pennsylvania's WPSX is a harbinger of things to come, the answer to your woes just might be Single-Frequency networks (SFNs).
More commonly discussed within the context of Europe's COFDM, SFNs are beginning to show signs that they might just work with America's 8-VSB modulation scheme also.
So what are SFNs?
Simply stated, single-frequency networks are when a broadcaster uses multiple transmitters to send the same signal over the same frequency. The idea is that in certain geographically-difficult areas broadcasters will have much better success if they can fill coverage voids by utilizing smaller, usually-less-powerful satellite towers in addition to their main tower.
It sounds like a simple idea, but, as always, the devil is in the details. Unlike the repeater towers that are sometimes used in western states (AKA translators), SFNs aren't simply receiving the signal, error-correcting, and retransmitting on a different frequency. That is an extremely inefficient use of the spectrum.
So what are they doing?
There are two main methods of creating a single-frequency network. The first is through the use of on channel boosters. On channel boosters quickly receive the OTA signal from the main tower and retransmit the signal on the same frequency. The problem is that there is no error correction. So, any errors in reception are simply retransmitted along with the slight echo caused by the booster itself. On channel boosters are also limited in their placement as they need reliable reception from the main tower.
So, when WPSX took the plunge into SFNs, it opted for option two, distributed transmission. Working with the New Jersey-based Merrill Weiss Group, WPSX has created one of nation's most technologically-advanced broadcasting systems.
With distributed transmission, the signal is delivered to each of its transmitters via fixed channels (land-based delivery). Then through the use of GPS-based reference clocks at each tower (for both time and frequency), the signals are synced so each can emit a perfect copy of both signal and symbol data. The result is a group of towers working in harmony.
So why isn't this done more often (at all)?
No matter how synchronized the output from the towers may be, there will always be some amount of multi-path to deal with, and until recently receiver hardware wasn't equipped to handle anything but very minor levels of multi-path. Multi-path? Yes, multi-path is the result of your receiver seeing the same signal more than once. This can be caused by a variety of things. For instance, in a crowded downtown area the signal can bounce off buildings many times before it hits your antenna. The problem occurs when some copies of the data arrive more quickly than others. This, in effect, can jam your receiver.
The same effect occurs within SFNs. Because waves from different towers have the potential to reach your antenna at different times, a receiver's ability to handle multi-path is paramount to its success.
With that said, LG has continued to make great strides in their ability to handle multi-path issues. Through the use of techniques such as adaptive equalizing they have even begun to turn a negative into a positive. First generation tuners had a multi-path window of -3 to +10 us. Fifth generation tuners have increased that window to -50 to +50. As always bigger numbers are way better.
So, if you're like me, unable to receive a signal despite near-heroic efforts. Have hope; there's technology out there that might just help one day.
But for now - we wait.