- Guy Bouchard Senior Manager for New Broadcast Technologies
The purpose of this article is to demystify Digital Television (DTV) reception considerations. As it is intended for a general audience, it is written in non-technical language and uses analogies familiar to everyone. Mind you, there will be a bit of arithmetic to illustrate certain phenomena, but nothing that requires you to go back to school.
Since July 2012, Canadians in several major cities have had access to an over-the-air digital television service. This transition has brought about a few changes in frequency and service contours. Add to that the fact that the virtual channel system (digital television) has replaced the physical channel system (analogue television), which helps feed into the public’s confusion about the service.
Same Old Physics, New Challenges
Digital television uses the same electromagnetic waves as analogue radio and television. These waves obey certain unchanging laws:
Electromagnetic waves weaken according to the distance between the transmitter and the receiver.
Attenuation phenomenon: An ambitious graffiti artist wants to cover the Earth in spray paint. What surface will he cover at a distance of X from the Earth? We could use the model of a can of spray paint as an isotropic paint source (a fancy way of saying that the device sprays equally in all directions). As such, we are talking about a sphere, the surface of a sphere at a distance of R being 4∏R2.
But the spray paint nozzle covers only a part of the sphere. Therefore, we have to add a multiplying factor proportional to the ratio between the surface covered by the nozzle and the total surface of the sphere. This ratio is called the broadcast gain.
- The same phenomenon applies to receiver antennas, with the same terminology.
- The intensity of an electromagnetic wave at a distance R from the transmission antenna is proportional to a factor: Gain/4∏R2.
Fact number 1: Electromagnetic waves weaken in inverse proportion to the square of the distance between the transmitter and the receiver.
On the UHF band, coverage is limited to a distance that corresponds to the sight line distance:
Electromagnetic waves propagate in a straight line. Imagine a baseball pitcher throwing a ball to the horizon at an infinite speed – the ball, instead of following the curvature of the Earth, will travel in a straight line. Eventually, the ball will leave the Earth and end up somewhere in ET’s back yard. The same phenomenon applies to electromagnetic waves. The straight-line trajectory limits the propagation to a visible horizon, called the radio horizon. The radio horizon varies according to the height of the transmission and reception points. For medium-height transmission towers, the radio horizon is limited to a distance between 30 km and 70 km.
Trying to get past the radio horizon by adding power is an investment with diminishing returns. It is a bit like trying to increase an airplane’s flight radius by increasing the capacity of the fuel tank. After a certain point, the aircraft becomes too heavy to take off, which hardly improves its range.
Fact number 2: The range of a UHF transmission station is limited by the terrain and the height of the transmission and reception sites. As a rule, the limit is around 70 km.
What differentiates outdoor and indoor reception?
- Passing through the walls of the house
- The indoor environment
Passing Through the Walls
Waves do not pass through all walls. Since the early ’70s, the National Building Code of Canada allows for the use of a metallic vapour barrier, which is completely impervious to electromagnetic waves, leaving only a few windows to let radio waves through. Interior reception then becomes a matter of chance, even when there is a short distance between the transmission site and us.
Basically, interior coverage is never guaranteed.
The Advanced Television Systems Committee (ATSC)[i] conducted a study on the margins required to pass through building walls on the UHF band. The study concluded that a margin of 13 dB (20 times the transmission power) was required. Since we are dealing with a statistical population with normal distribution, we need an average plus two standard deviations to reach 95% of the population. The standard deviation of the statistical population is 9 dB. As a result, we need a thousand times the transmission power, or (13 + 2 x 9) = 31 dB (times a thousand) to reach 95% of the population inside buildings.
Fact number 3: Interior reception cannot be guaranteed in all buildings, whatever the distance between the house and the transmission station.
Aggravating Indoor Factor: Micro-reflections
Since radio waves cannot penetrate buildings except through the windows, once they enter a room, the waves are reflected around the room hundreds of times, rather than travelling around the inside walls and escaping to the outside. As a result, there are thousands of micro-reflections within the room. Given that the waves travel at the speed of light (approximately 1 billion km/h) and that the distances travelled within the room amount to only a few meters, these micro-reflections are all very close together in time, which means that the receiver’s job of rejecting echoes is practically impossible.
Fact number 4: The many reflections of the signal inside the room diminish your digital TV receiver’s ability to receive the signal.
Aggravating Factor: Noise Due to Human Activity
Imagine that you are trying to follow a conversation right in the middle of a room packed with over 100 people talking at the same time. Your ability to decode what the person you are talking to is trying to say is diminished by all the other people shouting in your ears.
The same phenomenon is also true in digital communications. The sum total of all the other harmful signals is defined as noise.
Communications noise is usually caused by electron collisions within components. It is limited to a value defined as thermal noise (approximately 106 dBm for every 6 MHz).
In a house like the ones we live in today, the noise is completely buried by all kinds of interference generated by the devices around us: computers, DVDs, VCRs, electronic thermostats, smoke detectors, transmitters and receivers of all kinds and, especially, devices that use switching power supplies. These devices radiate mainly on the UHF band, which is also the band that carries digital television waves.
Fact number 5: The deterioration of the noise floor caused by human activity inevitably interferes with your receiver’s ability to receive digital television.
Implementing Theories for Maximising the Chances of Receiving DTV
Exterior reception will be by far the most reliable, since we have a certain amount of control over a series of factors that weaken the signal:
- Distance: Nothing can be done apart from moving closer to the transmission station. However, we can increase reception reliability by using a high-gain antenna.
- For the UHF band, there is no need to have a gigantic antenna to receive the signal – some very simple antennas are extremely effective.
An antenna of this size could be replaced by the examples below:
- Radio horizon: The radio horizon is a function of the height of the transmission and reception sites. Therefore, we can mitigate its impact by situating the receiver antenna as high as possible.
Exterior reception with a low-performance antenna will still be much more effective than interior reception due to the simple fact that the signal does not experience loss from penetrating walls.
If interior reception is the only option, here are a few rules to abide by:
- To reduce micro-reflections:
- Put the antenna in the window;
- Use a directional antenna (left antenna).
- To reduce noise caused by human activity:
- Move the antenna away from devices that may cause electric noise, such as:
- Televisions, receivers, DVDs, telephones, printers, computers, etc.
- Plug the television and decoder (if there is one) into the same electrical outlet.
- Move the antenna away from devices that may cause electric noise, such as:
Typical Antennas for Interior Reception
Digital TV reception is possible if we follow the rules set out above. Exterior reception, however rudimentary, will deliver superior results, especially in terms of reliability. Interior reception is possible, mainly if you live in close proximity to the transmission station, but it is never guaranteed.