Thursday, January 20, 2011

DAB 20 Times Greener Than FM

There are many reasons (21 Whys FM is almost History) why governments around the world should command broadcasters to switch off FM as soon as possible and go for DMB, DAB and DAB+ instead (do note that this blog post is slightly technical). One major reason is costs, as much fewer transmitters are needed, but also since DMB/DAB/DAB+ transmitters consume a lot less electricity than FM transmitters.

Why is that?
Mr. Tore Øvensen, is the former Director of Technical Development and currently a senior advisor for NRK (Norwegian Broadcasting Corporation) and one of the leading experts on network planning in Norway and beyond. He has helped me understand this through thorough explanations.  

The planning of DMB, DAB and DAB+ networks took place at the RRC-06 conference in Geneva in 2006. It was then decided that the reception sensitivity for a digital receiver (incoming voltage on the receiver) should be ca. 8,5 µvolt. This minimum value is based on mobile reception (reception while moving which is quite challenging) in areas with very high signal reflections from surrounding terrain, also known as the worst case scenario in radio planning.

Reception of FM in such areas is virtually impossible as the analogue distribution cannot cope with both signals that reflect or bounce off mountains or big buildings and the signals coming directly from the transmitters. The signals are there, but they cause interference with each other and are totally destroyed due to what Øvensen calls destructive reflections. 

For digital radio, all these bouncing signals do however strengthen each other, creating better reception conditions. This is being called constructive reflections. A great example of this was shown in the county of  Sogn of Fjordane, where there are loads of mountains. The DAB signals bounced so far up the valley of Lærdal that areas where there had never been FM reception before suddenly had good DAB coverage. FM, on the other hand, is depending on many smaller transmitters (repeaters or gapfillers) in order to strengthen the signal up the same valley. The local Volkswagen dealer who drove a lot in the area even said that he would never again sell a car without a built-in DAB radio.

In areas without mountains or big buildings, mobile reception is not a problem, given that your FM receiver gets incoming voltage of ca. 50 µvolt (typical figure for a very good stereo FM receiver - many receivers require more).

DAB is, as a system, both much better at receiving much weaker signals and capable of exploiting positive signals refected from the terrain. That is because the signal is digital and that it contains powerful error protection of the transferred data. 

Modulation
The type of modulation used does also directly affect the reach of the signal. The incoming voltage needed is much lower for digital radio due to DABs COFDM coding and the extremelly strong bit-error protection it possesses. That means that the receiver can decode very weak signals as well as exploit reflections from the terrain. The FM reception is easily destroyed by omnidirectional propagation and needs a directional antenna in order to surpress reflecting signals. That is the reason why FM reception is especially difficult in areas with mountains or in cities with high or big buildings. 


What is the difference? 
Based on the explanation above, FM transmitters need between 5 and 6 times more power than DAB transmitters (and that is with a good FM receiver). But there is more, especially in topographically challenging countries such as Norway where big mountains and forests interrupt FM coverage while strengthen DAB coverage. To counter this a lot more transmitters are needed for FM than for DAB.

A common main transmitter for FM is 10kW, while an FM transmitter can use 70% of the amplifier effect and will therefore draw around 14kW from the power grid. A common main transmitter for DAB is 2kW, but a DAB transmitter can only use 25% of the amplifier effect, thus drawing 8kW from the power grid.   

But, as mentioned in an earlier blog post (2034 Transmitters Are 1484 Too Many) one FM transmitter can only transmit one radio station while one DAB transmitter can transmit 10-30 stations (depending on quality and usage of DAB or DAB+).

The power consumption per main transmitter is as follows: 
FM: 14kW/1 channel = 14kW per channel. 
DAB: 8kW/10 channels = 0.8kW per channel.  

One radio channel transmitted via FM does in other words need 17.5 times more power per transmitter than a radio channel transmitted DAB station. Or put in a different way, one DAB channel uses only 5,7% of what is the case for one FM channel, per transmitter.

But that's not all
Since many more transmitters are needed for FM than what is the case for DAB (2034 vs. 5-600), the difference in power consumption between FM and DAB is even higher. Not all FM transmitters are however 10kW. The smallest ones are only 20W, whereas others are 50, 100 or 500W. For DAB the smaller ones are 40, 125, 250 or 500W. I have not had the time to go through all 2,034 transmitters, but to cut 1,484 transmitters will certainly save some wattage. We can safely assume that to cover Norway (99.8% of the population) with digital radio is more than 20 times more environmentally friendly per channel than to do the same with FM, when looking at power consumption. 

And finally, the chipsets used for DMB, DAB and DAB+ have improved greatly when it comes to power consumption and a modern DAB chip (25-60mW) now consumes the same amount of power as what is the case for an FM chip (30-60mW). And work is being undertaken to further cut power consumption of DAB. A chipset manufacturer I talked to said that their short term goal is 20mW, which will then be substantially below FM.  

DAB. Much Greener Radio.



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