Let us look at two different scenarios we know
In the Belgian case – the regulator decides how many transmitters can operate and which “Effective Radiated Power (ERP)” can be used for the transmitter sites. The radiostation will break the law when the radiostation uses an ERP power that does not conform to the license (used power too high).
In the Netherlands – the regulator will restrict the transmission area (instead of the number of transmitters and the ERP power). Here the radiostation will break the law when broadcasting outside the imposed transmission area. At a distance of 10km from the licensed service area, the field strength can be no higher than 50dBµV/m.
Both are opposite regulations!
Any information about DAB(+) regulations in other countries is welcome. We can share this information on localdab.org with readers worldwide. Everybody can learn about differences in law and what would be the best way to approach this.
For example, the following rules apply in the Netherlands, where the license defines the service area called the allotment area:
- At a distance of 10km from the licensed allotment, there will be a field strength no higher than 50dBµV/m
- At a distance of 30km from the licensed allotment, a field strength will be no higher than 40dBµV/m
Broadcasters and DAB(+) operators in the Netherlands will consider:
- custom antenna patterns
- the best-suited transmitter locations
- transmitter power levels
It is all about optimizing the service area under the imposed service area license. And no higher field strength is measured outside the fringe area of the allocated area. Additional SFN transmitter sites (gap fillers) are negotiable with the regulator or media government.
The facts in the field
The best solution for local radios in one region will not necessarily be the solution for local radios in another area. Sometimes a 5000 Watt transmitter with two 250 Watt gap transmitters can be the solution. In other situations, the best choice may be the combination of four 1000 Watt transmitters and a 250 Watt transmitter. Knowledge and practical experience will play an important role.
We can already share some theoretical facts with you:
1. FM power is rather simple. It is only a frequency change as modulation. The amplitude is constant. Which results in continuous transmitter power.
2. It is not the case with the DAB(+) transmitter. Although the transmitter will use Phase Modulation, comparable to FM, the amplitude can not change. However, we need to consider that we do not modulate one carrier but multiple carriers, 1536 in total. The 1536 carriers in the frequency domain result in the sum of all their amplitude in the time domain.
The carrier amplitudes can be positive (45° and 135°) or negative (225° and 315°). The addition of all the carriers’ amplitudes will be different at each moment. The DAB(+) transmitter power is variating strong and not comparable with the continuous FM power.
We will see that this strong variating amplitude can overdrive the DAB(+) transmitter power amplifier. The result will be harmonic distortion and spectral energy outside the DAB(+) channel. A mandatory filter between the transmitter and the antenna is needed.
Using 100% of its power all the time (as with FM) is not possible. Therefore a 1000 Watt DAB(+) transmitter will be installed with only 25% of its total power (average use). A 1000 Watt DAB(+) transmitter will use an average transmit power of only 250 Watts!
3. Power/ Bandwith ratio – If you start to modulate a carrier (sine wave), the spectrum of generated frequencies around the carrier frequency will become larger due to the modulation. The modulated information is not the carrier but the whole spectrum around the carrier. The more information per unit of time you want to transfer, the larger the bandwidth at both sides of the carrier.
The information in FM has a bandwidth of 15kHz. In high-quality Wideband FM, we tried to include up to the 4th Bessel harmonic, which means 4 x 15kHz is 60kHz. The reason why the spectrum is 75kHz on each side of the carrier. The total bandwidth for an FM channel is 150kHz. Of course, this is if radiostations are operating within the imposed legal limits of a 75 kHz swing. In practice, we see FM radiostations flirting with these limits pushing them to the limit and even above it. The well-known reason the famous war of loudness.
Assume a ratio of 100 Watts in FM with a maximum frequency swing of 75 kHz has a spectrum bandwidth of 150 kHz. If we divide 100 Watts (radiated power) by 150 000 Hz (bandwidth), we get a “power per Hz” of 0.000667 W/Hz or 0.667 mW/Hz.
Let us now consider a DAB(+) transmitter of 1000 Watts with a bandwidth of 1537 kHz ( 1536 carriers@1kHz distance). Dividing 1000 watts (radiated power) over 1537 000 Hz (bandwidth) and get a “power per Hz” of 0.0006506 W/Hz or 0.6506 mW/Hz.
A DAB(+) transmitter needs more than 1000 Watts to attain the same power/Hz as a 100 Watt FM transmitter. The spectrum generated by the modulation and the 1536 carriers is much wider than in FM.
The DAB(+) transmit energy is smeared over a much wider frequency spectrum.
Of course, comparing FM and DAB(+) is valid under similar, equal conditions. Consequently, you cannot simply compare the DAB+ reception range of the future one-to-one with the current range of the local FM radio transmitter.
4. In many countries, the FM band is highly congested. FM radiostations often will interfere with each other. The reason is the protection of 100 kHz between local transmitters even when the distance is much less than 60 km. These interferences are less likely to occur in the DAB(+) band, which will favor DAB(+) reception quality.
5. DAB(+) is insensitive to phase distortion, while FM is very sensitive to phase distortion. It is an advantage for DAB(+).
6. DAB(+) uses a higher frequency band (VHF Band III) than FM (VHF Band II). However, higher frequencies experience higher path attenuation. It could be a little advantage for FM.
7. The sensitivity of the receiver plays an important role. The FM receiver will have to capture a bandwidth of 150kHz, while the DAB(+) receiver has to catch a bandwidth of 1 537 kHz. It is about the tenfold of the FM receiver. It means that also the captured noise will be tenfold higher. The FM receiver will have a noise floor of –122,17 dBm and DAB(+) of -112,06 dBm (approx. 10 dB difference in noise floor).
8. Also important to remember that if a conformity certificate is requested, it will become more difficult to obtain it as the radiated power increases and the height of the mast remains low. Conformity is related to protecting humans against too strong electromagnetic fields in their living environment. So, high power needs to be on tall towers. Far away from ground levels where people are present.
Fortunately, a “Single Frequency Network (SFN)” is ideally suited to DAB(+). Instead of using 1 transmitter to cover a particular region, it is better with DAB(+) to use several transmitters at the same frequency. Of course, the location of the transmitter sites needs to be thought through very carefully.