Is a mask spectrum filter really necessary?
You can not operate without a filter! The reason is the unpredictable power of a DAB+ signal which is delivered by the 1.536 carriers and the associated 1.536 phase modulators. Some techniques are already used in the the encoding proces to spread the power of the transmitted bits, but still it remains a problem.
The bits (in fact phase modulation symbols) can, at a given time, all work against each other. Assume half of them use identical phase and the other half use also an identical phase, but which is opposite phase. The overal result wil be a low power signal.
However, imagine a major number of the modulators using all the same phase. Now the signal will be huge. In most cases the signal wil have an average peak level but at some moments in time a huge peak-power can arise.
We say the DAB+ signal has a high “Peak to Average Power Ratio (PAPR)”.
Even if the transmitter power amplifier is used at only 25% of the maximum power, the risk is still existing that the signal peaks go out of the linear amplification range of the transmitter. Unfortunately, if this happens the amplifier wil work in a non-linear zone or clip due to the power limit.
The result in both cases will be harmonic distortion at the output of the amplifier. The spectrum mask filter is to filter out these harmonics in order not to disturb adjacent DAB(+) channels. The need for better filtering will become important as more and more DAB+ transmitters will go on air.
DAB+ radiostations should be aware that differences exist in DAB(+) transmitters. Usually low-budget DAB(+) transmitters wil not use any special processing techniques to reduce the PAPR. Their output will contain a great percentage of harmonics. Due to the high percentage of harmonics a more selective (and expensive) filter will be required to protect the adjacent channels. The harmonic components also represent a part of the energy produced by the transmitter. This loss of energy will simply warm up the filter and is not used to transmit the DAB-signal itself.
Producing heath is not really the core business of a radio station, is it?
More expensive DAB+ transmitters will use several techniques to reduce the PAPR. Let’s mention one important technique “Digital PreDistortion (DPD)”. These transmitters have an input connector to create an outside feedback loop by feeding the output signal after the filter back to the transmitter.
Some very fast and expensive electronics are necessary for DPD. The technique has to act immediately on the power peaks (reason for expensiveness). Although these transmitters have a high efficiency which make them more energy conserving than others.
Remember that today (and probably also in the future), energy is (will be) a high cost for a radio station. A good rule is to make the correct business plan. Most of the time a higher CAPEX (Capital Expenditure) can reduce the Total Cost of Ownership (TCO) by reducing the monthly OPEX (Operational Expenditure) over a long operational period!