dk8cb
04-02-2006, 18:39
Hi,
recently, I notice a growing number of DRM tests on the 11m broadcast band.
Currently, we are in a minimum of the 11 year sunspot cycle and long-distance propagation is virtually non-existent on these frequencies. I very much doubt the significance of results obtained under these conditions and I'm afraid that results obtained now will be used in deciding whether to use this band for local DRM broadcasting or not.
What will happen when sunspot numbers are high again and such frequencies are used all over for local broadcasting?
I still remember the day, many years ago during a sunspot maximum, when I once sat in the lab and had a test receiver on, because we were testing a circuit, the kind of which I have since forgotten. A short piece of wire, perhaps half a metre long, was connected to the input of the test receiver, when I noticed a really strong carrier somewhere around 26 MHz. At first, we were convinced that because of the strength of the signal, the source of this signal would be inside the circuit we were testing or at least somewhere else in the lab. However, the carrier stayed on after we had removed the power to our circuit. Suddenly, there was AM modulation on the signal and it turned out to be Radio RSA, the Voice of Southafrica transmitting on this frequency. Of course, they were certainly using a highly directional antenna and a considerable power but the signal was also really strong, i.e. a in the tens of millivolts!
Now imagine propagation conditions like these and DRM stations all over broadcasting local programmes. What a chaos will result!
I'm writing this because it is my impression that in the evolution of DRM, other wrong decisions have already been made due to inadequate testing and because I want to prevent decision-makers from drawing false conclusions from tests that have been performed under idealised conditions. Currently available DRM modes also have not been tested thoroughly enough before putting DRM into regular service. Take e.g. mode B on 3995 kHz during nighttime hours which just doesn't work, even at ridiculously low bitrate. Also, other modes available do not provide any rescue since the quality that can be obtained is even worse. This is just an example, I could name more such examples and in my view, they are all a result of inadequate testing under real world conditions.
So don't draw any premature conclusions if it works now. Also think about whether it will work then.
Roland
recently, I notice a growing number of DRM tests on the 11m broadcast band.
Currently, we are in a minimum of the 11 year sunspot cycle and long-distance propagation is virtually non-existent on these frequencies. I very much doubt the significance of results obtained under these conditions and I'm afraid that results obtained now will be used in deciding whether to use this band for local DRM broadcasting or not.
What will happen when sunspot numbers are high again and such frequencies are used all over for local broadcasting?
I still remember the day, many years ago during a sunspot maximum, when I once sat in the lab and had a test receiver on, because we were testing a circuit, the kind of which I have since forgotten. A short piece of wire, perhaps half a metre long, was connected to the input of the test receiver, when I noticed a really strong carrier somewhere around 26 MHz. At first, we were convinced that because of the strength of the signal, the source of this signal would be inside the circuit we were testing or at least somewhere else in the lab. However, the carrier stayed on after we had removed the power to our circuit. Suddenly, there was AM modulation on the signal and it turned out to be Radio RSA, the Voice of Southafrica transmitting on this frequency. Of course, they were certainly using a highly directional antenna and a considerable power but the signal was also really strong, i.e. a in the tens of millivolts!
Now imagine propagation conditions like these and DRM stations all over broadcasting local programmes. What a chaos will result!
I'm writing this because it is my impression that in the evolution of DRM, other wrong decisions have already been made due to inadequate testing and because I want to prevent decision-makers from drawing false conclusions from tests that have been performed under idealised conditions. Currently available DRM modes also have not been tested thoroughly enough before putting DRM into regular service. Take e.g. mode B on 3995 kHz during nighttime hours which just doesn't work, even at ridiculously low bitrate. Also, other modes available do not provide any rescue since the quality that can be obtained is even worse. This is just an example, I could name more such examples and in my view, they are all a result of inadequate testing under real world conditions.
So don't draw any premature conclusions if it works now. Also think about whether it will work then.
Roland