Two transmitters, one frequency, better reception

"The first results are positive. With this single frequency network, we are less dependent on propagation conditions" says Deutsche Welle engineer Norbert Schall. His team of specialists is responsible for choosing the right combination of transmitter power, antenna direction and frequency for all of Deutsche Welle's transmitters so that listeners get optimum analogue reception across the globe and the best possible DRM reception in Europe.

Deutsche Welle engineers, who are working on the implementation of DRM, are concentrating on Europe. Ideally, European listeners should not be able to notice all the tests that are going on behind the scenes. The goal is perfect listening comfort without the hiss, interference or fading with which listeners to short-wave stations are all too familiar. It is a goal, though, that is not easy to accomplish. Single frequency networks are part of the solution.

With analogue transmissions, filling in those annoying 'gaps' in reception with a signal from a second transmitter is barely practicable. It is another story, though, with digital technology.

Reliable reception

The main objective is to cover a defined target area with consistently stable signal strength, which is essential if reception is to be free of interruption. If an analogue signal is weak, the listener hears the programme being swamped in a sea of noise; if the signal is digital, and falls below a certain strength, then it simply cuts out altogether. In other words, the broadcast is interrupted.

Deutsche Welle's single frequency network. The transmitters are at Sines, Portugal and Wertachtal, Germany. The point of synchronisation is Bonn, Germany (marked with a cross) Improved reception can be expected within the red circle-like diagram.

But the number of interruptions can be decreased significantly by transmitting a synchronised digital signal from two locations on the same frequency. The listener's DRM receiver stays tuned to one frequency, and - ideally - he can enjoy the programme being broadcast without any interruptions. As far as the receiver is concerned, it makes no difference where the signal is coming from at any given moment.

Synchronised to the millisecond

Ideally, at least one of the two signals is always strong enough for reception to work. If one signal fails because of screening, attenuation or poor propagation conditions, then the other one fills in the gap. Short-wave listening with digital technology becomes a real pleasure.

The engineers have to synchronise the two signals to the millisecond. As the two transmitters, Sines and Wertachtal, are at different distances from the target area, their respective signals take different times to reach to their destination. Synchronisation is achieved by delaying the Wertachtal signal by 4 ms. This ensures optimal synchronous reception at broadcasting centre in Bonn. However, because DRM receivers have a run-time tolerance of 5.3 ms, improvements to reception are possible throughout the entire target area.

Synchronised for the IFA in Berlin

The point of synchronisation can be set in exactly the right place with great accuracy. Norbert Schall and his team will be adjusting it in time for the Internationale Funkaustellung (IFA), Germany's radio and television exhibition, in Berlin.

Normally when trials are carried out the point of syncronisation is located in Bonn, which is where the DRM team work. However, it will be moved to Berlin for the IFA. With digital technology, this is easily done.

Test transmissions from Sines and Wertachtal

Deutsche Welle is testing its single frequency network at the following times:

Sines and Wertachtal 7265 kHz Mondays, Wednesdays and Fridays 0705 - 0855 UTC

Sines (on its own) 7265 kHz Tuesdays and Thursdays 0705 - 0855 UTC

Wertachtal (on its own) 7265 kHz Saturdays and Sundays 0705 - 0855 UTC

This means you can hear the single frequency network on Mondays, Wednesdays and Fridays and the individual sites on their own on Tuesdays, Thursdays, Saturdays and Sundays for comparison.

We welcome reception reports (with or without logfiles) Please send them to drm@dw-world.de