Digital on MW hits night-time difficulties

By James Welsh

The BBC would have to replan its entire transmission network to use digital radio technology DRM for domestic broadcasting on medium wave, it has been revealed.

In a post on the BBC Radio Labs blog, BBC Distribution's Tom Everest said that interference difficulties were encountered at night during a test of DRM on BBC Radio Devon's 855kHz medium-wave service in which a panel of listeners were given DRM radios and asked for feedback.

"For the most part, the panel's reaction to DRM was positive," posted Everest. "They enjoyed the generally improved audio quality and they took easily to their new radios. And for most of them, the reception was good; one or two glitches but normally ok."

A summary report into the trial found that panellists "rated the audio quality of DRM as comparable to FM", and that Radio Devon's medium-wave signal covered a far larger area during the day, but that interference problems caused disruption to night-time listening.

"The problem will be familiar to many listeners to medium-wave: at night, changes in the atmosphere mean that signals from distant transmitters reach our shores more easily. On medium-wave, you might hear this as cross-talk: a foreign voice underneath what you're trying to listen to, or the occasional 'splat' of another transmission. The issue we came across with DRM is that this interference causes the radio to stop decoding the signal: sometimes only momentarily, sometimes for a while longer. So rather than listening through the interference, it's like all digital systems: you either get it (and so get it at a consistently high quality) or you don't get it at all."

Everest said that the BBC's transmission network would need to be replanned "either by moving the frequencies around so that we use ones that aren't quite so damaged by interference, or by building higher power transmitters, or by simply building more of them".

One possibility offered by DRM in the event of a replan is the construction of robust single frequency networks, where the same content may be broadcast on the same frequency from different transmitters without causing interference.