DRM – The Good, the Bad and the Ugly
One of the most common complaints of HF listeners is that a station's fidelity is about as poor as a standard AM station. This issue attempted to be addresssed with the development of DRM, which stands for Digital Radio Mondiale. This name represents 2 things;
- A digital broadcasting standard that was designed to address the limited fidelity issue;
- And a consortium of countries that market the DRM standard.
It's beyond the scope of this introductory article to go into the technical standards and engine used in a DRM broadcast, and the reader is encouraged to check out the links referenced at the bottom. However, let's briefly examine some of the highlights
and issues that a DRM broadcast will entail;
When a DRM signal is received cleanly and without undue fading or interference, the audio is much more FM-like. Distortion is greatly reduced and music comes out sounding much clearer. It's much less fatiguing as well.
The software for decoding a DRM signal is widely available, and is used with a soundcard input, so no external interfaces are required.
A DRM signal is digital. Such a signal tends to create sidebands which would wipe out any weaker signal that is nearby.
For example, if you have a strong analog station on 6180khz, and a weaker one 5 khz away, you still might have a chance of hearing the weaker station using techniques such as Exalted Carrier Single Signal (ECSS) or similar techniques (to be described
in another article). If however the station on 6180 is using DRM, the chances of hearing the weaker station 5 khz away are vanishingly small, since the digital sidebands would be detected as 'noise' by most HF receivers.
In order to receive a DRM signal, there are only 3 ways currently available;
- Use a portable radio (usually marketed from Europe or Asia where DRM is much more widely heard) that has the ability to decode it
- Use a radio that has an output (usually 12khz) already built in (such as the Ten Tec RX320D, or WinRadios)
- Modify your radio to extract a signal from the detector
The modification to your radio is similar in principle to a discriminator tap from a scanner. The detector signal is fed from the receiver into your soundcard input, where the DRM software can decode it. It is sometimes necessary to add some manual attenuation (in the form of a small 10k pot in line between the radio and PC) as the software is very sensitive to being overdriven, and some soundcard software cannot reduce the level low enough. The outboard attenuation is also an attractive alternative if you are using the soundcard for other purposes, such as digital decoding.
Why are there few North American DRM Broadcasts?
HF as a broadcasting medium in North America is non-existant. The market has been saturated with FM and AM stations from the inception of broadcasting as a service. As a result, few average Americans even know that HF is a broadcasting medium, much
less what DRM is supposed to be used for.
- Receiver Modifications
Software Defined Radios Known to Support DRM