DAB Radio Experiment: A Non Starter?

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spotify95
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DAB Radio Experiment: A Non Starter?

Postby spotify95 » October 28th, 2019, 9:48 am

After the previous FM Radio reception test carried out a few years ago, I thought I'd revisit that experiment - with DAB digital radio as the test subject instead of FM. I have a portable pocket radio that does DAB (and DAB+), so should easily be able to pick up the digital stations - what could possibly go wrong?

The initial plan was to test DAB radio on both a Class 43 train and a Class 222 train, then compare them and see the results (like what happened on FM).

DAB digital radio gives more radio station choice than FM, and considering that local radio is now being morphed into various semi-national stations (Connect FM, a station that is local to my home area, is now Smooth East Midlands), it seemed to be a good solution (ignoring the concerns of low bit-rate streams). Today's initial test, however,. was another story.

The DAB test carried out did not match the results of FM - in fact, it wasn't even close to matching FM. There are various reasons why DAB reception is much more difficult to get on the move, compared to FM:

1) DAB (170 - 240MHz) is broadcast at a much higher frequency than FM is (88 - 108MHz). Therefore, as a result, indoor coverage is more severely affected, the radio signals don't travel as far at a comparable power rating, and the signal is more adversely affected by obstacles blocking the line of sight to the transmitter.
2) DAB signals are transmitted in vertical polarity only, compared to mixed polarity (horizontal and vertical) for the majority of FM stations. That means that your aerial needs to be pointing vertically, to get the best signal strength.
3) More transmitters are required to give the same, or similar, coverage as FM. Whilst there are more transmitters, they are often spaced too far apart to give a good continuous coverage on a portable radio. DAB is designed to work better with indoor radio sets and telescopic (or outdoor) aerials, not a portable with the headphone wire acting as an aerial.

The quick tests performed today mirrored the points above. The signal was much weaker on DAB, compared to FM, with the DAB signals dropping out in places that FM was fine. When sitting down in a train, it is difficult to get the aerial to be purely vertical in polarity - thus compromising the received signal.
The railway lines are often situated in locations that are not radio/telecommunications friendly - for example, tunnels, dips etc. Generally, there is a high possibility of obstructions in the pathway between the receiver and the transmitter. This does not bode well for good reception.

The Northamptonshire digital multiplex fared the best, from Wellingborough up to near Market Harborough. The local transmitters are situated in good locations and this helped to provide a decent signal. It must be admitted that the signal wasn't perfect, but it was by far the most reliable within its target coverage area.
The BBC multiplex was not that far behind - being a national multiplex, funded by the BBC, there are more transmitters, and therefore, a better signal, compared to other national or semi-national stations.
D1 Digital, the national commercial multiplex, fared much worse. The signal strength was much lower, due to fewer transmitters being used. Reception was often weak or unusable.
The Leicestershire multiplex comes in right at the bottom. Despite being a county wide multiplex, I was struggling to receive it even in Leicestershire, with the multiplex only being received properly from the outskirts of Leicester and in Leicester itself. Leicester has an in fill relay mast for DAB, with the main Leicestershire signals coming from Copt Oak. However, a quote from MB21:
DAB is right down on the bottom of the tower, very low, and probably explains why DAB coverage is quite so poor, if this is standard practice.

Unsurprisingly, DAB coverage from Copt Oak was very poor as a result. It is not standard practice to put the transmitting aerials in such a compromised location on the tower, but even so, this explains the poor coverage.

Sound Digital, the semi-national DAB commercial multiplex, was not tested - although given how weak the signal was even at my home location (it has the fewest amount of transmitters by far), it would have likely been almost impossible to receive.

And all of this was carried out on a BR Class 43 train - i.e. the train that actually worked very well on FM! Trying this out on a Class 222 train would have likely resulted in silence pretty much all the way along the journey.

I may try repeating this with other headphones, since the headphone wire act as the aerial - or alternatively, may try out another DAB receiver (I have two portables that I can use). Though at this rate, the entire DAB radio test will be cancelled due to the lack of coverage.

Conclusion: DAB is not likely to replace FM any time soon, due to the lack of coverage. It is fine in most household settings, assuming that the aerial is mounted vertically (or an external aerial is used). For other settings, such as in vehicles, the quality of the aerial and receiver will likely determine whether the signal is likely to be usable. DAB radio on the go is not a good solution, due to the lack of coverage - and given how cheap some of today's smartphones are becoming (as well as how cheap 4G mobile data plans are - there are some good value data plans to be had), it is likely better to use Spotify or Tunein Radio to stream your favorite music from a smartphone using 3G or 4G.

(This review does not take into consideration the quality of the DAB streams - it just focuses on coverage.)


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