Audio phase inversions can cause big problems to your FM broadcast signal!

Audio phase inversions are a relatively common problem in our experience.  In fact only today we just noticed a station near us that has one. We strongly suggest all station managers do some checks to ensure they do not have an Audio phase inversion. They lead to two pretty major impacts on FM.

Impacts of Audio phase inversions

• Stereo Listeners – not all ears are sensitive to it, but those that are hear a really weird stereo – a bit like ultra-wide stereo, but not in a good way. It can also make the stereo signal quieter.
• Mono Listeners – the mono elements in your programming (eg vocals on songs, voiceovers on adverts) can get almost or completely cancelled out.  It sounds like you are playing the karaoke version of songs, and commercial music beds with no voiceover message!

To explain these effects You need to go back to wave theory that you did in Physics at secondary school – constructive and destructive interference etc.  The addition of FM broadcast reception with its constant switching and blending between stereo and mono as reception conditions change, can make this a confusing problem to pin-down.    But let’s not go there – just think about making some checks and getting it fixed if you have the problem!

Causes of Audio phase inversions

The most common cause is a mis-wired audio connector, although some equipment has phase invert buttons that can be set incorrectly.  So a unbalanced RCA type connector may have the audio signal and earth wires reversed, or a balanced XLR connector may have its two signal wires swapped.

Fixing Audio phase inversions

The proper way to fix the problem is to inspect every audio connector in the chain to your FM transmitter and fix the offending beast.  If they are all correct, then start checking eack box in the chain.  The lazy way is to take any audio connector in the chain at random and swap the wires over. (not recommended!)

So we suggest you find yourself a FM radio that you can manually switch between stereo and manual and check you do not have this problem – make it your new years resolution!  Alternatively if you do not have access to such a radio, drive to a suitable area on the fringe of your reception area where you can drive between stereo and mono reception locations.

Many of you are probably familiar with the MCA maps that Ofcom publish for all Commercial and Community radio stations. These maps describe the formal administrative/legally licensed area of those stations.

Strangely though, Ofcom is unwilling or unable to publish MCA maps for the other 58% of FM transmitters that it licences – namely the BBC sites.  In a response to a Freedom of Information request they suggested contacting the programme provider (eg the BBC) to discover the licensed area of the transmitters that carry their services.

We have yet to find the right person in the BBC to get this information.

In the meantime we’ve decided to produce our own coverage maps for EVERY licensed FM transmitter in the UK.  These are designed to be as “MCA-like” as possible.  However they should be regarded as indicative only and not as a substitute for proper Ofcom-generated licensed area maps.  In particular note that Ofcom sometimes manually edits their maps to reflect target editorial areas, whilst our maps indicate the full extent of actual (radio) coverage.

You can browse them here.

Introduction

Ofcom have invited  stations to apply for improvements to existing coverage and coverage extensions for community radio services.  Surprisingly they have not charged an application fee. This makes it “worth a punt” for stations to submit speculative applications.

Even if you are not considering making an application, do not ignore this invitation. Think if there are any CR stations within say 50 miles of you on the same or adjacent frequency to you. If they get a power increase and you don’t, there is a risk your listenable area may reduce!

In their invitation, Ofcom has defined two types of coverage enhancement, with two different sections in their application form:

1. Coverage Improvement (closing date 23:59 hrs on 31 July 2018) – intended to mitigate interference, close coverage holes etc without changing the description of licensed area
2. Coverage Extension (closing date 23:59 hrs on 18 September 2018) – an increase to coverage area to encompass a neighbouring town/area

Of course this distinction is a bit of a fudge. Most technical changes to improve coverage will result in a coverage extension.  The distinction, we think,  is when a new town or area is covered, rather than just an incremantal extension to your coverage all round.

Methods of Improving coverage

Someone is bound to argue this, but roughly, on average in most situations, in order of effectiveness the changes worth considering are:

1. Site move.  Is your site in the best location possible?  Maybe that ideal site that was unavailable previously might now be available?  25w can go a long way when planned in the correct location.  Coverage predictions will help you determine the improvement
2. Check/renew existing antenna system.  If your system is a number of years old it may well have experiencd moisture ingress (if not installed properly) and/or corrosion.  This could drastically reduce your coverage
3. Additional infill transmitter. Arguably the best way to plug coverage holes, but can get expensive.  Coverage predictions will help you determine the improvement
4. Make sure you are using your full licensed ERP and antenna height
5. Antenna height increase.  Coverage predictions will help you determine the improvement
6. Power increase.  Coverage predictions will help you determine the improvement
7. Frequency change. If you are getting flattened by a big BBC transmitter or Classic then this could help a lot.  You could apply for a 1 day RSL to test a clear frequency to see if it makes a difference.  If you don’t play copyright music it will keep the costs down.  Coverage and Interference predictions will help you determine the improvement
8. Add horizontal polarised signal if licensed to do so.  I will get flack for this from people who earn a living from selling and installing antennas.  But at the end of the day, as Scottie used to say to Captain Kirk, “Ye cannae argue with the laws of physics”!  Horizontally polarised signals travel (virtually) identical distance to vertically polarised signals.  The extra signal just helps within your coverage area by making it less critical how the receiving antenna is oriented.  It cannot extend your coverage area (unless your current antenna system is faulty).   Coverage predictions cannot help you determine the improvement – horizontal signals cover the same area as vertical signals!

Declaration of interest.  Associated Broadcast Consultants sell consultancy know-how, not kit or installation services.  We sell coverage and interference predictions and can perfom complex coverage a frequency planning activities following Ofcom methodologies.

Key Considerations

• Have you already used your FM allocation to the maximum?  Are you using BOTH your vertical and horizontal power allocation, AND is your antenna at the height licensed?  If not, then you may already be heading for the “out” pile! (even though adding the horizontally polarised signal is unlikely to add much!)
• Make sure you read the guidelines carefully and address the points they raise – you need to provide EVIDENCE of poor coverage, or EVIDENCE of the need for an extension
• If your problem is interference, AND if you can stomach a frequency change, then that might be a more paletable option for Ofcom than granting a power increase.  Make sure you keep your options open if interference is your problem.  Our Ofcom Frequency Prognosis could help you shortlist candidate frequencies.
• If coverage holes are your problem then make sure you provide evidence of them, especially if Ofcom’s MCA map indicates the coverage should be OK.  Evidence might be coverage plots from a qualified broadcast engineer, but equally they could be short YouTube videos taken in each location showing a car radio tuned to the frequency and poor/no audio, and/or a simple table with Latitude & Longitude and notes about reception or interference.  Also evidence of listener/advertiser complaints would be compelling evidence.
• Consider cost-benefit ratio.  If you have a big 300w transmitter but are licensed for 25w, then a power increase could have low cost-benefit ratio (ie worth doing!).  But if you have a TX30 then maybe an alternative solution might be a better investment than a new transmitter (on the other hand your old transmitter can become a useful standby for emergencies).  Equally, additional sites can get expensive.

Read the Guidelines!!

Please read the guidelines in the invitation very carefully.  In particular for coverage improvements, note these requirements:

• You can show us that your radio service is suffering from significant and harmful
  incoming interference or other forms of poor coverage in your licensed area;
• and
• The request will not result in a significant extension of the coverage area. We will
  assess this by looking at the size of any extra area that will be covered and the size of
  the population in that area. We ask you to make a realistic judgement about this
  yourself when you apply.

For coverage Extensions, note these requirements:

• A description of the area you wish to extend existing coverage to include (e.g. the
  streets, council ward(s) or town), along with a map illustrating the area concerned (a
  map with the requested area marked/circled)
• The reason why you are requesting an extension into this area
• An estimate of the size of the population in the area you wish to extend into and the
  source of this information (e.g. local council statistics) and any evidence of demand or
  support for the service in the extended area
• Whether the area or locality into which you wish to extend coverage has a relationship
  or affinity to the existing licensed area, and a description of that relationship or affinity;
• Whether there are any exceptional circumstances which would justify an increase
  which would be reasonably considered to be “significant”
• A short description of how the station will provide a community radio service for the
  target community in the extended area, in accordance with the characteristics of
  service requirements set out in paragraph 2.48 above
• Any other reasons for the request that you wish us to consider

We can assume that failure to comply with the recomendations immediately above will give Ofcom an easy “get-out” to reject your application (and save them a ton or work)!

Next Steps

1. Complete Ofcom’s  application form following the advice above and the guidelines in their invitation
2. Submit a high quality, well reasoned application by or before the due date.
3. Wait.  We would be amazed if anything happens before end September 2018
4. Celebrate or:
5. If the decison goes the wrong way, challenge it and ask on what grounds.  If it is on technical grounds, ask for evidence and all details possible.   ABC can investigate this further to verify (or not) their claim, and propose modification/s to the design that could comply and be accepted.

Photo: aldi.co.uk

This post on Blocking and Adjacent Channel Interference in DAB started-off as a review of the DAB-FM convertor being sold in Aldi Supermarkets (right).  But halfway through the review we changed our minds on the emphasis of the review!

As a result we suggest that Blocking on DAB receivers may be the real reason why so many people complain of “crap” reception of DAB services!

Below we give a brief review of this product, and then explain some of the theory of Blocking and Adjacent Channel Interference.

Aldi DAB-FM Convertor Review

This unit currently retails for £34.99.  This is hardly an absolute steal, but still not bad value if your car does not have DAB and you want to listen to the (less than there ought to be) services that are uniquely available on DAB.

Unboxing

The unit was smaller than we expected and seems nice quality.  This deal includes an antenna, USB lead and cigarette lighter plug for power, and a nice windscreen suction clamp.  Setup was an absolute doddle – no need to read the instructions – it’s all really intuitive.  The FM frequency on ours was preset to 87.6MHz. We consider is the ideal channel to use – most likely to be clear unless someone else nearby has one of these, or there is a local temporary RSL station that uses 87.7MHz.

Two quibbles at this stage:-

1. The USB power cable is too short to (tidily) reach most cigarette lighter sockets we would have thought
2. The aerial cable seems excessively long (introducing needless signal loss).

DAB Performance

Performance seems very good.  We would have said “excellent” had it not been for the issue we uncovered, and lead to this extended post.   We have heard lots of talk about inadequate reception on DAB – despite all the transmitter infills that have been built, and power increases to existing services.

So to test the sensitivity of the unit we decided to place the antenna deep in the footwell of the car instead of “properly” placed high up on the windscreen.  Interestingly the DAB Coverage map for the area of our tests (Thornbury, South Glos) indicates it is not covered!  Nevertheless, reception was initally perfect, even with the antenna in the footwell!

Then we started travelling round – the audio quality was good, but not astounding.  We think that is a limitation of the transmtted signal, not a reflection on this unit.  We were listening to Absolute Radio that transmits in mono (what a retrograde step!).  As a result, although the music was generally enjoyable, it all sounded a but lifeless and flat!

FM Performance

FM performance (transmitting the DAB-derived audio on FM for the car radio to receive) was good.  Audio quality was very good – much better than most “mp3 transmitters”.  Clearly this unit had taken the trouble to use UK/EU de-emphasis of 50µs.  Many/most personal FM transmitters on the market use the US 75µs standard – and as a result sound tinny.   The modulation also did not have undue noise or hiss like many cheaper units have.   We had read that the FM range on this device was very good.  However when we tested it it seemed actually lower than most “mp3 transmitters” we have tried.  Maybe that is because this unit fully complies with UK law that limits ERP to 50nW (Yes, that’s 50 billionths of a watt!).  Despite their CE marks, we suspect many mp3 transmitters on the market might be designed for the US market with permits 100uW and hence have greater range, but risk that knock on the door!

Reception Problems

We first noticed reception problems when we got within around 1km of a mast that transmits a different DAB signal for South West Regional Multiplex.  It is a much smaller transmitter than the one we were listening to (1kWatts versus 9kWatts).  Furthermore its frequency is well removed from the signal we were listening to.  Nevertheless we found that Absolute Radio suffered from dropouts from about 1km from the mast.  This got worse and worse as we approached, rendering it unlistenable within 500m.  This fact despite the area being on high ground (and shown as covered on the coverage map).

Interference seemed to get even worse when we relocated the antenna from the footwell to the windscreen.  This might seem counterintuitive, but if you read-on you will find out why that observation is important!  The problem this unit has is not with it’s sensitivity.  It’s the selectivity that is the problem which is leading to a specific type of interference called “Blocking”.

What is Blocking?

Blocking, in terms of radio communication, is when reception of a wanted signal (generally termed the Wanted signal (in our case Absolute Radio) is impacted by another signal on a different frequency, often quite a long way away in terms of frequency.

The layman would blame the interfering transmitter (“well it’s fine when you turn your transmitter off”).  Actually the problem is the design of the receiver.  It should be good enough to “filter-out” the interfering signal.  It should also turn its internal amplifiers down as low as possible to receive an adequate signal (termed Automatic Gain Control AGC).  If it does not then it can cause intermodulation problems etc.

What is Adjacent Channel Interference?

Adjacent Channel Interference (ACI) happens when the Wanted signal is impacted by a signal immediately adjacent (or sometimes a second, 3rd or 4th adjacent in bad situations).  An analogy could someone complaining about the noise from next door.  They may simply have noisy neighbours, or perhaps the construction of the party wall is not adequate, or perhaps they are unduly sensitive to noise – a matter of opinion!.

In radio, ACI is caused by two main issues:

1. The technical design of the DAB system.  In order to squeeze in as many channels as possible (ie: increase spectral efficiency), there is only a very small gap between DAB channels (only 0.176MHz between each DAB block which are 1.536MHz wide).  For filter designers this is very challenging – it’s why “Mask Filters” are so big and cost so much on DAB transmitters to reduce ACLR (don’t ask, Google it!) and achieve that narrow gap.
2. Receiver Design – in theory all receivers could all have a filter as good as the mask filter on the transmitter.  But that would mean DAB receivers would be extremely expensive!   So a compromise is made to make the filters “good enough” for the “majority” of situations.

DAB Channel Raster

ACI is more likely when the Wanted signal is weak and the interfering signal is strong.  Like with blocking, the interfering signal may get the blame, but in reality it is most likely caused by the receiver being used (unless the transmitter’s Mask Filter has gone out of alignment.

Conclusion

Armed with that knowledge and returning to our review of the Aldi DAB-FM convertor, we think that the interference we experienced was “Blocking” caused by inadequate filtering and/or AGC control in the unit.  We rule-out ACI, because the interfering signal is quite a long way away from the wanted signal in frequency terms.  Furthermore, “blocking” seems likely due to the fact the interference got worse when the antenna was relocated to a “better” location.   If the issue were not blocking it would not make any difference because the ratio of the wanted and interfering signals would more or less stay the same – they would just both get stronger.  But in this situation, having stronger signals just made the problem worse.  This indicates a non-linear phenomenon such as Intermodulation which, when bad enough causes blocking.  (We won’t try to explain Intermodulation in this lesson – Google it!).

We have also noticed similar problems (although not so extreme) when using a Volkswagen factory DAB radio passing a low power “Small Scale DAB” transmitter in Bristol.  Although only a sample of two, it indicates that blocking and adjacent channel interfence may be a real problem for DAB reception.  Because it is so counter-intuitive, perhaps it explains the “DAB is crap” claims amongst typical listeners!  It could even be made worse by well-intentioned people who install bigger and better antennas to boost reception!

So we propose some points to consider for listeners, equipment vendors and broadcast regulator Ofcom!

DAB Lessons to Learn

Listener Lessons

1. If you have problems receiving a particular DAB service, don’t assume it’s due to a weak signal.  (It might be, but it might also be that signals are too strong!).  Try making the signal weaker – eg by moving the antenna so it has less view of the outside world or other such means.
2. If the problem seems bad then complain!

• Complain to the equipment vendor.
• Complain to the broadcaster.
• Complain to the Broadcast Regulator Ofcom.

If the problem is widespread, then they all need to know!

Equipment Vendor Lessons

1. Pay closer attention to the design of your radios to avoid selectivity problems which can lead to listener disappointment through mechanisms such as ACI and Blocking

Ofcom Lessons

Probably the wrong sub-title because Ofcom is full of very clever people, but we’d like to suggest the following:

1. If it has not been done already, test sensitivity and selectivity of a wide range of DAB receivers on the market – including the Aldi DAB-FM convertor and VAG factory-fitted radios.
2. If the results described here are typical, then it could cause difficulty for the new tranche of SSDAB multiplexes when they eventually rollout.  Considering when transmitters are positioned in populated areas, then they might cause problems.   One possible precaution might be to oblige SSDAB multiplex operators to reserve a percentage of capacity to relay interfered services if there are complaints.  Indeed, having such “Flagship” services on the SSDAB mux may aid uptake and lisenership of marginal services.

Pira P275 FM Modulation Analyser

Look what those jolly nice people at Broadcast Warehouse have sent us to review! A Pira P275 FM modulation analyser. You may have seen our previous review of the older model P175 on this blog. We’ll be putting this one through its paces soon and reporting back here soon!

Having openly declared this – let us say that our promise of independence remains:

“Here at ABC we offer independent help and advice for radio broadcasters large or small. We sell consultancy know-how, not kit – so you know our advice is independent.”

We’ve had LOADS of enquiries about FM frequency availability – it seems Ofcom want to “outsource” this activity whilst retaining the cards to make a final decision, and without informing consultants like us who could help.

We are sympathetic to the cause of new “challenger “broadcasters, but without the resources available to a big Government machine like Ofcom, it can seem difficult for stations to engage with them in a constructive way.

We believe the best way is to look at the situation through the “Ofcom periscope” and see the likelihood or not of a given request being viable according to their rules. BUT it’s difficult to give an honest, solid opinion because in addition to Ofcom’s tight technical restrictions (which we can model), they also have to apply judgement decisions – for example is a given interference area significant in terms of size or location? Also as a regulator they have to balance conflicting demand between (say) a Heart FM relay and a new local radio service for your area. We know where the balance should lie, but they have other external pressures!