Ofcom FM/VHF Protection Ratios Illustrated

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Ofcom are on the record stating that there is frequency congestion on the FM Broadcast band in many areas.  This can often cause frustration because to the layman it appears that there are plenty of unused frequencies on the dial.  The cause of this situation is the technical criteria that Ofcom use for their spectrum planning of this band.  There are many criteria, but in this post we focus on just one – the Protection Ratio, and in particular the co-channel (same channel) protection ratio.

It’s a bit of a dry subject, but has profound consequences.  Therefore we’ll use pictures and try and short-hand some of the technical language to make it more accessible.

Take by example our local Community Radio station – Bradley Stoke Radio (note we designed and commissioned their transmission system last year).  They broadcast with 25 watts on 103.4MHz from a low antenna attached to a community centre in North Bristol.   Their coverage area (the standard 54dBμV/m one that BBC and commercial stations use) is illustrated below – it’s that small green blob in the middle.

BSFM-54

Click to Enlarge. Underlay map by google.com

Now the cynics among you will probably ask why such a big map – perhaps we are trying to exaggerate how small their coverage is – but there is method in our madness – please read on.

The above-mentioned Ofcom technical criteria state (on page 12) that the co-channel protection ratio for stereo services is 45dB.  To the layman that’s just a number, but to a radio engineer they think “heck that means the interfering signal has to be 31623 times smaller than the wanted signal to avoid interference half the time!”.  Gut feel indicates it can’t be right, but Ofcom cannot plan their spectrum strategy on gut-feel – the figures are based on internationally-recognised recommendations published by the ITU (page 5 of this document).   Adding this factor on top of little Bradley Stoke Radio’s signal results in an interference area indicated below.

BSFM-9

Click to enlarge. Underlay map by google.com

That’s right – according to this protection ratio Bradley Stoke Radio’s lowly 25 watt transmitter effectively sterilises 103.4MHz across large swathes of south west England and south east Wales.  So in large areas from the southern edge of Birmingham, to Exeter, and Oxford to Cardiff it is not (apparently) possible to have another FM radio service on that channel.  This is borne-out by Ofcom’s actual frequency plan – only 11 transmitters on 103.4MHz across the entire UK.

UK Stations on 103.4MHz

UK Stations on 103.4MHz – Click to enlarge

This level of re-use is actually slightly above the national average of 9.3 (excluding the heavily under-utilised spectrum territory around 87.5-88.0MHz – presumably to ease the workload of RSL spectrum planning).

UK VHF/FM Frequency Re-use

UK FM channel re-use – Click to enlarge

To put that in context, each mobile phone network would probably have more than 50 20watt transmitters on the same frequency in Bristol alone and many hundreds across the same area of the UK.  However we are not comparing apples to apples there – the frequencies are different, and FM is an old analogue system that, in human terms, should be drawing its pension by now.   In contrast mobile phone networks are digital systems that have had over half a century extra human ingenuity in their development.  This is a major reason why Ofcom drives “re-farming” of spectrum from old to more spectrally efficient technologies (eg analogue to digital TV)

This is all caused by that 45dB protection ratio for FM.  It’s all too easy to stamp our feet and say “it must be wrong” – but as Scottie always said to Captain Kirk – “ye cannae change the laws of Physics“.  Nevertheless, there is a worthwhile goal here to achieve more listener choice and greater social gain – so could there be a third way?   Associated Broadcast Consultants think there could be – let’s outline a few possible approaches below:-

  1. Re-study the protection ratios.  We think this will not be a fruitful angle of approach.  Even with global warming, the laws of physics of radio propagation are unlikely to have changed over the last few decades.  True you could look at some of the assumptions (eg: that assumed receiving aerial height is 10m above ground level) – but in reality it could be difficult to get sufficient support to make this happen at ITU, and even if it were possible we’d probably all be pushing-up the daisies by the time the study finished!
  2. Band Re-plan.  An alternative approach is to re-plan the FM band to deliver a small amount of spectrum that is managed with lower protection ratios for use by stations at their own risk.  In practice this is not practical – too costly and disruptive to for wholesale frequency and power changes for a band/system with limited life.  Maybe if Radio 3 made an early exit from the FM band it could be an option to create a sub-band with tighter re-use of frequencies.
  3. Consider TSA instead of MCA – Many stations’ Target Service Area (TSA) is smaller than their Measured Coverage Area (MCA).  Therefore it seems rather a waste of spectrum to protect incumbent stations’ signals outside of their target area.  This approach could allow significantly denser packing of stations on the same channel.  Come to think of it, reduced protection may act as a gentle “nudge” to encourage incumbents to  transition to DAB should Ofcom want that.
  4. Review approaches of other regulators – There has been anecdotal evidence that other European regulators take a different approach to the ITU protection ratio recommendations (and other restrictions like the 10.7MHz image channel).  We are not aware of evidence of listeners in other countries complaining about FM interference any more than the UK.  Therefore we request Ofcom to consult their European colleagues and review what approaches to FM planning are taken in neighbouring countries.
  5. Pragmatic reduction in protection ratios – For example select a small number of stations that wish to have a power increase.  Carefully increase their power and see if there is any impact to the listen-ability of co-channel stations.  Most of those “interfered” stations will anyway be benefiting from increased protection since they came on-air because many will have alternative channels that could be used by listeners experiencing difficulties (eg DAB, satellite, DTV, mobile, internet…)

Associated Broadcast Consultants encourage Ofcom to consider options 3, 4 and 5 above. They are relatively small changes that should be within Ofcom’s power to change as part of its’ remit to efficiently manage the nation’s spectrum, and should not require primary legislation.

You are welcome to use the images and text in this post as long as you credit Associated Broadcast Consultants and give a link to this website.

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Keep this Frequency Clear!

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Real Radio Wales logo

Capital FM logo

Heart logo

The poor Heart listeners of North Wales have had a confusing time lately.  Their old Heart station has changed to Capital – not the kind of “More Music variety” that the average Heart listener likes!  But all is not lost because in the same area the old Real Radio has changed to Heart.  Confused – you won’t be after their slick marketing guys came up with some crafty communications to make everything clear.  They even avoid using the word “frequency” which is far too technical for their listeners to understand 😉

Phil Edmonds describes in more detail, with audio clips here.

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Radio Stations are like Pubs!

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Whilst reading the endless debate on Media UK about radio groups and takeovers it struck us that actually the Radio industry is much like the Drinks industry:

whitworthPhoto: Adam Bruderer

  • There are large breweries (radio groups), small breweries and plucky independents that run pubs (stations)
  • The industries follow fashions, and occasionally a pub needs a refurbishment or re-invention with the addition of mock beams, rows of old books or leather sofas (name, format, music etc).  Inevitably the regulars moan, but usually the update brings new punters.
  • People (especially “radio people”) are fiercely loyal about their pub/radio station preference – it’s a head not heart decision that no amount of debate can change.

 

Next we started wondering about what kind of pub each station would be – for fun we list some below:

Wetherspoons (Capital, Kiss…) Packed full of twenty-somethings drinking lager out of the bottle and downing Jagermeisters

Harvester (Heart, Smooth…) The safe bet where people go for breaded mushrooms or a prawn cocktail washed down by a nice glass of Chardonnay.

Yates’s Wine Lodge (Real Radio, GWR, BRMB, Piccadilly…) Although once very popular, now looking a bit run down and shabby and need of a refurb/reinvention

Old House at Home (BBC local) Patterned carpets and aging pine kitchen chairs.  Where people go for a good old gossip and chin-wag over a game of cribbage.

Busy Freehold Pub (Jackie, Sunshine…) Great atmosphere, friendly service, good range of well kept beers and good honest pub grub

Brewery Trip (Community radio) Where enthusiasts go to find out how beer is made and talk to people about making beer.  They do home brew with varying degrees of success.  Breweries have a limited distribution network (coverage) and are limited to 50% of their revenues selling beer.  They survive by begging and holding cheese and wine evenings (OK we made that last bit up!)

Village Local (Radio Caroline) Full of character with wonky old beams and an open fire.  Normally frequented by aging men with beards talking about the good old days.

Boose Cruise (Pandora, Spotify) Hire a van, drive abroad and load up with tons of whatever you want, siphoning profits and taxes away from the UK.

Please don’t take offence – it’s only a bit of fun.  See you in the Village Local for a pint!

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East and West Midlands – Attention! They are not joking!

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Today 1 April 2014 Ofcom invited applications for the third round of Community Radio licences in the East and West Midlands. Despite the date they are not joking!  The Ofcom definition of East and West Midlands is best shown in the map in their document:

Ofcom East West Midlands

As usual there is the list of areas where they will not accept applications due to “no suitable FM frequencies available” – these are:

  • City of Birmingham
  • Solihull
  • City of Coventry
  • Derby
  • Nottinghamshire (with the exception of Newark on Trent)
  • Nottingham
  • Lincoln
  • Leicestershire
  • Leicester

And the Black Country gets special treatment as follows:

Applicants wishing to provide a service to a locality within any of the following
districts of the West Midlands – Dudley, Sandwell, Walsall or Wolverhampton –
should note that there is only one suitable FM frequency available for a service in
these areas (in other words, FM applicants from any of these areas will be competing
with each other for the single available frequency).

The closing date for applications is 5.00 pm on Tuesday 24 June 2014. A non-refundable fee of £600 will be payable for each application submitted and considered. Full details here.

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Pira P175 Broadcast Analyser Review

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The Pira 175 Broadcast Analyser is designed by use for Broadcast Engineers.  It can be used on FM radio signals to measure and adjust peak deviation, pilot and RDS level and phase and even carrier frequency.

Pira P175 broadcast analyser

It looks a bit “home made” (indeed you can purchase just the electronics without the box), but in fact it performs very well indeed and is as accurate as much more expensive kit.

Below are some screengrabs from the “FM Scope” application that comes with the Pira 175 Broadcast Analyser device. We are displaying them at half size, so if you want a closer look, then right click and “file save as”. There is also an “FM Guard” software that lets you monitor one or more device and set alarms for various things like peak or min deviation, pilot deviation etc.

First off we have a simple bandscan. This is of limited technical value because the device is not a calibrated receiver, nor was it used with a calibrated antenna. Moreover it is relatively deaf (ie: insensitive) so it does not pick-out the weaker stations. Still interesting for an overall view of the FM environment:-

bandscan

Next we have the “Modulation Power” display. Modulation power in dBr is effectively a measure of the average “loudness” of the audio and gives an indication how much audio processing is used. For example for a given piece of music, without audio processing it might average at 0dBr. The same piece of music played on a transmission system with typical aggressive audio processing might show as 5, 6 or even 8dBr. The Modulation power tends to be lower during phases of speech than during music. This chart also shows peak deviation – displaying the peak deviation seen during a measuring time of 50 ms.

First we show Jack FM which is on the loud end of the scale, then Radio 2 which is not so loud, and finally Radio 4 which, on average, is one of the quietest stations on the dial along with Radio 3.

Jack FM 106.5MHz

Jack FM 106.5 Modulation Power

Radio 2 98.9MHz

Radio 2 Modulation Power

Radio 4 94.3MHz

Radio 4 Modulation Power

The Pira175 also has a spectral occupancy display. It’s not a spectrum analyser, and you cannot adjust its resolution bandwidth, but it is still an interesting display:-

Spectral occupancy display

Next we have a display of the MPX signal in time and frequency domains. It is most useful for checking pilot tone and RDS subcarrier level/phase.

MPX signal display

Finally some interesting plots showing PDF curves of frequency deviation. Below you see the results for several stations. You will see that deviation is never below 8 or 9 kHz – that is because even when there is no audio, you still have the 19kHz pilot and 57kHz RDS subcarrier giving some frequency modulation on the carrier. First Radio 4 which never reached peak deviation of 75kHz during the monitoring period and was noticably quieter than other stations on the dial (mainly speech with some music during measurement). Then Radio 2 which shows the impact of reasonably aggressive audio processing – all the samples are shifted to the right – more deviation = louder. After that Jack FM is louder still, with an even greater proportion of deviation focussed between 65 & 75 kHz. Finally there is “Station X” which seems to be over-deviating to some extent (actually the Radio 2 measurement had a tiny amount of over deviation too, but probably nothing to worry about and may even be measurement error). I’m waiting to see if some checks with proper calibrated equipment give the same result for “Station X”. In case you are wondering, Station X is an Ofcom licenced station. (Note – some months after this we used the P175 to commission a transmitter.  Ofcom confirmed the deviation was spot-on which is fairly rare in their experience – they normally expect to make some changes).  Thus we can say this device is as good as an Audemat device (that Ofcom use) for measuring and adjusting peak deviation.

Radio 4 94.3MHz

Radio 4 deviation PDF

Radio 2 98.9MHz

Radio 2 deviation PDF

Jack FM 106.5MHz

Jack FM 105.5 deviation PDF

Station X

Station X deviation PDF

This product can be used stand-alone with a basic LCD display, or connected to a PC using the FM Scope or FM Guard software. It has many other features – most notably the ability to report carrier frequency error relative to a trusted transmission.

You can find out more about the Pira Broadcast Analyser here. Please note that we do not supply, nor are not endorsing this product, nor do we receive any reward if you choose to purchase one.

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Audio Quality comparison of PCM, DAB, DAB+, FM and AM

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DAB Radio The purpose of this article is to give a reasonable audio quality comparison of uncompressed PCM audio, DAB, DAB+, FM and AM. The broadcast formats were simulated in the manner described in the following methodology – whilst not quite as good as using full transmission modulation and demodulation chains, it is hoped that these simulations are “good enough” for comparison purposes.

Photo by bods

Audio Comparison Methodology

First of all a sample of music was ripped from CD using dB PowerAmp software and saved as 16bit PCM Wav file – ie uncompressed audio. Then copies of this file were processed as following to simulate the various transmission formats:

  • DAB – file converted to MP2 with three formats using dB PowerAmp software. Formats selected were 80kbps Stereo, 80kbps Mono and 128kbps Stereo. These files were then converted back to PCM wav files for compatability reasons
  • DAB+ – file converted to AAC HE v2 (which we call AAC+). Formats selected were 56kbps Stereo, 32kbps Stereo, 16kbps Stereo and 12kbps Stereo. These files were then converted back to PCM wav files for compatability reasons
  • FM – File was band-limited to 15kHz. Another version also had audio processing applied using the “Broadcast” preset in the multi-band audio processor function of Adobe Audition 3.0
  • AM – File was processed using AM simulator preset on Reaper software

Associated Broadcast Consultants opinion is that 80kbps Stereo is inadequate quality, but becomes bearable at 80kbps Mono, albeit with total loss of stereo image. 128kbps MP2 seems to give a good approximation of FM quality both audibly and on the spectral charts. The AAC+ codec (used in DAB+) is remarkably good at lower bit rates – still acceptable at 32kbps, but becoming a bit “YouTubey” at 16kbps and horrible at 12kbps.  However we feel it still has “something missing” at 56kbps which is the highest bitrate possible for that codec (above that it becomes normal AAC).  Unsurprisingly AM sounds the worst, but this is probably exagerrated by the lack of proper AM audio processing which would make a professional AM broadcast sound much better.

Audio Quality Comparison files

The audio samples have been combined into one 6 minute comparison file. Ideally you should listen to the WAV file (123MB), but if you are in a rush and/or have poor broadband, the 320kbps MP3 file still clearly shows the difference in audio quality:-

WAV File (large – 123MB)

MP3 File (smaller – 14MB)

(Just click to play or Right click and file save or save target as to download the files)

Spectral Charts

To supplement listening tests, we also did screen captures of the audio spectrum of each file. This clearly shows the band limiting of the FM file. It also shows a surprisingly heavy band-limiting in the MP2 80kbps Stereo file (almost to AM quality). In contrast the 80kpbs mono MP2 file has much better audio spectrum, and this is clearly audible on the test file. The 12kbps AAC+ picture is interesting.  It appears to show aliasing – that is audio components between 0-4kHz are repeated from 4-8kHz and 8-12kHz.  This could be why it sounds so bad! It could be avoided by bandlimiting the audio to 4Khz before encoding.  We are not sure if this is a problem with the codec used, or a limitation of the AAC HE v2 codec specification.

Original Audio PCM WAV MP2 80kbps Stereo MP2 80kbps Mono FM Unprocessed FM Processed DAB+ (AAC+ or AAC HE V2) – 56kbps DAB+ (AAC+ or AAC HE V2) – 32kbps DAB+ (AAC+ or AAC HE V2) – 16kbps DAB+ (AAC+ or AAC HE V2) – 12kbps AM Europe (4.5kHz)

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Blog – Hello world!

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Well the SEO guys keep telling us that we simply MUST have a Blog on our website, so here it is!  We’ll try and post interesting stuff here from time to time.  If you have any suggestions, then please contact us via the Contact Us page on our website!

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