Turning off the FM signal

[An_Inspector_Calls]

R4 long wave will be switched off when the Droitwich transmitter becomes unserviceable, i.e. when the remnant stock of transmitting valves nears exhaustion. There are no plans to replace the transmitter. This was announced last year as part of the cuts.

There goes a perfectly serviceable NDB! Apparently you could take off from Gibraltar, tune 200 kHz (in those days) and fly home on the beacon!

[ardcarp]

R4 long wave will be switched off when the Droitwich transmitter becomes unserviceable, i.e. when the remnant stock of transmitting valves nears exhaustion. There are no plans to replace the transmitter. This was announced last year as part of the cuts.

This is tragic. Small boat owners who go offshore (and this isn't just yachties whom everybody thinks are rich) including fishermen rely VERY HEAVILY on the R4 longwave shipping forecasts which are updated 4X in a 24 hour period. There are other ways of receiving this information, the 'professional' one being NAVTEX. However, complicated systems are all prone to glitches and failure whereas a cheap battery-powered transistor radio tuned to 200kHz gives a totally reliable service and has coverage extending well beyond our shores (as in the Gibralter example given by A-I-C). I think R4 longwave is a MAJOR contributor to safety at sea, and if it is abandoned I hate to think of the consequences.

[An_Inspector_Calls]

Why on earth is my question biased? It is highly relevant to many millions of people who will be affected if the FM switch off goes ahead.

In passing I will comment on one of your points...The wireless spectrum is a finite resource and space is valuable No-one has come up with ANY commercial use (other than audio) for the FM spectrum. Can you?

Any number of industrial control wireless links disturbed by the arrival of 4G in the 860 Mhz band. Greater bandwidth for airfield ILS systems?

[Resurrection Man]

Any number of industrial control wireless links disturbed by the arrival of 4G in the 860 Mhz band. Greater bandwidth for airfield ILS systems?

You could well be right. I have no idea as I'm not immersed in this area and have no idea how technically or economically viable your suggestions are. Certainly at the time that PwC were looking into commercial uses of the FM spectrum, 4G and the problems associated with it were not on the horizon. However, I would have thought that any problems due to inadequate bandwidth for airfield ILS systems would have been known about, if indeed it actually is a problem. I will enquire.

[An_Inspector_Calls]

You could well be right. I have no idea as I'm not immersed in this area and have no idea how technically or economically viable your suggestions are. Certainly at the time that PwC were looking into commercial uses of the FM spectrum, 4G and the problems associated with it were not on the horizon. However, I would have thought that any problems due to inadequate bandwidth for airfield ILS systems would have been known about, if indeed it actually is a problem. I will enquire.

Neither am I, and I suspect PWC didn't have the industry-wide knowledge to pronounce on the subject either. Historically, whenever a part of the spectrum is released a feeding frenzy of potential users soon follows.

The ILS problem is historic. When the FM broadcast band expanded up to 108 MHz it started to interfere with the localizers operating in the band 108-110 MHz. This required modification to aircraft localizer receivers; in many cases the problem could only be solved by expensive replacement. Just an illustration of how jam-packed the spectrum is.

[Gordon]

Many people, including me, believe that FM as it was in the 60s and 70s was better than today's compressed, poor dynamic range, poor s/n ratio, version.

I would not disagree about the use of Optimod and its worst effects on dynamic range. However I would disagree about the performance of FM back in the late 60s an 70s. For a start read this:

http://stakeholders.ofcom.org.uk/binaries/consultations/dab-coverage-planning/annexes/annex-f.pdf

Especially the parts about the way the specifications for FM evolved since the 60s.

FM in Mono started in the early 50s and experimental stereo in 1958. Service in stereo started at Wrotham in 1966and didn't get to Rowridge IOW [eg] until 1971 and didn't get to 99% coverage until a year later. We can forget about going back to the 60s! The major bugbear at that early time was not thermal noise but ignition interference from unsuppressed cars and fridges etc and so a considerable margin of transmitter power had to be added to deal with this. It was due to this margin that some portables worked at all.

The Post Office provided the stereo links from BH. Those links were analogue telephone circuits whose performance was limited in frequency response and dynamic range but worst of all the matching of delay between L and R was murder to keep in alignment and so the stereo image drifted all over the place. This was partially solved by using more expensive dedicated analogue microwave links but not adequately solved until 1976 and the use of digital NICAM 1 transmitted using wideband TV circuits came in the early 70s. This gave 14 bit equivalent linear PCM performance in those links [Yes, FM is digitally distributed using a system that only uses 10 bits linear over short periods]. The rest of the country was of course late getting both stereo and NICAM. If I lived up North I'd not want to back to the 70s either.

The FM network as we know it now, but no Optimod, did not come into being until the 80s when a re-engineering exercise equipped transmitters with mixed polarisation aerials so that portables/cars would get a better signal at the expense of the roof top aerial. A service area is defined by the 48dBmicrovolt/metre field strength which would enable a competent receiver [ie one worthy of the name "HiFi"] to get to an audio S/N of the high 50s dB, referred to BBC PPM level 4 and weighted, for mono. Such a receiver is assumed in the planning to have a dipole on the roof to bring in enough signal from that field. See the figure here for the measured mono perfromance of an FM HiFi receiver taken from the document quoted in the link above, note the asymptotic S/N:



For stereo an additional 20dB or so is required because of the way that the Zenith GE multiplex works, placing the L-R component deep in noise. Pre-and De-emphasis helps to mitigate this noise somewhat but it also boosts considerably the high frequency components of the audio [6dB per octave from 3 kHz so a flat audio input would have its 12kHz components quadrupled in amplitude]. The assumption is that the audio will naturally have smaller amplitudes in the high harmonics. However, because of the way FM is bandlimited - unlike AM its natural spectrum is unbounded - the use of P/E in FM can lead to excessive distortion and so metering is vitally important as is making sure that high volume audio and/or lots of natural high frequencies is avoided. Despite its reputation Optimod usefully deals with some of this but perhaps is overdone. By the way 1970s analogue studio tape machines would struggle to get S/N's in the high 50s or low 60s.

The result of moving to stereo is a direct dB for dB reduction of S/N so that 48dBmicrovolt/m now only gives 30dB4W audio S/N which is hardly HiFi. To get some of this back a very much better aerial is needed together with reasonable proximity to the transmitter and a very good receiver noise figure. I live about 60 miles from a high power FM transmitter and so have an external 6 element Yagi giving about 2 millivolts to a good quality well known brand FM receiver which sounds well but has some audible hiss in quiet moments and especially when there is no programme and the R3 continuity desk fader is down. I reckon I'm getting about a S/N 55dB. A DAB receiver using a telescopic aerial is as quiet as the grave in those same quiet moments. I'll trade that for a bit of compression loss any day.

The ARCAM T32 has been well received but it's very coy about its FM spec: all it says is:

Sensitivity (IHF) typically 4μV
Signal to noise ratio 65dB at full modulation, 1kHz
THD+n 0.3% at full modulation, 1kHz

which is worse than useless. This sensitivity places the receiver just on threshold [4 microvolt == 12dBmicrovolt]. The IHF figure should refer to a usable audio S/N above threshold and it doesn't say if its stereo or not. I'd expect a quieting figure. Measuring distortion at such a low one tone only frequency is a waste of time. The S/N is claimed for full modulation at 1kHz which is fair enough but doesn't say what the level was, one assumes PPM6 to get 75kHz deviation?? One assumes that because the number looks good it is weighted but which weighting? [See document link above again]. This is already 8 dB better than quoting dB4W [BBC PPM scale is 4dB per step - http://en.wikipedia.org/wiki/Peak_programme_meter] so is nearer 58dB4W. This is getting nearer to credible HiFi performance. To get this S/N one requires a correspondingly high input signal from a decent aerial [see graph above] and so this S/N figure is only a potential maximum. It probably has a better noise figure by a few dB than my older receiver. One reviewer [who did no measurements at all by the look of his review] gave its DAB performance via R3 the thumbs up and didn't mention Bruckner's counterpoint once!!

Given all this one could be forgiven for questioning whether FM is a HiFi medium at all. Nevertheless it is like a comfortable pair of old slippers.

[An_Inspector_Calls]

Gordon: a fascinating post. I seem to remember that we didn't get stereo broadcasts until the 80s. I certainly had an FM aerial comprising two stacked 6 element Yagis in order to get stereo from Holme Moss - the house looked like GCHQ! But the early 80s were the best FM I ever had, until they started to add local radio stations and the birdies arrived.

Do you have any information on why they selected FM instead of AM?

[Nick_G]

I doubt you'll ever admit defeat, but points I would make are:

1. The bulk of BBC audio content is wirelessly broadcast via LW (R4 only), MW, FM, DAB, Freeview, Satellite, and now 4G; it is also distributed over the internet. That's SEVEN different broadcasts! The wireless spectrum is a finite resource and space is valuable. How much of this resource do you want? Some pruning of resource usage is required for use by other wireless users.
2. The FM transmitters are ageing, especially the masts. Any costs of a DAB replacement must be compared to the cost of maintenance of the FM system.
3. The FM system cannot be deveoped any further to improve sound quality. FM is the sound of the sixties, with poor dynamic range, poor signal to noise ratio, and considerable distortion. The digital systems offer decent sound quality now (as you say, it's already not an issue for most listeners) and can surely be developed in the future.
4. Strenuous efforts are being made to improve DAB coverage to a level that will probably far exceed that of FM. You cite the need for 1,000 DAB transmitters, but why is that when FM (another Band III service) seemingly requires just over 100? Probably because the FM coverage maps are assessed for roof-top aerials, not internal coat-hangers.

As for your biased question:
"Give me one benefit of DAB to the many millions of radio listeners who are perfectly happy listening on FM, perfectly happy with the quality of FM and perfectly happy with the choice of radio stations that are currently on offer and have no wish to spend money when they don't need to"

well I give you a question in reply:

"Give me one benefit of Freeview to the many millions of TV viewers who were perfectly happy watching analogue, perfectly happy with the quality of analogue and perfectly happy with the choice of TV stations that were currently on offer and had no wish to spend money when they don't need to". That would be the crowds tortured by the digital switchover into buying flat-screen TVs!

The reason that FM needs less transmitters to cover the population than DAB is because the frequencies are lower and aren't affected by obstacles such as hills and buildings as much. This is why high-powered TV transmitters on UHF had ERPs of 1000 kW with a smaller coverage area than the main BBC FM transmitters which were at a quarter of the power.

I think the decision over switchover rests on the conclusions of the next CBA, which is due to be published "this winter" according to the email sent to me after I responded on the public consultation. If anything, I would suggest that the balance is even less in DAB's favour now than in the last CBA published a couple of years ago. The recent Digital Radio progress report from Ofcom confirmed that DAB radio sales have flatlined over the last few years, most people listen to services that are already available on analogue, and only 9% of respondents said that they were very likely to buy their first DAB radio this year. Put simply, the costs outweigh the benefits.

Regarding 4G, the Ofcom requirements has a clause that states that 4G coverage should reach 97% of the population by the end of 2017. With the new IP protocols and 4G incorporating multicast as standard this will make it much easier and cheaper to deliver multiple streams to listeners. Give it 10, maybe 15 years and I think we'll be there, maybe sooner. Once this happens, then what would be the point of continuing with DAB?

I think in 10 years time DAB will be seen as one of radio's biggest and costliest mistakes.

[Gordon]

Do you have any information on why they selected FM instead of AM?

A long and complex story - FM was invented by Edwin Armstrong in the US and it cost him his life to promote it - simply put, the main feature of FM compared to AM is that there is a trade of performance - much better audio signal to noise and resistance to interference but at the expense of using more bandwidth. Mono AM on LW and MW in Europe uses a channel bandwidth of 9kHz - and for double sideband modulation which is most commonly used this gives an audio bandwidth of about 4.5kHz. If you remove one sideband, which is done in some instances like short wave and was used in a modified form for analogue TV, it complicates the receiver but allows more audio bandwidth. But all the unwanted noise and interference that gets into this 9 kHz bandwidth gets directly translated into the audio at the receiver. AM/DSB does however have a simpler receiver than FM.

FM as its name implies doesn't impose the audio on the carrier amplitude [in fact in theory FM is a constant carrier amplitude system] but on the frequency so that the louder the audio signal the more the instantaneous carrier frequency is displaced from its rest position [ie in silence].

The degree of carrier displacement is called deviation which spreads the radio frequency energy over a wider bandwidth but the noise added to the carrier does not get translated directly down to the audio but is attenuated depending on the Deviation. The agreed maximum FM deviation used in broadcasting FM is 75kHz, corresponding to the highest audio amplitude, and the audio bandwidth is nominally 15kHz [See ETSI EN 300 384]. The mathematics of FM is considerably more complex than AM and stereo complicates matters further compared to mono. However, the resultant bandwidth is approximately given by a rule due to Carson - the bandwidth is twice the sum of the deviation and the highest audio frequency. So the bandwidth needed for the FM system used is 2 x [75+15] = 180kHz, somewhat more than AM. That is for mono. The consequence is that the audio S/N now depends on this parameter Deviation, the greater it is the greater is the S/N. Stereo audio bandwidth is more than 3x higher than for mono and this unfortunately costs a S/N penalty.

Another reason is that AM is susceptible to distortion in the processing devices eg transmitter valves. This, together with the reasons above, is why satellite communications and broadcasting always used FM in the pre-digital era because they were always power limited and could only work is there was sufficient bandwidth. The constant amplitude feature of FM removes this limitation but does not exonerate FM from distortion.

#53: Whilst much of what Nick says I would broadly agree with, especially with regard to the CBA, have a look at this:

http://eandt.theiet.org/magazine/2012/10/rolling-out-mobile.cfm

not all in the 4G garden is rosy, there are issues. I'm not convinced that 4G will be able to cope with large scale multicasts.
There was an adjournment debate last week in which the Chair of the Select Committtee John Whittingdale was querying the approach taken to deal with DTT interference from 4G.

[Resurrection Man]

The idea that 4G is an answer to our economic recovery is about the daftest I have heard for a very long time. Being able to tweet faster isn't the solution.

Do you have a link to any minutes of that Select Committee meeting ?

[An_Inspector_Calls]

Thanks, your reply made me pick up my copy of Schwartz Information, Transmission, Modulation and Noise and study the comparison between AM and FM. The trade between bandwidth and deviation is interesting.

I disagree with Nick's claim that propogation at 225 MHz as against 88-108 MHz is the reason why DAB needs so many more transmitters. Both are VHF signals and transmission is a mix of both line-of-sight and ground wave; it is only be in the UHF band where we begin to see a move towards a requirement for line-of-sight.

Looking at the plans for my local DTT transmitter it would appear that the 4G take-over of DTT frequencies above 800 MHz is only a first step, and that that limit will be driven much lower by 2015, possibly to 600 MHz. Perhaps all the DTT multiplexes will have ot become 256 QAM to cope?

[Gordon]

Do you have a link to any minutes of that Select Committee meeting ?

It was a HoC short adjournment debate and is reported in Hansard. 4.30 - 5pm Tuesday 30 October 2012: John Whittingdale outlines his continuing concerns about the impact of 4G on DTT and Ed Vaizey says why he thinks everything is taken care of!!

[Gordon]

........Schwartz Information, Transmission, Modulation and Noise

I still have mine too, in a much thumbed early edition!! The treatment of FM is not particularly deep but it does the basics well enough.

Looking at the plans for my local DTT transmitter it would appear that the 4G take-over of DTT frequencies above 800 MHz is only a first step, and that that limit will be driven much lower by 2015, possibly to 600 MHz. Perhaps all the DTT multiplexes will have ot become 256 QAM to cope?

Yes this 800 MHz phase is the first of at least 2 phases. The second will occur in about 2018 [planning sooner of course] when the 700 MHz band will also be reclaimed for 4G and all DTT will be shifted down to the bottom end of 700 MHz and into the currently unoccupied 600 MHz slot. This willmean a second round of disruption to DTT viewers involving yet another filter to replace the one they will give out for the first phase. A pilot trial - "GoDigital" - is imminent to see how the mitigation measures for the DTT/800MHz issue will actually work.

To enable Freeview services to continue [and possibly expand eg more HDTV] some further spectrum efficiencies will have to be found. The OfCom consultation on this earlier this year

http://stakeholders.ofcom.org.uk/binaries/consultations/uhf-strategy/summary/spectrum-condoc.pdf

gave all the details of the various bands affected and also appended studies on how that spectrum efficiency might be achieved. Here's a diagram of the bands taken from this consultation:

[An_Inspector_Calls]

Which makes you wonder if 4G will become the mobile radio of the future, and not DAB? 4G is claiming a mighty chunk of the spectrum. I do hope the channel hopping stuff at 868 MHz is reilient to 4G otherwise a lot of industrial systems will need re-engineering, as will my weather station.

[Resurrection Man]

Thanks, Gordon. Just read it and it left me very depressed. I had always thought that Vaizey was a few kilocycles short of a full bandwidth and his response has done nothing to change that view. Seeing who they have selected to be the interim chairman of the mitigation company Digital Mobile Spectrum Limited - one Andrew Pinder - and looking at his track record, I am not optimistic on that score either. How depressing.

Mind you, it is all in a good cause as I am reliably informed that 4G is going to kickstart our economy and we are soon going to be the leading economic powerhouse of the world.

I feel for those in the 40,000 who are going to have to find an alternative to DTT. I can also imagine the hassle they will get when they give up paying for their TV licence as it has taken them so long to get sorted that they've given up watching TV.

Gordon, your post about downshifting DTT and bandwidth constraints. Radio 3 down-bitted to 48kbps perhaps on Freeview ?