DAB Report

[Dave2002]

The poor DAB reception that Dave2002 reports was probably due to tropospheric ducting bringing in another distant MUX on the same channel. There was a spectacular lift to southern Germany at the beginning of the week that only fizzled out on Wednesday evening. However, there is another very good one now towards NE France and as far as Germany tonight so it would be interesting to see if Dave has any drop-outs or bubbling mud tonight or tomorrow.

I'm not sure that DAB should be affected by signals from outside the UK. I suspect that DAB should be immune, unlike FM, which can definitely be affected by off shore stations under some weather conditions.

Unfortunately I missed checking the state of affairs last night, so I don't know if there were any problems then - and I think we listened on FM earlier rather than DAB - quite by chance.

Re Bryn's 4am observations, DAB this morning around 5.50am seems OK - with acceptable speech.

I don't know why there has been disruption. Weather is possible, but it could be that there are problems with the DAB infrastructure or settings.

[Dave2002]

re msg 27, this is the latest I could find on DRM - http://en.wikipedia.org/wiki/Digital_Radio_Mondiale

and http://www.drm.org/

I hadn't realised that there'd been a DRM trial in the UK (Devon, 2008 - according to Wikipedia), but overall there seems relatively little to go on.

I also thought that DRM wasn't only going to be based on AM, but also FM - though maybe I was mixing up with other systems. It does at least look as though DRM+ extends DRM into the bands which have been used previously (and still are) by FM.

The comparison test at http://www.drm.org/?page_id=25 is quite impressive. The AM sounds realistic enough i.e pretty dreadful, what you'd expect from AM, while the DRM sounds at first listen to be good. Whether it is good enough for high quality music is difficult to assess, but it doesn't sound too unpleasant, and should be at least good enough for speech programmes. Looking at the bit rates in the article suggests that it must be relying on a very efficient codec - most probably similar to some of the low bit rate aac+ formats (probably actually MPEG-4 HE-AAC) - so the quality on music would be adequate, rather than good.

Whether Ofcom are really interested is difficult to say. Again, according to the article, there was supposed to be consideration of DRM in 2012. If DRM is a viable and useful system, then maybe manufacturers could produce sets which are DAB, DAB_, DMB and DRM compatible, and that might also allow the available spectrum for DAB/DAB+ to be restructured to suit future needs, with perhaps a significant shift to DAB+ or better after the transition period.

It might make sense, if we are to be forced into digital, to adopt DRM as well as DAB, DAB+, and then delay FM switch off until all the systems are working, though maybe this would be too expensive for the UK. I really don't know, but it would at least seem to be an option.. I'm guessing that DRM or DRM+ might provide better coverage in more remote parts of the UK - such as Scotland - if adopted, and that could usefully be exploited.

I don't know how well DRM works in mobile applications compared with DAB or FM, for example in car use, or even use in tunnels. Indeed, I also don't know how well DAB works in tunnels. This is an aspect of broadcasting which I'd forgotten about, but I recall the first time I drove though a tunnel (Blackwall I think) and afterwards noted that the FM radio channel I'd been listening to had not cut off in the tunnel, presumably due to relays within the tunnel. I don't know whether DAB works in similar situations/locations.

[Nick_G]

I'm not sure that DAB should be affected by signals from outside the UK. I suspect that DAB should be immune, unlike FM, which can definitely be affected by off shore stations under some weather conditions.

Unfortunately I missed checking the state of affairs last night, so I don't know if there were any problems then - and I think we listened on FM earlier rather than DAB - quite by chance.

Re Bryn's 4am observations, DAB this morning around 5.50am seems OK - with acceptable speech.

I don't know why there has been disruption. Weather is possible, but it could be that there are problems with the DAB infrastructure or settings.

It's not necessarily overseas signals causing the problem. You were saying that it affected the BBC multiplex - well, it's probably co-channel interference from other sites greater than about 73 miles away. Any sites further than this distance exceed the 'guard interval' which causes problems with reception because the guard interval starts to get occupied by the interfering signal so the receiver cannot lock on to the MUX.

Conditions are still up this morning but not as intense as yesterday evening so that could easily explain why your reception has improved.

[Resurrection Man]

re msg 27, this is the latest I could find on DRM - http://en.wikipedia.org/wiki/Digital_Radio_Mondiale

and http://www.drm.org/

I hadn't realised that there'd been a DRM trial in the UK (Devon, 2008 - according to Wikipedia), but overall there seems relatively little to go on.

I also thought that DRM wasn't only going to be based on AM, but also FM - though maybe I was mixing up with other systems. It does at least look as though DRM+ extends DRM into the bands which have been used previously (and still are) by FM.

The comparison test at http://www.drm.org/?page_id=25 is quite impressive. The AM sounds realistic enough i.e pretty dreadful, what you'd expect from AM, while the DRM sounds at first listen to be good. Whether it is good enough for high quality music is difficult to assess, but it doesn't sound too unpleasant, and should be at least good enough for speech programmes. Looking at the bit rates in the article suggests that it must be relying on a very efficient codec - most probably similar to some of the low bit rate aac+ formats (probably actually MPEG-4 HE-AAC) - so the quality on music would be adequate, rather than good.

Whether Ofcom are really interested is difficult to say. Again, according to the article, there was supposed to be consideration of DRM in 2012. If DRM is a viable and useful system, then maybe manufacturers could produce sets which are DAB, DAB_, DMB and DRM compatible, and that might also allow the available spectrum for DAB/DAB+ to be restructured to suit future needs, with perhaps a significant shift to DAB+ or better after the transition period.

It might make sense, if we are to be forced into digital, to adopt DRM as well as DAB, DAB+, and then delay FM switch off until all the systems are working, though maybe this would be too expensive for the UK. I really don't know, but it would at least seem to be an option.. I'm guessing that DRM or DRM+ might provide better coverage in more remote parts of the UK - such as Scotland - if adopted, and that could usefully be exploited.

I don't know how well DRM works in mobile applications compared with DAB or FM, for example in car use, or even use in tunnels. Indeed, I also don't know how well DAB works in tunnels. This is an aspect of broadcasting which I'd forgotten about, but I recall the first time I drove though a tunnel (Blackwall I think) and afterwards noted that the FM radio channel I'd been listening to had not cut off in the tunnel, presumably due to relays within the tunnel. I don't know whether DAB works in similar situations/locations.

Seems to me that this is all a bit tinkering around the edges. Surely better to leave FM as it is. DAB can survive (or wither).

For those who want the best audio experience then bite the bullet and go for internet streaming (although in my experience the reliability and integrity of the streams leave a lot to be desired. For me, with my ears, the sound quality off Freeview is fine (and without any break-up in the stream).

[Resurrection Man]

It's not necessarily overseas signals causing the problem. You were saying that it affected the BBC multiplex - well, it's probably co-channel interference from other sites greater than about 73 miles away. Any sites further than this distance exceed the 'guard interval' which causes problems with reception because the guard interval starts to get occupied by the interfering signal so the receiver cannot lock on to the MUX.

Conditions are still up this morning but not as intense as yesterday evening so that could easily explain why your reception has improved.

Isn't there an inherent problem with national DAB because all the transmitters are on the same frequency? I copy this from the ten-myths-of-dab.co.uk website

The BBC network has already reached the stage where its coverage is, in places, being limited by self-interference. This is particularly obvious in the patchy coverage visible in Map A1 in the East Anglia area. As more transmitters are added to the network to improve coverage, the interference regime increases, paradoxically making coverage improvements harder.

[Dave2002]

Seems to me that this is all a bit tinkering around the edges. Surely better to leave FM as it is. DAB can survive (or wither).

For those who want the best audio experience then bite the bullet and go for internet streaming (although in my experience the reliability and integrity of the streams leave a lot to be desired. For me, with my ears, the sound quality off Freeview is fine (and without any break-up in the stream).

You may be right - presumably about the DRM system, but it might then restrict some possibilities. Those in power seemed determined to switch of FM anyway, given time, so not sure if your suggestion will work long term, though keeping FM doesn't seem totally unreasonable to me at the moment.

TBH I've lost track of what's going on with DAB too, but I was going to observe that at present one can take a car abroad to the continent for example, and still pick up radio (the foreign stations, and maybe sometimes the UK ones) on FM and also maybe AM. Some countries use DAB or a variant, but some have been retreating from DAB, and I don't know what they have been replacing it by, and this raises the possibility that if more countries switch off FM that there won't be any good way of receiving programs while abroad. DRM should, I think, be fairly easy to implement, and I don't think it would increase the complexity of sets very much, and might work with just a few transmitters to cover rural/sparsely populated areas areas, rather than trying to do coverage with DAB transmitters which I suspect may require more. It's only a suggestion.

[Dave2002]

Isn't there an inherent problem with national DAB because all the transmitters are on the same frequency? I copy this from the ten-myths-of-dab.co.uk website

The BBC network has already reached the stage where its coverage is, in places, being limited by self-interference. This is particularly obvious in the patchy coverage visible in Map A1 in the East Anglia area. As more transmitters are added to the network to improve coverage, the interference regime increases, paradoxically making coverage improvements harder.

There are some theoretical advantages in having a single frequency network (SFN), but the transmitters have to be carefully synchronised. If the synchronisation goes out, there can be problems in some reception areas. It is possible to revert back to multiple frequency operation, though loses some advantages in spectrum usage.

[Resurrection Man]

You may be right - presumably about the DRM system, but it might then restrict some possibilities. Those in power seemed determined to switch of FM anyway, given time, so not sure if your suggestion will work long term, though keeping FM doesn't seem totally unreasonable to me at the moment.

TBH I've lost track of what's going on with DAB too, but I was going to observe that at present one can take a car abroad to the continent for example, and still pick up radio (the foreign stations, and maybe sometimes the UK ones) on FM and also maybe AM. Some countries use DAB or a variant, but some have been retreating from DAB, and I don't know what they have been replacing it by, and this raises the possibility that if more countries switch off FM that there won't be any good way of receiving programs while abroad. DRM should, I think, be fairly easy to implement, and I don't think it would increase the complexity of sets very much, and might work with just a few transmitters to cover rural/sparsely populated areas areas, rather than trying to do coverage with DAB transmitters which I suspect may require more. It's only a suggestion.

They are using DAB+ for the most part, I believe, and most modern car DAB radios are both DAB and DAB+.

[An_Inspector_Calls]

Radio waves travel freely in empty space. Any material that occupies that free space will impair transmission in some way, usually by attenuating the wave and marginally changing its speed. Air does that. More importantly a volume of damp air will attenuate a radio wave and absorb some of the power because the water is conductive. If the wavelength is very small and comparable to the rain drop size the wave will also diffract and will scatter and cause it to lose its coherence.

A plane surface like a wet brick wall or a roof of tiles/slates - at least if the undulations in the plane are small compared to the wavelength - will act like a barrier and mirror to a radio wave because water does conduct electricity. Then the incident wave will be reflected, almost coherently rather than pass through the barrier as would happen if that same surface was dry.

The wetter the walls/roof, the more each conducts, and the closer the house resembles an earthed, Faraday cage.

As for attenuation of VHF signals by rain droplets, it'll be very small indeed. Forget it. Just as well, otherwise aviation VORs, ILS would be dodgy everytime it rained!

I doubt DAB reaching FM equivalence will actually be equivalence. DAB reception will be better than FM given the historic, lax definition of 'good' reception for FM, compared to the more closely defined definition for good DAB.

If this is not going to happen until 2020 then it might be worth remembering that by then most (equal to any delivery counts for FM or DAB) of the UK population will be on superfast broadband. What will be the point of either DAB or FM for fixed location reception? By then, all we'll need DAB for is radio on the move. 4G everyone? Why not plan to dump both in 2020?

[Gordon]

The wetter the walls/roof, the more each conducts, and the closer the house resembles an earthed, Faraday cage.

Agreed!!

As for attenuation of VHF signals by rain droplets, it'll be very small indeed. Forget it. Just as well, otherwise aviation VORs, ILS would be dodgy everytime it rained!

Agreed!! Rain attenuation due to droplets only happens when the wavelength is comparable to the drop size and that means around 1 mm or few. DAB wavelengths are about 1,400 mm and FM more than twice that. In my reply to Bryn's query I was being generic and perhaps not specific enough.

I doubt DAB reaching FM equivalence will actually be equivalence. DAB reception will be better than FM given the historic, lax definition of 'good' reception for FM, compared to the more closely defined definition for good DAB.

Agreed!! You are right that the definition of what FM coverage actually is, according to the planners, is very hazy and has changed over the years anyway. It is not a simple matter. The OfCom documents - here:

http://stakeholders.ofcom.org.uk/consultations/dab-coverage-planning/

that discussed DAB and FM coverage some while ago had technical appendices [F for FM and G for DAB - their relative file sizes tells you something]; the one discussing FM planning and it wasn't very good. The DAB document is far more rigorous and detailed. One got the impression that The FM document had been compiled by someone collecting and summarising some old BBC and CCIR documents but didn't really understand them. Also their cover of the FM RF to Audio noise conversion was very poor indeed, just a textbook quote of an equation.

Remember that there is no standard definition of an FM receiver performance and FM was planned for an external aerial 30 feet up in the open air NOT a tatty telescopic rod poking out from behind the fridge in a kitchen. That alone makes a very significant difference to the radio signal level reliably reaching a receiver. It's a miracle that FM works at all in a ground floor kitchen and is partly down to receiver design features. Many portables are mono anyway and so get an immediate lift of at least 15dB in S/N over stereo. There are other tricks available in analogue FM receivers that can be used by manufacturers: eg bandlimit the audio to, say 10 kHz or less, as many portables do, and another substantial S/N lift occurs. Bandlimit the RF channel a bit and get some more noise suppression at the expense of some more distortion. "Audiophile" stereo FM cannot be achieved without a substantial RF input signal and well proportioned receiver design.

However, consumers will judge the comparison with what they get; in the majority of cases FM mono portable reception will be compared with portable DAB whose coverage does not distinguish between mono and stereo [yes, I know that some DAB services are only in mono; if mono is good enough for consumers listening on FM portables, why not?]. Audiophiles will compare fully equipped FM reception with its like in DAB. How many audiophiles [given they accept DAB as a high quality source] have a DAB aerial mounted externally to exploit coverage?

If this is not going to happen until 2020 then it might be worth remembering that by then most (equal to any delivery counts for FM or DAB) of the UK population will be on superfast broadband. What will be the point of either DAB or FM for fixed location reception? By then, all we'll need DAB for is radio on the move. 4G everyone? Why not plan to dump both in 2020?

Agreed!! But one has to believe the tales put around about the extent and take up of 4G and superfast BB anyway! It seems that 4G so far has not been a runaway success but it is early days with the network build having a goodly while to go yet. I doubt that broadcasting as we know it will be a dominant thing beyond 2030 when 5G and even 6G might be around.

Ask yourself how many of the broadcast services, including HDTV, currently available can be accommodated in the 4G system. It might be comparable to the IPlayer or Streaming sevice being able to accommodate all the current services simultaneously. I know that my IPlayer experience is one of frequent failure especially when the IP network is busy, and I do have a very decent BB speed. Although 4G is supposedly planned for "broadcast" mode - they call it multicast, already a euphemism - it cannot necessarily and consistently support a very large multicast load unless the software transmission protocols and network topologies used favour those services over speech and text and the network resources are properly scaled. The speeds supported by 4G are not that great for individual subscribers especially when the system is busy - 3G offers up to 7 MBit/s but I doubt anyone gets that for any length of time.

High quality audio over 4G will need very good codecs AND sufficient network capacity and resilience. Jam tomorrow? Proof of that jam pudding will be in its eating - and we should taste before we buy. One reason why the internet companies want all the broadcast spectrum is because they know they will need it to to support an equivalent service. So what's new?

[An_Inspector_Calls]

Gordon

Your points about 3G/4G in the last two paragraphs are quite correct; I didn't think 4G through properly (but then I'm neither a 3G nor 4G user).

I'm not so sure about your slight negativity on the broadband front. My iPlayer experience on download speeds of 128 kbps are rock solid, and I only have a 4 Mbps ADSL service. The HD link run by the BBC is as you say a bit iffy. It'll be interesting to see what happens when superfast broadband arrives (if it arrives, I should say). I'd have thought those teething problems would have been licked well before 2020.

[Dave2002]

Your points about 3G/4G in the last two paragraphs are quite correct; I didn't think 4G through properly (but then I'm neither a 3G nor 4G user).

3G around the metropolis provides a patchy and erratic service for data. 4G is not available in many locations yet - and I have no experience of it yet. 3G in more rural areas tends to be even more suspect, so could be really hard to get a service which compares with either FM or DAB. Bring in a moving and mobile requirement, and performance becomes even more erratic. However, data drop outs on motorways do seem to be gradually becoming less of a problem.

I'm not so sure about your slight negativity on the broadband front. My iPlayer experience on download speeds of 128 kbps are rock solid, and I only have a 4 Mbps ADSL service. The HD link run by the BBC is as you say a bit iffy. It'll be interesting to see what happens when superfast broadband arrives (if it arrives, I should say). I'd have thought those teething problems would have been licked well before 2020.

128kbps audio is not particularly high quality, though one might argue it's in some ways comparable to FM. Actually I've lost touch with what the iPlayer uses for a codec at that bit rate - is it the HE version of AAC? If so, that should sound considerably better than the codecs which were used in the past. HD audio on the iPlayer, is significantly better I think, though there are perhaps several reasons why FM and DAB might still be considered more satisfactory by some. There are still some areas (probably rural, but not exclusively so) where broadband is either at too low a data rate, or too unreliable, so that the underlying ability, or lack of it, to carry data impacts significantly on audio streams. Further, there can be significant buffering delays in accessing channels.

I agree with the general hope that problems may have been eliminated by 2020, yet I'm not totally convinced that this will happen. To date different people in different parts of the country are having rather different experiences with broadband. There should be convergence towards a much improved service - we'll have to wait and see how well things pan out. I would actually expect an improved ubiquitous broadband service by 2016, but this might be a triumph of hope over experience.

[amateur51]

A shortish debate on BBC Radio 4's The Media Show this afternoon between Ford Ennals and Gillian Reyolds. Ennals was slippery as a fish but it was clear that there isn't going to be a switch-off campaign with a date as there was for television, at least not for many a year. In contrast Reynolds was incisive about why she feels that DAB is a rum deal. Hewlett did his best to be impartial

http://www.bbc.co.uk/programmes/b03brqlh

[DracoM]

Indeed, but very interesting that NO-ONE mentioned the fact that in a goodly number of places in the UK, both in towns and rural communities, digital reach is iffy at best and inoperable in others.

[Dave2002]

Indeed, but very interesting that NO-ONE mentioned the fact that in a goodly number of places in the UK, both in towns and rural communities, digital reach is iffy at best and inoperable in others.

I have hinted at such, but perhaps did not put it explicitly enough.