The Ten Myths of DAB

[Gordon]

No need Bryn, but thanks anyway.

As I'm here, people may like to have a browse at this lot, published last week sometime:

http://www.culture.gov.uk/publications/9733.aspx

should give this thread a pick me up!

[Resurrection Man]

......

should give this thread a pick me up!

Why, Gordon ? I found nothing to suggest that they are not still steamrollering along. £50 million for the marketing campaign?

Ludicrous waste of money. £40 million at the BBc for additional transmitters. That's £90 million. Upgrade a few FM transmitters for less.

There is nothing in the spec that I can see for the domestic device to say that any DAB portables must have a power requirement that doesn't need a spare set of rechargeables permanently available for when those in the set discharge after a few hours.

[Dave2002]

There is nothing in the spec that I can see for the domestic device to say that any DAB portables must have a power requirement that doesn't need a spare set of rechargeables permanently available for when those in the set discharge after a few hours.

Is it the case that batteries in a DAB set run down rapidly? Is this perhaps now also a myth?

According to http://www.culture.gov.uk/images/pub...rt_Issue_3.pdf the power for DAB is comparable with FM.

Did they do any testing with batteries? We have FM sets ready to go with batteries, and they usually last for weeks, maybe months. Is the same now true for DAB? I don't know.

In the disaster/emergency document they mention a short period. Is it not possible that a disaster/emergency could last weeks? Maybe the idea is that if the emergency is something like a nuclear strike it would be over so quickly, that consideration of battery life would become irrelevant, but there are other forms of emergency.

Another issue they have perhaps not considered is whether any people will have access to radio transmitters. This is an area I don't know about, and maybe things are not affected by national broadcasting. My understanding is that at present using AM it would be possible for private citizens to communicate with others, as there have been many people with small transmitters (radio hams etc. - think Hancock). I don't know whether any of the radio enthusiasts also broadcast using FM. I think it's unlikely that any/many will be able to broadcast using DAB - though I could be wrong. Is it an assumption that all communications will be mass communications - using networks, and from authorities (e.g government) to "the people"? It would seem that under some emergency conditions there could be a need for small transmitters, and mobile transmitters. Is this possible with DAB, and who would use/provide the services? At least with AM and FM it is likely that many people would be able to receive the channels broadcast, and certainly true that some would be able to transmit.

[An_Inspector_Calls]

Apologies to Gordon for my taking AIC's attribution of 'all or nothing' to him at face value. I should have gone back to Gordon's messages to check the validity of that attribution.

Which leaves me unconvinced that you've really grasped the concept of the digital cliff-edge. I can't find a decent reference to this (Digitalradiotech used to do this well, I think), so this rather grubby effort will have to do on the basis that a picture is worth a thousand words:

http://www.aerialsandtv.com/digitaltv.html#DigitalCliffEdge

[Resurrection Man]

Is it the case that batteries in a DAB set run down rapidly? Is this perhaps now also a myth?

According to http://www.culture.gov.uk/images/pub...rt_Issue_3.pdf the power for DAB is comparable with FM.

As far as I can see, that report does not draw any comparison of power consumption with FM radios. Although the table suggests that this is the case (viz FM only) when you look at the list of models measured, every single one of them is a DAB radio! I'd be delighted to be proved wrong. If I am right then it is yet another example of the hype and misleading BS being stuffed out to try and con us.

Did they do any testing with batteries? We have FM sets ready to go with batteries, and they usually last for weeks, maybe months. Is the same now true for DAB? I don't know.

Not as far as I can see which is a huge disappointment. Why didn't they address this and make it part of their specification? If it is a myth then you would expect them to take steps to try and disprove it.

In the disaster/emergency document they mention a short period. Is it not possible that a disaster/emergency could last weeks? Maybe the idea is that if the emergency is something like a nuclear strike it would be over so quickly, that consideration of battery life would become irrelevant, but there are other forms of emergency.
....

This emergency service is aimed as a "1 to many form" of communication (ie Govt ...or what's left of it....to us...or those that are left). It was initially driven by the Cold War and threat of nuclear annihilation. Still valid in todays troubled world but I don't think the concept of the lone survivor sitting in his homemade anti-Jihad bunker desperately transmitting on a home-made device is part of 'their' thinking!

[Resurrection Man]

On the subject of reception quality, my own experiences based on listening to a few FM and a few DAB portable radios is as follows...

FM...leaving aside birdies .....as the signal strength gradually falls, on a portable, it takes a very long time for any hiss to be (a) noticeable and (b) so bad as to make listening impossible. The switch from stereo to mono is neither here nor there since any stereo separation on a portable radio is going to be pretty minimal.

DAB...we seem to live in a marginal area and the signal fluctuates with some portables. So it's not so much a case of 'now you hear it...now you don't'. It's 'Now you hear it....now it's boiling mud..' DAB really does need a decent signal...and is much less tolerant ( as far as the listener is concerned) than FM.

[Gordon]

#302 what I meant by "pick me up" was a revitalisation of the debate here and another round of chat.

Yes, there is no mention of power supply - wonder why that is? A few other things aren't part of it either - like sound quality. I wonder why that is too.

#303 very broadly it is true that DAB sets consume power power than FM only sets, especially early DAB sets. Comparing like for like here is not straightforward though. SOME DAB sets can approach the same consumption as some FM sets. You can always find a simple analogue FM set that will be better than the fully functioned DAB set. In DAB/FM sets combined the FM part uses the same digital chip set as the DAB part [but possibly less of it] IOW it's a software decoder. Yes the sensitivity test will be done with batteries and mains where provided. Typically a battery powered DAB set will have a slighly better sensitivity than when powered from the mains because there self interference from the mains converter is removed.

The point about emergency transmissions is complex but in principle an FM transmitter isn't so hard to make - there are pirates in London that transmit FM - but an amateur DAB system might be a bit tricky and so there may have to be some portable ones that government [or Arqiva] have in their back pockets. If that is so I don't know about it. I think government's concern is that they can send out messages to the [remaining] public not that the public can do it.

#305 There is a detailed test specification to support these specs that has not yet been published but is being prepared by industry, including receiver manufacturers.

[Resurrection Man]

.....but in principle an FM transmitter isn't so hard to make ...

Neither is a UHF one! You just reminded me of a small one I made 30+ years ago to combat the noisy next door TV. Owned by a very BIG bloke, a gentle request to turn it down would most likely have been met with either a blank stare or a knuckle sandwich. So as I was working at a certain establishment with all the necessary test gear, I made up a tiny transmitter with which to try and 'programme' him into reducing the problem....along the lines of "something funny with my TV...whenever I turn the sound up, I get interference". Duly built, aligned and tested and taken to the flat. Switched on. Nothing. No effect. Lightbulb...I'd tuned to the Crystal Palace TX and not Sutton Coldfield - local to the flat. Back to work, quick re-tune, back to flat. Switch on. Again, nothing, but as I gently tweaked the tuning, I heard from his loudspeaker an increasing 'wah...wah...wah'. I heard him fiddle with the channel selector and had another brainwave ...as he fiddled, I turned my transmitter on and off..on and off....The TV was last seen being carted off to the repair man.

Silence...bliss.

[Gordon]

You are a naughty boy RM!! A friend of mine has a little box that does something similar in the mobile phone bands - very useful on trains.

[An_Inspector_Calls]

On the subject of emergency radios: surely we must retain our AM broadcasts on 198 kHz? I can appreciate that ordinary use of the service is declining (shipping forecast? - can we afford to ditch that?), but as an emergency broadcast it has no equal in terms of range (it can even be received underground because it has a strong ground-wave), and it's ability to 'get through' even when at low signal levels (a prime reason I believe why aviation voice broadcasts chose AM and not FM)? It's also ludicrously easy to make an AM receiver. And 198 kHz makes for a cracking long-range NDB (yes, it's modulated, which makes the ADF indicator twitch a little).

[Gordon]

Which leaves me unconvinced that you've really grasped the concept of the digital cliff-edge. I can't find a decent reference to this (Digitalradiotech used to do this well, I think), so this rather grubby effort will have to do on the basis that a picture is worth a thousand words:
http://www.aerialsandtv.com/digitalt...gitalCliffEdge

Thanks AIC not bad at all even if it is a bit "grubby", the graphic does work well enough to illustrate the point.

[Resurrection Man]

.....

Yes, there is no mention of power supply - wonder why that is? A few other things aren't part of it either - like sound quality. I wonder why that is too.

....

You make two very good points, Gordon. It is frustrating that there seems to be no 'champion' out there asking these sorts of questions.

[JFLL]

Neither is a UHF one! You just reminded me of a small one I made 30+ years ago to combat the noisy next door TV. Owned by a very BIG bloke, a gentle request to turn it down would most likely have been met with either a blank stare or a knuckle sandwich. So as I was working at a certain establishment with all the necessary test gear, I made up a tiny transmitter with which to try and 'programme' him into reducing the problem....along the lines of "something funny with my TV...whenever I turn the sound up, I get interference". Duly built, aligned and tested and taken to the flat. Switched on. Nothing. No effect. Lightbulb...I'd tuned to the Crystal Palace TX and not Sutton Coldfield - local to the flat. Back to work, quick re-tune, back to flat. Switch on. Again, nothing, but as I gently tweaked the tuning, I heard from his loudspeaker an increasing 'wah...wah...wah'. I heard him fiddle with the channel selector and had another brainwave ...as he fiddled, I turned my transmitter on and off..on and off....The TV was last seen being carted off to the repair man.

Silence...bliss.

By God, RM, you should market this. You'd make a fortune! (But Caliban will no doubt be along soon with his tiresome legal admonitions.)

[An_Inspector_Calls]

Gordon, yes but the one I had in mind had a nice display of the 80 dB high reception window, and the graph then illustrated the ~30 dB reception threshold, the 40 dB dynamic range of the RF section, leaving 10 dB of headroom. The beauty of that graph was it illustrated both the cliff edge, and the dangers of adding massive amounts of amplification to the aerial signal.

PS I'm pleased to see we're pushing DAB+ inclusion in the minimum spec.

[Gordon]

Gordon, yes but the one I had in mind had a nice display of the 80 dB high reception window, and the graph then illustrated the ~30 dB reception threshold, the 40 dB dynamic range of the RF section, leaving 10 dB of headroom. The beauty of that graph was it illustrated both the cliff edge, and the dangers of adding massive amounts of amplification to the aerial signal.

PS I'm pleased to see we're pushing DAB+ inclusion in the minimum spec.

Maybe I missed it in that link - was the graph you refer to there? I agree about the RF design issues, they tend to be buried and they are a bit of black art anyway. Massive amounts gain and DR but with a linearity requirement. Although applied to consumer electronics the front end design of DAB is near state of the art esp the required domestic sensitivity at 34.4 dB microvolts/metre and the CCI and ACI.

DAB+ was a struggle - manufacturers fought against it until the minister was persuaded [by whom I don't know] to rule in favour.