[cma-l] Ofcom announces trials to help small stations join digitalradio - 100w limit

Sun Mar 8 18:14:09 GMT 2015

I think you jumped in with a few too many assumptions Ian.

But you are right, even though I worked for a couple of years on VHF RF
propagation software models my pseudo equation didn¹t take into account of
all the nuances of the black art  my mistake.

The equation should be; 100W ERP equates to a small station MCA designed not
to annoy the commercial sector (which may or may not be 5km but depends on
encoding, modulation, propagation and a number of other factors which could
reasonably be estimated in software to arrive at the ITU recommendation of
field strength for DAB reception).

My point is 100W is not an arbitrary figure, someone at Ofcom has modelled
it. 

I was massively in favour of this test, although now I¹m coming around to
the opinion that the technical aspects can be fully tested without it
anyway. The idea came out of the lab¹ so one could imagine how they set
their budgets to move it the next stage.

Alan   

From:  "tlr at gairloch.co.uk" <tlr at gairloch.co.uk>
Reply-To:  "tlr at gairloch.co.uk" <tlr at gairloch.co.uk>
Date:  Sunday, 8 March 2015 16:14
To:  Apple <alan.coote at 5digital.co.uk>, "transplanfm at hotmail.com"
<transplanfm at hotmail.com>, Associated Consultants <info at a-bc.co.uk>
Cc:  "cma-l at mailman.commedia.org.uk" <cma-l at mailman.commedia.org.uk>
Subject:  Re: Ofcom announces trials to help small stations join
digitalradio - 100w limit

 Slow down there Ian - of course the factors affecting reception are
multiple and complex, and interact differently for FM and DAB, and of course
you can fit a higher gain antenna system into a given space at 200MHz than
you can at 100MHz. We all know those things, but they are irrelevant to what
I wrote... 

 All I was doing was musing on why Ofcom might have settled on a guideline
max 100w ERP. Nothing more nor less.

 Considering the power required to the carrier field strength and signal
reliability figures published by Ofcom for what it considers will generally
give adequate DAB coverage, versus the equivalent figures it uses for FM,
one does indeed find that this would support the idea that maybe they picked
the 100W figues as crudely equivalent to 25W FM in terms of meeting *their
definition* of adequate coverage.I thought I had clearly indicated that was
all I was doing, sorry if it wasn't clear.

 Re the RF end of things and the cost of a 300W unit, now it's my turn to
say "it's not so simple"! The transmitter's performance is heavily affected
by the nature of the modulation employed, and the 'RF end' must also include
any required filtering needed to meet regulatory limits. If you take a
typical 300W Band III amp design and run DAB through it at that power, its
non-linearity interacts so badly with the digital modulation that you would
need a lot more cooling of the amplifier and to spend a lot more on
expensive post-filtering of spurious products to the required DAB profile
(and probably find the amp subject to premature failure!). That is also a
very inefficient setup, with a lot of harmonic power being wasted as heat.

 I believe with current state of the market, it may turn out cheapest to
deploy a much higher power design of conventional amplifier and under-run it
a long way - eg use a 500W Band III TV amplifier and run it at 50-100W DAB.
Although the capital outlay is higher, the system is electrical more
efficient, and the transmitter runs much cooler and more reliably. Or you
can go for an inherently linear esign of amplifier, but these are far more
costly than traditional amplifiers. In the Ofcom engineer's Brighton trial I
believe he used a 100mW amplifier underrun at 5mW to achieve the required
linearity. 

 So at the end of the day, it is untrue to say there is no reason for a 300W
Band III unit to be much dearer than a Band II one. That is true for a
simple amplifier handling an FM or AM analogue signal, but for a digitally
modulated one the cost has to rise, either by the amp being more
sophisticated, or by fancy filters, greater cooling and a bigger electricity
bill. However, it's true the cost of achieving the desired ERP could be
minimized by use of a higher gain antenna array occupying the same space as
a lower gain Band II one would, and hopefully if this all takes off, there
will be market pressure to develop good digital amplifiers at lower cost.

 Reporting experiments by other EBU members, Ofcom also repo rted that a
prototype amplifier demonstrated that it would "not be too costly or
complicated to make an RF  amplifier for DAB which could deliver power
levels in the order of 100 watts" Hmmm... there's that 100W figure again.
Again, that could indicate part of the reason for Ofcom (who will be funding
the trial hardware) suggesting that the pilots be limited to 100W.

 Of course for a multiplexed service with all these costs shared by several
services the game changes, which is what can make it financially attractive
in densely populated areas. But for a lonely community station simulcasting
its output, the whole cost falls on them.

 Re your bit about signal strength and why Ofcom has looked at setting a
level of 72dBuv/m for DAB as opposed to 54dBuV/m for FM, it's hard to follow
your argument as you seem to be jumping between signal to noise ratios and
absolute power levels, but I think Ofcom has in fact explained its rationale
very clearly in technical publications.

 The FM 54dBuV/m figure is for urban areas with assumed very low levels of
electrical interference. The currently in force ITU-R BS.412-9
recommendation for stereo VHF FM is a median field strength of 54dBuV/m for
rural, 66dBuV/m for urban, and 74dBuV/m for dense urban.

 As it happens, Ofcom's guidance for DAB median field strength at 220MHz was
very similar: 54dBuV for cars and in rural environments, 63-68dBuV/m in
suburban areas, and 70-75dBuV/m in dense urban environments (at 95%
locational confidence). They have published extremely detailed tables of
data showing the exact derivation and assumptions underlying these figures.
The relevant ITU-R BS1660 recommended much the same, but they use a 99%
locational confidence and came up with a minimum of 58dBuV/m and a maximum
permissible interfering signal level of just 30dBuV/m..

 In discussing why they proposed moving to higher recommended field
strengths for DAB than originally planned, Ofcom's summary said:

 ----- 

  "Our aim was to make the most cautious assumptions possible as a starting
position for our analysis.
 [We found...] a very wide range of receiver performance but many receivers
met the standard assumed in the coverage planning model and so this
represents a practically achievable target....

   Given our cautious approach, the field strengths we propose using to
predict indoor reception of DAB are significantly higher than previously
used for DAB planning. (The previous value was 58dBV/m; we used 69dBV/m for
robust indoor reception in most areas, rising to 77dBV/m in dense urban
areas, but to retain 58dBV/m for in-vehicle reception.) We believe planning
to these field strengths will provide consumers with a better, more robust
listening experience than that available at present....
   In addition to planning for higher field strength, for in-vehicle
listening we have planned for reception in 99% of locations for 99% of the
time. This is a deliberately cautious approach at this stage which, in
practice, means we are planning coverage so that a listener would only lose
reception in marginal coverage locations if they happened to be sat in
stationary traffic during certain atmospheric conditions."

 ----- 

 One new thought occurs to me - since these trials as current constituted
seem to have fairly llimited potential for generating useful information
that can't already be demonstrated or deduced, perhaps they should be taking
the opportunity to make sure that some of the pilots use powers considerably
above and below 100W in similar circumstances to help assess the genuine
practical effects on coverage and interference!

 The sun has just come out for the first time in over a week of torrential
rain, so I'm off to get a life for a few hours now...!

 Cheers 

 Alex 

>  On 08 March 2015 at 14:01 Ian Hickling <transplanfm at hotmail.com> wrote:
>  
>   
>  It seem there's a lot of second-guessing going on here from people who may
> know a lot about administration and encoding but possibly not so about the
> black magic that is RF propagation.
>  There's no point in trying to relate 100W ERP to 5km for Band III DAB - just
> as it's equally irrelevant to relate 25W with FM to 5km - sorry.
>   
>  Topography, geology, refraction, refraction, foliation, antenna efficiency
> and launch conditions have far too large an influence.
>   
>  In terms of propagated signal transit, there's not a huge difference in
> practical terms between FM at say 100 MHz and DAB at 200 MHz when you take
> into account antenna size, efficiency, reflection and refraction.
>   
>  Because of the difference between demodulation formats, a  receiver can
> tolerate a much lower signal level on DAB than on FM to resolve an acceptable
> audio service. 
>   
>  This was originally proposed at 20dB from the point of view of transmitted
> power but then revised to 10dB - meaning that a DAB transmitter in Band III
> would need one tenth of the ERP of an FM transmitter in Band II to achieve the
> same audience. 
>   
>  Hence it is puzzling why Ofcom has set so high a required signal level for a
> DAB service area of the order of 72dBuV/m as opposed to 54 dBuV/m for FM.
> Beware - there is a distinct difference between a Power Decibel in
> transmission and a Voltage Decibel in reception!
>   
>    
>   
>  Let's not invoke DAB+ and DRM - Ofcom specifically rules them out in 2.30 and
> 2.32 
>   
>    
>   
>  Yes, Block 5A would be ideal as it's relatively clear, allocated and
> accessible to modern receivers - but Ofcom apparently doesn't accept that as
> it hasn't headed straight for it.
>   
>    
>   
>  As I've protested many times, there is technically nothing at all to prevent
> a standalone transmitter radiating a single programme stream to serve a
> discrete area either on DAB, DAB+ or DRM as far as I'm aware.  If I'm wrong
> I'd appreciate the exact reasons why.
>   
>    
>   
>  Looking at only the RF component in the transmission chain, several UK
> manufacturers could offer a 2U Band III 300W unit at around £2000 if the
> demand were high enough - no real cost differences from today's Band II units.
>   
>    
>   
>  Let's not get distracted - the encoding is software-defined - the actual RF
> transmitter is not!
>   
>    
>   
>  Ian 
>  
>   
>  
> 
> Date: Sun, 8 Mar 2015 11:13:25 +0000
> Subject: Re: [cma-l] Ofcom announces trials to help small stations join
> digitalradio - 100w limit
> From: alan.coote at 5digital.co.uk
> To: tlr at gairloch.co.uk; transplanfm at hotmail.com; info at a-bc.co.uk
> CC: cma-l at mailman.commedia.org.uk
>  
>   
>   
>  I can¹t help thinking that someone at Ofcom ran the simulations and came up
> with 100W = 5km radius.
>   
>    
>   
>  Therefore if small scale DAB became a reality it wouldn¹t annoy Radio Centre
> too much (they¹d still complain as that¹s their mentality) and at worst
> secondary legislation could make it happen.
>   
>    
>   
>  Kind Regards 
>   
>   
>   
> 
> Alan
> 
> 
> Hear Alan Every Week on Let¹s Talk Business The UK¹s Premier Radio Programme
> For Current and Future Entrepreneurs - Now Broadcast To Over 5 Million People
> <http://www.letstalkbusinessonline.com/>
> 
>   
>   
>   
>   
>    
>   
>  From:  " tlr at gairloch.co.uk" < tlr at gairloch.co.uk>
>  Reply-To:  " tlr at gairloch.co.uk" < tlr at gairloch.co.uk>
>  Date:  Sunday, 8 March 2015 00:45
>  To:  " transplanfm at hotmail.com" < transplanfm at hotmail.com>, Associated
> Consultants < info at a-bc.co.uk>
>  Cc:  " cma-l at mailman.commedia.org.uk" < cma-l at mailman.commedia.org.uk>
>  Subject:  Re: [cma-l] Ofcom announces trials to help small stations join
> digitalradio - 100w limit
>   
>    
>   
>   
>   
>  I simplistically presumed they settled on the 100W suggested limit on the
> basis that at the Band III frequencies of DAB it would give roughly the same
> coverage area (at 58dBuV/99%) as 25W on Band II (at 54dBuV/90%).
>   
>    
>   
>  NB the average  local DAB multiplex power is 1.3kW, not 2kW, but of course
> they tend to be from sites with much higher antennas than economically
> available to community stations, so the chances are the 100W represents an
> even tinier coverage area in comparison to current local multiplexes than
> might appear at first sight from a simple comparison of powers. But I can see
> it is much easier for Ofcom to control the allowed power than to get into
> arguments over exact percentages of area covered. Maybe 500W would have been
> more realistic if they wanted to take that simplistic approach, with a lower
> limit applied in the few cases where 500W coud cause difficulties.
>   
>    
>   
>  (I guess there is also the question that Ofcom is paying for the transmitters
> in the trial, and a band III amplifier running at , say, 250W is a lot more
> expensive than a 50W one, especially if one uses the technique of greatly
> underrunning a much higher power design to help achieve the necessary
> linearity.). 
>   
>    
>   
>  Seems to me that block 5A, (currently unused, but allocated for local DAB)
> could be used as a UK-wide frequency block for terrain limited single station
> services up to 500W to deal with all the areas where there is a low density of
> local stations (ie only one within the interference range of a 500W TX) and it
> could be done tomorrow, without any fancy trials or risk of interference,
> clearing out one whole tier of demand without any fuss, leaving trials and
> more complicated sharing and co-channel planning issues to be threshed out
> over time in the other seven frequency blocks allocated to local ensembles in
> areas of more dense demand. It's also much lower in frequency than the other
> blocks, which reduces the demands on the low-cost software defined
> transmitter. 
>   
>    
>   
>  Alex 
>   
>    
>   
>    
>   
>    
>   
>>  On 25 February 2015 at 13:04 Associated Broadcast Consultants <
>> info at a-bc.co.uk> wrote:
>>   
>>   
>>   
>>  We challenged the 100w limit in the consultation - suggesting that the "no
>> greater than 40% of the local commercial Mux area" was an adequate limit.
>> 100w is roughly 5% of the average existing DAB transmitter power, so
>> presuming community stations don't deploy their DAB transmitters using
>> tethered balloons or satellites etc they unlikely ever to get near 40% unless
>> they deploy multiple numbers of transmitters (thus undermining the low-cost
>> aim). 
>>   
>>    
>>   
>>  The standard consultation deflection response was invoked (ie: address a
>> different question) - stating that "it is not necessarily the case that
>> allowing a higher power will in all cases reduce the number of transmitters
>> needed". We never said it would in  all cases, but were suggesting that by
>> removing the 100w cap you retain some flexibility when it  would make a
>> difference in  some cases! Unfortunately though, consultations are single
>> shot - no possibility to clarify the point or challenge the response.
>>   
>>    
>>   
>>  I think we can all imagine the real (unstated) reason why they are limiting
>> it to 100 watts ;-)
>>   
>>    
>>   
>>  Don't get me wrong - 100w at 200MHz can still provide useful coverage if
>> planned correctly (other DAB coverage planning services are available!), but
>> in some cases more may be required. Otherwise we risk repeating the same
>> problem that analogue CR has - the paltry standard 25w power is often
>> inadequate and quite literally blasted off the dial by much stronger
>> commercial and BBC signals. And this problem is even worse with DAB (for
>> technical reasons that I will not go into here).
>>   
>>    
>>   
>>  Glyn 
>>   
>>  --  
>>   
>>  Glyn Roylance - Principal Consultant
>>  Associated Broadcast Consultants <http://www.a-bc.co.uk/>
>>   
>>    
>>   
>>    
>>   
>>    
>>   
>>    
>>   
>>    
>>   
>>   
>>   
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