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

Sun Mar 8 14:01:06 GMT 2015

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 can’t 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 wouldn’t annoy Radio Centre too much (they’d still complain as that’s their mentality) and at worst secondary legislation could make it happen. 
Kind RegardsAlan

Hear Alan Every Week on Let’s Talk Business The UK’s Premier Radio Programme For Current and Future Entrepreneurs - Now Broadcast To Over 5 Million People 

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

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