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

Mon Mar 9 10:37:45 GMT 2015

Alan - I was counteracting what I saw as too many assumptions from others - seen from my tiny specialised corner of expertise.I have grave reservations about software propagation models after having had the privilege of working for 4 years in practical range-based antenna development - simply because the human brain does not understand and therefore cannot predict or define how EMR functions in a real terrain environment.I can understand some of us here becoming disillusioned with the perceived purpose of this trial.I see it as an unexpected and very welcome initiative from Ofcom that some of us will actively support where we see a chance for development - and others will disregard as they wish.

Date: Sun, 8 Mar 2015 18:14:09 +0000
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
Subject: Re: [cma-l] Ofcom announces trials to help small stations join digitalradio - 100w limit

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 didn’t 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 I’m coming around to the opinion that the technical aspects can be fully tested without it anyway. The idea came out of the lab’ 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 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 Regards

          Alan

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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