[cma-l] Ofcom announces trials to help small stations join digitalradio - 100w limit
tlr at gairloch.co.uk
tlr at gairloch.co.uk
Sun Mar 8 16:14:41 GMT 2015
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
> <http://www.letstalkbusinessonline.com/>
>
>
>
> From: " tlr at gairloch.co.uk <mailto:tlr at gairloch.co.uk> " < tlr at gairloch.co.uk
> <mailto:tlr at gairloch.co.uk> >
> Reply-To: " tlr at gairloch.co.uk <mailto:tlr at gairloch.co.uk> " <
> tlr at gairloch.co.uk <mailto:tlr at gairloch.co.uk> >
> Date: Sunday, 8 March 2015 00:45
> To: " transplanfm at hotmail.com <mailto:transplanfm at hotmail.com> " <
> transplanfm at hotmail.com <mailto:transplanfm at hotmail.com> >, Associated
> Consultants < info at a-bc.co.uk <mailto:info at a-bc.co.uk> >
> Cc: " cma-l at mailman.commedia.org.uk <mailto:cma-l at mailman.commedia.org.uk> "
> < cma-l at mailman.commedia.org.uk <mailto: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 <mailto: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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