In article , Tony
wrote:
"Jim Lesurf" wrote in message
...
The problem here, I suspect (again as IIRC Hawker indicates), is that
the broadcasters and set makers were 'promoting' FM for many years and
this was uphill work [pun].
Eh? Sorry I am being thick here! Clue please.
Pun based on the preference for transmit and receive antennas being high.
The problem which I was meaning is that for many years the general public
tended to stick with AM and ignored FM. But the BBC - and set makers - were
trying to get them to buy FM sets.
This was fair enough as the competition in those days was AM, and so
FM was pretty likely to be better. But it may mean they glossed over -
and then forgot about - these problems and just how likely they are.
Again fair enough if the choice is FM with some multipath versus the
interference-ridden AM.
I don't remember Pat Hawker's articles, but I have done a bit of work
in the past with a multipath 'scope display. I have to say that I am a
bit surprised that the problem is as severe as you describe. I nearly
always found that multipath could be made acceptable with the right
aerial in the right direction. But I wonder if the change to mixed or
circular polarisation might have made multipath more difficult to get
rid of.
This is one of the points he deals with. He reports German work that showed
that moving away from H polarization to V or non-planar *does* tend to make
multipath worse. Quite interesting to read what he says as he makes clear
he is aware that because he worked for the IBA he might be felt to engaging
in BBC bashing to question their decision to move away from H polarisation
for VHF/FM.
As regards the engineering decisions, multipath is not nearly so much of
a problem in mono as in stereo, as it is usually worst for high audio
frequencies with a large S content.
Agreed. This also means it is easier for people to overlook if they aren't
familiar with what FM can sound like when there is no multipath. This was
also something Hawker discussed.
So I think the problem mainly arose with the change to stereo rather
than the change from AM to FM.
The problems probably grew worse when stereo was introduced.
There was pressure on the BBC to do stereo, and I suppose they thought
that anyone who was interested enough to get a stereo receiver would
also get (or already have) a good outside aerial, which would nearly
always be needed anyway for adequate signal to noise ratio. But I'm
sure the engineers never forgot multipath, which is one reason why they
thought DAB was such a good idea.
My recollection is that they were aware of fading and flutter problems for
FM, but I can't recall those involved saying much about multipath
distortion on FM being an audible problem. The multipath resistance of
digital transmissions was, I think, mainly to ensure reliable reception
cover without fades (or ignition interference). Of course, the goalposts
moved here as we have gone from DAB for cars to DAB for general use!
TBH I am not sure about the engineers. May depend on the era you have in
mind. Only aware that the broadcasters essentially fell silent on the
matter. The articles by Hawker are the only ones I have found thus far.
I've been reading mags like the audio ones and WW on and off for decades
and I can't off-hand recall any other articles that examine multipath in
anything more than general terms - and assume it is a minor problem.
Interestingly, Hawker does mention some research the BBC engineers did, but
this was never officially published! That, I think, also says something
about the attitude at the time, but it is hard to know who made the
decision to not publish the results as a normal BBC paper.
Also my experience is that when I have in the past read about or asked
about multipath the 'standard response' has been along the lines of your
own initial comments. i.e. That it isn't much of a problem, and can
generally be cured by a good RX and carefully aligned antenna.
However, I now seriously doubt that is so for many people. Hence my
suspicion that the early engineers did know about this, but felt it didn't
matter, or wasn't something to bother people about. Then - later on - as
stereo and FM grew the mindset may have been established that multipath
wasn't a problem. So the then current engineers tended to assume it wasn't
a problem as FM had been going for years and no-one had told them it *was*
a problem.
As with my 'uphill' pun, I think the mindset was to promote FM and then
Stereo, not to make a meal of any snags. ( ahem You might like to draw a
parallel with DAB here. ;- )
Part of the problem is that a general analysis of this would have been
quite difficult before computers were commonly available. Easy now to
forget this and that the world before about 1970 was different to today in
this respect. General modelling of FM can be quite difficult.
Another part is that a statistically useful survey of the problem takes a
significant amount of time and effort. So not something engineers would do
unless they already thought there was a good reason.
Perhaps the history here is a parallel with 'how to cook a crab' stories...
:-)
Slainte,
Jim
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Dynamics and level compression - FM vs DAB
Hybrid Mode
