Re: Is the BBC deliberately degrading FM?

Doug McDonald wrote:
DAB sounds worse than FM wrote:


You are comparing apples to oranges. If you use multiple parallel
links, the bandwidth is there sum, as used in Shannon's theorem,
which is what matters. You can't do otherwise ... it's the laws of
physics. Shannon's law is a consequence of the Second Law of
Thermodynamics. You MUST multiply the bandwidth by the number
of channels to compare transmission efficiency. Note that
you could use multiple VSB channels and the same fudge
would apply.


In the MIMO literature they refer to the overall capacity, they do
not divide by the minimum number of antennas at each end. The whole
purpose of MIMO is to increase the overall capacity, so they're
hardly going to want to use the normalised figure, are they?

For example, someone designing a system where they know how many
antennas are going to be used is going to use the overall capacity
value, not the normalised capacity value, because they want to see
how much data they can squeeze through in total, not how much they
can squeeze through per antenna. Sorry, but you're just looking for
excuses because you said that
OFDM "IS inarguably inefficient", when in fact OFDM is nothing of
the sort,


You use "OFDM". No, "OFDM" without the pilot carriers and without
guard bands is not inefficient. COFDM without pilot carriers and
without guard band has identical average power efficiency to 8-VSB with
the same error
correct level and no pilot carrier. Note that that is AVERAGE power
level, which is what goes into Shannon's Law. COFDM **IS* less efficient
than 8-VSB
when considering "wall plug" power compared to ATSC, because of the
higher peak/average power ratio.

I originally said "DVB-T COFDM" and that's what I mean. What is
inefficient is the use of pilot carriers and guard band. DBV-T style
COFDM is inarguably inefficient compared to 8-VSB.

It requires more
power to deliver the same number of bits per second per Hz. And that
includes the 10% power inefficiency due to the (unnecessary) carrier
in the US ATSC 8-VSB.
Using multiple antennas does not change that ... if you use DVB-T on
one set of antennas and ATSC 8-VSB on that same set, the efficiency
ratio remains the same.


Tell it to someone who's interested, because quite frankly I'm not. OFDM is
the dominant modulation scheme used on modern wireless comms systems,
because there's far more to a modulation scheme than merely the power
efficiency - OFDM inherently mitigates against multipath, which is the
biggest problem in wireless comms, and 8-VSB does not. OFDM inherently
allows SFNs, 8-VSB does not, etc.


--
Steve - www.digitalradiotech.co.uk - Digital Radio News & Info


.

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