[BC] Digital Coverage

[Reader]

One more crosspost:

"Ira A. Wilner" <iwilner at monadnockradiogroup.com>

Hi Chris,

I just reviewed a USADR white paper.  It has a chart depicting first
adjacent interference between two FM IBOC stations showing one digital
sideband of each buried within the other's analog sidebands.  I think
their contention is that the alternate sidebands of both stations would
have sufficient redundancy to permit digital decoding.  But, elsewhere
they talk about the FEC being able to more robustly decode the signal
when both sidebands are available.

So, this begs the question, what is the performance difference when only
one side of the carrier is clean space?  It would seem from early real
world experience that it isn't working.  The BER quickly becomes
unmanageable.

OFDM is supposed to provide the necessary redundancy to assure
sufficient data recovery with selective fading/distortion of parts of
the subcarrier range of an IBOC signal.  The assumption being that only
some of the OFDM carriers will be affected at any given time.
Interleaving provides some time diversity for the symbols being sent so
that error dropouts will be of shorter duration.

QPSK modulation is sensitive to timing errors, jitter.  And guess what
multipath does!

In a fixed link transmission system, point to point with highly
directional antennas much of this environmental phase noise can be
eliminated.  Not so for moving receivers or stationary receivers with
omnidirectional antennas.

Another eye opener is part of the 8 second analog delay has nothing to
do with digital processing propagation delay.  Rather it is done to
provide some time diversity so a big noise hit taken by the digital
signal will not coincide with the analog.  That way when an IBOC
receiver blends to analog there will hopefully be usable analog audio!

So, unless the IBOC side of receivers become far more multipath proof,
there will always be a need for analog fallback with time diversity!
Ironically, without the analog sidebands the digital side could be
expanded to include sufficient interleaved redundant symbols to provide
its own time diversity.

--Ira Wilner
Chief Engineer
Monadnock Radio Group
Saga Communications, Keene NH