Re: [IRCA] NAB wants it both ways

["Craig Healy" <bubba@xxxxxxxxxxxxxxxxxxxx>]

> > Plus, don't the dual carrier sets provide a sort of diversity reception?
>
> The important diversity element is the time diversity between the analog
and
> digital signals, which of course doesn't apply to the all-digital system.
> The frequency diversity that exists between the digital sidebands on the
> upper and lower adjacent channels is a win on FM, where there is selective
> fading in mobile reception, but it doesn't do much for AM.

Why not on AM?  There are many times when the LSB or USB provides far better
reception.  Why wouldn't that apply here?  It's not selective fading, but
interference.  One sideband is almost always better except for very close
locals.  Why is the time diversity important on hybrid?  I thought the delay
in the analog channel was just to match the delay needed to process and
create the digital sidebands.  I was unaware there was any other reason.  Do
they use some sort of Forward Error Correction that needs time to work?
Every other signal I've seen with serious FEC doesn't need 8 seconds.

> The big
> difference between the hybrid and all-digital systems is that the primary
> digital sidebands get folded into the central 10 kHz of the channel in the
> all-digital system. This gets the digital signal that carries the core
audio
> stream out of the strong skywave interference on the adjacent channels,
and
> into the protected space of the assigned channel.  This would make a huge
> difference to the digital coverage (mono audio only though, since the
stereo
> enhancement stream still depends on the sidebands in the adjacent
channels).

I think in some cases it would be worse.  For example, if WNBH-1340 went all
digital, it's entire signal would be within the 1340 channel.  That's shared
by a bunch of other stations around here.  If it were in hybrid mode, the
digital information would be on 1330 and 1350, both far more clear than
1340.

> If I were to hazard a guess, I'd say if a station flipped to all-digital,
they
> could equal their analog coverage with maybe 6 dB less power (25%).  Of
> course, the digital signal doesn't degrade gracefully - when it falls
below
> the decoding threshold, the audio is gone, so you need to allow some extra
> margin for that when you assess the coverage.

That same concentration of signal also makes for an increase in
interference.  We all know the devastating effects of the 100% digital hiss.
It goes much further than analog.  Look at the difference in the US local
frequencies when they allowed them to stay at 1,000w nights.  Overall it
made little difference.  All it did was raise the signal above some ambient
noise.  It really didn't improve reception.

> > That makes the case for using the analog allocation system much more of
an
> > issue.  Consider the WYSL-1040 matter.  Having a 3400w station with a
two
> > tower array gain aimed right at them a few hundred miles away would
*never*
> > be allowed under analog allocation rules.  That makes this much worse
than
> > the 500w I had written earlier.  If they allow it, then it really voids
the
> > whole allocation procedure.
>
> Yep - I figure that the WBZ digital sideband on 1040 is actually
equivalent in
> its potential to cause interference to a non-directional analog signal of
> about 10 kW, as far as WYSL is concerned (same deal for KDKA on 1020,
which
> must have them a wee bit concerned!).  I've also calculated that WYSL's
> current NIF contour of 13.7 mV/m effectively rises to 28.2 mV/m when WBZ
has
> their IBOC on.  They have a strong case, if the FCC can shake off the
> politics and be made to see reason.

I don't think the FCC will do the right thing.  It will be up to the courts,
or perhaps the Congress.  It may even be the State Department if some of
your countrymen get your government to act on the interference.  There are a
number of stations in Canada suffering because of this.  Doesn't the
government act on such things?

> > Given the earlier Harris IBOC exciters don't support all-digital - or so
> > I'm told - I have to wonder if the receivers do.  I've never seen it
> > mentioned. With the all-digital at least a decade away, why would they
> > bother?
>
> It would cost little or nothing to include the all-digital functionality
in
> the chipset.  I guess it really depends on whether the licensing agreement
> with the receiver manufacturers forces them to implement that mode.  I'd
be
> very surprised if it didn't - after all, what if, against all odds, IBOC
> caught fire and became wildly popular?  Some stations might want to
convert
> to all-digital within a few years.  We both know that isn't going to
happen,
> but some wild-eyed optimists did think that way.

If someone were to write the firmware, yes.  It would still require extra
code and memory to include the selection algoritm, plus the decoder.  It
probably is somewhat different than the hybrid version.  Extra cost, and
hardly worth it for something that's at least ten years off.  I simply don't
know what's under the hood.  I did ask one of the local engineers and he
said they couldn't do all-digital mode.  So, I have no real way of testing
my receiver.  If I run into the other IBOC station engineer, I'll ask him.

> > Even more superior if they put it on another band.  No matter how good
> > Digital Radio Mundial is compared to IBOC, it still has to battle the
> > inherent problems of the frequency range.  Added noise, inability to
> > penetrate buildings, skywave propagation.  Those three difficulties
along
> > are enough to kill it.  And no amount of blather and bluster from
iBiquity
> > will change the physics.
>
> DRM has a very tough job on long-haul HF channels, but at MW, for
groundwave
> and one-hop skywave service, I think it should work quite well.  One of
the
> keys to success would be to pull in the bandwidth to 10 kHz, so it would
> truly be "on-channel".  The BBC and others in Europe are doing MW DRM
trials
> on 9 kHz channels, so it will be interesting to see how they make out with
> that.

It may be on-channel, but the noise goes up as well.  European DXers aren't
at all happy with their version of the Digital Disease.  Hiss-teria!

No matter what they do, it will not cure the problem of poor building
penetration, skywave interference, or higher ambient noise levels.  And this
doesn't even address the horribly dictatorial business model IBOC users are
forced to buy.  All in all, this is a really poor way to go digital.  They
appear to have taken the worst choice at every turn.  How could it even be
worse than this?

Craig Healy
Providence, RI

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