Re: [IRCA] Critical flaw in HD sub-channels?

[Barry McLarnon <bdm@xxxxxxxxxx>]

On Friday 13 October 2006 07:45, Dan Strassberg wrote:
> Someone on another list to which I subscribe commented on his experience
> with an HD car radio that he had just purchased as a replacement for a car
> radio that had died. He was asking about the lack of sensitivity on FM in
> the digital mode. I gather that, on the main HD channel, the mode flipped
> to analog in areas where the analog signal was still quite strong, so that
> if he considered the reception range to extend to where the analog signal
> was no longer listenable, the sensitivity was about what he expected.
> Although he wasn't especially happy about the mode flipping back and forth,
> he had no complaints about the radio's overall sensitivity on the main HD
> channels.
>
> BUT on the HD subchannels (mostly, if not entirely, HD-2), the flip was, of
> course, to just silence. In other words, the HD subchannels had very
> limited range--such limited range that their viability in a car would seem
> to be very much in question. When I read his posting, my immediate reaction
> was "Of course; why didn't everybody foresee this fatal flaw?"

It's pretty simple, really... this flaw wasn't foreseen because HD2 wasn't 
foreseen.  When IBOC was developed in the 1990's, it was seen purely as
a simulcast technology.  With the audio codec technology of the day, it was
thought that the full 96 kbps throughput of the FM IBOC system would be needed 
to get one decent quality audio stream, and even that was a stretch.  Besides 
which, it was assumed that consumers would flock to the new technology simply 
because it was digital, even though it offered no new content (except maybe 
some data like song titles and the like).  Of course, experience with DAB in 
countries other than the UK has subsequently shown that digital simulcasts 
are a non-starter as far as interest from the radio-buying public is 
concerned.

HD2 came along a few years ago because NPR (and related pubcasters) were 
concerned that FM IBOC wasn't shaping up as they had hoped.  There was only 
one audio stream, and nothing equivalent to the FM subcarrier (SCA) services 
that they used for reading services for the blind, etc.  They started a 
project called Tomorrow Radio to test the idea of carving up the 96 kbps into 
two or more audio streams.  In listening tests, they found that with the 
latest IBOC codec (thanks to a licensing agreement iBiquity made with Coding 
Technologies in Germany), quality was still pretty good at 48 kbps.  It 
started to get dicey at 32 kbps, but it was still good enough for some types 
of programming.  The commercial guys took note of this, and they were also 
aware of the dismal performance of DAB in countries such as Canada, where it 
was tried as a simulcast service.  They seized on the Tomorrow Radio idea, 
dubbed the secondary audio service HD2, and got behind it as the potential 
killer app that would give some value-added appeal to IBOC and allow it to be 
more competitive with the satcasters.  The FM IBOC spec was updated to allow 
multiple audio streams (up to 8!), and HD2 services started to roll out... a 
lot of them are automated jukeboxes, or simulcasts of AM stations, of course, 
but that's another story.

Amidst the hoopla, one troubling aspect of the Tomorrow Radio experiments was 
ignored.  In tests with four different stations, NPR found that reasonably 
reliable digital coverage didn't extend all that far - it ranged from the 60 
dBu to the 70 dBu contours for the four locations.  On average, the digital 
coverage was found to extend to the 66 dBu contour.  For most FM stations, 
this doesn't come anywhere close to duplicating the analog coverage.

> Most radio listening, I'm told, is done in cars. If the HD subchannels are
> usable only in areas of very high signal strength, hardly anybody will
> listen to them in cars, which means that Rich's scenario about a glut of
> spot inventory depressing ad rates nationwide will never come to pass. It
> would seem that availabilities on signals that hardly anybody finds
> listenable are not availabilities at all. Moroever, if the HD Dominion is
> pinning its hopes on the availability of signals that are largely
> unlistenable, isn't the Dominion pinning its hopes in the wrong place?

Yep, but what choice have they got?  Other than offering new services that
aren't available with conventional radios, what does HD have going for it?

The limited coverage of the IBOC digital signals is no doubt a major reason 
why the car manufacturers aren't exactly embracing this technology with open 
arms.  None of them have committed to offering HD receivers as factory 
equipment (except for BMW, whose receiver doesn't support HD2 reception).
They don't want cars returned by customers who are frustrated with radio 
reception that keeps dropping out.

The only solution is to apply bandaids.  I notice that several manufacturers 
are gearing up to offer FM IBOC repeaters that can extend and fill in holes 
in the digital coverage.  The downside is that this can only be done at the 
expense of damaging analog coverage in some areas.

There are tough days ahead for the HD Dominion... not to mention the fiasco 
called AM IBOC.

Barry

-- 
Barry McLarnon  VE3JF  Ottawa, ON
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