HD Radio receivers and HD-n (n>1) subchannels

[Aaron Read]

Yes, that was me who originally mentioned it, and Scott does an 
excellent job breaking it down.  There's just two things I'd like to add 
to it:

FIRST: the number of possible multicast channels is primarily limited by 
the available bandwidth.  IIRC, the standard will support up to an HD-9 
if you're in all-digital mode (not authorized by the FCC or iBiquity). 
However, since the digital streams get real crunchy at less than 24kbps, 
the effective limit is HD-4.  That said, there were some test stations 
trying to run an HD-4 stream (extremely low bitrate, I think 12kbps, for 
radio reading service purposes...sounded like crap but it was 
intelligible)  and they ran into a software bug in the radios that would 
cause them to lock up and require a "reboot" (power on/off) at random 
times if the HD-4 stream was present on the station in question; the 
listener didn't even have to be tuned to the HD-4 stream!

Obviously that was a huge problem, and I know iBiquity was trying to 
find a software solution on the transmitter end to get around it.  I 
don't know if they ever did but I assume so because this was a few years 
ago, and it was a pretty major issue.

SECOND: the alleged problems with extended hybrid modes interfering with 
SCA's on 92kHz is controversial because it's a station-by-station 
problem.  Just because it works fine at WDUQ doesn't, unfortunately, 
guarantee that it'll work fine everywhere.  It's because the problem is 
more with the fragility of the analog SCA signal; if the terrain or 
adjacent-channel situation is unfavorable...then ext.hybrid can take a 
marginal 92kHz SCA and, potentially, "push it over the cliff".

That said, I do wish WXXI would give it a try...the 48-24-24 scheme for 
their HD-1, -2 and -3 makes for crummy listening to WXXI-AM (on the 
HD-2) and the shows on the HD-3.  24kbps doesn't work nearly as well for 
talk as it does for music.  If it weren't for the purists who no doubt 
make up the bulk of WXXI's classical listenership, I'd say they ought to 
go 32-32-32, or go ext.hybrid and go 48-48-24 to at least make one of 
the HD-n's sound better.

-- 
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Aaron Read                  |  Finger Lakes Public Radio
friedbagels@gmail.com       |  General Manager (WEOS & WHWS-LP)
Geneva, NY 14456            |  www.weos.org / www.whws.fm

As for interference between the extended partition and the analog
signal, there's a great deal of dispute about whether it exists in the
real world. My employer here in Rochester won't use extended partition,
not because of concerns about the main-channel analog signal but because
there's a risk of interference to our 92 kHz analog SCA, which hosts a
popular radio reading service that we run. Yet in Pittsburgh, WDUQ uses
extended partition AND runs a reading service on the 92 kHz SCA,
apparently with no ill effects.