HD Radio receivers and HD-n (n>1) subchannels
Thu Feb 19 10:36:11 EST 2009
> I think (but I'm not positive) that it was Aaron Read who pointed out
> to me (and the post may not have been on this list) that, in addition
> to the FM-band HD Radio subchannels that share the main 96-kbps
> bit-stream, the system allows for one additional 24 kbps bit-stream
> that uses OFDM subcarriers closer in frequency to the analog
> modulation (that is, closer in frequency to the channel's center
> frequency). This separate bit-stream would be ideal for talk
> programming. The problem with it, though, is that the proximity of the
> subcarriers that convey the additional information to the main analog
> channel is likely to cause audible interference (hiss, I assume) on
> the analog channel.
There are a number of misconceptions here, and now I'm thinking I should
really ship one of my "extra" HD tuners off to Arlington to give Dan a
chance to play with one firsthand.
As several on the list have already noted, there are no "pushbuttons"
for each stream. HD radios are entirely software-based, and the
subchannels are simply displayed in order after the main analog signal.
If there's no "HD-2," as in the case of WODS a week or two ago, it will
simply skip from "HD-1" (which, by custom and regulation, but not
technical necessity) is always a simulcast of the analog audio) to
"HD-3." Those are just labels assigned to virtual program streams in the
same way that "4.1" is displayed as the label for a program stream on
WBZ-DT's channel 30 RF transport stream.
The FM HD transport stream can be divided in more ways than Dan
imagines, too. Working just with the 96 kbps primary datastream, the
most common permutations are 48/48 (HD1/HD2), 48/24/24 (HD1/HD2/HD3) and
64/32 (HD1/HD2), but those values are almost infinitely adjustable, and
if a station wanted to do, say, 60/36, there's nothing in the system to
stop them. Nor is three subchannels a fixed limit - if you wanted to do
24/24/24/24 and have HD1-2-3-4, you could. I think that's part of what
ESPN's experimental HD FM in Bristol is testing.
About the only real restriction is that a stream can't be divided across
the partition between the primary 96 kbps stream and that additional 24
kbps "extended partition," so you can't treat it as a complete 120 kbps
of available datastream. It's nevertheless possible to do, say, 48/48
for HD1/HD2 on the primary stream and 24 for HD3 on the extended, or 96
for HD1 on the primary and 24 for HD2 on the extended. It's all
transparent to the end-user - the software in the radio figures out
where everything is and presents it to the consumer with no distinction
between primary and extended.
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.
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