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

Dan.Strassberg dan.strassberg@att.net
Thu Feb 19 09:01:47 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.

Are existing HD receivers able to decode this extra bit-stream? If so,
how does the listener select it? My understanding is that HD receivers
are equipped with HD-1, HD-2, and HD-3 pushbuttons. If there is no
HD-4 button, is one of the main-bit-stream subchannels sacrificed if
you tune to a station that is transmitting the extra bit stream? If
yes, is the sacrificed channel always HD-3? That is, if a station
chooses to use the extra bit stream, is the normal HD-3 channel always
the one sacrificed? Or is there, perhaps, just one HD-channel
pushbutton that, through repeated presses, cycles through all of the
HD subchannels that the selected station is transmitting?

Also, my understanding is that the HD-1 channel can use either the
full 96 kbps of the main bit-stream, or 48 kbps, or 32 kbps. And the
HD-2 bit-stream can use either 48 kbps, 32 kbps, or 24 kbps, whereas
the normal HD-3 bit stream must use either 32 kbps or 24 kbps. Thus,
using only the main bit-stream, the possible combinations are limited
to HD-1=96 kbps, HD-2=0, HD-3=0; HD-1=48 kbps, HD-2=48 kbps, HD-3=0;
HD-1=48-kbps, HD-2=24 kbps, HD-3=24 kbps; or HD-1=32-kbps, HD-2=32
kbps, HD-3=32 kbps. No other combinations are permitted. Is my
understanding correct?

In addition, does the receiver determine the selected subchannel's bit
rate and automatically adapt to it?

Finally (for now, at least) are any stations transmitting the extra 24
kbps bit stream and does it really cause unacceptable interference
with the main analog channel?

Dan Strassberg (dan.strassberg@att.net)
eFax 1-707-215-6367

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