AM-band HD Radio bit rate

[Aaron Read]

With HD Radio, the total bitrate includes everything: audio, headroom, 
subsidiary services like PSD/PAD (and many others), multicast channels 
(if any are possible), etc.

For AM, the total bits is 40kbps, but with all the other stuff on there, 
the maximum for audio is about 32kbps, as you suspected.  Not enough for 
any multicasting, BTW.

http://en.wikipedia.org/wiki/Hd_radio

The codec is a "black box" that iBiquity fought long and hard to 
(successfully) keep out of the final NRSC-5 standard.  So nobody outside 
of iBiquity knows *exactly* what the HDC codec is.  In reality we all 
know that, originally, the codec as a pretty version of PAC (Perceptual 
Audio Codec) which was a dismal failure; sounded terrible at low 
bitrates.  After much scrambling, iBiquity (allegedly) put a wrapper 
around AAC and turned that into the new "HDC" codec.  So basically, it's 
AAC, and stations generally process & pre-process their audio expecting 
the usual quirks of AAC (at least as a starting point).

For a long time, many HD AM stations sounded like crap because AAC works 
better for music than it does for talk...as a lossy codec there's more 
"places to hide the missing bits" in music than there is in talk.  It 
took a while before processing companies like Neural, Orban and Optimod 
figured out the ideal ways to process news/talk for HDC.  Even now, not 
every station does it right; I've noticed that local to me, WHAM still 
sounds a lot crunchier/swishier than it should (although it's not all 
that bad) but WDCX sounds just awful.  WHTK seems to have gotten it 
pretty right, though.

Also worth noting, when you're multicasting on FM in "regular mode", 
you've got 96kbps total to play with.  So WODS is probably splitting 
their HD to 33/33/33 or maybe 48/24/24.  If the latter, it's entirely 
possible that WBZ-AM analog will sound "better" than the WODS-HD3.  The 
HD3 will probably have a lot more high end, but may also sound more 
crunchy/swishy...WXXI-AM vs. WXXI-HD2 here in Rochester is like that; 
their AM sounds just flat-out incredibly good (put that bad boy on a 
wideband receiver and I'm in heaven) but the 48/24/24 split on WXXI-FM 
means the HD2 sounds just so-so.  But as they promote it: no pesky 
pattern changes at sunset!  :-)


-- 
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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



Not counting subsidiary services (for example, whatever you call the
DAB equivalent of RDS), how many bits per second do they squeeze into
the AM-band HD Radio datastream? Is it 32 kbps? At the moment, I am
listening to what is reported as a 32-kbps Web stream from WHAT (AM)
Philadelphia. The content consists of adult standards music and
occasional voicetracked voice. I can't imagine anyone having any
complaints about the audio quality of what I am listening to with this
(rather mundane) content. In other words, 32 kbps through what I'm
assuming is a lossy CODEC seems just fine for routine suff such as
Frank Sinatra backed by Nelson Riddle (at least as good as the best
I've heard recently on AM) but would it cut it for classical music or
something else intended for an audio connoisseur?

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