[BC] Bitrates

[Barry McLarnon]

On Sunday 22 April 2007 14:12, Craig Bowman wrote:
> Both AM and FM Ibiquity systems place the digital carrier fully in the
> adjacent channel. ?It "fits within the mask" is the excuse people use to
> call it IBOC but the (FM) mask was developed for this purpose to allow
> such encroachment. ?Both systems are IBAC plain and simple. ?IBOC is not
> a proprietary system it is an acronym (as below) for In Band On
> Channel. ?Ibiquity has a proprietary system called HD Radio. ?I look at
> it the same as real estate. ?You might have a right-of-way ?for the
> state or town three feet from the curb but when you neighbor dumps his
> garbage there the excuse that it is on state right-of-way (within the
> mask) does not cut it.

It's true that the FM system is 100% IBAC.  The AM system is mostly IBAC, but 
it differs from the FM system in that some of the digital carriers share 
spectrum with the analog signal - namely, the "tertiary" digital sidebands 
are at 0-5 kHz on either side of the carrier.  Then there are the "secondary" 
digital sidebands at 5-10 kHz from the carrier, and the "primary" digital 
sidebands (containing most of the digital power) at 10-15 kHz from the 
carrier.  See http://topazdesigns.com/iboc/AM-IBOC-Parameters.html for all 
the gory details.

As for the bit rates in the AM (hybrid) system: the primary sidebands provide 
a 20 kb/s "core" mono audio stream.  The secondary and tertiary sidebands are 
combined to provide an additional 16 kb/s "enhanced" stream for parametric 
stereo.  The latter stream is less robust, so there will be occasions when 
only mono audio is received (and, of course, many occasions when it falls 
back to analog!).

Barry

-- 
Barry McLarnon  VE3JF  Ottawa, ON