On Saturday 02 September 2006 12:37, Chuck Hutton wrote:
> Is the main AM carrier locked to GPS with IBOC, or is GPS used to set the
> subcarrier frequencies, spacing, guard time, etc? My impression is the
> latter, at least for the installed base of legacy equipment. I bet Barry
> McLarnon can verify that.
The answer to both questions is "probably". :-) In IBOC parlance, the key
parameter is called "synchronization tolerance". There are two possible
levels of this, called (surprise, surprise) Level I and Level II. If Level I
is implemented, then all of the carrier frequencies (including the analog
carrier) and digital timing are locked to a GPS reference. For the carrier
frequency in AM IBOC, this means an absolute error of no more than +/- 0.02
Hz. If only Level II is implemented, then things are considerably looser,
and the tolerance on the carrier frequency becomes +/- 2.0 Hz. The use of
Level I specs is not mandatory, but it's recommended in the standard. Since
it wouldn't be very expensive to implement in most cases, I expect that most
stations would be Level I, so IBOC stations in general should make excellent
frequency references - especially if they're groundwave signals. Alas,
that's about the only thing IBOC signals are good for! BTW, if you're using
skywave signals for calibration, it's best to avoid the periods near sunrise
and sunset (or during propagation disturbances), when ionospheric Doppler
shifts may introduce some errors.
Barry
--
Barry McLarnon VE3JF Ottawa, ON
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