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The "Radio Crazy" Well-earned demise of AM IBOC.
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July 16th 04, 06:45 AM
Robert Orban
Posts: n/a
In article ,
says...
In article ,
Mark Roberts wrote:
On a synchronous detector, it sounds worse...like a mosquito buzzing
in the background.
That's because it's in quadrature, which is "invisible" (modulo
transmission artifacts and channel noise) to envelope detectors. Your
sync. detector is only detecting one sideband at a time, so the IBOC
carriers don't cancel out.
You have this exactly wrong. Envelope detectors are not sensitive to
the phase of the carrier, responding instead to the square root of the sum
of the squares of the I and Q components of the modulation:
SQRT(I^2 + Q^2)
(The carrier must be present in order to properly bias the envelope
detector, but, upon demodulation, the carrier simply appears as a DC term.)
A synchronous detector will detect either the I channel or the Q channel (or
some linear sum of the two) depending on the phase of the regenerated
carrier. It can be configured to reject the Q channel entirely by ensuring
that the regenerated carrier is precisely in-phase with the original
carrier.
To detect single sideband, you need two synchronous detectors whose carrier
inputs are in quadrature and whose IF inputs are also in quadrature. A
single synchronous detector will _not_ detect single-sideband.
See any textbook on modulation theory.
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