FD,
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You may have something here.
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IIRC - WWV runs a semi-suppressed Carrier with Dual Side Bands.
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I all of the International Broadcasters did the same and all
new Shortwave Radios had AM-SYNC. Then that just may be the
better 'improvement' for the Media of Shortwave Broadcasting.
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now why didn't i think of that ~ RHF