Re: [IRCA] HD Question

[Charles A Taylor <MWDXer@xxxxxxx>]

At 01:35 PM 9/14/2007 -0700, you wrote:
>Why does the HD hash QRM the lower frequency more than the upper one?
>In the case of 650-KSTE, the QRM is worse on 640 than 660 (splits, 639
>is worse, 666 is clearer), in the case of KEX 1190, 1180 is worse and
>1200 is clearer (splits 1179 is trashed, 1206 in clearer).
>
>73,
>
>Patrick

Patrick,

The IBOC hash ought to be equal on both sidebands; however most
tube transmitters use Pi output tuning networks which tend to
attenuate the upper sidebands slightly more than the lower
sidebands. The math is simple: a Pi network, as usually con-
figured for a broadcast station, is a low-pass network with
the carrier frequency being the cutoff frequency.

Since HD is wideband, anything above the rolloff, or cutoff
frequency is excessively attenuated. This requires a redesign
of the output networks for symmetrical bandpass.

Once everyone goes IBOC, this won't be a problem anymore; or
at least not nearly as big a problem.

But, of course, IBOC will never be universal unless someone
prices HD (the latest nonce-word to avoid the four-letter word
"IBOC") low enough.

Ain't going to happen. Too many stations are right on the
financial edge. A good idea in the wrong place.

73 de Charles 


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