"Joe Analssandrini" wrote in message
oups.com...
Dear "Telamon,"

I hear this "note-flattening" on music fairly frequently when listening
to music via a station whose signal is variable in strength (when
listening on my Satellit 800 with AGC set to "slow," sync circuit
"on,." using 6.0 kHz [nominal] filter or the 4.0 kHz one, and the
station's frequency set to "spot-on"). Obviously, I cannot tell others
when they will hear this effect but it is very noticeable here on the
east coast when the VOR (whose signal varies in strength) is playing
their interval signal. Try listening to that. On the Satellit 800 (and
the two Drake radios I heard - R8B and SW8), a sustained note will
appear to change frequency, "flattening" momentarilyand then returning
to its proper frequency. This happens often.

It never happens with my AOR AR7030 Plus receiver.

Best,

Joe

Perhaps I could throw my two cents into the discussion. Qualify them by
saying I have no experience with any of the radios mentioned. What I have
experimented with is add-on synch AM detectors (both homebrew hardware synch
AM and software DSP with a 455kHz/15kHz downconverter) and Radio Shack
DX-394A and DX-394B radios as tuners (OK, OK, don't tune out just yet!).
With deep, rapidly fading signals, both detectors would warble, wow, groan
when fed by the 394A. Changing the RF Gain control setting on a stable
signal had a similar effect.

Some time later, I found that the detectors did not warble, wow or groan
with the DX-394B. The difference turned out to be that the B has a buffer
between the 2nd local oscillator and the 2nd mixer whereas the A does not.
This isolates the oscillator from variable loading caused by changing the
gain of the 2nd mixer. The 2nd mixer is on the AGC line so varying signal
strength affects its gain/loading and the AGC voltage is also affected by
the RF Gain control. I added a buffer to the A and now it's as stable as the
B.

Maybe there is something similar happening to a slight extent with those
radios on which you hear slight pitch changes. Rather than a warping
upstream oscillator with gross effects as in my case, maybe the PLL
oscillator in your higher class radios is being pulled by changes in the
strength of the input signal. You mention that it is most noticeable when
you use slow AGC - that would cause the most extreme and prolonged variation
of signal strength at the input of the detector. Why not use fast AGC? That
seems to be the norm for AM reception. Do you use slow or fast AGC on the
AOR7030?

Tom