Let's try that again.............AM receivers don't have a deemphasis
circuit. You can state that the composite response of an AM receiver implies
a deemphasis circuit, but that is not really true if you consider a
deemphasis circuit as having a standard turnover frequency and a standard
rolloff characteristic (so many dB per octave). (I do understand your point,
though).
A deemphasis circuit, as applied after the detector in FM receivers is
designed to have a specific rolloff characteristic that is the complement of
the preemphasis characteristic applied at the transmitting end.
This holds true for broadcast stations as well as land mobile equipment.
Take a look at any of the more recent stereo FM tuners/receivers and you
will see that this characteristic is determined by the external components
in the line amp of the stereo decoder chip.
A TDA1591 data sheet gives a good example of this circuit.
And you are right.....it would be very difficult to have a standard
preemphasis curve for AM stations, because there are so my receivers with
different characteristics because of different I.F. bandwidths and different
rolloff characteristics in the audio chain.

Pete

"Richard Fry" wrote in message
...
"Pete KE9OA"
As far as AM receivers, they do not have a deemphasis circuit
following the detector. With FM receivers, it is a different story.
___________

But in effect, almost every AM receiver has de-emphasis -- it is the
result of the "haystack" amplitude response of their RF/IF circuits
driving the 2nd detector. Higher modulating frequencies are rolled off,
sometimes very severely.

The problem with trying to compensate for it at the AM tx is that the
amount needed for "flat" system response varies considerably from rx to
rx, and even with carrier frequency. Even so, many MW/SW AM stations do
boost their highs, trying to get some of it to pass through the narrowband
receivers typically in use these days.

RF