"Pete KE9OA" wrote in message
...

[snip]

AM stations do not use any preemphasis so a deemphasis network after the
detector in your receiver is not required.

It's my understanding that there's no FCC standard for AM stations to
preemphasize their audio, but nearly all do in order to compensate for the
normal roll off. I have an old school high fidelity AM receiver, and AM
stations usually sound rather shrill on the "Hi-Fi" IF bandwidth position.

A quick google search brings up:

"In 1977 Orban Associates introduced "Optimod-AM." This unit contained a
high-slope receiver equalizer to pre-compensate for the highly rolled-off
radios of the time, "

http://www.bext.com/histproc.htm

"and Omnia exclusives like a pre-emphasis section placed behind the
multi-band limiters to create a more consistent, natural sound."

http://www.omniaaudio.com/am.htm


It is true that digital modes such as IBOC are being use on the MW band,
but
this is a totally different technique.

Pete


I brought up IBOC because Clear Channel wants all AM radio stations to limit
their audio bandwidth to 5 - 6 kHz. One of the reasons for the change was
given:

"1. Increased modulation efficiency. By eliminating the broadcast of the
high-frequency energy, we can increase the amount of energy that is in the
20 Hz to 5 kHz region. Let's not forget that due to pre-emphasis, higher
frequencies are boosted and will have a more profound effect on total
modulation than lower frequencies will."

http://www.rwonline.com/reference-ro...andwidth.shtml

I don't know if the IBOC stations use a different preemphasis on the analog
channel than the non IBOC stations.

Frank Dresser

Hi Frank,

I didn't realize that..........I thought that the older AM stations took
greater pains to have flatter high frequency response. I do remember
listening to stations such as WGN on my old McIntosh MR-55, and they sounded
very good. The same stations on my Dynaco AF-6 tuner in its wide bandwidth
sounded shrill, as you describe.
As far as AM receivers, they do not have a deemphasis circuit following the
detector. With FM receivers, it is a different story.
Thanks for the info!

Pete

"Frank Dresser" wrote in message
...

"Pete KE9OA" wrote in message
...

[snip]

AM stations do not use any preemphasis so a deemphasis network after the
detector in your receiver is not required.

It's my understanding that there's no FCC standard for AM stations to
preemphasize their audio, but nearly all do in order to compensate for the
normal roll off. I have an old school high fidelity AM receiver, and AM
stations usually sound rather shrill on the "Hi-Fi" IF bandwidth position.

A quick google search brings up:

"In 1977 Orban Associates introduced "Optimod-AM." This unit contained a
high-slope receiver equalizer to pre-compensate for the highly rolled-off
radios of the time, "

http://www.bext.com/histproc.htm

"and Omnia exclusives like a pre-emphasis section placed behind the
multi-band limiters to create a more consistent, natural sound."

http://www.omniaaudio.com/am.htm


It is true that digital modes such as IBOC are being use on the MW band,
but
this is a totally different technique.

Pete


I brought up IBOC because Clear Channel wants all AM radio stations to
limit
their audio bandwidth to 5 - 6 kHz. One of the reasons for the change was
given:

"1. Increased modulation efficiency. By eliminating the broadcast of the
high-frequency energy, we can increase the amount of energy that is in the
20 Hz to 5 kHz region. Let's not forget that due to pre-emphasis, higher
frequencies are boosted and will have a more profound effect on total
modulation than lower frequencies will."

http://www.rwonline.com/reference-ro...andwidth.shtml

I don't know if the IBOC stations use a different preemphasis on the
analog
channel than the non IBOC stations.

Frank Dresser

"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

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

"Pete KE9OA"
....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.
______________

In the US, broadcast AM pre-emphasis is defined by a voluntary standard of
the Nat'l Radio Systems Committee. The tx audio response is a modified 75
us curve. The curve has an 8700 Hz break frequency to reduce adjacent
channel interference.

The NRSC standard expects the amplitude response of the narrow RF/IF
bandwidth of "typical" MW broadcast receivers to restore ~ flat system
response, not that a network complementary to that at the tx be added to
audio circuits following the 2nd detector. However that is not
prohibited -- it is just more expensive. Also, that approach to
implementing AM pre/de-emphasis would not be "backward compatible."

RF (retired broadcast field/systems engineer -- RCA & Harris Corp)

Visit http://rfry.org for FM transmission system papers.

I agree...............still, you don't see a deemphasis network actually
following the detector in AM receivers. Are there many stations actually
using the curve?

Pete

"Richard Fry" wrote in message
...
"Pete KE9OA"
....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.
______________

In the US, broadcast AM pre-emphasis is defined by a voluntary standard of
the Nat'l Radio Systems Committee. The tx audio response is a modified 75
us curve. The curve has an 8700 Hz break frequency to reduce adjacent
channel interference.

The NRSC standard expects the amplitude response of the narrow RF/IF
bandwidth of "typical" MW broadcast receivers to restore ~ flat system
response, not that a network complementary to that at the tx be added to
audio circuits following the 2nd detector. However that is not
prohibited -- it is just more expensive. Also, that approach to
implementing AM pre/de-emphasis would not be "backward compatible."

RF (retired broadcast field/systems engineer -- RCA & Harris Corp)

Visit http://rfry.org for FM transmission system papers.

"Pete KE9OA"
Are there many stations actually using the curve?

Probably at least 1/2 of them are (there's no official record that I know
of).

Nice website!

Thanks.

RF

Nice website!

Pete

"Richard Fry" wrote in message
...
"Pete KE9OA"
....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.
______________

In the US, broadcast AM pre-emphasis is defined by a voluntary standard of
the Nat'l Radio Systems Committee. The tx audio response is a modified 75
us curve. The curve has an 8700 Hz break frequency to reduce adjacent
channel interference.

The NRSC standard expects the amplitude response of the narrow RF/IF
bandwidth of "typical" MW broadcast receivers to restore ~ flat system
response, not that a network complementary to that at the tx be added to
audio circuits following the 2nd detector. However that is not
prohibited -- it is just more expensive. Also, that approach to
implementing AM pre/de-emphasis would not be "backward compatible."

RF (retired broadcast field/systems engineer -- RCA & Harris Corp)

Visit http://rfry.org for FM transmission system papers.