Pete KE9OA wrote:

FM stations in the United Stated use a 75uS deemphasis while European FM
stations use a 50uS deemphasis. The reason for this is because the FM
transmitting stations use a preemphasis, boosting the transmitted treble
response above a certain frequency. The end result of this technique is a
better signal to noise ratio at the receiving end.
Without a deemphasis network in your FM receiver, the received audio would
sound trebly, similar to, but not exactly not like listening to a dolby
encoded tape on a non-dolby tape playback machine.
AM stations do not use any preemphasis so a deemphasis network after the
detector in your receiver is not required.


Actually, since NRSC II, yes they do. Part of this was to
accomodate AM stereo and make the audio competitive with FM on
receivers with sufficiently narrow IF bandwidth to accomodate
largely interference free reception. NRSC II also brickwalled audio
below 10K, so preemphasis helped overcome the top end losses from
shoddily designed IF strips.

On a vintage wideband receiver, NRSC II sounds unusually bright.
And on an NRSC II receiver the audio quality isn't really what
anyone would call high fidelity. Or even medium fidelity. It was a
feel good response to cutting the nuts off AM's potential for decent
audio in order to accomodate interference free listening as
nighttime powers were increased for local AM's.

Neither the AM stereo nor the better receiver audio developed as
expected. Tell me you're surprised. With the exception of some
dedicated receiver manufacturers focussing on AM audio quality, like
Fanfare, or builders who produced the limited number of AMAX
certifed receivers, most AM manufacturers took NRSC II as a license
to build cheaper receivers letting the preemphasis at the
transmitter compensate for poorer circuit designs.








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

Pete

"Frank Dresser" wrote in message
...

wrote in message
roups.com...

It is late and I have been up a little too long.
I have spent an hour wading through my reffrence books,
and the ITU web pages.

Earlier today I found a reffrence that states that the
"standard Pre/De-emphasis is 75uS", and implies that
this is also true for HF.

I thought that 75uS was for non-Dolby FM,50Us for US TV and
European FM and that MW/AM and HF/AM had no pre/de-emphasis.
Have I lost what is left of my mind?

It's not quite as simple as that. All broadcast audio is processed.
Here's
a brief history:

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

Clear Channel is adopting IBOC and has developed a coincidental interest
in
reducing the bandwidth of AM audio:

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


I have been trying to decide on the best pivot point for my
tone-tilt. Craig at Kiwa used 700Hz. The best diagram I found
was from a guitar link and was designed to be used with spring
reverbs and had the pivot at 1KHz. I have been experimenting
and think I may have to have two, one for 700Hz for male voices,
and one about 1.4KHz for females.

There might also be a station to station difference. Stations can process
their audio differently.

Frank Dresser