caf (02/21/83)
The hills around Portland generate enough FM multipath to make AM
stereo an interesting subject. The following exchange (from the
Compuserve communications interest group bulletin board) may prove
interesting.
#: 505 Sec. 0 - General Interest
Sb: #AM Stereo
31-Jan-83 02:22:11
Fm: MARTY SOEHRMAN 70105,137
To: ALL
For what its worth, working for a station that has had both Kahn and Harris
stereo on the air, my subjective judgement is that Kahn sounds like AM radio
with 2 channels and Harris sounds like FM plus a little noise at our downtown
studio site 30 mi. from our xmtr site. True, Kahn can be successfully received
using two radios, one tuned high and one tuned low, but of course it sounds
terrible and who is going to go to the bother to do this anyway. Kahn is not as
loud and results in substantial spurious emmissions. My initial knee jerk
reaction was that the Kahn system would seem sensible, but it can be
demomstrated both in theory and in practice the linear Harris stereo is the
only one that does not prevent high fidelity AM reception. Harris does have
disadvantages such as higher distortion when using an envelope detector, but on
a 3 inch speaker, who'll notice. Any comments? Marty Soehrman[70105,137]
#: 508 Sec. 0 - General Interest
Sb: #505-#AM Stereo
31-Jan-83 18:44:45
Fm: DON HAINZL 76703,240
To: MARTY SOEHRMAN 70105,137 (X)
MARTY, IS YOUR STATION DIRECTIONAL AND IF IT IS HOW CRITICAL IS YOUR ANTENNA
SYSTEM. I HAVE HEARD THAT THE ANTENNA SYSTEM PLAYS AN IMPORTANT ROLE IN BEING
ABLE TO TRANSMIT AN AM STEREO SIGNAL? IS THAT TRUE? THANKS - DON
#: 515 Sec. 0 - General Interest
Sb: #508-AM Stereo
01-Feb-83 00:49:36
Fm: MARTY SOEHRMAN 70105,137
To: DON HAINZL 76703,240
Don, Our station is non-directional. Yes, antenna system performance must be
good for optimum stereo performance, but good antenna system performance is
also necessary for high fidelity mono transmission. Many non-directional AM
stations have antenna system problems such as narrow bandwidth that seriously
degrade even monaural performance. Anything that would attenuate sidebands or
carrier or create some kind of assymetry must be minimized. Directional systems
just create more opportunities for these difficulties to crop up. Also,
listeners in deep nulls might be apt to have reception problems due to these
effects. Other previously ignored transmitter system problems such as
incidental phase modulation degrade stereo performance. This is a problem with
our Harris transmitters. A good transmitter and a clean antenna system that
have worked well in mono should work well with stereo too. Marty
Soehrman[70105,137]
#: 519 Sec. 0 - General Interest
Sb: #505-AM Stereo
01-Feb-83 02:11:04
Fm: Chris Hays 70205,746
To: MARTY SOEHRMAN 70105,137
Your comments are most interesting as you have experienced both systems on one
station....a couple of your comments I will respond/rebute.... Using
KFI(Harris) and KRLA (Kahn) I would have to say that after a week or two of
observation I must question a couple of your observations.... First, KRLA
unquestionably has a cleaner sideband spectrum than KFI. KRLA has a mexican
neighbor a mere 20 khz away and we are causing them no trouble.... KFI has been
observed to occasionally spit on KOGO san diego...a whopping 70 khz away!!!
This may or may not have anything to do with the stereo system...at the moment
we are using very similar audio processing equipment. Second observation....the
quality of mono reception with the KAHN is noticeably cleaner (remember...it
will be MANY years before there are a significant number of stereo receivers
available and even then, most listening may still be done on inexpensive mono
receivers). I cannot comment on the stereo quality, as I do not have any
receiver which will receive Harris and only an early kludge prototype KAHN
receiver.. One final note on the KAHN system... an often overlooked fact about
the KAHN system is that it creates much more actual sideband power when
broadcasting stereo than 'normal mono'. It may be that reports of distortion
are more due to the inability of the transmitter to cope with this than the
system producing distortion....might also explain your comment about
loudness....question.... how important is stereo coverage to you? I personally
don't think Harris can create the coverage of the Kahn system...we can't afford
to lose an INCH of coverage to stereo...we have enough problems in some areas
as it is!!!!! Let's keep the dialog going.. this kind of discussion is what
will let us choose the best system..... One final comment....if a multi mode
receiver becomes available (Sansui is working on one) maybe two systems is the
best solution....one for stations who can afford to be hi-fi and escoteric and
another for those who have coverage problems....FM has had to put up with a 20
db degradation in signal to noise when the stereo light comes on...... we can't
afford ANY..... Chris
#: 684 Sec. 0 - General Interest
Sb: #AM Stereo
17-Feb-83 00:18:59
Fm: MARTY SOEHRMAN 70105,137
To: Chris Hays
Sorry that it took so long for me to respond to your last comments. I am still
here. Yes, it's true that we have not progressed much beyond the crystal set
stage in AM broadcast receiver design. I guess most of the public just do not
care so there is no incentive to apply current technology to AM receiver
design. I'm excited now because stereo has the potential to get the ball
rolling. By the way, WGN had Harris on the other day for a little while. I was
not impressed. Maybe the receiver I was using had a problem. Normally WGN has
been using Kahn. As you might predict, I have not been too impressed with that
either although their mono still seems to be OK. We have done a home brew
conversion to a Dymek for both Harris and Kahn and are now completing
modification of a Potomac which seems to be working very well. Also, Harris
supplied us with a modified Sansui but it has some bugs (like the stereo lite
never goes off - the 55hz notch is a 55hz oscillator). As far as the xmtr
proof, the rules are really pretty clear i.e. 73.40 (b)(2) "... When
stereophonic transmission is used, the distortion must be observed in the left
and right channels seperately modulated using a suitable stereophonic
demodulator". Noise measurements must also be made using this "suitable
demodulator". Trouble is, Kahn seems to be having trouble supplying a "suitable
demodulator" that works well enough. On your skywave interference problem which
is fortunately not a problem for us, don't forget that the synchronous detector
handles fades very well. Personally, I would rather sacrifice separation for
low distortion under these adverse conditions. Question - What proportion of
your rating survey area is subject to this 20% skywave interference? Also, be
advised that a test of Motorola on WLS is in the works. We expect we'll still
prefer Harris, but it'll be interesting. Marty Soehrman[70105,137]
#: 688 Sec. 0 - General Interest
Sb: #684-AM Stereo
17-Feb-83 02:12:18
Fm: Chris Hays 70205,746
To: MARTY SOEHRMAN 70105,137 (X)
VERY INTERESTING...THERE IS ONE CURIOUS MYTH, THAT HARRIS HAS PERPETUATED, AND
THAT IS THAT THE KAHN SYSTEM IS NOT COMPATIBLE WITH SYNCHRONOUS DETECTION....
JUST PLAIN NOT TRUE!!!!....SPOKE TO MR KAHN TODAY, AND HE CONFIRMED MY FEELINGS
THAT THE PREDISTORTION COULD BE REMOVED FROM THE SYSTEM WHEN THERE ARE A
PREDOMINANCE OF SYNCHRONOUS DETECTORS, AND THE SYSTEM WILL STILL WORK...IN
FACT, HE SUSPECTS THAT THAT'S THE WAY THINGS WILL, IN FACT, GO...AS NEAR AS I
CAN TELL, HIS RECEIVERS ARE SYNCHRONOUS ANYWAY! HE CLAIMS HIS NEW RECEIVER
SOUNDS BETTER THAN THE SO CALLED 'GREAT SOUNDING' SANSUI RECEIVER THAT THE
HARRIS USERS ARE RAVING ABOUT...SO WE WILL SEE....SPEAKING OF STATE OF THE
ART....SOME ENGINEERS ARE PREDICTING THAT WAY DOWN THE ROAD, AM MAY BECOME A
DOUBLE-SIDEBAND MEDIUM (REDUCED OR SUPPRESSED CARRIER)....WHERE WOULD THAT
LEAVE A CARRIER-DEPENDENT SYSTEM? CHRIS