Following our discussion about “bad language” on certain programs, KDX activated a low low power transmitter on 1550kHz to allow shutting down our full-low-power transmitter at 1680 during periods of bad language. But the 1550 transmitter puts whistles at 1530 and 1570kHz.
Awhile back this situation was discussed here on this site, and the problem was found to be from our FM studio-to-transmitter link. Evidently a 19kHz stereo-pilot tone is appearing on the audio lines from the Technics FM tuner to the ssTran3000 transmitter. Removing jumper S6, treble boost, in the transmitter, does not prevent the whistle.
This whistle was first observed using a C.Crane FM transmitter to send our signal to the AM equipment location, and of course the stereo generator cannot be switched off in the Crane. So we installed a Wholehouse 2.0 which does allow turning off the stereo, but it would appear that it does not cancel the pilot-tone signal at 19kHz.
We’ll probably need to build a low-pass filter that chops the 19kHz by perhaps 20dB, but I bring the problem to the table for any ideas you might have for stopping the whistle.
Passing 19Khz gas….
LOL!!! Oh boy do I remember this one from those early days of figuring out what was what!!!
I first discovered this “passing” of the 19khz pilot frequency from a received stereo FM signal via a stereo receiver and piping that output into a mono FM transmitter. Though the 19khz pilot lit up, there was no “stereo” audio on the second FM transmitted signal simply because there was no
32khz sub carrier being passed as well…which in FM the L-R information rides on the 32khz sub carrier!!!
But boy was it exciting to see my little ol one transistor FM oscillator making FM receiver’s stereo light turn on!!!
Ahh those were the days eh….discovery and fascination….cant beat that with anything these days!
Your experiencing the 19khz pilot signal embedding onto the audio being fed into the AM unit and producing the appropriate side bands from the 19khz pilot signal modulating the carrier. Thus the whistling on either side of your carrier. Best way to eliminate this is to switch the FM receiver to “mono” reception prior to feeding the audio into the AM unit….no need for any filter, or use a simple mono FM receiver.
But to get really creative in a solution…use a cheap equalizer, say a 5 band unit and just dump everything above 10khz all the way down to minimum..that should work and remove the 19khz.
RFB
i have to agree with RFB on this
i have to agree with RFB On this, i think he’s right about the sub carrier and the L to R information on the 32KHZ.
Can’t
Unfortunately this tuner is total stereo with no mono option. It was made as a rack-mountable futuristic hi-fi super-slick tuner even with an S-meter, and what un-house-broken character would dream of switching such a tuner to mono? Can’t do it.
Maybe I just need a cheaper FM tuner. Don’t have one of those equalizers.
C Crane pilot
You can easily kill the pilot in the c crane by removing or shorting out the pilot crystal,38khz. You could even add a switch if you like. Mono also has better range as well.
low pass filter
As already suggested, just cut off everything audio above 10k. There are any number of ways.
The main one would be a low pass shelf filter you can set the high end to cut @ 10k, between the FM receiver output and the TX audio input. E.g., My current ol’ Tascam mixer has a shelf filter. If I turn it down to 10k, it should get rid of the pilot tone and carrier, but I don’t rebroadcast anything from FM right now.
Anyways, if you are passing the FM audio through your mixer, just do that and see what happens.
Plans A and B
The advice is all good, and here are two plans both on the front burner.
Plan A is to build an audio low-pass filter to get that whistle stopped, especially at 1570 where an actual station across the river is weak but very good with talk programming.
Plan B is to follow through with a stereo-off switch on the C.Crane, because it’s true, a mono FM signal is good over a larger area.
Unfortunately the Wholehouse is built like it was never meant to come apart, so it may not be possible to modify it without breaking the case with a nut-cracker and building the whole thing into a different housing. That’s a play on words, if you look hard enough.
There’s always another small problem in the world of low power radio.
Supposed To Be Done
Actually,
Receivers are supposed to filter out those tones,
On my AM, which is fed by an FM tuner, there is no whistle present 19 khz up.
FM Receiver filtering
Most usually do filter out nicely…very sharp cut off at around 15Khz internal to the domod chip. Usually found in very well designed units…and some of the other less pricy units.
But…lets take into the account that a majority of tuners audio outputs frequency range goes from 30hz up to 20Khz and sometimes a bit more…a little bit of that 19K will be there just enough to wreak havoc in certain situations..such as feeding that audio over to another FM oscillator or an AM oscillator and put the 19Khz modulation onto the carrier and produce the appropriate side bands. Since the 19Khz signal is constant and not changing amplitude like regular audio does even at 19Khz, the “whistle” will be consistent and no doubt also beat frequency with stations on a nearby frequency.
Remember the bandwidth of an AM channel is 10Khz, NRSC curve calls for a cut off at that same point..leaving only 5khz either side of that carrier. Shoving 19Khz through the audio onto an AM carrier..well we end up with small 19Khz side bands just sitting there while below that the regular audio side bands doing their normal thing too.
And it really does not take much 19Khz injection at all to produce the whistling issue when there is a nearby station close enough that its outer edge of its carrier would beat frequency with your outer edge carrying the 19Khz side bands.
“Just whistle while you work……”
A little birdie once told me that if you whistle while you work…the day ends sooner. 😛
Let me make a correction to my earlier post before I get the all to familiar email from our “RF” friend….the actual sub carrier frequency in stereo FM transmission is 38Khz, not 32 as I noted before. Sorry bout that “LITTLE” discrepancy…a typo….I was only on my 2nd cup of morning mud…so the number 2 stuck for some reason….nothing to see here move along….all is well….we are in green mode….DefCon-5.
RFB
SOLVED BIGTIME
Thank you for all the considerable input to the whistle situation.
By a stroke of fantastic luck IT IS SOLVED.
I went looking for the slim manual that came with the tuner, but it is hiding very well.
Then I used STARTPAGE to search Technics and found a vintage tuner site that everyone will love knowing about.
There was a super technical review of the tuner, ST-9030, plus vivid pictures. Then I saw those magic words: “The tape output has a 19kHz filter!” Oh boy! So I hooked it up, and no more whistle!
I’d buy drinks all around, but liquid cannot be sent on the internet, so instead here is that great link…
http://www.fmtunerinfo.com/technics.html
A little whistle solves a long way home to solution
Ahh very good!! Yes that tape record output would be very well filtered so that the same pilot frequency wont pass through to a tape recorder and cause the same beat frequency problem on magnetic tape…as most tape bias usually ran around 120Khz….close enough when divided down to cause havoc and this would be heard on tape as a nasty hiss…adding to the natural noise floor of the analog circuits and tape itself.
Glad the obvious solution was available to you right there already in the tuner unit itself! See….miracles DO happen when minds of good nature collaborate.
Now..where is my drink!!! :p
RFB
I have a Technics 9030!
I have a Technics 9030! Opps, I already
said that.
Unrelated to that, I knew a guy who had
a pirate FM station in about 1971. It was
never caught by the FCC. He just decided
to stop operating. Eventually he went on
to become a really great broadcast engineer.
That pirate station wasn’t stereo, but he hooked
up an audio signal generator into the transmitter’s
audio input in parallel with the program line.
Then he tuned the generator to
19 kHz and then everybody’s stereo lights lit
up. He wasn’t a ham operator, but he knew
some morse code. So he put a telegraph key
in series with the signal generator and started
sending us simple morse code messages! We
had to look at the stereo lights on our radios
and try to figure out what he was saying!
Unrelated to that, there was an FM station that
I could get that was still mono in 1977 or so.
It played jazz on Saturday nights and I would
listen. If there was drumming that used cymbals
and such, the stereo light would light up at
those times. I guess the guys that were running
that station didn’t have any kind of filter to
stop that from happening. This was when I didn’t
have any good FM BCB equipment. So I sat outside
in my dad’s 1971 Oldsmobile Torinado. It had a
really good FM radio. Plus, the inside was almost
big enough to be a living room!
Oh, I bought my Technics 9030 FM tuner from WBZ TV,
channel 4 in Boston. Some of you might recognize
that callsign. It’s still being used, and it’s on the AM
band on 1030 in Boston, 50 kW.
Best Wishes,
Bruce, Dog Radio Studio 2
Old Rumors and Anecdotes
Well that’s all interesting. Morse code by stereo pilot would make a great gimmick in a spy novel.
An old rumor was put in my ear way back when our engineer, Mr. Ed Bench, mentioned that he and the engineer at WBZ-FM were the developers of stereo FM ! I think he said that WBZ-FM was the very first to go stereo, and KCFM, 93.7 St. Louis, was the second. Either that or I got the story wrong. But I’ll keep telling it anyway.
Also, our FM station had been in the business of commercial background music at first using phone wires, but then on the freshly developed FM sub-carrier up around 60-something-mHz. The trouble with me was that I’d sneak in the occasional cha-cha record with the piercing cow bells and upper-frequency percussion. It would rip into the sub-carrier and people in grocery stores and dentists offices would hear racket static and I’d get a lot of phone calls. I still play those cha-chas only now it’s the after-life and I’m in low power paradise.