AM Stereo questions

[KA1ZGC]

So I've got this Gonset GSB-100 I'd like to also use as an exciter for the Junkyard Dawg. Well, I was staring at the thing a few months ago, when I noticed the mode switch has a "PM" position, and The Light Dawned over Marble Head!

I know Pete, WA1SOV is really the guy I should be asking this stuff, but I'll throw it out there for the crowd anyway.

I know all I really need to do is feed L+R to the Junkyard Dawg's modulator and L-R to the Gonset's phase modulator.

That leaves me to wonder: how much L-R is the right amount? I don't have a spectrum analyzer handy, nor do any of my AM stereo receivers cover any freqs the Gonset operates on, but I could always Farnsworth it by ear with a product detector and come pretty close if I know the correct deviation value.

I've looked online and found nothing particularly helpful, so I'm hoping someone here has engineered at least one AM Stereo station in their lives and remembers the correct L-R phase deviation value.

Also, is there any other artifact of an AM Stereo signal that tells the receiver to flip to stereo reception (analogous to the pilot tone of FM stereo), or are AM receivers just listening for output energy on their phase detectors and flipping to stereo when they find some? I'm assuming the latter, but please let me know if I'm wrong.

Thanks in advance!

--Thom
Kilowatt Amplifier One Zero Grid Current
p.s. To anyone at the FCC reading this: of course I'd be doing this into a dummy load, don't be silly!

[N3DRB The Derb]

GO GO GADGET GONSET!

[flintstone mop]

The stereo AM system that was never accepted but was in a lot of GM cars used a very low freq pilot..........20 or 30 HZ? I heard WKBW one night in the late 70's in Northern Va. and it was a beautiful sound. Even though it faded the stereo indicator always stayed lit on that nice Delco radio. The separation was amazing for an AM sound!!! It was a rich full sound without the current mess we have now and the need for a "digital radio"
And I know that Mr Khan will never get credit for the system he still proposes. Money talks unfortunately

fred

[Steve - WB3HUZ]

If you are talking about C-QUAM, the L+R signal amplitude modulated the carrier as normal. The L-R modulated a quadrature carrier (90 phase difference from the L+R carrier). Then this signal was run through a limiter to remove all the amplitude variations, leaving only phase variations. This was the C part of the C-QUAM where C stands for Compatible. Without the amplitude limiting, a standard diode detector would not demodulate the signal without severe distortion. The two carriers were then summed. Finally, a pilot carrier was added in the form of phase modulation of the L-R carrier at 25 Hz (actually, I think this was done before the limiting and summing).

So, you need to do more than just phase modulate the L-R info. It must be on a quadrature carrier and the amplitude variations removed. Otherwise, to phase modulate and amplitude modulate the same carrier will result in distortion to both L+R and L-R and no good way to demodulate them on the receive end.

IIRC, SOV was experimenting with independent sideband for stereo (i.e. left channel on the lower sideband and right channel on the upper sideband). Dunno if he ever put it on the air. Jerry, WA2FNQ did put a stereo signal on the air. Recording at the URL below.

http://www.amwindow.org/audio/htm/wa2fnq.htm

[Jeff W9GY]

I did design work on the origianl Delco receivers back in the day...  Even if you could generate the requisite L+R and L-R signals prior to introduction to the transmitter, I doubt that the Gonset would have low enough residual phase noise to do the job.  I remember conducting field tests and finding "Mom and Pop" mono stations that used vintage transmitters which had so much oscillator jitter that they tripped the stereo circuitry.

AM stereo...I could go on and on...  Pretty neat feature when signals were strong and stable.  We used a pretty wide band IF, and had decent fidelity.  There was even a 10 KHz notch filter in the recovered audio to prevent hearing the beat note from a 1st adjacent signal. BUT, when signals got flaky (like multipath from skywave and ground wave interference, and assymetrical sidebands due to path differences, etc), then wierd things would happen...Like platform motion where the stereo image moves around and around like the musicians are on a rotating merry-go-round.  HI

[flintstone mop]

Hi Jeff and Others
WOW, I never heard that when I was pulling standby and driving the only company truck with the AM stereo, listening to WKBW. But dealing with Ma Nature, there are times that the selective fading is severe on 160M. I have found that selective fading is not so bad on 160M, except in the early morning. Everyone in QSO has a weird sound to their signal, especially as 'Ole Sol rises.
fred

[KA1ZGC]

Ah, okay. Now I understand better.

Well, I'm pretty sure there's a limiter in line after the phase modulator in the Gonset when in PM, but I'd have to go grab the schematic and look again.

Quadrature phase, huh? That should be doable, since the Gonset is a phasing sideband transmitter that just happens to do AM and PM also. So quadrature phase shift is already being done in the rig, but (knowing my luck) it's probably 45 forward on one side and 45 back on the other.

But, if I were to luck out and find a point after the modulator that's 90 deg. out from the carrier oscillator, then the requisite modifications should be fairly straightforward from there.

Phase noise - good point, I never even considered that. The Gonset's VFO is a fantastic piece of work, but I doubt that's reflected in the phase noise quotient.

Still, some things to cosider. Thanks, guys, and Merry Christmas

Killer Agony One Zipper Got Caught