FM stereo?

Thanks, Phil!

So the FM stereo signal will still get decoded through the demodulator? So my demodulated signal should be in the range of 1Hz u to 50kHz and then my stereo receiver should be able to separate the channel signal centered around 38kHz?

Thanks again, Phil!

Do you know what it means when a datasheet mentions that the RF signals are fed into two balanced inputs? I'm not quite sure. As you can tell, I"m a newbie.

:-)

Correct

'Fraid you're wrong, Phil. The +38KHz stereo (L-R) signal is a suppressed carrier, double sideband signal, not part of the FM. You can actually see it on a spectrum analyzer hooked up to an antenna, quite separate from the cluster of FM sidebands.

That's why the pilot tone - to locally regenerate a demodulation carrier. Unlike SSB, DSB needs the reinserted carrier to be phase locked.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

demodulator?

and

signal

*sequentially* at

switch the

Oh man, you're way off base. The L+R signal is sent on the main carrier just like any mono FM broadcast. The L-R signal is a supressed carrier double sideband signal FM modulated onto a 38kHz subcarrier. The 19kHz pilot tone is doubled in frequency inside the receiver and then used to demodulate the subcarrier signal to retrieve the L-R signal. This is then mixed with the L+R signal to seperate the L and R information.

Actually there are subcarriers commonly used at 67kHz and 91kHz pretty much using up the rest of the channel allocation. Sometimes data is sent on a 71kHz subcarrier. If you have a shortwave receiver that does FM and tunes below 100kHz, you can take the base band audio from the discriminator output and feed it into the antenna jack and tune it to

67kHz and see for yourself. It's called SCA and I figured you'd at least heard of it before.

"MRW" gmail.com

** We have fair warning......

** No.

The 19 kHz stereo pilot tone is a frequency modulation.

** No again.

Modulations of an FM carrier are about the centre frequency.

They are not separate frequencies.

......... Phil

"MRW"

** Wow.

** Stereo FM is a bit complicated.

In reality, the original Lch and Rch signals are separately *analogue sampled * and used to modulate the VHF carrier frequency *sequentially* at the rate of 38,000 times per second.

The exact 19kHz "pilot tone" is sent to make it possible for the receiver's decoder to lock onto this sequential sampling and then switch the recovered audio to Left and Right audio outputs at the exactly right moments.

The upshot is that a stereo FM receiver must have at least 53 kHz of recovered modulation bandwidth - requiring a 200 kHz radio spectrum bandwidth.

Care to ask a more precise Q ?

........ Phil

carrier

carrier

used

You're right on it being an AM process to create the L-R sidebands using a 38kHz carrier which is then suppressed, but the results of that process is FM modulated onto the main carrier just like the L+R stuff.

switches

of

decoders.

Learn something new every day I guess. Never before heard that just switching back and forth at 38kHz would give the same results. I'm still not convinced on that as the datasheets I've seen so far (for the devices you mentioned) seem to work in the so-called "classic" way. I'd like to see something a little less vague that "simply switches between the left and right channels". What, precisely, is being switched?

supressed

subcarrier.

then

suppressed

using

stuff.

together to

just

I understand that, they just use a ~100kHz wide audio signal to FM the main carrier.

is less

the

I'd

between

directed) to L

That doesn't sound like it would do anything usefull at all. How will that make the L-R information relocate itself down to human audible frequencies from the 38kHz carrier? It's Seems to me that you'd at least have to demodulate the L-R stuff to convert it back to its original form and then mix that with the L+R stuff. I presume that the switching is phase locked to the 19kHz pilot tone which you'd have to filter off as well. If that's all the case, then it sure doesn't sound all that much easier.

Good luck on that anger management.

!!!!

You really should try to grow up a little phil.

I have. The only thing I've seen so far that agrees with you (about the rapid switching of the baseband signal) is the one link that you provided. I don't suppose you could supply something more technically detailed could you? Like I said in the stuff that you so cleverly snipped away, AFAICT there is no way that simply switching the output of the discriminator rapidly will magically transpose the freqeuncy shifted L-R stuff down to recover the stereo channels. It, surely, must be much more complex than you are inferring.

That's not what I'm finding after scads of searching.

** No - YOU are totally wrong here.

** Complete nonsense !!!!

It arrives at part of the recovered output from the FM detector.

Ergo, it was part of the *modulation* signal.

** Makes it a high frequency modulation - not a separate signal. ** FM stereo receivers made since the mid 70s use *switching* decoders.

The LM1310 and LM1800 and Asian equivalents.

Better go look them up.

......... Phil

Fred & Anthony are correct...

From the LM1800 test data:

"Note 2: The stereo input signal is made by summing 123 mVrms LEFT or RIGHT modulated signal with 25 mVrms of 19 kHz pilot tone, measuring all voltages with an average responding meter calibrated in rms. The resulting waveform is about 800 mVp-p."

This explains it quite well.

It probably doesn't help using the term "multiplexing" in a slightly confusing way.

"Anthony Fremont"

= a net stalking, wanking, bloody IDIOT !!!!

** WRONG !!!

See:

" In addition a difference signal (Left - Right) is generated and then used to modulate a 38 Khz subcarrier using Double sideband suppressed carrier (DSBSC) modulation. This is an AM modulation of the subcarrier. "

-------------------------------------------------

" This is the 'Classic' method. An easier to build method simply switches between the left and right channels at a 38 Khz rate. The mathematics of this produce a signal the same as the classic method but there is less likelyhood of differences between channels sneaking in. "

** FM stereo receivers made since the mid 1970s use *switching* decoders.

The LM1310 and LM1800 and several Asian equivalents.

Better go look them up and learn something.

FUCKHEAD !!!!!!!

........ Phil

"MRW"

** One would have to work out exactly what was meant by knowing the context.

Can you post a link to the sheet?

.......... Phil

"Anthony Fremont" = a net stalking, wanking, bloody IDIOT !!!!

** The L+R, 19kHz tone and the DSBSC signals are simply added together to form a composite signal and then used to frequency modulate the transmitter's carrier.

Same goes for any additional supersonic signals for extra stuff - just added into the modulation signal.

** The signal coming out of the FM detector is switched ( ie directed) to L and R outputs alternately.

Go do you own Google search for the details - f*****ad.

....... Phil

"Anthony Fremont"

= a vile, net stalking, public wanking, autistic bloody IDIOT !!!!

** Bad luck - you dumb as shit, criminal FUCKHEAD.

Go do your own Google search for the details.

Plenty there.

FUCKWIT !!

........ Phil

Consider how DSBSC is recovered in the first place, the 19 kHz pilot tone's role in that - and then realize that "sampling" a signal at 38 kHz and amplitude modulation of a fixed-amplitude 38 kHz carrier are to a great degree exactly the same thing. That should make both the encoding and decoding processes much clearer for all concerned.

There seems to be a huge argument going on here about two apparently different things which really do not differ at all. "Sampling left and right channels alternately at 38 kHz" results in PRECISELY the same signal as the original "L+R at baseband, plus L-R DSBSC around 38 kHz" - they're just essentially two ways of doing the same thing. Either way, you take the result and use it (with a 19 kHz pilot tone added in) to FM the VHF carrier. At the receiving end, the output of the FM demodulator is as it always was - L+R at baseband, and L-R DSBSC up around

38 kHz. Sampling THAT whole shebang at 38 kHz equates to "demodulate the DSCSC L-R signal and combine the recovered signal with L+R to get L and R separately." (Note that sampling the total signal at only 38 kHz is actually UNDERsampling it, per Nyquist - which you are counting on, since the aliasing of the various components is what's going to bring those together and properly combine them, mathematically, and so result in L and R separated and at the correct audible frequencies.)

Bob M.

"Bob Myers"

** A small point to keep in mind is that the FM stereo signal is at no time actually " sampled " in the way that digital audio samples the waveform.

The Nyquist theorem strictly applies to a stream of *instantaneous* samples of a waveform, ie fixed voltage samples taken at exact moments in time.

FM stereo involves a different method known as "analogue sampling" - where a signal is switched at some rate onto a link for a small increments of time during which it can and does vary in voltage.

........ Phil

kHz

and

way,

FM

demodulator

kHz

frequencies.)

Thanks Bob, I think I'm getting it now. I had never before seen this described as a plausable method to encode/decode stereo. I still can't help but think that this method would introduce allot of intermod type distortion.