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From: Robert Orban on 10 Feb 2010 20:51
In article <hki7js$cav$9(a)naig.caltech.edu>, gah(a)ugcs.caltech.edu says...
>
>
>Jerry Avins <jya(a)ieee.org> wrote:
>(snip)
>
>> I don't think that the RF spectrum is relevant. Compatible FM stereo
>> consists of L+R in the main band where a mono detector will reproduce
>> it, and L-R multiplexed in a way that can, for this discussion, remain
>> mysterious. A stereo receiver combines (L+R) and (L-R) to produce L and
>> R. The problem addressed here occurs at the transmitter. If one of the
>> channels [L, R] is inverted before the modulator gets it. The main FM
>> channel will consist of L-R. An announcer speaking into a single mic
>> that feeds both channels might as well have stayed home. *All* of his
>> voice will be in the (L-R) channel that a mono receiver doesn't see.
>
>Yes. But with all the power in the subcarrier, that is, above 19kHz,
>I think that changes the spectrum of the transmitted signal.
>With 75kHz deviation and a large amount of 38kHz signal, you might
>get a significant amount outside the 200kHz wide band.
>
>As well as I understand it, assumptions were made that,
>on average, most of the power isn't that high. The only way to
>know would be to read the FCC rules in detail. With highly
>compressed rock music, the amplitude could be pretty high.
>If, in addition, the signal had a large component close to
>15kHz (maybe there are FCC rules on that, too), that makes
>it even worse.
>
>>>> The "wrong" (incorrect phasing) way will work fine on a stereo
>>>> receiver, but it will not work fine on a mono receiver.
>
>>> It is convenient that as (L-R) increases (L+R) decreases
>>> such that the sum doesn't get too large. (Assuming L and R
>>> stay in range.)

The FCC Rules limit the peak frequency deviation of the FM carrier to +/-
75 kHz. It turns out that the peak modulation of the FM carrier is the
larger of the left or right input signals due to an interesting and
slightly non-intuitive property of the FM stereo multiplex signal called
"interleaving." You can prove it by proving the equivalence of (1)
generating the multiplex signal by summing L+R and double sideband
suppressed-carrier amplitude-modulated L-R and (2) alternately sampling the
L and R signals at the stereo subcarrier rate (38 kHz). (The proof just
requires some trig identities.)


Without the 19kHz pilot tone, the peak modulation produced by a pure L+R
signal is the same as that produced by a pure L-R signal -- flipping the
polarity of one channel does not change the peak modulation at all.

The presence of the 19 kHz pilot tone, which is phase-locked to the 38 kHz
subcarrier, slightly breaks the interleaving rule. It turns out that in the
presence of a pilot tone at 9% modulation, a pure L+R signal modulates the
FM carrier 2.7% higher than a pure L or pure R signal with the same
content.

From: Jerry Avins on 10 Feb 2010 21:38
Robert Orban wrote:

...

> The FCC Rules limit the peak frequency deviation of the FM carrier to +/-
> 75 kHz. It turns out that the peak modulation of the FM carrier is the
> larger of the left or right input signals due to an interesting and
> slightly non-intuitive property of the FM stereo multiplex signal called
> "interleaving." You can prove it by proving the equivalence of (1)
> generating the multiplex signal by summing L+R and double sideband
> suppressed-carrier amplitude-modulated L-R and (2) alternately sampling the
> L and R signals at the stereo subcarrier rate (38 kHz). (The proof just
> requires some trig identities.)
>
>
> Without the 19kHz pilot tone, the peak modulation produced by a pure L+R
> signal is the same as that produced by a pure L-R signal -- flipping the
> polarity of one channel does not change the peak modulation at all.
>
> The presence of the 19 kHz pilot tone, which is phase-locked to the 38 kHz
> subcarrier, slightly breaks the interleaving rule. It turns out that in the
> presence of a pilot tone at 9% modulation, a pure L+R signal modulates the
> FM carrier 2.7% higher than a pure L or pure R signal with the same
> content.

That's interesting detail that I didn't go into because it isn't related
to the gist of this discussion. Briefly, a mono receiver is sensitive to
the FM portion of the stereo signal, normally L + R. If that should by
inadvertence be L - R, a mono signal fed to the transmitter will be lost.

Jerry
--
Engineering is the art of making what you want from things you can get.
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