In article , The Natural Philosopher
scribeth thus
On 17/04/2019 14:53, John Rumm wrote:
On 17/04/2019 13:44, The Natural Philosopher wrote:
On 17/04/2019 13:33, John Rumm wrote:
On 16/04/2019 23:24, wrote:

Does it give a better S/N ratio?

IIUC, for reasons of backward compatibility, a stereo FM transmission
encodes stereo image into a pair of sum and difference signals[1]. So
mono receivers can simply process the sum signal and get both
channels combined into a single mono channel. A stereo receiver will
need to add/subtract the difference signal from the main combined
channel to get the separate L & R channels.

The sum signal is transmitted using FM and the difference signal is
amplitude modulated onto a sub carrier shifted up from the main
carrier. (IIRC there is also a pilot tone included just above the
baseband audio to signal the received that its a stereo
transmission). Since the AM modulation will suffer more in poor
reception conditions it can also introduce hiss. Many radios hence
include a Stereo/Mono switch to elect for mono with no hiss. Some of
the posher car radios actually use a mixer for the stereo decoding,
so they can switch the stereo in and out in gradual way depending on
how much noise is being detected.


[1] Conceptually not unlike the way colour was added to mono TV

Not quite correct.
https://en.wikipedia.org/wiki/FM_broadcasting#Stereo_FM

is definitive.

..and actually appears to be a fairly close match to my overview above.

the reason for worsening S/N is quite simply that more bandwidth is
needed to get stereo and that simply lets in more noise.

From your source...

"The (L+R) Main channel signal is transmitted as baseband audio limited
to the range of 30 Hz to 15 kHz. The (L0 onto a 38 kHz double-sideband suppressed-carrier (DSB-SC) signal
occupying the baseband range of 23 to 53 kHz."

later:

"for a given RF level at the receiver, the signal-to-noise ratio and
multipath distortion for the stereo signal will be worse than for the
mono receiver."

AM or FM doesn't really make much odds here.

Its the AM modulated difference signal (i.e. the bit that carries the
stereo information) that suffers the poorer SNR. Whether one argues that
is because or its modulation technique or its the lower bandwidth is a
bit moot (I would say both are a factor, but AM is still the better
choice for narrowband applications).

However I will concede a better wording may have been "Since its the AM
modulated part of the signal that will suffer more in poor reception
conditions..."



No, that still doesn't wash.

Its the 38kHhz subcarrier part that suffers worse.
The FM S/N is only better because it occupies a humongous bandwidth -
400kHz - channel. Narrow band FM is just as noisy as AM

The stereo subcarrier is only 38khz wide.

https://en.wikipedia.org/wiki/Shanno...artley_theorem

is what yopu need toi understand.

AM is restricted to eseentially a channel as wide as the audio bandwidth
being transmitted. FM may *or may not* use a wider channel,

That's why it's possible to get lower S/N.


The key thing is that its lower bandwidth.






FWIW i had a new type of specialist FM receiver on demo/evaluation
recently its a BW Broadcast encore series rebroadcast receiver which
apart from being very good at what it does has lots of other needed
bells and whistles like diversity reception, two tuner and multipath
cancellation the capability to receive distant stations whilst ones on
the same mast where it is may be knocking out kilowatts on close by
channels on FM, DAB or TV etc.

However its main purpose is to pick up a distant transmitter and provide
a high quality signal for local rebroadcast transmission.

Now its getting quite fashionable to use digital signal processing in
such devices, they do now regenerate the 19K stereo pilot tone, regen
the RDS etc but i had this one and put it on, you do have the Read the
bloody manual to get it to do what you want it to do, but i had it tuned
to a very good quality FM transmission pair of phones plugged in and
just a few inches of wire plugged in the back as a makeshift aerial.

Went and had tea and came back, phones on very impressive audio, sounded
very close to CD very quiet background all fine.

Further investigation revealed that what they call FMSI was enabled this
is where the incoming difference signal is sampled out of band and a few
other interesting things happen to it. Now they do have a measurement
mode where you can listen to the original signal to check that under "as
is" conditions when that was enabled the Almost CD quality disappeared
in a very mushy hiss!

Hooking it up to a better aerial alleviated that but it was very
interesting to see or hear rather just how good the signal processing
was you can select conventional stereo "blend", well a slightly more
advanced version etc, but switching the signal processing in the
difference was remarkable even back on the bit of wire aerial.

At the prices these are its not intended as a domestic product and is a
PITA to tune in different channels etc but just shows what can be done
in the receiver to improve a deficiency of the original GE Zenith pilot
tone system!..


https://www.bwbroadcast.com/bwbroadc...ore/64/product


--
Tony Sayer


Man is least himself when he talks in his own person.

Give him a keyboard, and he will reveal himself.