FM stereo

I wear an armband FM radio for my outdoor running, w/ digital tuning.

It has a noise level threshold, which switches from stereo to mono when the signal gets weak or noisy. This diminishes the noise effectively, at the cost of listening depth.

What is it about mono decoding, which reduces the noise?

--
Rich
Reply to
RichD
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For FM sterio broadcast the left and right channels are summed and to that is added a 38kHz subcarier AM modulated by difference between left and right. the result is then used to modulate the FM carrier. (see wikipedia)

Mono decoding discards all information (and the noise) in the 30Khz band occupied by sterio subcarrier.

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  When I tried casting out nines I made a hash of it.
Reply to
Jasen Betts

I seem to recall the signal broadcast is not left and right channels. It i s left plus right and left minus right. To recover the left and right chan nels you add the two received signals or subtract them. By only using one received signal you are introducing less noise into the output.

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  Rick C. 

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Reply to
gnuarm.deletethisbit

** Mono FM signals using the broadcast standard ( +/-75 kHz deviation from the centre frequency plus high frequency pre-emphasis) has inherently a ver y large signal to noise ratio - in the order of 80dB.

In order to accommodate stereo broadcasting, while occupying the same RF ba ndwidth, a system employing a supersonic sub-carrier was introduced. The su b carrier carries a L-R signal, amplitude modulated and phase locked to a 1

9kHz pilot tone frequency modulated onto main carrier with about +/- 7kHz d eviation.

In any modern FM receiver, there is a switching decoder that steers the out put of the FM detector to L and R outputs at a 38kHz rate to create two cha nnel audio. Switch this circuit off and the result is the mono ( L+R sum) o n both outputs.

The extra post FM detector bandwidth needed for the sub carrier signal plus the use of amplitude modulation for the L-R information creates additional noise which is most obvious when the RF signal is weak.

..... Phil

Reply to
pallison49

Not quite AM, double sideband suppressed carrier.

It's been said the FM stereo signal is effectively 23 dB weaker than mono. I don't know how they figured that out but it is obviously because of the i ncreased bandwidth.

The sidebands on FM act differently than AM, if you lose them on AM it acts as a filter, on FM you get distortion. The VERY best they can do with it i s something like 0.07% THD and -3dB at about 17.5 KHz. That requires a PLL to generate a pilot cancelling signal. Most FMs do have a good low end resp onse, you can see it when you turn the knob and the woofer cones move. ther e is no technical limitation but the lower limit cannot be 0 Hz.

And those limits I described, you get those in like a Revox, or actually I' m working on a Marantz Model 19 and it is pretty much like that. You should see the IF strip. In a decent FM you can expect 0.5 % THD and flat within

3 dB up to 15 KHz. To get much better than that costs MONEY.

Mono FM can do up to about 30 KHz with about 0.03% THD. Actually even bette r, but nobody seems to want mono.

Reply to
jurb6006

In demodulated FM signals, the SNR is very good at low modulating (audio) frequencies, however the noise density increases very fast when going into higher modulating frequencies. Even in monophonic broadcasting, the high audio frequencies (up to 15 kHz) the noise would be strong on weak FM signals.

Since the amplitude of high frequency audio frequencies typically drops quite fast, so in the transmitter, the high frequency audio is amplified by + 6dB/octave above 3.1 kHz (50 us pre-emphasis).

In the receiver, high frequency is attenuated by -6 dB/octave, restoring a flat audio response. However, the high frequency audio noise is attenuated by -6 dB/octave above 3.1 kHz by the same amount.

In stereo, the difference signal S=L-R is DSB modulated on a 38 kHz subcarrier, occupying the 23 to 53 kHz FM demodulator band. The noise is much worse than even at 15 kHz and worsens with frequency, so the upper sideband (38 to 53) is badly affected if the FM signal was weak.

This explains why the stereo signal is significantly worse than mono, so the 23 dB sounds reasonable.

When the baseband M mono-signal and the S subcarrier is combined in a matrix into L and R, the differential channel S noire will pollute both L and R channel. For this reason, at weak FM signals, the S subcarrier is cut and both L and R contains the same mono M signal, restoring the SNR.

Some manufacturer use a gradual switching from stereo to mono, by either attenuating the S-signal, producing a gradually reduced channel separation (narrower stereo image) or band limit the S-signal to say 3 kHz, thus dropping the signal and noise especially from the upper sideband at 41-53 kHz. The end result is that there is a good channel separation below 3 kHz, but no separation above 3 kHz (mono).

Reply to
upsidedown

** Modulation has nothing to do with it.

Background noise in broadcast FM signals is constant long as the received signal strength is the same.

** The background is essentially " white noise " with energy content proportional to bandwidth. So the band up to 7.5kHz has the same noise energy as that from 7.5 to 15kHz.
** The use of pre-emphasis and de-emphasis improves the measured s/n ratio by about 10 dB, so instead of 80dB mono FM would become 70dB without it.

In both cases background noise is audibly silent in the presence of typical program material. With wide range stereo program, a very soft hiss may be noticeable in some conditions.

However, the OP is using a ridiculous wristwatch radio - that he has any s/n ratio at all is a damn miracle.

.... Phil

Reply to
pallison49

The difference is about 22 dB. I provided the derivation in this comp.dsp thread about 20 years ago:

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(Please use a fixed point font to make sense of the ASCII graphics.)

Regards, Allan

Reply to
Allan Herriman

snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@googlegroups.com:

PA... err.. Phil A Got it right... Twice.

Weak signal in FM stereo is noisey. Set any FM station to 'mono' (set your receiver to mono), and all received carrier gets through the circuit well above any noise. Demod of stereo signal is noisier to start with. So a weak signal will get deconstructed and said noise will have a higher value and (hearability) in the 'finished', received 'product'.

Reply to
DecadentLinuxUserNumeroUno

After an SSB (or AM) detector, the noise density {W/Hz] is constant regardless of which audio frequency is used for measurement. After an FM demodulator, the noise increases with the square of audio frequency.

The original mono FM system with 15 kHz audio bandwidth and 75 kHz deviation had very nice modulation indexes (about 5) and hence very little noise in the passband and most noise above the 15 kHz. The noise above 15 kHz was simply filtered out. This was an early form of "noise shaping" i.e. push the noise above wanted passband.

Unfortunately with MPX stereo, the audio passband extended to 53 kHz and the modulation index was just about 1. With the differential signal in the high noise spectrum with modulation index, much of the original mono SNR was lost.

The interstation noise comming from the speakers of an FM receiver is considered "white" (constant spectral power density) by most people. Since the de-emphasis circuit is connected also during interstation noise, the noise after the FM detector must have been bluish not white.

Getting 80 dB SNR on mono would require an extremely clean local oscillator (not much phase noise).

The worst problems with wristwatch radios is the multipath issues, which can cause nasty distortion.

Reply to
upsidedown

snipped-for-privacy@downunder.com wrote in news: snipped-for-privacy@4ax.com:

'Good' FM stereo reception requires, ideally, a very good signal. That is best derived by a nicely tuned antenna. So, the little jobs inside the little receiver units pull in a very small amount by comparison.

Modern tuners deal with multipath a lot better.

But a weak signal is a weak signal and getting locked onto it does not guarantee good stereo demod. So switching any of them onto pure mono is going to get a cleaner output. Even switching to mono on a well received, clean sounding stereo signal will sound 'better'.

Reply to
DecadentLinuxUserNumeroUno

** Really ?

That is not what you posted before.

** Really - who the f*ck are " most people " ?
** Massive red herring. Interstation noise comes from the internal circuits of the receiver, so has nothing to do with FM broadcast signals.
** Really - IME any decent FM receiver gets close to that number in mono. .
** Bollocks. The worst problems are the lack of an antenna, crap receiver design and the fact that the wearer is moving about.

Go away you tedious BS artist.

.... Phil

Reply to
pallison49

Yeah but some of them sound like shit even without the multipath.

Reply to
jurb6006

Partly because shit is being broadcast...

Reply to
Clifford Heath

Clifford Heath wrote in news:jiKzE.2360$Lc4.907 @fx31.iad:

Much easier to set my smartphone VLC media player app to pull in ZENITH classic rock from Ireland.

No shit broadcast there.

Great '70s classic rock stream, zero signal issues. All the while pulling in a new movie title of 2.8GB at 1MB/s. Modern technology is amazing. Oh and I could also play Euchre online with three other people from around the world also at the same time, typing chat messages back and forth.

Reply to
DecadentLinuxUserNumeroUno

Most FM radio is listened to in automobiles while driving. The ambient noise is likely higher than most received noise as well as the driver not being able to truly focus on what is being listed to in the first place.

How good does FM need to be really?

--
  Rick C. 

  + Get a 1,000 miles of free Supercharging 
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Reply to
gnuarm.deletethisbit

snipped-for-privacy@gmail.com wrote in news:a3dc482f-c659-4bc5- snipped-for-privacy@googlegroups.com:

A hell of a lot better than any I have seen since the antenna on the car days. Even satellite FM is a PITA.

It is a digital age. Seen any cassette tapes lately? How about a pay phone?

Like I said... my phone is what I pump my amps with wherever they are. (my PC at the house). Or I DL high rs FLAC files or make my own. MP3s are so lo res fuzzy jpegish. I also get full videos of a song I may like from youtube as they too are pretty nice, if not original renditions of the song I was looking for.

Like... say... Bombay Calling...

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Reply to
DecadentLinuxUserNumeroUno

snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@googlegroups.com:

That is called 'diversity'. They get used at race tracks so the video feeds always get the best signal, not merely the strongest. I used to make diversity receivers.

Reply to
DecadentLinuxUserNumeroUno

You play that ? I think I have a couple times but forgot everything, but it is in the pinochle/spades/66 family of games.

I never took to 66 but I played auction freezout pinochle well which makes bridge look like spades, and I am a master at spades.

Euchre might be fun to learn.

Reply to
jurb6006

this is the best tutorial re FM stereo that I have seen on the internets

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One interesting part is that a full scale LEFT only or RIGHT only signal = 100% modulation.

AND

Full scale LEFT + RIGHT is also 100% modulation.

m
Reply to
makolber

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