FM stereo

[RichD]

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

 

[Jasen Betts]

On 2019-05-04, RichD <r_delaney2001@yahoo.com> wrote:

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?

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.

--
When I tried casting out nines I made a hash of it.

 

On Saturday, May 4, 2019 at 6:44:31 PM UTC-4, RichD wrote:

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?

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

--

Rick C.

- Get a 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

RichD wrote:

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?

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

In order to accommodate stereo broadcasting, while occupying the same RF bandwidth, a system employing a supersonic sub-carrier was introduced. The sub carrier carries a L-R signal, amplitude modulated and phase locked to a 19kHz pilot tone frequency modulated onto main carrier with about +/- 7kHz deviation.

In any modern FM receiver, there is a switching decoder that steers the output of the FM detector to L and R outputs at a 38kHz rate to create two channel audio. Switch this circuit off and the result is the mono ( L+R sum) on 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

 

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 increased 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 is 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 response, you can see it when you turn the knob and the woofer cones move. there 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 better, but nobody seems to want mono.

 

On Sat, 4 May 2019 19:45:42 -0700 (PDT), jurb6006@gmail.com wrote:

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 increased bandwidth.

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).

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 is 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 response, you can see it when you turn the knob and the woofer cones move. there 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 better, but nobody seems to want mono.

 

[Allan Herriman]

On Sat, 04 May 2019 15:44:26 -0700, RichD wrote:

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?

The difference is about 22 dB.
I provided the derivation in this comp.dsp thread about 20 years ago:
https://groups.google.com/d/msg/comp.dsp/a8omWrNY13o/qun1tAzfVq0J

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

Regards,
Allan

 

upsid...@downunder.com wrote:

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 increased bandwidth.


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.

** Modulation has nothing to do with it.

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

Even in monophonic
broadcasting, the high audio frequencies (up to 15 kHz) the noise
would be strong on weak FM signals.

** 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.

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.

** 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

 

pallison49@gmail.com wrote in
news:bdde047c-724b-4729-ba76-d6b04384b413@googlegroups.com:

upsid...@downunder.com wrote:




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 increased bandwidth.


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.



** Modulation has nothing to do with it.

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


Even in monophonic
broadcasting, the high audio frequencies (up to 15 kHz) the noise
would be strong on weak FM signals.



** 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.



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.


** 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

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'.

 

On Sun, 5 May 2019 03:06:35 -0700 (PDT), pallison49@gmail.com wrote:

upsid...@downunder.com wrote:




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 increased bandwidth.


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.



** Modulation has nothing to do with it.

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.

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


Even in monophonic
broadcasting, the high audio frequencies (up to 15 kHz) the noise
would be strong on weak FM signals.



** 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 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.

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.


** 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.

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

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.

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

.... Phil

 

upsidedown@downunder.com wrote in
news:ta1ucehviolp46eu1qvt5rlc5s3hjn4dri@4ax.com:

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

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

'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'.

 

>Modern tuners deal with multipath a lot better.

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

 

upsid...@downunder.com wrote:

** Modulation has nothing to do with it.

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.

** Really ?

That is not what you posted before.

** 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 interstation noise comming from the speakers of an FM receiver is
considered "white" (constant spectral power density) by most people.

** Really - who the fuck are " 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.

** Massive red herring. Interstation noise comes from the internal circuits of the receiver, so has nothing to do with FM broadcast signals.

** 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.

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

** Really - IME any decent FM receiver gets close to that number in mono.
..

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

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

** 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

 

>Some manufacturer use a gradual switching from stereo to mono, by ...

They have gone farther than that. I believe one scheme was called a Schotz. I am not sure of all of this but it is all somewhere true. These things not only variably controlled channel separation, also by signal strength varied the IF bandwidth.

Except for a few things like the Revox, or a Marantz Model 19, a few others, are not. Even user selectable IF bandwidth is not a cheap feature. To have it do it itself is more.

Some of the better FMs were in Delco car radios in the 1980s, 90s. Variable stereo switching, IF bandwidth and DNR. For those who don't know, DNR is a noise reduction scheme that requires no encoding.

Those Delco tuners were actually just like Haflers, and the design had Hughes Aircraft involved. Why ? Don't ask me. Maybe they just plain old had a better design. In those years, amps and some of the speakers were Bose, and though everybody knows about Bose, they actually come in handy in a car. Those things sounded good.

Then their type "D" tape deck was a Blaupunkt. Not sure about the rest of them but they all sounded good. Type "A" was the elcheapo, "B" and "C" were intermediate decks which really weren't too bad. Type "D" was the good one, it actually ejected the tape when you turn the key off.

They also had dynamic power limiting, at 8.9 watts per channel that made them sound louder.

OTHER car radios went to greater extremes. I have seen them that use like four antennas and not only that, have four identical front ends and it switches to whatever one is giving the best signal - based on signal strength AND multipath. Mostly foreign.

Still, there are some FMs that are simply superior. Take a Marantz 19, the IF strip looks like it could be a bomb. It uses a real discriminator, not a ratio detector. It has like four limiter stages. In the IF strip there are five adjustments per stage.

Now THAT'S a fucking tuner. And the Revox outdoes it bigtime. that is not easy. People who designed this stuff really DID pay attention in school.

 

[Clifford Heath]

On 6/5/19 7:04 am, jurb6006@gmail.com wrote:

Modern tuners deal with multipath a lot better.

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

Partly because shit is being broadcast...

 

Clifford Heath <no.spam@please.net> wrote in news:jiKzE.2360$Lc4.907
@fx31.iad:

On 6/5/19 7:04 am, jurb6006@gmail.com wrote:
Modern tuners deal with multipath a lot better.

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

Partly because shit is being broadcast...

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.

 

On Sunday, May 5, 2019 at 5:00:55 PM UTC-4, jurb...@gmail.com wrote:

Now THAT'S a fucking tuner. And the Revox outdoes it bigtime. that is not easy. People who designed this stuff really DID pay attention in school.

Can't you blow all this away with a good SDR? ADC are pretty durn good these days and once in the digital domain with enough headroom, you can do pretty much anything you want.

--

Rick C.

+ Get a 1,000 miles of free Supercharging
+ Tesla referral code - https://ts.la/richard11209

 

On Sunday, May 5, 2019 at 1:24:49 PM UTC-4, DecadentLinux...@decadence.org wrote:

upsidedown@downunder.com wrote in
news:ta1ucehviolp46eu1qvt5rlc5s3hjn4dri@4ax.com:

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

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


'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'.

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
+ Tesla referral code - https://ts.la/richard11209

 

jurb6006@gmail.com wrote in
news:5dc3cc07-d671-4079-8bb6-c5fb27c9b2d5@googlegroups.com:

I have seen them that use like four antennas and not only that,
have four identical front ends and it switches to whatever one is
giving the best signal - based on signal strength AND multipath.
Mostly foreign.

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.

 

gnuarm.deletethisbit@gmail.com wrote in news:a3dc482f-c659-4bc5-
b45b-fd3df2315a2b@googlegroups.com:

How good does FM need to be really?

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...

https://www.youtube.com/watch?v=jEKg9qyEQmw