will upgrade to 12 bit sdrplay help me?

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Heiner08
Posts: 1
Joined: Sat Aug 29, 2015 3:04 pm

will upgrade to 12 bit sdrplay help me?

Postby Heiner08 » Tue Sep 20, 2016 10:15 am

Hi all,
at the moment I use a rtl dongle RT820.
I like the SDRuno software, but my phenomenon is software independent.
I like to listen to FM broadcast. Even the strongest signals produce noise (not hissing but in the upper part of the spectrum) in FM stereo mode.
This completely disappears if I select mono.
Will this noise be gone if the noise floor is due to the 12 bit about 20dB lower?
Or maybe the SDRplay is just more quiet?
The signals are big but not saturating the tuner.

Heiner
Last edited by Heiner08 on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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F1BJB
Posts: 108
Joined: Wed Jul 15, 2015 4:24 pm
Location: Beauvais France

Re: will upgrade to 12 bit sdrplay help me?

Postby F1BJB » Sat Oct 01, 2016 6:18 am

Hi
FM reception with the SDRplay is definitely better than with any RTLdongle.
Now noise is a well known drawback of FM stereo so don't expect miracles.
The 12bits dynamic range mainly improves reception of weak signals close to strong ones.

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KM4JOJ
Posts: 10
Joined: Fri Aug 26, 2016 1:34 pm

Re: will upgrade to 12 bit sdrplay help me?

Postby KM4JOJ » Sun Oct 02, 2016 5:25 pm

Noise in FM stereo is somewhat related to the SNR of the FM signal, but not entirely; there is a point in which the SNR of your signal doesn't improve. It's not a result of the receiver; but how the technology works. FM Stereo is also known as "FM MPX", even though MPX itself stands for multiplex and FM Stereo is just one use of Multiplexing technology for an FM signal.

So...here is what's going on and why the noise is going to be unavoidable.

In a typical FM Stereo transmission there are three signals involved; main audio, a pilot tone/carrier, and a sub-carrier containing a difference signal. The main audio is a left+right sum of the stereo audio, with the difference being a left-right of the audio. This is the standard mid/side (or sum/difference) matrix; one of the few lossless matrix solutions since you're extracting two channels from two signals. This same matrix is pretty standard in digital audio compression like FLAC or MP3 as well. When you tune to an FM stereo signal; the radio detects this 19khz pilot and decodes stereo by "tuning" the difference signal and applying the matrix.

But does that explain why FM Stereo has noise that FM Mono doesn't? Not yet, I'm getting there; because I gave you the dead simple method of how FM Stereo works...it starts getting a bit complicated. To understand why you get noise; you have to start looking at how an FM signal is constructed. First of all, you need to try to grasp the concept that these signals are in fact multiplexed within one carrier. While I've met some that think the sub-carriers are actual separate transmissions from the main carrier; they're not. Everything is merged in to a "baseband signal"; this baseband signal has a bandwidth of about 100khz and is used to frequency modulate a carrier.

Your main (mono) audio programming occupies the first 15khz of spectrum in the baseband and is plain analog audio. All the sub-carriers are essentially pre-modulated at specific frequencies and then mixed with this signal.

For stereo operation, you have a pilot tone/carrier at 19khz. This signal historically does two things; it signals to the receiver that there is stereo content, and through it's second harmonic of 38khz, provides a carrier to decode the difference part of the stereo matrix. This matrix signal is double-sideband amplitude modulated carrier suppressed signal centered on 38khz; occupying something in the range of 23khz - 53khz.

The key to all of this is the fact the difference signal is amplitude modulated; and I'm sure you know how and why AM signals are prone to noise and interference. But the other key is that as you go up in frequency in the baseband, your SNR drops. As I said, this is dependent somewhat on signal strength of the FM signal itself; as the signal level decreases the upper-end of the spectrum starts to exhibit more noise. You also have to add to this the power levels of the sub-carriers is much weaker than the main program audio.

Of course, if one hasn't noticed; the noise is exaggerated by the fact it is 180 degrees out of phase. This phase reversal is a by-product of the matrix.

One of the features of SDRUno is "FM Stereo Noise Reduction"; which applies a noise reduction routine on the difference signal before applying the matrix decoding. The tradeoff is a little "warbling" or "flanging" distortion as opposed to actual noise.

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DaveB
Posts: 132
Joined: Sun Feb 01, 2015 6:04 pm

Re: will upgrade to 12 bit sdrplay help me?

Postby DaveB » Tue Oct 04, 2016 11:42 am

A difficult question to answer - but obviously FM stereo needs a far higher S/N ratio to become noise free. I'm currently listening, on headphones, to BBC R3 classical music in 250 kHz bandwidth. According to SDR# the s/n ratio is 54-55 dB and it is practically noise free. An alternative frequency gives around 42dB s/n ratio and I can hear some hiss. These figures come from an Airspy mini (12 bit ADC) running SDR#. I get the same audible results on the SDRPlay - but I can't display the s/n ratio.

So the first thing to check is - how strong a single are you getting on the RTL dongle? If it well below 50 dB it is not strong enough to give hiss-free FM stereo. The problem is that you can't get more than 50 dB s/n ratio on an RTL dongle because it is only an 8-bit ADC so you can't achieve a high enough s/n ratio to eliminate the hiss completely. You end up overloading it - which just adds hiss. A 12 bit ADC in theory will give some 70 dB dynamic range and so can handle a stronger signal and hence get better s/n ratio. I certainly cannot run the RTL-dongle and reach a s/n ratio of 55dB on the same station.

Another aspect affecting the noise on the signal is multi-path distortions - I can see all sorts of weird and wonderful drop-outs within the bandwidth of the signal as I rotate my aerial and apart from weak signal hiss you can can get distortion as a result.

David

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DaveB
Posts: 132
Joined: Sun Feb 01, 2015 6:04 pm

Re: will upgrade to 12 bit sdrplay help me?

Postby DaveB » Sat Oct 08, 2016 1:21 pm

I need to correct something in my previous post regarding the S/N ratio that can be achieved with an RTL dongle. I don't know why but running SDR# 1430 I've found that some of my local radio stations are showing S/N ratios up near the 60dB mark without the dongle completely overloading. There are probably underlying spurious signals being generated but I found that on a quiet piece of classical music the hiss was really hard to hear.

So how does one get 60 dB S/N ratio when the ADC only has a dynamic range of 50dB? Must be some magic hocus-pocus in the software.

David

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F1BJB
Posts: 108
Joined: Wed Jul 15, 2015 4:24 pm
Location: Beauvais France

Re: will upgrade to 12 bit sdrplay help me?

Postby F1BJB » Mon Oct 10, 2016 10:20 am

Hi
There is no mystery as it is frequency modulation well known for having a demodulated SNR better than the RF SNR.
The improvement is related to modulation index and sadly can be quite low in FM stereo .

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