Forums

FM Demodulation

Started by Henry VIII July 5, 2008
"John E. Hadstate" <jh113355@hotmail.com> writes:

> "Henry VIII" <NoBodyHome@SSSpamavoidance.com> wrote in message > news:C1hck.14800$N87.6262@nlpi068.nbdc.sbc.com... >> "Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in message >> news:xPack.1005$zv7.194@flpi143.ffdc.sbc.com... >> >> Why deal with all that pre-detection bandwidth if we don't care >> about the audio? So is it realistic to simply detect the pilot's >> spectral sidebands at RF - no discriminator - and demodulate the >> pilot from them? Say we can achieve filters of a few tens of Hz >> bandwidth centered on the pilot's Bessel components. That's a noise >> bandwidth improvement of roughly 30 dB or so over conventional >> discriminator. Does this approach make sense? > > Not to me. The pilot's spectral components are all mixed up with > components that belong to other features of the signal (as well as > noise).
I'm with you, John. In my opinion, this entire engineering problem (I know exactly what Henry is attempting) is ill-formed. An alternate approach, such as diversity combining, would be more fruitful. -- % Randy Yates % "She tells me that she likes me very much, %% Fuquay-Varina, NC % but when I try to touch, she makes it %%% 919-577-9882 % all too clear." %%%% <yates@ieee.org> % 'Yours Truly, 2095', *Time*, ELO http://www.digitalsignallabs.com

Henry VIII wrote:

> Yes, but can the discriminator be eliminated altogether to achieve even > better C/N performance? The great majority of the FM Carson's Rule > bandwidth is due to the high modulation index audio. Why deal with all that > pre-detection bandwidth if we don't care about the audio?
The efficient bandwidth of the optimal maximum-likelihood FM demodulator is about the bandwidth of the modulation or smaller then that.
> So is it > realistic to simply detect the pilot's spectral sidebands at RF - no > discriminator - and demodulate the pilot from them?
To do that, you have to unscramble the nonlinearly scrambled eggs. Although it seems possible to exploit the short term correlation in the spectrum with the period of the pilot tone frequency without demodulating the whole thing.
> Say we can achieve > filters of a few tens of Hz bandwidth centered on the pilot's Bessel > components.
No use.
> That's a noise bandwidth improvement of roughly 30 dB or so > over conventional discriminator. Does this approach make sense?
No it doesn't. The pilot sidebands are smeared by the other components of the signal. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Henry VIII wrote:
> "Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in message > news:xPack.1005$zv7.194@flpi143.ffdc.sbc.com... > > >> What hardware is available? I.e. how big of the DSP you can afford? > > Will be all-new design from scratch. There is no specific DSP cost > limitation at this time. > > >> The practical solution would be optimizing the discriminator and using the >> narrowband filter with a good PLL to pick up the pilot at the >> discriminator output. There could be an improvement of ~10dB over the >> trivial solution just by doing that. > > Yes, but can the discriminator be eliminated altogether to achieve even > better C/N performance? The great majority of the FM Carson's Rule > bandwidth is due to the high modulation index audio. Why deal with all that > pre-detection bandwidth if we don't care about the audio? So is it > realistic to simply detect the pilot's spectral sidebands at RF - no > discriminator - and demodulate the pilot from them? Say we can achieve > filters of a few tens of Hz bandwidth centered on the pilot's Bessel > components. That's a noise bandwidth improvement of roughly 30 dB or so > over conventional discriminator. Does this approach make sense?
No. The sidebands of an FM signal are not in general symmetric, and they vary in amplitude depending on what else is modulated onto the carrier. Once there are other signals sharing the carrier, you need to rethink the whole scenario. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
> > > Yes, but can the discriminator be eliminated altogether to achieve even > > better C/N performance? &#2013266080;The great majority of the FM Carson's Rule > > bandwidth is due to the high modulation index audio. &#2013266080;Why deal with all that > > pre-detection bandwidth if we don't care about the audio? &#2013266080;So is it > > realistic to simply detect the pilot's spectral sidebands at RF - no > > discriminator - and demodulate the pilot from them? &#2013266080;Say we can achieve > > filters of a few tens of Hz bandwidth centered on the pilot's Bessel > > components. &#2013266080;That's a noise bandwidth improvement of roughly 30 dB or so > > over conventional discriminator. &#2013266080;Does this approach make sense?
to the op: are you trying to simply determine if there is a pilot tone present, or are you trying to actually recover the tone, i.e. recover the exact phase and frequency of the pilot tone to use it for stereo decoding? Mark
to the op:

are you trying to simply determine if there is a pilot tone present,

or

are you trying to actually recover the tone, i.e. recover the exact
phase and frequency of the pilot tone to use it for stereo decoding?

-----------------
Just want to detect the presence of the tone, whether it's present or not in 
order to control a stereo reception light.



> > are you trying to simply determine if there is a pilot tone present, > > or > > are you trying to actually recover the tone, i.e. recover the exact > phase and frequency of the pilot tone to use it for stereo decoding? > > ----------------- > Just want to detect the presence of the tone, whether it's present or not in > order to control a stereo reception light.
please elaborate.... what is the point of using threshold extension to operate the stereo indicator lamp, if you cannot actually decode the stereo? Mark