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Sample audio from my FM stereo receiver

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Sample audio from my FM stereo receiver

Marcus D. Leech
http://www.sbrac.org/files/fm_stereo_test.ogg

This is tuned to a station that's approximately 100km from my location.

--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org



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Re: Sample audio from my FM stereo receiver

Andrew Davis-9
Wow, sounds good, I've got a bunch of 50KW stations about 2 miles away
and I cannot get any to sound good at all. What receiver code are you
using?

On Sun, Mar 11, 2012 at 7:46 PM, Marcus D. Leech <[hidden email]> wrote:

> http://www.sbrac.org/files/fm_stereo_test.ogg
>
> This is tuned to a station that's approximately 100km from my location.
>
> --
> Marcus Leech
> Principal Investigator
> Shirleys Bay Radio Astronomy Consortium
> http://www.sbrac.org
>
>
>
> _______________________________________________
> Discuss-gnuradio mailing list
> [hidden email]
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

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Re: Sample audio from my FM stereo receiver

Marcus D. Leech
On 03/11/2012 08:40 PM, Andrew Davis wrote:
Wow, sounds good, I've got a bunch of 50KW stations about 2 miles away
and I cannot get any to sound good at all. What receiver code are you
using?
The code that I put on on CGRAN yesterday:

https://www.cgran.org/svn/projects/simple_fm_rcv

And the supporting RDS code:

https://www.cgran.org/svn/projects/radio_data_system

I'm using a TVRX2 on a B100, and this was with 8-bit baseband samples at 320Ksps input rate to the FM demod.

Your problem might actually be that you have a *bunch* of 50KW stations close by.   Although with 70+ dB of dynamic range
  even at 12-bit ADC sampling, with the gain set right, you shouldn't have too many problems.
-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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Re: Sample audio from my FM stereo receiver

Johnathan Corgan-2
On Sun, Mar 11, 2012 at 17:50, Marcus D. Leech <[hidden email]> wrote:

> ...with the gain set right...

This is where I've run into a lot of issues.  Too much gain, the lower
end of the dynamic range is filled with IMD.  Too little gain, the
noise figure is too high.  It's hard to get even 12 bits of ADC
dynamic range in the presence of strong signals.

Johnathan

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Re: Sample audio from my FM stereo receiver

Marcus D. Leech
On 11/03/12 10:54 PM, Johnathan Corgan wrote:
> This is where I've run into a lot of issues.  Too much gain, the lower
> end of the dynamic range is filled with IMD.  Too little gain, the
> noise figure is too high.  It's hard to get even 12 bits of ADC
> dynamic range in the presence of strong signals.
>
>  
It's the "dirty little secret" of SDR--analog considerations continue to
be important.  Although I suppose
  with a direct-sampled 28-bit ADC running at a few Gsps, you'd have
enough dynamic range and oversampling
  to cope with anything--as long as whatever gain stages you had in
front of the ADC would remain nice and
  linear :-)

Some broadcast FM receivers (even analog ones) have a tracking filter in
the front end, to eliminate the
 IMD problem before it becomes a problem.   That eliminates the "broad
as a barn door" front-end issues.

I'm using the TVRX2, which uses the TDA18272 chip, which has a built-in
4-band band-select filter, so that
  may improve things quite a bit in terms of keeping subsequent stages
linear.



--
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org



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Re: Sample audio from my FM stereo receiver

Rafael Diniz-2
In reply to this post by Marcus D. Leech
Marcus,
Have you done any improvements to the good old RDS code in cgran?


Best regards,
Rafael Diniz

> On 03/11/2012 08:40 PM, Andrew Davis wrote:
>> Wow, sounds good, I've got a bunch of 50KW stations about 2 miles away
>> and I cannot get any to sound good at all. What receiver code are you
>> using?
> The code that I put on on CGRAN yesterday:
>
> ** https://www.cgran.org/svn/projects/simple_fm_rcv
>
> And the supporting RDS code:
>
> ** https://www.cgran.org/svn/projects/radio_data_system
>
> I'm using a TVRX2 on a B100, and this was with 8-bit baseband samples at
> 320Ksps input rate to the FM demod.
>
> Your problem might actually be that you have a *bunch* of 50KW stations
> close by.   Although with 70+ dB of dynamic range
>    even at 12-bit ADC sampling, with the gain set right, you shouldn't
> have too many problems.



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Re: Sample audio from my FM stereo receiver

Marcus D. Leech

Rafael:

 

I added methods for resetting the state machines on the demodulator and the decoder, but apart from that, no.

My code relies on those updates so I pushed the updates up to CGRAN for radio_data_system

 

-Marcus

 

On Mon, 12 Mar 2012 06:09:11 -0700, Rafael Diniz wrote:

Marcus,
Have you done any improvements to the good old RDS code in cgran?


Best regards,
Rafael Diniz

On 03/11/2012 08:40 PM, Andrew Davis wrote:
Wow, sounds good, I've got a bunch of 50KW stations about 2 miles away and I cannot get any to sound good at all. What receiver code are you using?
The code that I put on on CGRAN yesterday: ** https://www.cgran.org/svn/projects/simple_fm_rcv And the supporting RDS code: ** https://www.cgran.org/svn/projects/radio_data_system I'm using a TVRX2 on a B100, and this was with 8-bit baseband samples at 320Ksps input rate to the FM demod. Your problem might actually be that you have a *bunch* of 50KW stations close by. Although with 70+ dB of dynamic range even at 12-bit ADC sampling, with the gain set right, you shouldn't have too many problems.
_______________________________________________ Discuss-gnuradio mailing list [hidden email] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

 


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Re: Sample audio from my FM stereo receiver

Marcus D. Leech
In reply to this post by Rafael Diniz-2
On 03/12/2012 09:09 AM, Rafael Diniz wrote:
> Marcus,
> Have you done any improvements to the good old RDS code in cgran?
>
>
Just updated the simple_fm_rcv  code to use 250e3 sample rate, since
that's a common rate between B100/USRP1/E1XX and N2XX family, and
   all the math "works out".  That will more lightly load the cpu as
well, compared to 320e3.




--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org



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Re: Sample audio from my FM stereo receiver

Marcus D. Leech
On 03/12/2012 09:01 PM, Marcus D. Leech wrote:

> On 03/12/2012 09:09 AM, Rafael Diniz wrote:
>> Marcus,
>> Have you done any improvements to the good old RDS code in cgran?
>>
>>
> Just updated the simple_fm_rcv  code to use 250e3 sample rate, since
> that's a common rate between B100/USRP1/E1XX and N2XX family, and
>   all the math "works out".  That will more lightly load the cpu as
> well, compared to 320e3.
>
>
>
>
And here's an audio sample using the new "common" sample rate:

http://www.sbrac.org/files/fm_stereo_test2.ogg


--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org



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Re: Sample audio from my FM stereo receiver

Andrew Davis-9
Doesn't sound as good to me, maybe it's just a bad sample. I for some
reason ( I know the math tells me otherwise ) I find it sounds better
the faster I sample, like 500M. I feel like it has something to do
with more exact following of the FM signal curve or something.

So how exactly does the software FM Demod work ( crossing detection )?
And why can my $5 toy FM radio sound so much cleaner?

On Mon, Mar 12, 2012 at 9:09 PM, Marcus D. Leech <[hidden email]> wrote:

> On 03/12/2012 09:01 PM, Marcus D. Leech wrote:
>>
>> On 03/12/2012 09:09 AM, Rafael Diniz wrote:
>>>
>>> Marcus,
>>> Have you done any improvements to the good old RDS code in cgran?
>>>
>>>
>> Just updated the simple_fm_rcv  code to use 250e3 sample rate, since
>> that's a common rate between B100/USRP1/E1XX and N2XX family, and
>>  all the math "works out".  That will more lightly load the cpu as well,
>> compared to 320e3.
>>
>>
>>
>>
> And here's an audio sample using the new "common" sample rate:
>
> http://www.sbrac.org/files/fm_stereo_test2.ogg
>
>
>
> --
> Marcus Leech
> Principal Investigator
> Shirleys Bay Radio Astronomy Consortium
> http://www.sbrac.org
>
>
>
> _______________________________________________
> Discuss-gnuradio mailing list
> [hidden email]
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

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Re: Sample audio from my FM stereo receiver

Marcus D. Leech
On 12/03/12 10:08 PM, Andrew Davis wrote:
> Doesn't sound as good to me, maybe it's just a bad sample. I for some
> reason ( I know the math tells me otherwise ) I find it sounds better
> the faster I sample, like 500M. I feel like it has something to do
> with more exact following of the FM signal curve or something.
>
>  
I had the bass turned up, which matches my speakers well, but now that
I'm listening to the
  "Baba O'Reilly" sample on my headphones, it's quite a bit too bassy.


> So how exactly does the software FM Demod work ( crossing detection )?
> And why can my $5 toy FM radio sound so much cleaner?
>
>  
It's a quadrature demodulator using ATAN2--not a simple crossing
detector. It should be quite high
  fidelity.

Some of these "listening tests" are notoriously subjective as well, and
the source material varies wildly
  in quality.

The other thing is this receiver doesn't do any deemphasis processing,
choosing instead to have a 3-band
  equalizer with settable gains that allow you to tweak the audio
spectrum of the output a bit.  Some keener
  could make it more fine-grained to tune the resulting sound to suit
personal preferences/room/etc.






--
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org



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Re: Sample audio from my FM stereo receiver

Eric Brombaugh-3
In reply to this post by Marcus D. Leech
Thanks for posting this - it appears to work just fine with an
Ultra-cheap RTL2832U-based USB dongle. Sounds pretty good too.

Eric

On 03/12/2012 11:53 AM, [hidden email] wrote:

> Rafael:
>
> I added methods for resetting the state machines on the demodulator and
> the decoder, but apart from that, no.
>
> My code relies on those updates so I pushed the updates up to CGRAN for
> radio_data_system
>
> -Marcus
>
> On Mon, 12 Mar 2012 06:09:11 -0700, Rafael Diniz wrote:
>
>> Marcus,
>> Have you done any improvements to the good old RDS code in cgran?
>>
>>
>> Best regards,
>> Rafael Diniz
>>
>>> On 03/11/2012 08:40 PM, Andrew Davis wrote:
>>>> Wow, sounds good, I've got a bunch of 50KW stations about 2 miles
>>>> away and I cannot get any to sound good at all. What receiver code
>>>> are you using?
>>> The code that I put on on CGRAN yesterday: **
>>> https://www.cgran.org/svn/projects/simple_fm_rcv And the supporting
>>> RDS code: ** https://www.cgran.org/svn/projects/radio_data_system I'm
>>> using a TVRX2 on a B100, and this was with 8-bit baseband samples at
>>> 320Ksps input rate to the FM demod. Your problem might actually be
>>> that you have a *bunch* of 50KW stations close by. Although with 70+
>>> dB of dynamic range even at 12-bit ADC sampling, with the gain set
>>> right, you shouldn't have too many problems.
>>
>>
>>
>> _______________________________________________
>> Discuss-gnuradio mailing list
>> [hidden email]  <mailto:[hidden email]>
>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>>
>
>
> _______________________________________________
> Discuss-gnuradio mailing list
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Re: Sample audio from my FM stereo receiver

Alberto Albiol
In reply to this post by Marcus D. Leech
I found a bug with de-emphasis, the coefficients in de denominator should change their sign in gnuradio....

One question. I have tried your simple_fm_rcv and a similar version that I made on my own. In both cases I have found that there is a phase shift in the pilot tone. In the image the red curve is the L-R modulated signal and the green line is the recovered 38Khz carried recovered after squaring the pilot tone. It looks like the zero crossings are pi/2 rad shifted so the demodulated signal is almost zero.



By delaying the pilot by one sample (which is almos pi/2) the problem solves, but I do not see any reason for doing this :-(

Of course, I have been careful and all the filters to split the mux signal have the same delay and have linear phase

Any idea?
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