Bug in wfm_rcv_pll and fix, who has stereo FM specs/info a

GNU Radio
1

Hi,
I found a nasty bug in the wfm_rcv_pll.py code.

The pilot tone pll should return the recovered carrier,
For this you should use gr.pll_refout_cc, not gr.pll_carriertracking_cc.
pll_carriertracking returns the input transformed to baseband using the
recovered carrier.
pll_refout returns the recovered carrier.

With the fixed code I now actually get stereo.
(Before you would get a kind of attenuated, distorted, spectrum inverted
stereo)

I tested it with a stereo-coder I wrote in gnuradio, and now I seem to
actually get stereo seperation.
(stereo modulater is attached)

But now I do get a lot of noise when decoding stereo.

I also don’t quite understand why double side band suppressed carrier
Left-Right audio is attenuated and needs a factor 20 for gain.
(It seems to be correct or at least close)

The code comments say 10 dB
The actual code uses a factor 20 which corresponds to 20 *log10(20)= 26
dB.
This 26 dB seems to be quite accurate, but where is this number based
on?

can you give me a link to more info or the stereo FM specs on this.

        # Pick off the double side band suppressed carrier

Left-Right audio. It is attenuated 10 dB so apply 10 dB gain

        stereo_dsbsc_filter_coeffs =

gr.firdes.complex_band_pass(20.0,

Greetings,
Martin

fix for wrong pll is below:
— gnuradio-core/src/python/gnuradio/blksimpl/wfm_rcv_pll.py
2007-06-11 18:12:55.000000000 +0200
+++ gnuradio-core/src/python/gnuradio/blksimpl/wfm_rcv_pll_fixed.py
2007-06-17 08:07:43.000000000 +0200
@@ -131,8 +131,8 @@
max_freq = -2.0math.pi18990/audio_rate;
min_freq = -2.0math.pi19010/audio_rate;

  •        self.stereo_carrier_pll_recovery =

gr.pll_carriertracking_cc(alpha,beta,max_freq,min_freq);

  •        self.stereo_carrier_pll_recovery.squelch_enable(False);

  •        self.stereo_carrier_pll_recovery =

gr.pll_refout_cc(alpha,beta,max_freq,min_freq);

  •        #self.stereo_carrier_pll_recovery.squelch_enable(False)

#pll_refout does not have squelch yet, so disabled for now

         # set up mixer (multiplier) to get the L-R signal at

baseband

2

Martin D. wrote:

Hi,
I found a nasty bug in the wfm_rcv_pll.py code.

The pilot tone pll should return the recovered carrier,
For this you should use gr.pll_refout_cc, not gr.pll_carriertracking_cc.
pll_carriertracking returns the input transformed to baseband using the recovered carrier.
pll_refout returns the recovered carrier.

Great catch, Martin.

With the fixed code I now actually get stereo.
(Before you would get a kind of attenuated, distorted, spectrum inverted stereo)

Can you check it in?

I tested it with a stereo-coder I wrote in gnuradio, and now I seem to actually get stereo seperation.
(stereo modulater is attached)

But now I do get a lot of noise when decoding stereo.

I also don’t quite understand why double side band suppressed carrier Left-Right audio is attenuated and needs a factor 20 for gain.
(It seems to be correct or at least close)

I would try this with the regular (non-PLL) FM demod. I think the
rolloff of the PLL tracking loop is what is giving you the lower gain.

3

Matt E. wrote:

Great catch, Martin.

With the fixed code I now actually get stereo.
(Before you would get a kind of attenuated, distorted, spectrum inverted stereo)

Can you check it in?
Sure,
I will change the example(s) too, because they try to enable the squelch
which the pll_refout does not have.
(I will disable the squelch for now. Probably implement squelch in
pll_refout later)

I am working on an improved FM-stereo-receiver and a stereo-fm
transmitter too.
So I will get back on this when it is finished and add it to the
examples.

I would try this with the regular (non-PLL) FM demod. I think the
rolloff of the PLL tracking loop is what is giving you the lower gain.
Yes, that seems to be the problem.
For now I switched to using the non-PLL FM-demod.

Do you know if/how I can make a PLL which does not have this high
roll-of.
I played with the pll-parameters but that doesn’t seem to help.
It is critically damped, so that is probably the best we can get with
this pll.
Would I need another type of pll with a higher order loop-filter or
something like that.

Greetings,
Martin

4

I think it’s just a question of bandwidth. Have you confirmed that the
PLL has at least 60 kHz of bandwidth?

Matt

5

Matt E. wrote:

Would I need another type of pll with a higher order loop-filter or something like that.

I think it’s just a question of bandwidth. Have you confirmed that the
PLL has at least 60 kHz of bandwidth?
Bandwidth should be 200 kHz.

But when I look at the FFT of the output I see about 10 dB/octave
roll-of for the pll_demod.
The quadrature_demod has a straight freq response (as expected)

See testcode below:

I wrote a small loopback test fm_mod ->fm-demod
http://www.olifantasia.com/pub/projects/gnuradio/mdvh/testcode/wfm_loopback_test.py
The main code is:
#mod
max_dev=75e3
mod_gain = 2 * math.pi * max_dev / self.sample_rate
self.fm_mod = gr.frequency_modulator_fc (mod_gain)
#demod pll
bandwidth = 200e3
alpha = 0.25bandwidth * math.pi / self.sample_rate
beta = alpha * alpha / 4.0
max_freq = 2.0math.pi*100e3/self.sample_rate
self.fm_demod_pll = gr.pll_freqdet_cf
(alpha,beta,max_freq,-max_freq)
#demod quadrature
fm_demod_gain = 1.0/mod_gain
self.fm_demod_quadrature = gr.quadrature_demod_cf
(fm_demod_gain)

When I look at the FFT of the output I see about 10 dB/octave roll-of
for the pll_demod.
http://www.olifantasia.com/pub/projects/gnuradio/mdvh/testcode/wfm_loopback_screenshot.gif
Should this be right?

The quadrature_demod has a straight freq response (as expected)

Martin

6

Hew How Chee wrote:

Hi,

This document might be useful.

RECOMMENDATION ITU-R BS.450-3
Transmission standards for FM sound broadcasting at
VHF
Is it downloadable somewhere?
I already found that this is the standard I need, but could not find a
place where I could download it.

Greetings,
Martin

7

Hi,

This document might be useful.

RECOMMENDATION ITU-R BS.450-3
Transmission standards for FM sound broadcasting at
VHF

Regards,
Hew

— Martin D. [email protected] wrote:

can you give me a link to more info or the stereo FM
specs on this.

Greetings,
Martin

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8

Hi Martin,

Goto
http://www.itu.int/rec/R-REC-BS.450-3-200111-I/en
and click on the electronic bookshop. After that
register for 3 free download.

Regards,
Hew

— Martin D. [email protected] wrote:

I already found that this is the standard I need,
wrote:

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