Forum: GNU Radio Waveform after AM demodulation looks weird

7bf646b8deffe05ccebc8f2dd2787957?d=identicon&s=25 Anton Komarov (Guest)
on 2014-04-25 14:24
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Attachment: grc_airband.grc.png (80 KB)
Attachment: 126000000.0_2014_04_25-16.wav (300 KB)
Hi, you can find in attach AM demodulator grc file. Everything is
straight
forward with demod but audio file generated looks weird. Only positive
values of amplitude. In FFT i do see symmetric picture. I
enabled/disabled
DC offset option in osmosdr block, but it did not help.
Originally file is generated as raw audio and after processed with sox
(-t
raw -r 16k -b 16 -e signed-integer). I have also tried to use wav sink
block - the same result.

Find wav file in attach.
104b263a577688e6cf59f06843a38e2d?d=identicon&s=25 Michael Ossmann (Guest)
on 2014-04-25 15:13
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On Fri, Apr 25, 2014 at 04:14:30PM +0400, Anton Komarov wrote:
>
> Hi, you can find in attach AM demodulator grc file. Everything is
> straight forward with demod but audio file generated looks weird. Only
> positive values of amplitude.

The particular definition of "amplitude" adopted by GNU Radio (and
pretty much everyone else in SDR as far as I know) is "absolute value".
It is correct for it to always be positive.
7bf646b8deffe05ccebc8f2dd2787957?d=identicon&s=25 Anton Komarov (Guest)
on 2014-04-25 15:27
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But when we have only positive values that means in fact we have 0.5 DC
offset, and that is bad. Moreover i've made test recording with GQRX -
waveform is fine! Looking into the code, nothing special, am
demod->low-pass->wav-sink.

--
public PGP key
http://pastebin.com/dqZqgyVE
104b263a577688e6cf59f06843a38e2d?d=identicon&s=25 Michael Ossmann (Guest)
on 2014-04-25 15:42
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On Fri, Apr 25, 2014 at 05:25:54PM +0400, Anton Komarov wrote:
>
> But when we have only positive values that means in fact we have 0.5
> DC offset, and that is bad. Moreover i've made test recording with
> GQRX - waveform is fine! Looking into the code, nothing special, am
> demod->low-pass->wav-sink.

See "DC removal":

https://github.com/csete/gqrx/blob/master/dsp/rx_d...
7bf646b8deffe05ccebc8f2dd2787957?d=identicon&s=25 Anton Komarov (Guest)
on 2014-04-25 15:59
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Thanks a lot. It does work!

--
public PGP key
http://pastebin.com/dqZqgyVE
558c40b97bd1af8d912424757714bda9?d=identicon&s=25 Marcus Leech (Guest)
on 2014-04-26 09:07
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7d89a70df32c0ae27c1235016f9e5441?d=identicon&s=25 Marcus Müller (Guest)
on 2014-04-26 10:29
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Actually I'm a little confused - Is seeing an AGC in an AM demodulator
something you usually do?
Especially the feedforward AGC should be rather err jumpy with a scope
of but 256 samples, unless I'm missing something; f_sample seems to be
1024ksps, thus 256 samples max search and rescaling should behave very
badly if there are baseband signal contents below 4kHz -- and that
should be the case for audio, usually?

Greetings,
Marcus

On 25.04.2014 16:03, Marcus Leech wrote:
> -- public PGP key http://pastebin.com/dqZqgyVE
> The particular definition of "amplitude" adopted by GNU Radio (and
>
>
>
> _______________________________________________ Discuss-gnuradio
> mailing list Discuss-gnuradio@gnu.org
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>
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3edd9a0f297625c9c4318c74ec076615?d=identicon&s=25 Anton Komarov (Guest)
on 2014-04-26 10:40
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Marcus, w/o AGC audio sometimes is creapy. I have tested AGC2, tried to
play with attack and decay, but at last when there is a plane in the
vicinity dongle overloads and sound become very loud. Then i tested FAGC
and it fits well! 1024M/64 gives 16k and 256 samples equals - 62 ms of
signal. Works ok for me.
7d89a70df32c0ae27c1235016f9e5441?d=identicon&s=25 Marcus Müller (Guest)
on 2014-04-26 10:59
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A shoot, I overlooked the 64 decimation rate... Sorry about that.


On 25.04.2014 17:36, Anton Komarov wrote:
> Actually I'm a little confused - Is seeing an AGC in an AM
>>>> An AM demodulator is a squaring function.  It can only
>>>>
>>>> The particular definition of "amplitude" adopted by GNU Radio
>>>> (and pretty much everyone else in SDR as far as I know) is
>>>> "absolute value". It is correct for it to always be
>>>> positive.
>>>>
>>>>
>>>>
> --------------------------------------------------------------------------------
>>>>
>>>>
>
_______________________________________________ Discuss-gnuradio
>>
>> _______________________________________________ Discuss-gnuradio
>> mailing list Discuss-gnuradio@gnu.org
>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>>
>
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739a038d5a03d5448114b3615e2caedc?d=identicon&s=25 Activecat K. (activecat_k)
on 2014-04-28 11:25
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When I looked into the attached graphical flowgraph in the first email,
the
signal was multiplied by zero (constant multiplier) before feeding into
file sink and audio sink.  In this case what you get was probably
amplified
noise.
Was it zero or something else?
7bf646b8deffe05ccebc8f2dd2787957?d=identicon&s=25 Anton Komarov (Guest)
on 2014-04-28 11:54
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This, is a squelch, variable changes between 0 and positive gain value.

--
public PGP key
http://pastebin.com/dqZqgyVE
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