Re: [Discuss-gnuradio] Waveform after AM demodulation looks weird

[Marcus Müller]

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A shoot, I overlooked the 64 decimation rate... Sorry about that.


On 25.04.2014 17:36, Anton Komarov wrote:
> 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.
> 
> 
> On Fri, Apr 25, 2014 at 7:25 PM, Marcus Müller <address@hidden>
> wrote:
> 
> 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:
>>>> An AM demodulator is a squaring function.  It can only
>>>> *ever* produce positive values.  You can remove DC offset by
>>>> using a high-pass filter after it. on Apr 25, 2014, *Anton
>>>> Komarov* <address@hidden> 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.
>>>> 
>>>> -- public PGP key http://pastebin.com/dqZqgyVE
>>>> 
>>>> 
>>>> On Fri, Apr 25, 2014 at 5:11 PM, Michael Ossmann
>>>> <address@hidden <mailto:address@hidden>> wrote:
>>>> 
>>>> 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.
>>>> 
>>>> 
>>>> 
> --------------------------------------------------------------------------------
>>>>
>>>>
> 
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