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Re: [Discuss-gnuradio] First time user - fm radio tutorial has choppy au

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] First time user - fm radio tutorial has choppy audio
Date: Wed, 14 Jan 2015 18:52:45 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0

Hi Chris,
On 01/14/2015 06:44 PM, Chris Hallinan wrote:
> Greetings, I'm a first time user of gnuradio.
> I've prototyped an FM receiver as described in the tutorial at the
> bottom of this page:
> http://gnuradio.org/redmine/projects/gnuradio/wiki/Guided_Tutorial_Hardware_Considerations
> However, when I run it, and set the center frequency to a close-by FM
> station, although I can hear actual broadcast audio snippets, meaning
> that it's actually receiving a coherent signal, it is very choppy,
> with audio snippets being played in a very choppy, almost rhythmic
> fashion.  The console window on gnuradio-companion shows a continuous
> series of aUaUaU, etc.
> I'm guessing this might be an underrun situation somewhere, but that
> is speculation.
But speculation that's probably correct :)
aU is audio underrun, which means you're most probably supplying samples
at a rate that's lower than the audio sink is configured to accept.
> I'm using parameters provided by the tutorial including a sample rate
> 250K.  I've tried moving the sample rate around, but without much
> improvement.  Dropping it down significantly destroys all hints of
> intelligence in the signal ;)  Am I overtaxing the capabilities of
> this rtl-sdr dongle?
Probably not, since you're doing so well this far! Basically, use a
sampling rate that works, and decimate to exactly the sampling rate you
need for your audio sink.

> I have a reasonable (and growing) understand of the core concepts, but
> could use some help on the learning curve.
> Any advice appreciated.
see above :)

Greetings, and happy hacking,

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