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Re: [Discuss-gnuradio] Stuck at WBFM Project :

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] Stuck at WBFM Project :
Date: Tue, 8 Aug 2017 10:52:06 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

Hi Ash,

This might be a bit of a DSP problem more than a GNU Radio-specific one!
So, you have this wave file that was sampled at 48 kHz. Then you interpolate it by 20 – so, now, if you follow that logic, that stream is /meant/ to represent an audio signal sampled at 960 kHz. Now, after that, you parameterize the WBFM as if the signal was 400 kHz, and have a 1:1 input/output rate (that's what's important for my argument).

So, you have what's meant to be 960 kHz, and then you interpolate it again, by 20, to a signal that is meant to be sampled at 19200 kHz = 19.2 MHz. Then you go ahead and "play back" that signal with a hardware device through the Osmocom sink, but at 8 MS/s – so, everything transmitted is played back at /less than half speed/!

I don't really understand what your motivation with the specific parameterizations of the two (that's definitely one too many) resamplers, or the WBFM was, but you should probably just take a step back, and ask yourself "what does this block do?" for /every/ block. This flow graph looks like it is a random combination of tutorials and Youtube videos – that's not how engineering works ;). (By the way, there's other things wrong with the flow graph, but since it needs a bit of restructuring anyways, I'm not going to address those here)

If you haven't, I'd strongly recommend reading the "Guided Tutorial" on


It doesn't specifically address the sampling rate confusion you were a victim of, but it does (hopefully) give you a bit of a feeling about what generally happens, so I think this is the very most time-efficient method of approaching GNU Radio.

Hey, and keep up the good work! Starting into SDR is always a bit of a rocky path, because you need a threefold set of talents – Radio, Software, and Signal Processing. Don't be demotivated if things don't work right off the start!

To help you with your application: Remember, a Resampler has a non-1 (output rate)/(input rate) ratio. In case of the rational resampler, that ratio is (interpolation)/(decimation)

What about

Wav File Source -> WBFM (audio rate 48k, quadrature rate 192k) -> Rational Resampler (interp/decim=(Osmocom sink sampling rate)/(192k)) -> Osmocom sink

Best regards,


On 08/08/2017 04:55 AM, Ash SDR wrote:
Hello Everyone ,

I am a GNU Radio beginner trying to implement Wideband FM Project and transmit it to my FM receiver at a very lower power.  I am choosing a frequency in a FM band such that my receiver is not picking anything from that station/..

Attached is GRC file which i am working on .and the music file which i am trying to transmit..

Can anyone please tell me where I am messing up ?

As a beginner , my learning curve is from implemented projects , University lectures which are available on-line .. This one is from a book which i am using to get basic understanding ..


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