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Here is how the FM Receiver I made using the GNU Radio Companion.

(Basically, I was following this tutorial by VYE6Y on youtube.)

FM Receiver

FM Receiver

  • Problem: Unfortunately for me, the GNU Radio gives no output. It opens a window as you can show below but no matter what I click or change, there is nothing to be heard. I think, the signal is being processed as the Bandpass spectrum can be seen(as shown in the 2nd image below, which shows the characteristic peak at the center-frequency of the channel).

I am using GNU Radio Companion 3.7.11.1, which is fairly new, on Windows 8.1.

What cannot be a problem:

  1. The RTL-SDR Dongle works. I can run it, for example, without any problems on CubicSDR, where it produces crisp clear sound.
  2. Also, I can run a different FM Receiver also made on GNU Radio Companion (though the parameters and scheme of the receiver are different and not quite tuned.)

Output

  • Any ideas why there is no sound?
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  • $\begingroup$ Ok. Apparently, I was too sleepy last night to notice what I was doing. There seems to be a very small mistake there. The Multiply const block is accepting ints, whereas the blocks before and after Multiply const expect floats. (Notice the green color). Probably that is the only mistake. Will test tonight with the Dongle. $\endgroup$ – Duck Dodgers Feb 15 at 9:25
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    $\begingroup$ Aaaaah! Normally, you'd get red arrows when there's a type mismatch, but since int is 32 bit and float is 32 bit, GNU Radio doesn't care (in fact, doesn't even know) that there's something going wrong here. $\endgroup$ – Marcus Müller Feb 15 at 9:27
  • $\begingroup$ Yes, I noticed that too. There were plenty of red arrows and red boxes at first. But then slowly I fixed all the mistakes and there were none anymore. And I thought why is it now not working. Anyways, thanks for the explanation @MarcusMüller. :) $\endgroup$ – Duck Dodgers Feb 15 at 9:29
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    $\begingroup$ Do try 48 kHz for the Audio sink, which means that you need to adjust the interpolation/decimation in the rational resampler block, too. $\endgroup$ – Marcus Müller Feb 15 at 9:35
  • $\begingroup$ Ok. I will try that. Thank you for the hint, Marcus. $\endgroup$ – Duck Dodgers Feb 15 at 9:39
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Indeed, as I suspected this morning, it was the Multiply Const block. After changing it to accept&work with floats instead of ints, I can hear sound.

Also as Marcus suggested, I changed the Sample Rate for the Audio Sink to 48KHz from 24KHz. But then I noticed that I could only hear every other second. I realized then that the Rational Resampler Interpolation rate nees to be double also from 24 to 48.

Honestly, I don't hear a big difference between 48KHz and 24KHz, but then again, I am not much of an audiophile. :)

Image of fixed flow-graph

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    $\begingroup$ there shouldn't be an acoustical difference, since your FM radio station will not have a high audio bandwidth, but my suspicion was that since you heard nothing, the audio system of your operating system simply couldn't internally resample the 24 kHz to whatever your sound card hardware uses as rate; 24 kHz is relatively uncommon compared to 48 kHz. $\endgroup$ – Marcus Müller Feb 16 at 13:00
  • $\begingroup$ @MarcusMüller While some sound hardware might be limited in sample rate, from a hardware POV, single chip solutions for audio which permitted using all semi-common audio sampling rates from 8 kHz to 48KHz have existed since the early 90s and became standard in audio hardware since shortly thereafter (i.e. inexpensive and user-expected capability). It might be that a software layer wasn't properly telling the driver which sample rate to use, but the hardware is almost certainly capable of converting many different sample rates, including 24 KHz. $\endgroup$ – Makyen Feb 23 at 17:15
  • $\begingroup$ @Makyen that might be true, but modern non-studio audio hardware typically only has one rate at a time, so, if you want to use your sound card with multiple software applications simultaneously, something in software will have to do the resampling to a single rate. $\endgroup$ – Marcus Müller Feb 23 at 18:31

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