Sample Rate & Hardware Considerations Tutorial: GNURadio.org

Morning All,

I have a question after following the sample rate tutorial and its implementation on the Guided Tutorial Hardware Considerations section on the official site: https://wiki.gnuradio.org/index.php/Guided_Tutorial_Hardware_Considerations

After the first portion of the lesson, one is asked to add new blocks shown below. 

• 1x Rational Resampler
• 1x WBFM Receiver
• 1x Audio Sink
• 2x Variable

For the Variable: samp_rate we are told to add the following:

• ID: samp_rate
• Value: 250e3

Now, I know the bandwidth of FM is 250 kHz. What I don't know is why we were asked to make this our sample rate. Coincidence? 

When I run the example (with a few changes for my hardware, and creating a few more variables) I get a signal stream that seems out of sync. 

The sound I hear is a constant drop or miss in signal about three times a second. Think waa-waa-waa, each second, in between the radio playing. 

I launched SDRSharp to compare output and noticed the sample rate is default at 2.4M 

I then changed this in my Variable samp_rate and low and behold, the stream is steady.

I have two questions. 

1. Why was a sample rate of 250K chosen in the example?
2. What is the goal of the multiplier?
• In your (Barry Duggan's) example the multiplier helped to remove his static
• In my file, setting this as a variable slider, and adjusting while playing, I see no difference to the sound

Below is my flowgraph, I also attached the .grc file. 

Any help is greatly appreciated. Thanks in advance! 

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Thanks,
James G Hayek

Youtube.com/JamesHayek