Re: [Discuss-gnuradio] Transmit volume very low

[Marcus Müller]

Hi Santos,

as you're transmitting FM, and not the audio amplitude itself (that'd be AM), the amplitude you're receiving is really just what the NBFM receiver interprets from the incoming signal. Just multiply with a constant, and you're done. That very much is like the amplifier knob on your analog radio receiver -- it scales the audio signal. Information-theoretical, multiplying your signal so you can hear it louder doesn't change SNR, and hence is a lossless operation.

I do see a problem with your flow graph, though:
Your NBFM transmitter produces a that has 10 times the sampling rate of your audio signal, and your NBFM receiver reduces the incoming sample rate to one tenth; that would be fine, but you're using a rational resampler in between; so your RF samples aren't at the NBFM transmit sampling rate, but at 1.05 times that rate. In other words: all frequencies now appear to be 5% lower if you'd expect the sampling rate to be the original one.
Now, in FM, that will scale your output amplitude (but you probably won't be able to hear that).

Why are you using the rational resampler? All your downstream blocks seem to expect a 220.5kHz rate, not 231.525kHz.

Best regards,
Marcus

On 28.03.2016 19:56, Santos Campos wrote:

Hello, all! Newer user here and even newer to the mailing list!

I'm trying to transmit a short .wav file via a b200 board (usrp). Before I broadcast I was trying to hear what the audio would sound like after going through the block diagram and I ended up with the diagram attached. It "works" but the volume is very very low. Any ideas? Previously I'd just put a multiply const block right before the audio sink, but I want the sample to be already transmitting with a higher magnitude. Any help is much appreciated!​ 

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