Re: [Discuss-gnuradio] Transmit and receive suing GMSK

[wayne roberts]

I was messing with the same thing myself.

First off, i'm not sure the packet decoder outputs in a real-time fashion to drive an audio sink.  Try a scope sink to see how often the packet decoder output updates: is there a better way?

On the GMSK demodulator side, I would think its best to observe the signal going into the demodulator, from uhd rx source.  To see it in time domain, you can use Quadrature Demod -> throttle -> scope sink, making sure signal is centered at 0 and is reasonable distortion free.  To be sure what its supposed to look like, you can put on the transmitter side Quadrature Demod -> throttle -> scope sink on the output of GMSK modulator.

And of course you can put an FFT sink right on the UHD source to see it in frequency domain.  On the transmitter side, to UHD sink, I have found the GMSK modulator outputs signal level near the maximum, meaning some transient peaks could cause clipping on USRP transmitter, so it might be prudent to put a multiply const (with 0.99 or so) just before going to UHD sink.

On Wed, Apr 25, 2012 at 1:41 AM, Jamie Wo <address@hidden> wrote:

Hi all,

I am currently working on tranmiting and receiving data using 2 USRPN210s. I got some ideas from here and did this task using GMSK in GRC. 

The transmit side: File source --> Packet encoder --> GMSK Mod --> UHD sink.

The receiver side: UHD source --> GMSK Demod --> Packet decoder --> File sink.

Data in file source can be received by the receiver. However, I am not sure whether the data has been received correctly. Does anyone know how to test it ? 

My method is to use  wav source and audio sink like this:

The transmit side: Wav source --> Packet encoder --> GMSK Mod --> UHD sink.

The receiver side: UHD source --> GMSK Demod --> Packet decoder --> Audio sink.  (samp_rate 48K). 

I think if the sound received on the receiver is the same as the wav file, the data may be correctly received. However, after I tried this, the sound received was disjointedly, not as smooth as the wav source,  and "audio underrun" happened sometimes.

However, when I put all the blocks in one flow graph without UHD like this:

wav source --> Packet encoder --> GMSK Mod -->  GMSK Demod --> packet decoder --> audio sink.(samp_rate 48K),  it works very well. 

Does anyone meet the same problem  or know how to solve it? Is my way to test how to receive correctly right? 

To do what I want, is there any other blocks that need to be added in the GRC? 

Please give me some guidance.

Thanks in advance.

Jamie

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