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

[Jamie Wo]

Hi Vivian,

I do not know how to measure the overhead. You may refer to Wayne's reply.

Please see the flow graphs attached which may be able to solve your problem.

Jamie

On Sat, Apr 28, 2012 at 12:25 AM, Vivian Paola Triana Galeano <address@hidden> wrote:

Hi Jamie, My name is Vivian, I am working with DPSK and GMSK mod/demod too.  Do you know how to measure the overhead of the packet encoder-decoder ?

I could see that in the first images that you attached you weren't  receiving a good signal at the receiver. But in the other images you are receiving a good signal. What changes did you do? Can you attached the flow graph, I mean the grc squematic. I have been working on this for a long time and I haven't could be able to solve this problem because I cant receive anything. I would aprreciate any help you could give me.

Also, I have found the same problem with the wave file that you mention here.

Thanks a lot.

Vivian.

2012/4/27 Jamie Wo <address@hidden>

Hi Tom,

Thanks for your reply. It works.

I met another strange problem.  When I use Wav file source --> audio sink, I can hear the sound in the wav file clearly. However, when I want to hear the file and transmit it using UHD sink at the same time, audio underrun "aUaU" occur. The flow graph is like:

Wav file source --> Package Encoder --> GMSK Mod --> UHD sink.
                       --> Audio sink

Do you know what is the problem?

Jamie

On Fri, Apr 27, 2012 at 3:53 AM, Tom Rondeau <address@hidden> wrote:

On Thu, Apr 26, 2012 at 12:08 AM, Jamie Wo <address@hidden> wrote:
>
>
> On Thu, Apr 26, 2012 at 5:12 AM, wayne roberts <address@hidden>
> wrote:
>
> Hi Wayne,
>
> Thanks for your reply. My responses are:
>
>
>>
>> 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?
>
>
>
> I  am not sure the real-time fashion to drive an audio sink either, but
> there should be a way to support it, I guess. I ran the flow graph and saw
> the update time interval from a scope sink is 3 - 5s . Is this too long?  Do
> you know what does this update time mean?

Yes, these run in "real-time." If they didn't, you wouldn't be
properly able to transmit or receive on an actual piece of hardware.

>> 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.
>
>
> I used Quadrature Demod -> throttle -> scope sink on the receiver side to
> see the signal going into the GSMK demod after uhd source. Also I
> used Quadrature Demod -> throttle -> scope sink after the output of GMSK
> mod. The time and frequency domain outputs are attached.  Theoretically,
> these two output should be similar or  the same, However, they are quite
> different after travelling over the air. Can you see any problems from the
> figures?

Don't use a throttle when you have a block that's already setting the
rate of your flowgraph. The USRP receiver is controlling that. Having
two rate-controllers in there is at best not needed and at worst
actually screwing up your receiver.

Tom

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