Re: [Discuss-gnuradio] Channel model - the identity channel

[Martin Braun]

This is not the first time this has come up, and yes, patches are
welcome. The simplest fix might be to add a factory-time argument that
disables the resampler entirely -- this will guarantee no sample
offsets, ever, but of course will not allow to change the timing offset
at runtime.

M

On 06/28/2016 07:47 PM, Raj Bhattacharjea wrote:
> Thanks for the hints for where to look and why this happens from a code
> perspective. I don't think it makes sense from a semantic perspective
> (if the user says taps=1, she should get the unity gain delay free
> channel) but I'm guessing "patches are welcome" is the appropriate
> response to this minor usability issue.
> 
> On Tuesday, June 28, 2016, Andrej Rode <address@hidden
> <mailto:address@hidden>> wrote:
> 
>     Hey Raj,
> 
>     > introduces a delay that can be corrected by applying the taps
>     [0,0,0,1].
>     > See the attached flowgraph that subtracts the signals before and after
>     > the channel model; if you let taps = 1, the two signals don't
>     cancel. If
>     > you use taps = [0,0,0,1], they do.
> 
>     The delay you are seeing is introduced by the fractional_resapmler
>     inside the channel_model block. The FIR filter inside the fractional
>     resampler needs to build up and therefore introduces a initial delay of
>     three samples.
> 
>     If you want to know more about the insides of the channel model I
>     suggest you look at the sources [0][1] yourself.
> 
>     Greetings,
>     Andrej
> 
>     [0] ./gr-channels/lib/channel_model_impl.cc
>     [1] ./gr-channels/lib/channel_model_impl.h
> 
> 
> 
> -- 
> Raj Bhattacharjea, PhD
> Georgia Tech Research Institute
> Information and Communications Laboratory
> http://www.prism.gatech.edu/~rb288/ <http://www.prism.gatech.edu/~rb288/>
> 404.407.6622
> 
> 
> 
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