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Re: [Discuss-gnuradio] possible to synchronize two input streams?

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] possible to synchronize two input streams?
Date: Tue, 15 Apr 2008 10:23:19 -0700
User-agent: Mutt/1.5.17 (2007-11-01)

On Tue, Apr 15, 2008 at 10:55:02AM -0400, George Nychis wrote:
> Hi all,
> What I need is the output of GNU Radio's FIR filter (gr.fir_filter_ccc) to 
> be synchronized with the raw samples from the USRP into the input of a new 
> block.
> Like this, where the input to the new block ensures that input[0][N] is 
> related to input[1][N] ... such that input[M][N] is always valid for both 
> input streams:
> http://skrbl.com/72178738
> Then, the output streams need to be synchronized also... but I can ensure 
> this in work() as long as my input streams are synchronized.
> I re-went through the how to write a block guide, and I'm still a little 
> iffy on whether or not this is possible.  Forecast() specifies the 
> relationship between input and output of the streams, but I'm wondering if 
> it can force the block to wait for an equal number of input items on both 
> streams?
> I'd greatly appreciate any feedback.
> Thanks!
> George

They'll be synchronized.  There's nothing special that you need to do
assuming that "new block" derives from gr_sync_{block,interpolator,

However, because of the group delay in the FIR, there'll be a constant
offset between the two streams.  The easiest way to fix that is to use
gr.delay in the path that is currently straight through.


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