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[Discuss-gnuradio] Confusion about delay block

From: Anderson, Douglas J.
Subject: [Discuss-gnuradio] Confusion about delay block
Date: Thu, 16 Oct 2014 20:24:31 +0000

Hi all,

A little while back I submitted an issue (http://gnuradio.org/redmine/issues/693) regarding the inability to instantiate the delay block with a negative int (to consume samples) despite the code comments and documentation.

Tom submitted a commit and clarified that you have to instantiate with a int >=0, but that you can set a negative delay after the block is instantiated via set_dly().

I am still unable to set a negative delay and I'm wondering if maybe I'm misunderstanding something.

In a topblock, I do...

        self.delay = blocks.delay(gr.sizeof_gr_complex, 0)

...and connect it to a USRP source.
Then in a main loop which I use to run the flowgraph, I do...

    while True:

... I'd like to skip 1 fft_frame worth of samples for each run of the flowgraph (after a frequency retune).

I get the following error, with both Ettus Research's stable binary of gnuradio/uhd and with pybombs gnuradio and uhd, built today:

terminate called after throwing an instance of 'std::invalid_argument'
  what():  buffer_add_reader: nzero_preload must be >= 0
Aborted (core dumped)

It seems that the delay block still can't accept a negative delay (to consume samples) even after instantiating at 0, or am I missing something simple?

Thanks in advance!

Douglas Anderson | Intern
DOC/NTIA/ITS-T | 325 Broadway St., Boulder, CO 80305 | P: 303 497 3582

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