Reset streaming vector/block

We have an application which uses the uhd.usrp.sink3 block, with
io_type.COMPLEX_FLOAT32. The complex vector is converted from a
text-based I-Q file. It is then sent to the sink with the assignment
self._src = gr.vector_source_c(z, True).

As I understand the documentation, the Boolean True parameter means that
USRP should continually/repeatedly re-transmit the same signal.

Transmission is started using tb.start(), time.sleep(1), and streaming=
True statements.

Transmission (called “streaming”) is stopped with tb.stop() ,
tb.wait(), tb.set_vector(), and streaming=False statements.

(Note tb=my_top_block(), which is gr.top.block class).

  1. Not sure why the time.sleep(1), or the tb.wait() statements are even
    necessary.

  2. But more importantly, it seems that if we change the parameters which
    affect the vector (including the filename used as the source), the
    tb.set_vector() statement seems to have no effect on the signal when
    transmission is resumed. It appears that once the vector has been sent
    to the USRP (once), it cannot be reset except by a complete re-start of
    the entire program/script. In other words, tb.stop is really tb.pause .

Is there a better way ? (Note: changing other parameters, such as
preamp and gain, does have the proper effect. I surmise that the stream
is buffered in USRP memory and cannotsimply be over-written. Is that
true ?

George Kinal
Lead Engineer
MITRE E531 Signal Processing and Communications Analysis
Phone (703) 983-7585


On 08/08/2012 03:28 PM, Kinal, George V. wrote:

Transmission (called “streaming”) is stopped with tb.stop() ,
tb.wait(), tb.set_vector(), and streaming=False statements.

(Note tb=my_top_block(), which is gr.top.block class).

Basically, you do not want to use gnuradio like this. Starting and
stopping the flow graph is not a practical thing. And vector source is
really only for test purposes.

It sounds like you want to perform controlled sample bursts. For this,
you will want to create a source block that produces the desired samples
with metadata tags.

There is a good tags demo example to demonstrate this usage:
http://code.ettus.com/redmine/ettus/projects/uhd/wiki/GNU_Radio_UHD#Using-UHD-with-GNU-Radio

And more about tags here:
http://gnuradio.org/redmine/projects/gnuradio/wiki/BlocksCodingGuide#Stream-Tags

-josh

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