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Re: [Discuss-gnuradio] pipe trick

From: Chuck Swiger
Subject: Re: [Discuss-gnuradio] pipe trick
Date: Sun, 10 Apr 2005 09:40:34 -0400

At 03:02 AM 4/10/2005 -0400, you wrote:
How are you _reading_ the data from the pipe?  Even if you're flushing
the data into the pipe after every write, the reader could be
buffering it.

Tks Dave - yes, yesterday I put a vector_sink_b after the file_descriptor_source
of the pipe and verified that it is reading ALL the date sent across it.
That is, after sending 20 \0xaa characters, the vector contains
(170, 170, 170, 170  ...  170, 170)   all 20 of them.

The little trickle of data going out to the usrp every byte written is interesting
and just now measured this:

bytes written to pipe   size of output to usrp  meaning
-----------------------------   ------------------------------- ------------
2                       49144                   6143 complex samples  49144 / 8
                                                6143+1 = 3 * 2048
2048 is size of bytes_to_syms interpolation

3                       65528                   8191 complex samples
                                                8192+1 = 4 * 2048

4                       81912                   10239 complex samples
                                                10239+1 = 5 * 2048


Every byte written to the pipe is /supposed/ to produce 8 * 2048 * 25 = 409600 samples the output rate to the usrp is 1.6msps (usrp interpolates by 80 to get 128e6) so it only has to write about 4 characters / second to the pipe to keep it stuffed ( or approximately 3.906 as Spock would say ). Then, as soon as I close the pipe after getting these little 'ticks' of data, boom, it takes off and processes the data at full speed and produces
the correct output and prints text on a nearby receiver and psk31 software.

This didn't happen with a pipe head and a sound card sink - only with the usrp at the tail. Even changed the bytes_to_syms interpolation back to 256 as in the sound card version,
but still get the same issue with the usrp.

Interestingly, the X25 interp fir filter doesn't show up mathematically in the output, even tho it's in the stream. It's like the entire gnuradio stream is being clocked by the pipe and the
X2048 bytes_to_syms interpolator at the beginning.


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