[Discuss-gnuradio] Issues with extra samples after an tx_eob

[Uher, Jason J.]

Hello,

I'm trying to send FM modulated packet data using the message burst
source and it's mostly working.  

I am able to see the tx_sob and tx_eob tags using the tag_debug block
hooked up to the same final block the USRP sink is (ie, the tags are not
getting lost somewhere along the chain).

When I examine the output file, the actual data for the bursts do indeed
start and end at the offsets listed in the tag_debug output, and there
are no samples in between.

If I receive the transmitted burst with a second USRP and plot the data,
you can see the received waveform at http://imgur.com/ZNuNWjt

The tx_sob and tx_eob are pointed out in the diagram, but there is a
long period of just a blank carrier at the end of the data being added
by the USRP N210.  This works for some of the receivers, but is not
interoperable with the COTS hardware I'm working with.


I've attached the output of the block debug and my USRP instantiation
below.  You can see that the tx_sob and tx_eob are listed, but that it
still gives an underflow indication (I thought the tx_eob was supposed
to prevent that).

How can I get rid of this extra energy the USRP is adding to the end?

Thanks,
Jason


The output of the tag debug is this:
<start output section>
Using Volk machine: avx_64_mmx_orc
-- Opening a USRP2/N-Series device...
-- Current recv frame size: 1472 bytes
-- Current send frame size: 1472 bytes
U----------------------------------------------------------------------
Tag Debug: print_tags
Input Stream: 00
  Offset: 0  Source: message_burst_source1     Key: tx_sob   Value: #t
  Offset: 3875  Source: message_burst_source1     Key: tx_eob   Value:
#t
----------------------------------------------------------------------
U
<end output section>


My USRP instantiation is:
<start code section>
        # TX Sink
        self.uhd_usrp_sink_0 = uhd.usrp_sink(
            device_addr="",
            stream_args=uhd.stream_args(
                underflow_policy="next_burst",
                cpu_format="fc32",
                channels=range(1),
            ),
        )
        self.uhd_usrp_sink_0.set_samp_rate(samp_rate)
        self.uhd_usrp_sink_0.set_center_freq(cfreq, 0)
        self.uhd_usrp_sink_0.set_gain(15, 0)
        self.uhd_usrp_sink_0.set_antenna("TX/RX", 0)

        self.connect((self.unclip, 0), (self.uhd_usrp_sink_0, 0))
<end code section>

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