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GR 3.8 overflow, dropped samples with 2 channels from B210


From: Michael Wentz
Subject: GR 3.8 overflow, dropped samples with 2 channels from B210
Date: Sun, 19 Jan 2020 14:02:36 -0500

Hi,

I recently updated to GNU Radio 3.8 and noticed some code that receives 2 channels from a B210 is now always printing a single overflow (O) followed by two drops (DD) right after the flowgraph starts. This happens regardless of the sample rate and only when using 2 channels. It also doesn't happen outside of GNU Radio; the UHD examples benchmark_rate and rx_multi_samples never show overflows or drops. I've tested under macOS 10.14 and RHEL 7, both with UHD 3.15. I also tried downgrading to GNU Radio to 3.7.13.5 and problem goes away, so it seems like a GR issue and not UHD.

In debugging with a file meta sink I've found that the first header for both channels always shows 38760 items before it gets a new tag. I did some more testing with a very simple flowgraph of just a USRP source and null sink. With 1 channel I can stream 20 MSPS without problems. With 2 channels I tried rates 250 KSPS to 4 MSPS and saw the same behavior every time.

I thought that maybe the device was starting to stream before the flowgraph was ready, so tried setting a start time several seconds in the future, but that didn't make a difference. I also tried resetting the B210 and the host USB subsystem.

Any idea what could be going on? A snippet of how the 2 channel B210 is set up by GRC is below.

Thanks,
Michael

---

self.uhd_usrp_source_0 = uhd.usrp_source(
            ",".join(("", "")),
            uhd.stream_args(
                cpu_format="sc16",
                args='',
                channels=[0,1],
            ),
        )
        self.uhd_usrp_source_0.set_center_freq(1e9, 0)
        self.uhd_usrp_source_0.set_gain(0, 0)
        self.uhd_usrp_source_0.set_antenna('RX2', 0)
        self.uhd_usrp_source_0.set_center_freq(0, 1)
        self.uhd_usrp_source_0.set_gain(0, 1)
        self.uhd_usrp_source_0.set_antenna('RX2', 1)
        self.uhd_usrp_source_0.set_samp_rate(1e6)
        self.uhd_usrp_source_0.set_time_unknown_pps(uhd.time_spec())

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