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Re: [Discuss-gnuradio] libusrp C++ inband transmission

From: Brian Padalino
Subject: Re: [Discuss-gnuradio] libusrp C++ inband transmission
Date: Wed, 2 Sep 2009 12:00:05 -0400

On Wed, Sep 2, 2009 at 11:39 AM, Andrey A<address@hidden> wrote:
> Hello, colleagues.
> I'm developing an application in pure C++ using libusrp to interface to
> USRP1 device. The rbf file in use is inband_2rxhb_2tx.rbf from 3.2.2
> release.
> My test application sends bursts of samples to libusrp specifying write
> timestamps. The problem is that I want to start transmitting as quickly as
> possible upon application startup, but actual transmission is delayed for
> about a minute.
> Since inband code takes into account the write timestamp in USB packet I
> think it might be the case when I start with wrong write timestamp. Since
> USRP clock runs at 64e6 Hz and timestamp is 32 bit integer the clock should
> turn around approximately every 67 seconds. So the maximum time I need to
> wait for transmission if I specify wrong timestamp is about a minute.
> I understand that some time is needed to load the rbf to USRP. (Assume it is
> about a few seconds)
> I tried to determine the right timestamp by firstly reading a few samples,
> acquiring current timestamp and using it as initial write timestamp
> (additionally I added a fixed timestamp to account for delay between reading
> and writing to USRP). This still did not have an effect on actual
> transmission start time.
> Could you advise on how to start transmitting as quickly as possible?

Try the "now" timestamp:



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