[USRP-users] Retune stream tag arriving before retune Is complete
Marcus D. Leech
mleech at ripnet.com
Tue Sep 9 20:23:27 EDT 2014
On 09/09/2014 08:17 PM, Jacob Gilbert wrote:
> Thank you Marcus. I had erroneously assumed the tag was an indication
> that the samples following it were post-retune as opposed to it being
> more of an acknowledgement that the tune request had been performed by
> the driver. It makes sense though given how quick DSP tuning is.
> If I were to move to timed commands to perform more deterministic
> retuning, do you know if a tag be generated at the time of retuning?
> And if so, would that have some level of assurance that subsequent
> data is corresponding to the newly tuned frequency?
B2xx don't support timed commands anyway, as far as I know.
What magnitude of latency are you observing, and at what sample rates?
> On Tue, Sep 9, 2014 at 8:05 PM, Marcus D. Leech via USRP-users
> <usrp-users at lists.ettus.com <mailto:usrp-users at lists.ettus.com>> wrote:
> On 09/09/2014 08:01 PM, Jacob Gilbert via USRP-users wrote:
>> I am having some trouble with a USRP B210 and quickly retuning it
>> for a wideband survey application. I have an OOT GR block which
>> retunes (DSP only, since the LO retune takes quite a while) the
>> USRP via a call to the usrp_source block's retune method, waits
>> for the stream tag, passes some data forward, then repeats.
>> The issue I am currently having is on occasion the stream tag
>> indicating the retune will arrive well before the data is
>> actually valid for the frequency the tag is indicating it just
>> tuned to. I had originally thought the uhd source block would
>> insert a stream tag when the USRP indicated the retune had been
>> completed, however in looking into usrp_source_impl.cc:145, it
>> looks like the tag is appended (or indicated it should be
>> appended) immediately after the tune call is made regardless of
>> whether or not the tune in the FPGA has actually completed.
>> Is this correct? And if so, is there a better way to know when a
>> DSP-only tune has completed and the data stream is valid for the
>> new frequency?
>> USRP-users mailing list
>> USRP-users at lists.ettus.com <mailto:USRP-users at lists.ettus.com>
> The FPGA-only tune happens so quickly that the way it is currently
> written is "correct", modulo latencies in Gnu Radios internal
> buffering mechanisms,
> which are definitely outside the scope of what the hardware can
> There will be ambiguous data after a retune, since you don't
> really know how many samples are "in flight" after doing that.
> Marcus Leech
> Principal Investigator
> Shirleys Bay Radio Astronomy Consortium
> USRP-users mailing list
> USRP-users at lists.ettus.com <mailto:USRP-users at lists.ettus.com>
Shirleys Bay Radio Astronomy Consortium
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