Re: [Discuss-gnuradio] How do I capture of the time of USRP N210 samples with host computer system time?

[Josh Blum]

On 01/16/2013 06:26 PM, LD Zhang wrote:
> Hi,
> 
> Please see my response below:
> 
>>>
>>>> How are you communicating the same start time to each device in your
>>>> setup? Suppose there were two devices, would it not be more like this:
>>>>
>>>> self.uhd_usrp_source_0.set_time_now(uhd.time_spec_t(time.time()))
>>>> self.uhd_usrp_source_1.set_time_now(uhd.time_spec_t(time.time()))
>>>>
>>>> #start stream time common for all N devices
>>>> start_time = uhd.time_spec_t(time.time() + 0.5)
>>>>
>>>> self.uhd_usrp_source_0.set_start_time(start_time)
>>>> self.uhd_usrp_source_1.set_start_time(start_time)
>>>>
>>>> -josh
>>>>
>>>>
>>> The 2 USRP is each connected to a different computer. Each computer is
>>> sync'd in time via NTP update. Since NTP time is accurate to ~ 1ms, I
>>> consider the 2 computers sync'd right after the NTP update. There is
>>> network communication (socket signal) between the 2 computer so that they
>>> note their system time immediately after the socket signal and schedule
>>> (round forward to a future integer 10 second point) to perform the same
>>> action (data gathering) at the same time in the future. That is, each
>>> schedule an amount of time and each watch its clock, when it gets to that
>>> scheduled point, it immediately launches the top_block.py script in which
>>> the same set_time_now and set_start_time command are performed. There is
>> no
>>> "_0" and "_1" distinction because each is operating independently. I am
>>
>> The code snippet was just supposed to help demonstrate. You need to
>> share the *same* start time for both flow graphs for this to work. The
>> fact that you are using different start times and scheduling the
>> execution of the flow graph and creation of device objects is
>> introducing all this extra variability.
>>
>> -josh
>>
> 
> My understanding is that the approximately *same* start time is enforced at
> the 2 computer that have just been sync'd to NTP, because the top_block.py

Once you sync the USRP time to NTP. The same start time is enforced by
actually using the same start time. I highly recommend doing this.

#syncs to NTP with some small error
self.uhd_usrp_source_0.set_time_now(uhd.time_spec_t(time.time()))

#tells device to stream at time X
self.uhd_usrp_source_0.set_start_time(start_time_for_both_devices)

> is executed at the approximately (~ 1 ms accuracy) the same time. And
> inside top_block.py the same code is applied to the 2 units. So I would
> expect approximately the same start time is used. Maybe not. Maybe the

By executing the flowgraphs at the same time, instead of actually using
the same time, you adding the variability of how long it takes to open
python, construct blocks, and construct the usrp object (which is quite
long).

> better way to do this is to get away from the top_block.py script and start
> using the rx_samples_to_file command, and get the metadata structure info
> and correct for the difference in the metadata timestamp. My question is
> what are the files to modify and how I save the metadata, whether in a
> separate file or in the same file as the sampled data.

The rx_samples_to_file example does not write out the metadata to file.
But it can be modified.

But don't you think its harder to do it this way? have two unaligned
streams, process the start time, remove some samples for the offset in
time..... When you could just have two streams at the same time?

-josh