How to get multipe rx_time tags while receiving continuously

GNU Radio
1

Hi,
As far as I know rx_time tag is associated with the first sample of a
receive stream. If I wanna get multiple rx_time tags while receiving
continuous data, should I stop and issue a new stream again and again
for getting more rx_time tags.
Thank you.

2

On Thu, Oct 31, 2013 at 3:46 AM, Harry Z. [email protected] wrote:

Hi,
As far as I know rx_time tag is associated with the first sample of a
receive stream. If I wanna get multiple rx_time tags while receiving
continuous data, should I stop and issue a new stream again and again
for getting more rx_time tags.
Thank you.

Harry,

We want to minimize tags through the flowgraph since it adds overhead.
The UHD driver only sends an rx_time tag whenever one is necessary.
That means that if there is a chance that the host has become
unsynchronized, it sends an updated tag. So there’s one at the
beginning of the stream to set the initial time. Then, if a dropped
packet or overflow are detected, it sends a new rx_time tag.

Between time tags, you can count samples and you know the sample rate,
so you know the time of every sample based on that initial rx_time tag
(to within the tolerance of the sample clock on the USRP).

Tom

3

Tom,
Thanks for your reply.
I got a weird problem when using rx_time tags. I have three nodes,
node A sends 10 packets within 0.2 sec ,stops for 1sec sends 10 packets
, stops…, sends…,stops… . Node B and C receive it and record
the receive time using (rx_time+ sample_count*sample_rate).
Considerating the clock offset between B and C, the difference of B and
C’s receive time must remain stable. But every time after A stops for
1sec, the receive time’s difference varies several hundreds of
microsecond. I’m stumped by this problem.
Could you give me some advice. Thank you in advance.

Harry

4

Dear Miklos,
Yes, I do never stop receive program. I’m using USRP N210, WBX,
Ubuntu12.04 and UHD3.5.3. I’m writing a document in detail, which will
be sent to you later today.

Best,
Harry

5

Hi Harry,

You never stop the receiver on node B and C, right? You should not
observe anything like that if you do not have dropped packets. Are you
using USRP2’s?

Miklos