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Re: [Discuss-gnuradio] Source Block - Flow Control

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Source Block - Flow Control
Date: Thu, 16 Oct 2014 14:53:47 -0700

On TX, if you need your first sample to be valid and everything settled in the radio, you should zero pad ahead of it.  If you need your last sample to go out clean, you should zero pad after it.  In both cases, a few microseconds worth is usually fine.  This flushes buffers and filters, but also gives physical devices like antenna switches to settle.


On Thu, Oct 16, 2014 at 2:46 PM, David Halls <address@hidden> wrote:

-------- Original message --------
From: Matt Ettus
Date:2014/10/16 22:42 (GMT+00:00)
To: John Malsbury
Cc: David Halls ,GNURadio
Subject: Re: [Discuss-gnuradio] Source Block - Flow Control

On Thu, Oct 16, 2014 at 8:40 AM, John Malsbury <address@hidden> wrote:
I'm happy this is working for you, David.  So do I understand correctly, that the adding tx_time finally made the MIMO case work?

I'd never done burst transmission with multiple USRP outputs.  I'm not totally surprised but I wasn't sure what would happen.

Maybe someone from Ettus can comment on this?  Since UHD is trying to do time alignment in the multi_usrp class, what happens to time alignemt in the non-continuous-streaming case if you *don't* include a tx_time tag?

Noncontinuous streaming means there is some amount of down time.  If you don't include tx_time tags, the device doesn't know when to start back up again, so each antenna is going to start at a different time.


That seemed exactly the kind of behaviour I saw where sync between the two streams was lost. Matt, can you comment on whether I need to put dummy padding items before the first burst? to flush buffers and is there a delicate way to do this?

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