# Re: Source Block - Flow Control **URL:** \ **Category:** GNU Radio\ **Created:** [October 16, 2014, 11:47pm UTC](https://www.ruby-forum.com/t/re-source-block-flow-control/232591 "2014-10-16T23:47:45Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [October 16, 2014, 11:47pm UTC](https://www.ruby-forum.com/t/re-source-block-flow-control/232591/1 "2014-10-16T23:47:45Z")
-------- Original message -------- From: Matt E. Date:2014/10/16 22:42 (GMT+00:00) To: John M. Cc: David Halls ,GNURadio Subject: Re: [Discuss-gnuradio] Source Block - Flow Control On Thu, Oct 16, 2014 at 8:40 AM, John M. \ 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. Matt 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? * * * NOTE: The information in this email and any attachments may be confidential and/or legally privileged. This message may be read, copied and used only by the intended recipient. If you are not the intended recipient, please destroy this message, delete any copies held on your system and notify the sender immediately. Toshiba Research Europe Limited, registered in England and Wales (2519556). Registered Office 208 Cambridge Science Park, Milton Road, Cambridge CB4 0GZ, England. Web: www.toshiba.eu/research/trl ---
**Author:** ![divotdave](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/bbe5ce/32.png) [@divotdave](https://www.ruby-forum.com/u/divotdave)\ **Post date:** [October 16, 2014, 11:55pm UTC](https://www.ruby-forum.com/t/re-source-block-flow-control/232591/2 "2014-10-16T23:55:29Z")
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. Matt On Thu, Oct 16, 2014 at 2:46 PM, David Halls [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) ---
**Author:** ![divotdave](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/bbe5ce/32.png) [@divotdave](https://www.ruby-forum.com/u/divotdave)\ **Post date:** [October 17, 2014, 12:02am UTC](https://www.ruby-forum.com/t/re-source-block-flow-control/232591/3 "2014-10-17T00:02:02Z")
Thanks Matt. is this likely to be the cause of the Us at the beginning? How can I pad simply in GNU radio? can i add padding to every burst? if so that seems straightforward. I can just mux some zeros in. Also should the padding be included in the burst length? -------- Original message -------- From: Matt E. Date:2014/10/16 22:54 (GMT+00:00) To: David Halls Cc: John M. ,GNURadio Subject: Re: [Discuss-gnuradio] Source Block - Flow Control 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. Matt On Thu, Oct 16, 2014 at 2:46 PM, David Halls \ wrote: -------- Original message -------- From: Matt E. Date:2014/10/16 22:42 (GMT+00:00) To: John M. Cc: David Halls ,GNURadio Subject: Re: [Discuss-gnuradio] Source Block - Flow Control On Thu, Oct 16, 2014 at 8:40 AM, John M. \ 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. Matt 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? * * * NOTE: The information in this email and any attachments may be confidential and/or legally privileged. This message may be read, copied and used only by the intended recipient. If you are not the intended recipient, please destroy this message, delete any copies held on your system and notify the sender immediately. Toshiba Research Europe Limited, registered in England and Wales (2519556). Registered Office 208 Cambridge Science Park, Milton Road, Cambridge CB4 0GZ, England. Web: www.toshiba.eu/research/trl[http://www.toshiba.eu/research/trl](http://www.toshiba.eu/research/trl) * * * This email has been scanned for email related threats and delivered safely by Mimecast. For more information please visit [http://www.mimecast.com](http://www.mimecast.com) * * * * * * NOTE: The information in this email and any attachments may be confidential and/or legally privileged. This message may be read, copied and used only by the intended recipient. If you are not the intended recipient, please destroy this message, delete any copies held on your system and notify the sender immediately. Toshiba Research Europe Limited, registered in England and Wales (2519556). Registered Office 208 Cambridge Science Park, Milton Road, Cambridge CB4 0GZ, England. Web: www.toshiba.eu/research/trl ---
**Author:** ![divotdave](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/bbe5ce/32.png) [@divotdave](https://www.ruby-forum.com/u/divotdave)\ **Post date:** [October 17, 2014, 9:08pm UTC](https://www.ruby-forum.com/t/re-source-block-flow-control/232591/4 "2014-10-17T21:08:34Z")
You’ll probably want to make a block to do the padding, or mux in zeros. Matt On Thu, Oct 16, 2014 at 3:00 PM, David Halls [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)