# A few questions about subclassing gr\_block **URL:** \ **Category:** GNU Radio\ **Created:** [May 3, 2013, 1:05am UTC](https://www.ruby-forum.com/t/a-few-questions-about-subclassing-gr-block/228682 "2013-05-03T01:05:30Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Mark\_Cottrell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/c5a1d2/32.png) [@Mark\_Cottrell](https://www.ruby-forum.com/u/Mark_Cottrell)\ **Post date:** [May 3, 2013, 1:05am UTC](https://www.ruby-forum.com/t/a-few-questions-about-subclassing-gr-block/228682/1 "2013-05-03T01:05:30Z")
Hello, I am trying to create a block that detects sync patterns at baseband tagging the first sample of the pattern using stream tags, then using the tags down stream as part of demodulation. I have made a few assumptions about how gnuradio works that I would like to validate: - a sync pattern could span two blocks of samples passed to general\_work - I need to keep SYNC\_PATTERN\_LENGTH - 1 samples to get around this, so I should be able to use a general block and not output all of the items - you can’t tag historic samples (i.e. samples obtained using set\_history), so I can’t use that are these all reasonable? Currently I have an implementation of the block, but I am having trouble understanding the relationship between ninput\_items and noutput\_items. When I feed the block from a file source consisting of 720 samples, I get ninput\_items[0] = 720 and noutput\_items = 512. Does this value for noutput\_items mean I can only consume and copy 512 of the input samples? And do I need to implement forecast if I want to output more? Thanks in advance for any help, Mark – * * * This email, including any attachments, is only for the intended recipient. It is subject to copyright, is confidential and may be the subject of legal or other privilege, none of which is waived or lost by reason of this transmission. If you are not an intended recipient, you may not use, disseminate, distribute or reproduce such email, any attachments, or any part thereof. If you have received a message in error, please notify the sender immediately and erase all copies of the message and any attachments. Unfortunately, we cannot warrant that the email has not been altered or corrupted during transmission nor can we guarantee that any email or any attachments are free from computer viruses or other conditions which may damage or interfere with recipient data, hardware or software. The recipient relies upon its own procedures and assumes all risk of use and of opening any attachments. ---
**Author:** ![Mark\_Cottrell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/c5a1d2/32.png) [@Mark\_Cottrell](https://www.ruby-forum.com/u/Mark_Cottrell)\ **Post date:** [May 3, 2013, 9:09am UTC](https://www.ruby-forum.com/t/a-few-questions-about-subclassing-gr-block/228682/2 "2013-05-03T09:09:56Z")
On Fri, May 03, 2013 at 11:04:40AM +1200, Mark Cottrell wrote: > I am trying to create a block that detects sync patterns at baseband tagging > the first sample of the pattern using stream tags, then using the tags down That’s the correct way to do it, if you ask me. Just to clarify: Before you get a sync pattern, are you producing anything? Or are your copying all of the input to the output, plus the tag? If the latter is the case, you should use a sync\_block. > stream as part of demodulation. I have made a few assumptions about how > gnuradio works that I would like to validate: > > - a sync pattern could span two blocks of samples passed to general\_work It depends on your sync pattern. If it’s not too long, you can avoid that. > - I need to keep SYNC\_PATTERN\_LENGTH - 1 samples to get around this, so I > should be able to use a general block and not output all of the items > > - you can’t tag historic samples (i.e. samples obtained using set\_history), so > I can’t use that You can also use ‘future’ samples to do signal processing. An example are the peak\_detector blocks, which find the beginning of a peak. Simply don’t consume them. > Currently I have an implementation of the block, but I am having trouble > understanding the relationship between ninput\_items and noutput\_items. When I > feed the block from a file source consisting of 720 samples, I get ninput\_items > [0] = 720 and noutput\_items = 512. Does this value for noutput\_items mean I > can only consume and copy 512 of the input samples? And do I need to implement > forecast if I want to output more? That depends. Do you _really_ want to output more? How many? What if your have 1440 items at the input? Basically, this is telling you there’s space for 512 items in the output buffer. If you can use a sync\_block, you’ll never run into these problems, because there’s only one value. MB – Karlsruhe Institute of Technology (KIT) Communications Engineering Lab (CEL) Dipl.-Ing. Martin B. Research Associate Kaiserstraße 12 Building 05.01 76131 Karlsruhe Phone: +49 721 608-43790 Fax: +49 721 608-46071 www.cel.kit.edu KIT – University of the State of Baden-Württemberg and National Laboratory of the Helmholtz Association ---
**Author:** ![Mark\_Cottrell](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/c5a1d2/32.png) [@Mark\_Cottrell](https://www.ruby-forum.com/u/Mark_Cottrell)\ **Post date:** [May 3, 2013, 9:11am UTC](https://www.ruby-forum.com/t/a-few-questions-about-subclassing-gr-block/228682/3 "2013-05-03T09:11:06Z")
On Fri, May 03, 2013 at 09:08:34AM +0200, Martin B. (CEL) wrote: > Basically, this is telling you there’s space for 512 items in the output > buffer. If you can use a sync\_block, you’ll never run into these > problems, because there’s only one value. Also, there’s lots of other functions to control the output buffer size, e.g. set\_output\_multiple(), set\_min\_output\_items()… MB – Karlsruhe Institute of Technology (KIT) Communications Engineering Lab (CEL) Dipl.-Ing. Martin B. Research Associate Kaiserstraße 12 Building 05.01 76131 Karlsruhe Phone: +49 721 608-43790 Fax: +49 721 608-46071 www.cel.kit.edu KIT – University of the State of Baden-Württemberg and National Laboratory of the Helmholtz Association