# Need help to understand code of sync:decimator block **URL:** \ **Category:** GNU Radio\ **Created:** [March 8, 2014, 8:50am UTC](https://www.ruby-forum.com/t/need-help-to-understand-code-of-sync-decimator-block/239639 "2014-03-08T08:50:17Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\ **Post date:** [March 8, 2014, 8:50am UTC](https://www.ruby-forum.com/t/need-help-to-understand-code-of-sync-decimator-block/239639/1 "2014-03-08T08:50:17Z")
Hello, I am using GNU Radio version 3.6.5 and trying to write a block to calculate average of 5 samples. I want to produce one average output corresponding to 5 input samples. I am using set\_history(5) to remember previous data. I used gr\_modtool script of type decimator to create file. eg. for Vector source {1,2,3,4,5,6,7,8,9,10}, it should generate average {3,8}. Instead I am getting {.2, .6}. My requirement is to produce output, it should take 5 inputs. Please suggest me where I need to do modification. In function gr\_sync\_decimator(“average”, gr\_make\_io\_signature(1, 1, sizeof(float)), gr\_make\_io\_signature(1, 1, sizeof(float)), 5) what role value 5 will play? Below is code snapshot. #ifdef HAVE\_CONFIG\_H #include “config.h” #endif #include \ #include “average\_impl.h” namespace gr { namespace howto { ``` average::sptr average::make() { return gnuradio::get_initial_sptr (new average_impl()); } /* * The private constructor */ average_impl::average_impl() : gr_sync_decimator("average", gr_make_io_signature(1, 1, sizeof(float)), gr_make_io_signature(1, 1, sizeof(float)), 5) { set_history(5); } /* * Our virtual destructor. */ average_impl::~average_impl() { } int average_impl::work(int noutput_items, gr_vector_const_void_star &input_items, gr_vector_void_star &output_items) { const float *in = (const float *) input_items[0]; float *out = (float *) output_items[0]; in +=4; for(int i=0; i **Author:** ![kunal\_sankhe](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/9e8a1a/32.png) [@kunal\_sankhe](https://www.ruby-forum.com/u/kunal_sankhe)\ **Post date:** [March 8, 2014, 10:54am UTC](https://www.ruby-forum.com/t/need-help-to-understand-code-of-sync-decimator-block/239639/2 "2014-03-08T10:54:39Z") -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Hello Kunal, You don’t want to use set\_history in this case: you want to produce 1 output for 5 input, so you use a decimator; that’s all you need to do. Drop the set\_history. For reference: history is when for every input item you need the last 5 items before, [http://gnuradio.org/redmine/projects/gnuradio/wiki/OutOfTreeModules#set\_history](http://gnuradio.org/redmine/projects/gnuradio/wiki/OutOfTreeModules#set_history) Greetings, Marcus * * * Sent from my Android device with K-9 Mail. Please excuse my brevity. -----BEGIN PGP SIGNATURE----- Version: APG v1.0.9 iQFABAEBCAAqBQJTGuigIxxNYXJjdXMgTfxsbGVyIDxtYXJjdXNAaG9zdGFsaWEu ZGU+AAoJEBKNLRrpJvfoIRUH/0Orh65WsPeli99oQrbMpb40FVG+cytjmuUS8mKX s0PHoDcQ/HXC8KIKxAOo+JQ26X/AK8Qnf3WCgfYB2/dvA7sz7JmYUbXgGFry1S9D dtPUzAW2D0AASXG19rFBtCq6u1nhVDsIwxQ1yASNwx2wKuQEZyRFA1tjHiUqjpFf 0V+S1nB81jWw0WUJTpRMRGOe1tvhG0IDKmeBptMbKw6ShM979pPM3JgDsFIhn2MS UWIM7Qjvnul4g8dnq/t7DpNe+lguHFgFes3hB8cHtTRGBvy+8QPJWn5lnCRBZz+4 ZKFrkWdsslzKjUHP67ZIQ5hn104P6MQZER+RpFdtcO3BJqg= =9KAs -----END PGP SIGNATURE----- ---
**Author:** ![kunal\_sankhe](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/9e8a1a/32.png) [@kunal\_sankhe](https://www.ruby-forum.com/u/kunal_sankhe)\ **Post date:** [March 8, 2014, 12:05pm UTC](https://www.ruby-forum.com/t/need-help-to-understand-code-of-sync-decimator-block/239639/3 "2014-03-08T12:05:02Z")
Hello Marcus, Sorry for my vague explanation. I want to write a block which will calculate moving average for a block of samples. I have only one input in this case. I am providing my data serially using vector to stream converter. I want to calculate average of a block of 5 samples. eg. from vector source {1,2,3,4,5,6,7,8,9,10}, i will get stream of integers. Now I want to calculate average for first 5 samples i.e. 3 and then it should calculate average of {6,7,8,9,10} which is 8. So i should get desired output as {3,8}. So I should get output for a block containing 5 samples. It should again consider block of 5 next samples and calculate average. Please let me what changes do I need to make. Thanks, Kunal ---
**Author:** ![kunal\_sankhe](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/9e8a1a/32.png) [@kunal\_sankhe](https://www.ruby-forum.com/u/kunal_sankhe)\ **Post date:** [March 8, 2014, 12:14pm UTC](https://www.ruby-forum.com/t/need-help-to-understand-code-of-sync-decimator-block/239639/4 "2014-03-08T12:14:10Z")
Hi Kunal, Use a decimating FIR filter with taps [0.2, 0.2, 0.2, 0.2, 0.2] and decimation 5. It will be faster than anything you are about to write. Miklos ---
**Author:** ![kunal\_sankhe](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/9e8a1a/32.png) [@kunal\_sankhe](https://www.ruby-forum.com/u/kunal_sankhe)\ **Post date:** [March 9, 2014, 2:53am UTC](https://www.ruby-forum.com/t/need-help-to-understand-code-of-sync-decimator-block/239639/5 "2014-03-09T02:53:31Z")
Dear Kunal, This is not “moving average”, this is normal “average”. I agree with Marcus’ explanation, you don’t need the “set\_history()”. Moving average is: 1st output = average of 1,2,3,4,5 2nd output = average of 2,3,4,5,6 3rd output = average of 3,4,5,6,7 4th output = average of 4,5,6,7,8 Obviously what you want is not “moving average”. If you really want moving average, then only you need “set\_history()”. Regards, Activecat ---
**Author:** ![kunal\_sankhe](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/9e8a1a/32.png) [@kunal\_sankhe](https://www.ruby-forum.com/u/kunal_sankhe)\ **Post date:** [March 8, 2014, 12:26pm UTC](https://www.ruby-forum.com/t/need-help-to-understand-code-of-sync-decimator-block/239639/6 "2014-03-08T12:26:17Z")
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Sorry I was unclear: “5 inputs” means “5input items from the same (only one) input stream”. Now, Miklos is most probably very right, the GNU Radio filters are highly optimized. But let’s take this as an exercise in understanding decimators: A decimator is just a normal block that “knows” that for every N input items, it produces 1 output item. So if you produce but 1 output, GNU Radio will now that you’ve seen input 0…4, and you will be presented with 5…9 next time. You’re work’s for loop does something like a moving average for _each_ input item, so that’s what’s wrong with it. So, as I said: just drop the set\_history call, and change your for loop like following and your block should work: for(int i = 0; i/5 \< noutput\_items; i += 5) out[i / 5] = (in[i]+ in[i+1]…+in[i+4]) / 5.0f; return i/5; Greetings, Marcus On 08.03.2014 12:03, kunal sankhe wrote: > [http://gnuradio.org/redmine/projects/gnuradio/wiki/OutOfTreeModules#set\_history](http://gnuradio.org/redmine/projects/gnuradio/wiki/OutOfTreeModules#set_history) Greetings, > Marcus > > > -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 Comment: Using GnuPG with Thunderbird - [http://www.enigmail.net/](http://www.enigmail.net/) iQEcBAEBAgAGBQJTGv3kAAoJEBQ6EdjyzlHtEXkH/i3Z8NnM9sQP3lZLFRU7SEsT NedPJD7hcCGVlDUY16h7fdtuweHesDIP51vzrLJfmbC/swRS2gWhps5KrY7w8+vq LwHZjBNSa1tBZ5e0OlrW+mCv2PXlrbC0kGOdMSnyoptWNVm8NNJgTw3VxVKHpWFK FEpBfaIdoNt873qZdWnSxaCDeusrdeIDmNiE68bGwRd3solH4CSQe4Dve7Q7N45a wEUn9UhHk/jOKRZ9hEqDM/sF1Bp2JL2zmZOj46YT82tvFjGNpFyftu2wei/LByst /s0n8h8w0l0TjkEIE7ehIwYIEzBtuUv7Op8ePrDtKEBRy7BBcw/25e8ChlOKe1U= =HpzM -----END PGP SIGNATURE-----