# Implementing a FIR Filter **URL:** \ **Category:** GNU Radio\ **Created:** [August 18, 2015, 10:27pm UTC](https://www.ruby-forum.com/t/implementing-a-fir-filter/243059 "2015-08-18T22:27:38Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Rafa\_F](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/958977/32.png) [@Rafa\_F](https://www.ruby-forum.com/u/Rafa_F)\ **Post date:** [August 18, 2015, 10:27pm UTC](https://www.ruby-forum.com/t/implementing-a-fir-filter/243059/1 "2015-08-18T22:27:38Z")
Hi all, Would someone please recommend a good starting point in C++ source code that demonstrates how one uses the kernel::fir\_filter. I started following the pfb\_clock\_sync method of implementing it, but then realized this had been customized a bit for the polyphase approach. I’m not doing anything fancy, I’m building a modulator and need to pass symbols through a shaping filter. The shaping filter is what I need the FIR filter for. Appreciated, Rich ---
**Author:** ![reback](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@reback](https://www.ruby-forum.com/u/reback)\ **Post date:** [August 19, 2015, 12:36am UTC](https://www.ruby-forum.com/t/implementing-a-fir-filter/243059/2 "2015-08-19T00:36:09Z")
Rich, hilbert\_fc.cc and filter\_delay\_fc both use kernel::fir\_filter in a relatively easy to understand way. Jeff ---
**Author:** ![reback](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@reback](https://www.ruby-forum.com/u/reback)\ **Post date:** [August 19, 2015, 9:06pm UTC](https://www.ruby-forum.com/t/implementing-a-fir-filter/243059/3 "2015-08-19T21:06:29Z")
Thank you Jeff. Those are perfect. Rich ---
**Author:** ![reback](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@reback](https://www.ruby-forum.com/u/reback)\ **Post date:** [August 20, 2015, 2:00pm UTC](https://www.ruby-forum.com/t/implementing-a-fir-filter/243059/4 "2015-08-20T14:00:01Z")
On Wed, Aug 19, 2015 at 2:17 PM, Richard B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > ``` > kernel::fir_filter_ccf *d_shape_filter; > > ``` > > To my CMakeLists.txt file I added FILTER to the dependency list: > set(GR\_REQUIRED\_COMPONENTS RUNTIME FILTER) > > I’m not sure where to go from here. What else needs to happen to call > kernel::fir\_filter\_ccf? > > Thanks, > Rich Probably a namespace issue. You’ve included the fir\_filter.h, I’m assuming. Now, instead of referencing it as “kernel::fir\_filter\_ccf” use the full namespace “gr::filter::kernel::fir\_filter\_ccf”. Tom ---
**Author:** ![reback](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@reback](https://www.ruby-forum.com/u/reback)\ **Post date:** [August 20, 2015, 10:17pm UTC](https://www.ruby-forum.com/t/implementing-a-fir-filter/243059/5 "2015-08-20T22:17:45Z")
I have a fundamental misunderstanding of the filter system in gnuradio. I would appreciate some help. I wrote a simple C++ testbench to help me figure out what’s going on, but it continues to not behave as I expect. I am using fir\_filter\_fff with 29 taps that I set. I send an impulse into the filter and want to see the filter taps at the output in response. The first sample that comes out is correct, but every sample after the first is either the smallest or largest number representable by float. The code that does this is below. float impulse[29] = {1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0}; // 29 samples long float filter\_taps[29] = {0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1} // input to filter shape\_filter = new gr::filter::kernel::fir\_filter\_fff(1, filter\_taps); for(int i = 0; i \< 29; i++) { filter\_out[i] = shape\_filter-\>filter(&impulse[i]); } Printing filter\_out to the screen produces: 0.1 0 8.12753e-44 1.61149e-43 -4.16205e-20 -8.32411e-20 -1.24862e-19 -1.66482e-19 1.13358e+21 When I expect to see 0.1 0.2 0.3 0.4 etc…, the taps of the filter that I set above You can see only the first output matches the expected. _What am I doing wrong here?_ Thanks, Rich On Wed, Aug 19, 2015 at 12:02 PM, Richard B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) ---
**Author:** ![reback](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@reback](https://www.ruby-forum.com/u/reback)\ **Post date:** [August 20, 2015, 2:05pm UTC](https://www.ruby-forum.com/t/implementing-a-fir-filter/243059/6 "2015-08-20T14:05:52Z")
That fixed the problem. Thank you. Rich ---
**Author:** ![reback](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@reback](https://www.ruby-forum.com/u/reback)\ **Post date:** [August 21, 2015, 1:53am UTC](https://www.ruby-forum.com/t/implementing-a-fir-filter/243059/7 "2015-08-21T01:53:20Z")
I got this working. For completeness, I’ll answer my own questions and sum up what I’ve learned about filtering. 1. You need to call set\_history(num\_filter\_taps) so that block boundaries are handled smoothly 2. You input samples one at a time to fir\_filter\_xxf 3. fir\_filter\_xxf does not store data in it’s own memory space, it uses the address of the array element you give it to figure out the rest of the data it should use to process around it, this is why history is necessary The last problem I was having was related to converting an array of char to float before feeding it into the filter. Not realizing how fir\_filter\_xxf figured out what data to use, I was typecasting each char to float sample by sample as the loop progressed and assigning the single float to a new variable that was not an array. I then passed the address of this variable to filter. This would cause the filter to use values in addresses around that variable in its calculations, leading to ridiculous results. Don’t do this! Rich On Thu, Aug 20, 2015 at 12:37 PM, Richard B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) ---
**Author:** ![reback](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/b5ac83/32.png) [@reback](https://www.ruby-forum.com/u/reback)\ **Post date:** [August 21, 2015, 2:03am UTC](https://www.ruby-forum.com/t/implementing-a-fir-filter/243059/8 "2015-08-21T02:03:16Z")
I’m having some trouble getting my code to compile. I get the following error: error: ‘kernel’ does not name a type kernel::fir\_filter\_ccf \*d\_shape\_filter; To my \*\_impl.h file I added the following: #include \ private: kernel::fir\_filter\_ccf \*d\_shape\_filter; To my CMakeLists.txt file I added FILTER to the dependency list: set(GR\_REQUIRED\_COMPONENTS RUNTIME FILTER) I’m not sure where to go from here. What else needs to happen to call kernel::fir\_filter\_ccf? Thanks, Rich On Wed, Aug 19, 2015 at 9:39 AM, Richard B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)