5 Replies Latest reply on Oct 13, 2019 3:14 PM by colporteur

    Looking for insight into RTL-SDR using Raspberry Pi


      What resources would you recommend to a person just starting out in RTL-SDR?


      I have been bench testing configuring a Raspberry Pi 3B+ to support  RTL-SDR. I have successfully displayed a local FM radio station signal using the CubicSDR application and had it play audio. The audio is rather choppy but it does play. I consider this a milestone of a working RTL-SDR setup on a Pi. I have tried using GQRX but became frustrated at failing to accomplish the same objectives I did on CubicSDR. Can GQRX play audio? How can I improve the audio quality produced from CubicSDR? Maybe using this software is just beating a dead horse so to speak.


      I had an epiphany after reading some of the use cases posted for the new Pi4 offering View the full discussion. It appears a few of the members have some affinity for RTL-SRD. I am looking to accelerate my learning by tapping your experience.


      What would you recommend to a person just starting out? What documentation have you found useful? What software public licensed or commercial do you use? What hardware have you used? What pitfalls would you have avoided?


      To give you some you some context for responses, I have a background in radio frequency technology. I spent seventeen years maintaining ground based aviation navigation and communication ground based equipment before moving from electronics systems to computer systems. Yeah, it can be argued they are both electronics.


      I don't know what I don't know! I am also looking to short circuit my learning curve. I have exhausted google searches for information. Much of the information is dated. Some software has been apt-get developed eliminating the need to compile. I have found little that is current to today.  Ideally, I would like someone looking over my shoulder while I execute and discuss scenarios. What about manuals for the opensource software, do they exist? I am reluctant to further invest in software and hardware without gaining some knowledge. Having a working RTL-SDR setup to me is a starting point. My goal is to expand what I have done so far to include obtaining broadcasted aircraft data signals. I see reference to DUMP1090 software.


      Do you have some RTL-SDR knowledge you are willing to share? I would like to sponge off you. Care to make a post?



        • Re: Looking for insight into RTL-SDR using Raspberry Pi

          Hi Sean,


          I cannot offer much insight, except that a lot of the application software seems a mess, particularly in the usability area. I am still looking for a receiver application just for analog demod schemes in my case, but I don't think I will find one that is half as user-friendly as a physical receiver.


          If you do not want to use RTL-SDR as a traditional receiver demodulating to audio, then there are other dedicated applications too. For instance if you want to use it like a spectrum analyzer with no audio capabilities at all, then QSpectrumAnalyzer could be used. I used it here: Molex 2.4GHz / 5GHz Antenna Kit - Review

          Again it has its limitations, it's not something I could use everyday, it would drive me insane. But it's free, the author has made effort, and the source code is available for it too.


          If you wish to do completely custom things, like your own demodulation or filtering scheme, then GNURadio's GUI is worth considering.

          It is popular, and it works with RTL-SDR. I've tried it a few times with a different SDR, but it usually crashes for me : )

          It is not a dedicated end-application like a receiver, rather it is a set of software oblects like filters, signal generators (oscillators) and modulators/demodulators and sound card connectors, and you connect them together, to build your end application. There's a learning curve, since you're effectively building a radio (or other signal processing application) by connecting those objects together, so knowledge is required of what an oscillator is, what a filter is, what a mixer is, and so on. It's not like using a radio where the operator only needs to know what the control knobs do.


          There's also software called Pothos, which is compatible with RTL-SDR too (I've not  used it with RTL-SDR though). It works well, but is not as full-featured as the GNURadio features in the GUI, and the limitations may rule it out for you, depending on what you want to use it for.

          Pothos screenshot:

          Basically the way it works, is that you have a palette of objects, and you can connect them to create a flow. That flow defines what your SDR will do. So, you could have a source object for instance, that is the RTL-SDR source (doesn't have to be, it could be a source sinewave or any other signal - depends on the object you choose). Then you could manipulate the signal, (e.g. demodulate) and direct it to a sink object (e.g. the audio sink will send to the soundcard). Everything is sample based, so you pick a sample rate that works for your expected modulation/demodulation, and the source/sink objects seem to resample to match the connected hardware (but if there is an example sample rate someone is using and is known to work, then its best to use that). So, in the screenshot above, the sample rate is 2e6 (2MHz) which I knew was a value my SDR could handle.

          I wouldn't say it's trivially easy, especially due to the limited functionality, and limited documentation, but is still interesting to try, it may meet your needs. I use it as a simple signal generator (my SDR can transmit). So, I can set up a flow in Pothos, with the SDR as the sink object rather than a source object.


          The user documentation for nearly all these projects is quite bad unfortunately. I had to go through lots of web searches and youtube videos to even get just a basic flow working : ( Some of the stuff is just magic numbers. You can see in the screenshot above, top-left, that the Antenna parameter is set to LNAL for instance. That's just a magic thing that you cannot guess. It happens to be in the code for the driver to the SDR board I was using, so I have to use that value. RTL-SDR would use a different value there.

          6 of 6 people found this helpful
            • Re: Looking for insight into RTL-SDR using Raspberry Pi

              My tardiness in replying to this post was not of my own making. It appears I was reply inhibited for this post and other due to a technical issue. I am told the issue has been resolved.  I try and make a concerted effort to respond, as soon as possible, after I receive replies. My intent is to demonstrate I am actively seeking an answer and to let the responder know, I value their input.


              Thank you for the response shabaz. Your commentary regarding the software, usability and documentation validated what I discovered during my short foray into get it working on the Pi. I questioned whether my expectations were to high after what I found. I respect much of the stuff is opensource done by volunteer hands but it wasn’t pretty, especially if your a novice. I am comfortable in RF and working with Pi's but much of the documentation was what I refer to as technical hen scratches.


              I have been scraping the Internet pulling down bits and pieces. Fragmented, incoherent and largely dependent on processes or understanding not fully explained, that left me bewildered at times. When I first started exploring the topic, the eye candy was great. But that is all there appeared to be for the most part. I’m surprised that I actually have a working Pi unit.


              Your suggestion of GNURadio piqued my interest. After looking at GNURadio I came up short on this task. My goal is to develop a step by step tutorial to configure Pi hardware and software to support GNURadio being used to display output from a demodulated signal.


              I'm stumped on the assembling a block in GNURadio configuration to demodulated an FM radio frequency. Do you have a simple tutorial to assemble a Scratch Type programming process? Pick this, plug in this value, connect this output. I don't need to build a quadrature phase multiplexer demodulator. Just a simple FM receiver is fine.


              Anyone have a simple GNURadio layout or point to a resource that might help?



                • Re: Looking for insight into RTL-SDR using Raspberry Pi

                  Hi Sean,


                  No worries, I figured it was due to the commenting issue.

                  Pothos has a FM demod block, and that seems like a subset of GNU Radio functionality, so that may have it too (but I've failed in getting GNU radio to run with my SDR so cannot offer much advice there : (

                  If it's like Pothos, then you'll need to define a source block which will be the SDR, and you'll be able to type in a frequency into the configuration menus for that block. And then a single connection to FM Demod, and then another connection out from there to a sink block which will need to be set to your soundcard device in the menu for that block. That should be a minimal implementation, with no display or tuning buttons.

                  If I find a decent GNU Radio resource with the detail (since I'm sure some of the stuff to configure in the menus will definitely not be obvious : ( I'll post it here. I'll have a look tonight.

                    • Re: Looking for insight into RTL-SDR using Raspberry Pi

                      Much appreciate your understanding on the tardiness of replies. If I ask a question, I will do my damnedest to stay on top of responding.  I want you to responder, to feel like the most important person in my world. "Hey look at that, I responded to his post and he sent a note right back." I'm looking for an answer, I am willing to invest because maybe you are the person that has it.


                      I napkin'ed a block diagram of an FM receiver. I got lost wondering the Pothos site. After finding my way back out, I was taking a rest before making another attempt.  Shite, that last sentence made me laugh, I substituted Pathos for Pothos. Yeah I am in a pity state at the moment.


                      I'm not much of a trial and error guy when research something " I don't know what I don't know". It evokes a feeling of driving through one city, using a map from another city. I understand what I need, I'm just not sure I would recognize it, if I found it. If after many long hours of research someone says, it right where I'm looking, that drives my frustration up exponentially.



                • Re: Looking for insight into RTL-SDR using Raspberry Pi

                  Thanks shabaz  for your insight.


                  Over the summer I set out to develop some knowledge RTL-SDR. software defined radio (SDR), is a form of radio that replacing the traditional electronic circuits that modulate and demodulate radio frequency signals for broadcast. Inexpensive USB dongles are doing the heavy radio frequency (RF) lifting and converting the signal to digital. Digital signal processing done by applications on the PC extract the contents from the signal and enable it to be played or displayed on a desktop PC.


                  I wanted to close out this post by sharing some of the resources I uncovered. If any members are interested in digger deeper into the technology, these links should give you a good start.


                  A reference source of all things RTL-SDR https://www.rtl-sdr.com I went as far as purchasing their book and the dongle from their store. I wasn't disappointed in the wealth of information provided or the product. I was impressed with the step-by-step practical examples to get projects working.


                  Two great video's for the beginners:




                  Hardware reference:



                  I did some extensive research on GNU Radio. GNU Radio is an open-source software development toolkit (SDK) that can be used to replace physical test equipment and electronic circuitry to generate and process the analogue signal using software.


                  Wiki pages for GNU Radio:



                  A ten part video course on GNU Radio using a HackRF SDR device:


                  5 of 5 people found this helpful