Category Archives: rtl-sdr

Taming the Comb; Spurious/Sideband Troubleshooting with the RTL-SDR (MAJOLSURF.NET)

Previously I wrote about the bring-up of the MRF49XA Shield.  Recall that instead of the standard FSK spectrum I was met with an asymmetric comb about the carrier.

the problem

Decreasing the span we see are able to measure the comb spacing, about 80kHz.

SDRsharp Analysis

When viewed in this way, we see more of a traditional harmonic series from the carrier.  I’ll assume for now that the series is not an odd function; the waveform is not square or compressed.  Typically symmetric sidebands that reside xdBc from the carrier are indicative of A.M. distortion.  I’ll also chalk up the asymmetry seen in at wider bandwidths is due to heterodyning.

The first place I like to look when experiencing undesired A.M. spurs is the power supply.


Setting the DMM to AC mode and turning on the frequency counter, I find a dominant 78kHz AC component on my 3V3 supply.


That’s close enough for me to be a smoking gun.  A bypass capacitor is placed near the MRF49XA supply…


And the major sidebands are gone.  We can still see spurs about 35dBc at 200kHz; for now I’ll assume they are Frac-N PLL spurs and move forward.

SDRsharp Analysis cap installation


Controlling with an Android Device, iPad or iPhone and TouchOSC (

Midi to the Rescue

One of the big advantages and disadvantages of Software Defined Radio is the ability to control the software with virtual buttons and knobs via an computer user interface. Some hams view this as an advantage, but others would rather have more direct control of the radio software through a hardware interface. We have already discussed this on Ham Radio Science using the Hercules MP3 E3 interface. The MP3 E3 uses midi to connect to PowerSDR to allow the user to operate PowerSDR through various hardware rotary encoders and sliders. The big advantage of using midi as an interface protocol is that there are many hardware devices it can be used with. Midi has been an industry standard in the music production area for a long time. Most of the hardware controllers are more geared to toward music production software rather than ham radio applications. This usually means that the knobs and controllers are not marked for easy recognition of what they control in the software. There are some midi controllers that will allow you to label the controls via an LCD display. However, because of this desire to have direct hardware control of SDR software, some developers are adding midi as an controller option. This allows the ham radio operator to map the various software functions to the midi hardware.

One such program is The author  (Simon Brown) has included some very basic midi commands to allow you to use a midi controller with the software. allows you to control several SDR radios such as the RF-Space SDR radios, FUNcube dongle, and SoftRock kit SDRs. The midi control of is very basic at this time, but maybe he will expand it in the future to include more control and even midi out to trigger various leds and status displays. Y0u can use various existing midi hardware devices such as the Hercules MP3 E3 device to control, but wouldn’t be nice to be able to use a custom designed touch screen midi controller? Well you can if you own an iPad, iPhone, iPod Touch, or Android device.

Enter TouchOSC

TouchOSC is a software package that allows you design custom midi touchscreen controllers for iPads, iPhone, iPod Touch, and Android devices. The software can be purchased through the Apple App Store ($4.99) or downloaded  from the Android Store. The program includes an editor that can be run on Windows, OSX, or Linux and can be downloaded for free from The editor program is a very easy to use tool that allows you to create or edit touch midi interfaces for your device. You can easily add sliders, rotary encoders, buttons and labels to your interface with the editor. You can also quickly and easily modify existing templates that others have designed.

Getting Connected

Once you have your interface designed, you will need to connect your device to send midi commands to your computer running Fortunately there are several easy ways to do this. You can do this wirelessly or hard wired.

This the easiest and best way to go if you have your device and your computer on the same wireless network in your home. You will need to download and install the TouchOSC Midi bridge software from the website. This provides a wireless midi interface between the TouchOSC app on your device and your computer. Once you install TouchOSC midi bridge, be sure to start the midi bridge software application on your computer. Then go to the device you will be using as a controller and start the TouchOSC app. Go to the TouchOSC app and look for the setup screen (if you don’t see it, touch the i in the upper right hand corner). Look for Connections and select midi bridge. Enable it and you should see your PC as the host if TouchOSC has found it. If not you can try manually entering the PC ip address.Thats it, you are connected!

Wired (Apple Devices Only)
You can also use a usb adapter to plug your device directly into your computer. One way to do this by using Apples Camera connection kit. This will use the CoreMidi setting in the Connections setting of TouchOSC. Simply plug in a usb cable between the adapter and the computer and you should be good to go.

Another more eloquent solution but pricer is the Alesis IO dock for the iPad (my personal favorite). The Alesis IO Dock gives you a very nice console to mount your iPad in as well as  a professional audio interface and usb midi out. The Alesis IO Dock would be very useful to those who were going to use their iPad as a midi controller a great deal or was interested in using advanced audio processing software.

TouchOSC also supports the Line 6 Midi Mobilizer which an adapter that plugs into to your Apple device and sends midi out to a standard midi din connection. If you were to use the Line 6 Midi Mobilizer. you would probably also need an additional midi din to usb converter / adapter if you don’t have a way to get midi in to your computer via a din connector.

One note, I don’t think you need to install the midi bridge driver if you are using Core Midi. That was my experience with the Alesis IO Dock.

Getting Started

Once you have your device connected via TouchOSC to your computer, here is what you need to do to get it talking to

  1. We are going to assume you have installed TouchOSC to your device and Midi Bridge on your PC and TouchOSC sees your pc.
  2. Download and install the TouchOSC editor compatibel with your OS.
  3. Download the iPad or iPhone template from the downloads area at the bottom this page. You may edit these templates if you want, jut be sure not to change the controller numbers or the midi.xml file that is provided in the download area may not work correctly with
  4. Start the TouchOSC editor on your computer and load one of the desired templates.
  5. Select the Synch button in the editor.
  6. Start TouchOSC on your device and go to the setup menu.  Under Layout, select Add. You should see your PC as an Editor Host. Select your and the editor should transfer the template to your device.
  7. Download the midi.xml file to your PC. The midi.xml file simply contains the key mappings that match the template controls.
  8. Start and go to the Program Options setting. Look for the Midi settings menu.
  9. In the midi settings select import and then select the midi.xml file.
  10. You should be good to go!

One note is that I noticed that if is not stared (processing IQ signals), that touching the Volume control on the touch controller will cause to lock up with an error message that can only be cleared by exiting and restarting the program. Maybe Simon can fix this in the near future.

If you come up with your own TouchOSC design template and would like to share it, just attach your TouchOSC, Midi.html file, and your call, to an email and I will host it here.

In a follow up article we will discuss using TouchOSC to control other Ham Radio Programs that do not have midi built in.

TouchOSC Templates

– iPad Interface Screen 1

-iPad Alternate Interface Screen

-iPhone Screen 2

-iPhone Screen 1

Demo Video 

File Downloads 

iPad TouchOSC Template

iPhone TouchOSC Template