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Community Documents
Description
In order to demodulate a broadcast FM signal, it's important to understand the structure of this signal, as there is much more information contained in the FM signal than just the mono audio information. The following image should make things clearer:
As you can see, the mono audio is only occupies a relatively small part of the channel bandwidth from 30Hz to 15kHz. This is the portion that we will be focussing on.
As the name implies, FM, or frequency modulation, uses a baseband signal to modulate the frequency of a carrier signal. The amplititude of the signal stays the same. This is an advantage over amplitude modulation, where the information to be transmitted is contained within the modulated amplitude of the carrier. If the AM signal is weakend significantly, information is lost. Since the information for FM is contained in the zero-crossings of the signal, theoretically only a small signal level is required to reconstruct the entire baseband information.
The basic structure of the used demodulator is shown in the image below:
Here is a short description of the chain:
- The signal is received from the rtl-sdr device as IQ data. This is converted to a complex signal and the phase is extraced.
- The phase correction removes phase discontinuities.
- The key demodulation component in the chain is the phase derivative. The phase derivative takes the phase of the signal and creates a second signal that is composed only of the changes in frequency. This is then the demodulated signal.
- The low pass filter is used to filter out frequencies above 15kHz, which do not contain the desired information.
- The rational resample takes the signal, which is still at the sampled rate (in the examples case 286650Hz) and resamples it to something the sound card can handle. In this case, we are using a decimation factor of 13, which results in a 22050Hz audio stream. Actually, I worked this out the other way around. I wanted a 22050Hz audio stream and checked which sample rate would give me an integer decimation while keeping the RF sampling rate as low as possible.
The attached project provides an application which performs exactly these steps, allowing the user to tune to a frequency and listen to the mono part of the fm broadcast. The demodulation is handled in a subvi, aptly named FM_Demodulation.vi, which looks like this:
As you can see, all the steps for fm demodulation are present. I've tried to comment the code as good as possible, and to adhere to the Labview coding guidelines. There are some parts which could probably be further split into subvi's, but I wanted the software to be easy to read and be understandable to everyone.
Hello Albert,
Thank you for this excellent tutorial on the RTL-SDR. I have been playing around with the attached Labview code and wondered if you could help me to answer something. I am wondering why the FFT of my demodulated audio shows the noise floor moving up when the radio transmitter is powered off instead of showing the 6KHz moving down? I have disabled AGC within the RTL-SDR and successfully prevented the same behavior in my RF spectrum FFT, but I'm not quite sure why it happens in the audio spectum. Any ideas?
Thank you,
Aaron Rice
Hello, I hope you could help me. I was very interested in this project of yours. So i tried to run the project to learn and study about this fm demodulator. I already follows all the steps and download all the files needed. I also bought the RTL-SDR devices. Unfortunately, when I run the FM_Demodulation.vi, there is an error that I could not understand. Would you mind explained it to me and where should I correct it? I already try my best to fixed the problem but i failed terribly. Here I attached a picture of the said error. Thank you in advanced.
