RF Spectrum Sweep

RF Spectrum Sweep

Identifying RF Channels to monitor can be a time consuming effort. Using a standard non PC radio receiver can take a long time to identify a transmitting channel. Using RTL_Power and SDR FFT Spectrum Logging provided here (http://kmkeen.com/rtl-power/), makes the effort less challenging. RTL_Power will sweep through a range of radio frequencies at given steps and log the RF power levels detected by the USB SDR receiver. It does this over and over at set intervals to create a log…

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RTL-SDR Radio Scanner

RTL-SDR Radio Scanner

In this post I’ll be giving some background and install steps on RTL-SDR. SDR stands for software defined radio and has been in use since the 1970s. Traditional radio processing is handled by hardware components like oscillators, regulators, and mixers. SDR replaces these functions with digital computer processing. RTL-SDR is a device that was originally designed to receive digital television transmissions, namely DVB-T TV using the RTL2832U chipset. The device is typically contained in a USB dongle. The RTL2832U chipset…

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Decoding DTMF with MultiMon-NG

Decoding DTMF with MultiMon-NG

Dual Tone Multi Frequency signaling, commonly referred to as DTMF, is mainly used for phone dialing systems. But this isn’t the only use for DTMF signaling. Another common use is security access systems. In this post, I’ll be covering how to decode DTMF using MultiMon-NG. SoX will be used to record the DTMF signals generated on cell phones and keyless entry systems. I’ll be using a high gain directional microphone with a secondary microphone to cancel out background noise. SoX…

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Sonic data analysis with SoX

Sonic data analysis with SoX

In this post I’ll be using SoX to export audio to data values that can be processed outside the audio realm. The example I’ll use here is a breif recording that will be plotted with GNUPlot. SoX and other programs provide spectrograms, which are frequency domain images of audio files. I’ll be using GNUPlot to create an image of an audio file in the time domain. First, I’ll use this command to convert the audio wav file into a text…

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Noise Cancelled Microphone with SoX

Noise Cancelled Microphone with SoX

In this post I’ll be using two USB microphones to record audio, one for the intended sounds I wanted to record, the other to capture background noise that I want to filter out. I’m using two cheap USB embedded microphones that I bought for less than $10 each, http://a.co/eIsnp3Z. The machine I’m recording on is a Raspberry Pi 3 running Ubnutu Mate. Since the Raspberry Pi doesn’t have an audio in device built in, I selected the USB microphones. These…

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Ultrasonic SoX

Ultrasonic SoX

This post will be quick and to the point. We’ll be using SoX to bring high frequency audio, commonly referred to as ultrasound, down to a range that can be heard. First thing I did to validate the process was generate an ultrasonic data file using Audacity. The problem with the standard 44.1k sampling rate is ultrasound frequencies will be dropped. I created my data files with a sampling rate of 192k. play “/home/local/Desktop/Ultrasound/Audacity192k_33khz.wav” Playing audio from this sample is…

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Infrasound processing with SoX

Infrasound processing with SoX

In this post I’ll cover how to use SoX to make low frequency sounds audible in a range that can be listened to. Infrasound is a low frequency sound wave that ranges from 20 hertz down to .1 hertz. Monitoring audio frequencies in this range is typically done to detect seismic activity that is caused either by nature or artificially, https://www.isla.hawaii.edu/sounds/ocean-sounds/. I have a sample of video that was shot with a Mobius camera. The audio range of the camera’s…

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Noise Filtering with SoX

Noise Filtering with SoX

In this post I’ll cover how to remove noise in an audio file using SoX. I’ll be using a Raspberry Pi which does not have an audio input by default. To get audio, I used an simple USB audio microphone to capture audio. The OS I’m running on my Pi is Ubuntu Mate and there was no need for anything to get the microphone to run. Once it was plugged in, I ran this command to view the properties of…

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Generating DTMF with SoX

Generating DTMF with SoX

In this post I’ll show how SoX can generate DTMF, (Dual-tone multi-frequency signaling) which is commonly used in phone and security systems. These dual tone signals are the foundation from which digital communications are based. The technology around DTMF is not complex. Lets cover the history of DTMF. DTMF was developed by Bell Labs and put in production in the early 1960’s. It was mostly used to replace pulse dialing systems that had limits on how far a phone could…

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SoX Noise Generation

SoX Noise Generation

In my last post I suggested creating noise and using a band of it to fill in a band gap. SoX provides a way to generate noise. There are different types of noise that SoX will generate. Lets take a moment to look at these types and what applications noise generation is used for. There are 3 main types of noise that SoX will generate, they are white, pink, and brown. White noise is a random signal that has equal…

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