![]() This is desirable as we aim only at detecting the presence of the tone. Note that this signal has double frequency of the original and, will have also some extra harmonic content due to the discontinuity around zero, so this signal is suitable only to evaluate average power, for later processing the original signal minus the DC will be used.Īfter that we apply an AGC (Automatic Gain Conttrol), this doesn't enhance the SNR, since it's done after the D/A, anyhow it allows to keep the signal level constat, so that the output of the following stages is independent of the captured signal level. The signal after rectification looks like this, and the red line represents the average: ![]() If we didn't rectify it the average would be zero. After removing the DC we are left with the signal centered aroud zero:Īt this point we rectify it and calculate an average level. This is good as the ADC can sample only levels above zero but we need to get rid of the DC to be able to evaluate the amplitude of the signal. You can see a video of the device in action at: Signal detectionĪfter sampling the first thing we do is to remove the DC component, it should be remembered that the pramplifier is a class A amplifier and is DC coupled with the A/D, so the signal swings around VCC/2, the input signal looks like in the plot below: The current code expects audio at 700Hz and a morse speed around 13 WPM. The hardware is a very simple board I used in many other my DSP projects, just an Arduino Nano with a microphone and a pre-amp. ![]() The goal of this project is to develop a device able to listen for audio containing morse code and decode its text to serial port. ![]()
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