Using low cost USB Audio devices for WSPR

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G4IKZ
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Joined: 2009/12/07 - 18:03
Using low cost USB Audio devices for WSPR

Potential users of low cost USB audio dongles for WSPR might be interested in what I learned while testing some. Those that I bought were detected automatically by Windows XP SP3. A level control slider appears in a box called 'Capture', which is accessed by clicking on the Volume button for USB AUDIO input at Control_Panel / Sounds_and_Audio_Devices / Audio / Sound_Recording. It looks just like the 'Microphone Volume' slider that appears if the on-board audio device is selected. But they are not the same. The difference is easy to see by looking at the sampled waveform using DL4YHF Spectrum Lab oscilloscope.

When I apply a 1kHz tone to the on-board microphone input the 'Microphone Volume' slider adjusts the amplitude of the signal in the oscilloscope display. If I raise the slider up high the signal will clip. In my netbook the clipping occurs for levels of about +/- 18,000. If I lower the slider the clipping stops. This is like a volume control *before* the A-D converter. When I apply a 1kHz tone to the USB AUDIO device the 'Capture' slider also adjusts the amplitude of the signal in the oscilloscope display. However the position of the slider does not affect the clipping. Raising and lowering the slider changes the waveform amplitude but any clipping remains. This is like a volume control *after* the A-D converter.

I don't know the electronic details of each circuit but my guess is that the slider for the USB audio is controlling a digital numerical multiplication of the sampled values before passing them on to the end application. This might scale the sampled values but it will not ensure that the audio input makes best use of the ADC dynamic range. Nor will it stop the ADC overloading if the input is too big. By contrast the slider for the analogue input seems to adjust a genuine analogue control before the ADC. That one can be used to make best use of the ADC range while preventing overload.

Both the sliders will affect the displayed Rx Noise level in the lower left corner of the WSPR software. But if my guess about the USB device is correct then adjusting its slider for a 0dB here will not help minimise the quantisation noise of the ADC. The only way to do that would be to ensure the input level to the ADC is as high as it can safely be. I used a resistive divider to ensure the maximum audio level from my receiver cannot overdrive the USB dongle ADC. I found a strong carrier well above the AGC threshold and chose resistors to give an oscilloscope amplitude about 70% of the ADC input clipping threshold. It works fine.