I have found something in the operation of the digital bandpass filters I don't quite understand and hope someone will be able to set me straight. When listening to a CW signal, the bandwidth is narrowed. I have a 2.7 kHz and 400 Hz roofing filters installed. As I narrow the bandpass with the digital filter, the background noise heard is less as is expected and the signal loudness heard through speaker and/or headphones remains the same. This happens until the digital filter bandwidth gets to 100 Hz. When the bandwidth is changed from 150 Hz to 100 Hz, there is an audible "click" heard and the loudness of the signal heard is significantly reduced. At 50 Hz bandwidth audio level as at 100 Hz. The audible "click" noise may not be significant as I do hear this when switching between other bandwidths, but until recently only noticed it when reducing the bandwidth to 400 Hz (switching to 400 Hz roofing filter).
The only reason I can think this might be happening is if the width of the transmitted signal is wider than 100 Hz, but this appears to be happening even on unmodulated carriers. Can anyone explain what might be happening? I appreciate any and all explanations. Thank you, John/K8AZT
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