In this video I use the manual notch filter and the contour filter to quiet down the ringing sound that you sometimes hear when you tighten the DSP filter width to less than 150Hz on CW. Before I go any further, let me explain what causes this ringing sound. There are two popular filter types, or shapes, used in radio receivers. There is the Butterworth and the Chebyshev. A Butterworth filter will slope, or roll off, more smoothly away from the center point. Due to this smooth slope, it will allow more signal through the passband than a Chebyshev filter will allow. We affectionately call this slope the “skirt”. For a tighter passband there is the Chebyshev filter. A Chebyshev filter features a very steep skirt, which will yield a tighter passband. But there is a trade-off. A characteristic ripple that produces something akin to a ringing sound can be heard when the noise floor is much above S zero. The first thing you want to do when you wish to remove the background noise from a CW signal is attenuate the noise. Utilize such things as Attenuation, IPO, and RF Gain to reduce the noise floor. It might not always require all three. Experiment with the three until you find the combination that yields the best signal-to-noise (S/N) ratio. The filters in the IF stage of your receiver will be able to do a better job of cleaning up the signal if you first knock the noise floor down in the front end of the receiver.
With rigs like the Yaesu FTdx5000MP you typically employ the Audio Peak Filter (APF) to accentuate the CW signal you are focused on, and effectively yank it up from the noise floor. But, as an alternative, you could employ a manual notch filter, and possibly even add the contour filter, if you have a radio that includes a contour filter.
I usually set my side tone to 550Hz. That’s just my personal preference. In this video I will adjust the manual notch to filter to center up at 520Hz. This will effectively knock down the ringing effect you typically hear when you narrow the DSP to 150Hz or less. I will also employ the contour filter and adjust it to 700Hz. Note: The reason I didn't adjust the contour filter closer than 700Hz is because it isn't as narrow as the notch filter, thus it will begin to knock down the ripple effect from 150Hz away.
So, I am effectively using these two filters to knock down the ringing (ripple) effect that is customary with a Chebyshev filter. I call this a CW sandwich.
73, de N4HNH
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