Today we are going to introduce a high-level filter adjustment technique, RPM Cross Fading. This parameter is very important for 6" and larger long-range and cinematic drones. Our RPM filter has a cutoff frequency, and the cutoff frequency can be adjusted lower for larger-sized airplanes to achieve a better filtering effect of the RPM filter. This is especially true for the low-frequency noise of large paddles and motors. A detailed explanation of this noise can be found in Betaflight's wiki interface (http://betaflight.com/docs/wiki/tuning/4-3-Tuning-Notes#rpm-crossfading), which has a more detailed explanation and also provides the corresponding case code.
"set rpm_filter_min_hz=80" This is the cutoff frequency of the RPM filter; "set rpm_filter_fade_range_hz=50" means the range of RPM cross fading.
For example, if you use Betaflight's default parameter 100 for the cutoff frequency, and Betaflight's default parameter 50 for the following RPM cross fading, it means that your rpm filtering frequency in the range of 100-150 is the same as that of our TPA, which is a gradual enhancement of the intensity of the rpm filtering frequency from 0 to 100. This process is a first order primary function. For example, if you are using a larger airplane, and your rpm cutoff frequency is very low, and you are using Betaflight's default rpm cross fading, this will result in a range that is too large for a large airplane, which will result in your motor's low frequency noise not being filtered out as well as it could be. Because your filtering strength is weakened as it gradually increases from 0-100, it is recommended for large airplanes 8" and above to set a narrower cross fading range while turning down the rpm cutoff frequency. Normally we would set this range to about 40% of the cutoff frequency. For example, if our airplane has a cutoff frequency of 70, the cross fading range would be set to 30.
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