PT1 filter gain formula about rotorflight-firmware HOT 9 CLOSED

@rotorflight definately something has to be done, If any first order lpf will be set above nyquist/2, bad things might happen. Also pt1 filter might have it's own limitCutoff() function.

from rotorflight-firmware.

The formula used in BF is incorrect. They have discussed it, and concluded that they did not want to fix it, as people are used to have it that way. Everybody would have needed to fix their configs after that.

The BF formula is for a continuous domain first order filter. When a continuous filter is converted into a discrete filter with Tustin/bilinear transform/zero order hold/etc, frequency warping happens. The cutoff formula must do frequency pre-warping to compensate.

Further, a discrete first order filter can't have cutoff above Nyquist / 2. (unlike the continuous first order).
If you try to use a freq higher than this, the gain will be incorrect.

from rotorflight-firmware.

See https://en.wikipedia.org/wiki/Bilinear_transform
Section "Frequency Warping"

from rotorflight-firmware.

@rotorflight thanks fo quick response, so that limitCutoff() is probably wrong, there is

    // 95% of Nyquist
    return fminf(cutoff, 0.475f * sampleRate);

but should be

    // 47.5% of Nyquist
    return fminf(cutoff, 0.2375f * sampleRate);

I'm not sure if UI account this, it shouldn't allow to set any LPF above nyquist/2 too.

from rotorflight-firmware.

Only the first order LPF has this limit. Higher order LPFs can go up to Nyquist.

from rotorflight-firmware.

But we can add a limit in the 1st order gain calculation so that it is never higher than 0.99 or so.

from rotorflight-firmware.

I'll keep this issue open until the limit has been added.

from rotorflight-firmware.

Correction. Only the filters that are converted to digital domain with the "rectangular rule" (zero-order hold) rather than the "trapezoidal rule" or bi-linear transform has this problem. PTx filters and EWMAx are the only ones in RF like this.

Both types require the frequency pre-warping. However, PTx filters just can't go over Fs/4.

from rotorflight-firmware.

Fixed in devel branch.

from rotorflight-firmware.