FFT_ parameters

15 parameters in the FFT_ family. ArduCopter 4.7 only — other vehicles share many of these names but not always the same ranges or meanings.

FFT_ATT_REF

FFT attenuation for bandwidth calculation

FFT attenuation level in dB for bandwidth calculation and peak detection. The bandwidth is calculated by comparing peak power output with the attenuated version. The default of 15 has shown to be a good compromise in both simulations and real flight.

Range	0 to 100
User level	Advanced

FFT_BW_HOVER

FFT learned bandwidth at hover

FFT learned bandwidth at hover for the attenuation frequencies.

Range	0 to 200
User level	Advanced

FFT_ENABLE

Enable

Enable Gyro FFT analyser

Reboot required	Yes — the change takes effect after a reboot.
User level	Advanced

Values

Value	Meaning
0	Disabled
1	Enabled

FFT_FREQ_HOVER

FFT learned hover frequency

The learned hover noise frequency

Range	0 to 250
User level	Advanced

FFT_HMNC_FIT

FFT harmonic fit frequency threshold

FFT harmonic fit frequency threshold percentage at which a signal of the appropriate frequency is determined to be the harmonic of another. Signals that have a harmonic relationship that varies at most by this percentage are considered harmonics of each other for the purpose of selecting the harmonic notch frequency. If a match is found then the lower frequency harmonic is always used as the basis for the dynamic harmonic notch. A value of zero completely disables harmonic matching.

Range	0 to 100
Reboot required	Yes — the change takes effect after a reboot.
User level	Advanced

FFT_HMNC_PEAK

FFT harmonic peak target

The FFT harmonic peak target that should be returned by FTN1.PkAvg. The resulting value will be used by the harmonic notch if configured to track the FFT frequency. By default the appropriate peak is auto-detected based on the harmonic fit between peaks and the energy-weighted average frequency on roll on pitch is used. Setting this to 1 will always target the highest energy peak. Setting this to 2 will target the highest energy peak that is lower in frequency than the highest energy peak. Setting this to 3 will target the highest energy peak that is higher in frequency than the highest energy peak. Setting this to 4 will target the highest energy peak on the roll axis only and only the roll frequency will be used (some vehicles have a much more pronounced peak on roll). Setting this to 5 will target the highest energy peak on the pitch axis only and only the pitch frequency will be used (some vehicles have a much more pronounced peak on roll).

User level

Advanced

Values

Value	Meaning
0	Auto
1	Center Frequency
2	Lower-Shoulder Frequency
3	Upper-Shoulder Frequency
4	Roll-Axis
5	Pitch-Axis

FFT_MAXHZ

Maximum Frequency

Upper bound of FFT frequency detection in Hz. On smaller vehicles the maximum motor frequency is likely to be significantly higher than for larger vehicles.

Units	Hz
Range	20 to 495
User level	Advanced

FFT_MINHZ

Minimum Frequency

Lower bound of FFT frequency detection in Hz. On larger vehicles the minimum motor frequency is likely to be significantly lower than for smaller vehicles.

Units	Hz
Range	20 to 400
User level	Advanced

FFT_NUM_FRAMES

FFT output frames to retain and average

Number of output frequency frames to retain and average in order to calculate final frequencies. Averaging output frames can drastically reduce noise and jitter at the cost of latency as long as the input is stable. The default is to perform no averaging. For rapidly changing frequencies (e.g. smaller aircraft) fewer frames should be averaged.

Range	0 to 8
Reboot required	Yes — the change takes effect after a reboot.
User level	Advanced

FFT_OPTIONS

FFT options

FFT configuration options. Values: 1:Apply the FFT *after* the filter bank,2:Check noise at the motor frequencies using ESC data as a reference

Reboot required	Yes — the change takes effect after a reboot.
User level	Advanced

Bitmask

Value	Meaning
0	Enable post-filter FFT
1	Check motor noise

FFT_SAMPLE_MODE

Sample Mode

Sampling mode (and therefore rate). 0: Gyro rate sampling, 1: Fast loop rate sampling, 2: Fast loop rate / 2 sampling, 3: Fast loop rate / 3 sampling. Takes effect on reboot.

Range	0 to 4
Reboot required	Yes — the change takes effect after a reboot.
User level	Advanced

FFT_SNR_REF

FFT SNR reference threshold

FFT SNR reference threshold in dB at which a signal is determined to be present.

Range	0.0 to 100.0
User level	Advanced

FFT_THR_REF

FFT learned thrust reference

FFT learned thrust reference for the hover frequency and FFT minimum frequency.

Range	0.01 to 0.9
User level	Advanced

FFT_WINDOW_OLAP

FFT window overlap

Percentage of window to be overlapped before another frame is process. Takes effect on reboot. A good default is 50% overlap. Higher overlap results in more processed frames but not necessarily more temporal resolution. Lower overlap results in lost information at the frame edges.

Range	0 to 0.9
Reboot required	Yes — the change takes effect after a reboot.
User level	Advanced

FFT_WINDOW_SIZE

FFT window size

Size of window to be used in FFT calculations. Takes effect on reboot. Must be a power of 2 and between 32 and 512. Larger windows give greater frequency resolution but poorer time resolution, consume more CPU time and may not be appropriate for all vehicles. Time and frequency resolution are given by the sample-rate / window-size. Windows of 256 are only really recommended for F7 class boards, windows of 512 or more H7 class.

Range	32 to 1024
Reboot required	Yes — the change takes effect after a reboot.
User level	Advanced