Frequency buckets

cjn566 · June 26, 2019, 7:42pm

What are the frequency ranges in each of the 32 buckets of data?

wizard · June 27, 2019, 5:26pm

Roughly:
37.5 (single)
50.0
75.0
100.0
125.0
162.5
195.3
234.4
312.5
390.6
468.8
585.9
703.1
859.4
976.6
1171.9
1367.2
1562.5
1796.9
2070.3
2382.8
2734.4
3125.0
3593.8
4101.6
4648.4
5312.5
6015.6
6835.9
7773.4
8789.1
9960.9

You can see the bucket values used here:

github.com

simap/pixelblaze_sensor_board/blob/master/src/output.c


#include "main.h"

//an fft of 512 gives us 256 buckets of frequency info. at 20khz, each has ~38Hz
//we don't really want all 256 buckets of frequency info
//if we compressed this linearly, we'd lose a lot of low/mid tone info
//also, in order to capture low frequency stuff below 38Hz would require a much larger fft
//so we can combine a downsampled 400hz using a smaller fft for low frequency stuff with the 20khz stuff

//the first 6 buckets are dedicated to low frequency audio from 12.5-162.5 Hz
const uint8_t lowFrequencyMap[6] = {
		3, 4, 6, 8, 10, 13
};
//the next 26 buckets are dedicated to higher frequency audio from 195Hz to just under the nyquist limit of 10khz
const uint8_t highFrequencyMap[26] = {
		5, 6, 8, 10, 12, 15, 18, 22, 25, 30, 35, 40, 46, 53, 61, 70, 80, 92, 105, 119, 136, 154, 175, 199, 225, 255,
};


char outBuffer[100];

This file has been truncated. show original

The low frequency FFT bin size is 12.5Hz, and the high frequency FFT bin size is 39.0625Hz. Different maps of these are combined into the 32-bucket output usable in patterns. Each mapped range takes the local maximum for the range up to and including that position. So for example, the output at 75Hz includes 75Hz and 62.5Hz. At the very top, the bucket for 9,960Hz includes 30 bins of FFT data starting at 8,828Hz

Scruffynerf · October 14, 2020, 2:08am

Thanks for the table of frequencies… Saved me some effort.