## no-std microfft

Embedded-friendly Fast Fourier Transforms

### 7 unstable releases

 0.4.0 Apr 3, 2021 Nov 9, 2020 Mar 8, 2020 Mar 8, 2020 Mar 7, 2020

#77 in Algorithms

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# microfft

microfft is a library for computing fast fourier transforms that targets embedded systems. It provides an in-place implementation of the Radix-2 FFT algorithm. All computations are performed directly on the input buffer and require no additional allocations. This makes microfft suitable for `no_std` environments, like microcontrollers.

Speed is achieved mainly by maintaining a pre-computed sine table that is used to look up the necessary twiddle factors. By replacing arithmetic operations with simple memory lookups, we reduce the number of CPU cycles spent. Unfortunately, the pre-computed table also claims a considerable amount of memory, which might be a deal-breaker for some embedded projects (see Memory Usage).

microfft also implements a specialized algorithm for FFTs on real (instead of complex) values. Naively one would calculate a real FFT simply by converting the input to complex values (leaving the imaginary part empty) and running a CFFT. microfft's RFFT algorithm instead packs pairs of real values into a single complex one each, then computes a CFFT of half the original input size, followed by some recombination magic. This has the effect of roughly halving the number of CPU cycles required, as can be seen in the [benchmark results][bench/README.md].

## Example

The following example demonstrates computing a 16-point RFFT on a set of samples generated from a sine signal:

``````use std::convert::TryInto;
use std::f32::consts::PI;

// generate 16 samples of a sine wave at frequency 3
let sample_count = 16;
let signal_freq = 3.;
let sample_interval = 1. / sample_count as f32;
let mut samples: Vec<_> = (0..sample_count)
.map(|i| (2. * PI * signal_freq * sample_interval * i as f32).sin())
.collect();

// compute the RFFT of the samples
let mut samples: [_; 16] = samples.try_into().unwrap();
let spectrum = microfft::real::rfft_16(&mut samples);
// since the real-valued coefficient at the Nyquist frequency is packed into the
// imaginary part of the DC bin, it must be cleared before computing the amplitudes
spectrum.im = 0.0;

// the spectrum has a spike at index `signal_freq`
let amplitudes: Vec<_> = spectrum.iter().map(|c| c.norm_sqr() as u32).collect();
assert_eq!(&amplitudes, &[0, 0, 0, 64, 0, 0, 0, 0]);
``````

## Requirements

Requires Rust version 1.51.0 or newer.

## Sine Tables

microfft keeps a single sine table to calculate the twiddle factors for all FFT sizes. This removes some memory overhead compared to keeping a separate table for each FFT size, as there would be duplication between those tables.

The default sine table supports a full 4096-point FFT. If you only want to compute FFTs of smaller sizes, it is recommended to select the appropriate `maxn-*` feature, to not waste memory. For example, if your maximum FFT size is 1024, add this to your `Cargo.toml`:

``````[dependencies.microfft]
default-features = false
features = ["maxn-1024"]
``````

This tells microfft to not provide functions for computing FFTs of sizes larger than 1024 and to keep only the 1024-point sine table.

## Bit-reversal Tables

The optional feature `bitrev-tables` enables the use of pre-computed tables of bit-reversed indices required for the reordering of input values performed at the start of each FFT. If this feature is disabled (the default), the bit-reversals are computed at runtime instead.

Note that enabling bitrev tables significantly increases the memory usage of microfft. While it can speed up FFT computation on some systems, there are also architectures that provide dedicated bit-reversal instructions (like `RBIT` on ARMv7). On such architectures, switching on bitrev tables is usually detrimental to performance.

## Limitations

microfft has a few limitations, mostly due to its focus on speed, that might make it unsuitable for some embedded projects. You should know about these if you consider using this library:

### Memory Usage

The use of pre-computed sine and bitrev tables means that microfft has considerable requirements on read-only memory. If your chip doesn't have much flash to begin with, this can be an issue.

The amount of memory required for tables depends on the the configuration of the `maxn-*` and `bitrev-tables` features:

`maxn-*` without `bitrev-tables` with `bitrev-tables`
`maxn-4` 0 8
`maxn-8` 4 20
`maxn-16` 12 44
`maxn-32` 28 92
`maxn-64` 60 188
`maxn-128` 124 380
`maxn-256` 252 764
`maxn-512` 508 1,532
`maxn-1024` 1,020 3,068
`maxn-2048` 2,044 6,140
`maxn-4096` 4,092 12,284

In addition, the code size also increases with FFT size.

### Supported FFT Sizes

microfft only supports FFT point-sizes that are powers of two, a limitation of the Radix-2 algorithm. Additionally, the maximum supported size is currently 4096, although this limit can be increased in the future as necessary.

### `f64` Support

This library currently only supports single-precision floating-point inputs. Similarly to the FFT size limit, this is a restriction that might be lifted in the future, should the need arise.