Compute 'multitaper' spectral densities of time-series data

## Usage

```
multitaper_config(
data_length,
fs,
frequency_range = NULL,
time_bandwidth = 5,
num_tapers = NULL,
window_params = c(5, 1),
nfft = NA,
detrend_opt = "linear"
)
multitaper(
data,
fs,
frequency_range = NULL,
time_bandwidth = 5,
num_tapers = NULL,
window_params = c(5, 1),
nfft = NA,
detrend_opt = "linear"
)
```

## Arguments

- data_length
length of data

- fs
sampling frequency in 'Hz'

- frequency_range
frequency range to look at; length of two

- time_bandwidth
a number indicating time-half bandwidth product; i.e. the window duration times the half bandwidth of main lobe; default is

`5`

- num_tapers
number of 'DPSS' tapers to use; default is

`NULL`

and will be automatically computed from`floor(2*time_bandwidth - 1)`

- window_params
vector of two numbers; the first number is the window size in seconds; the second number if the step size; default is

`c(5, 1)`

- nfft
'NFFT' size, positive; see 'Details'

- detrend_opt
how you want to remove the trend from data window; options are

`'linear'`

(default),`'constant'`

, and`'off'`

- data
numerical vector, signal traces

## Value

`multitaper_config`

returns a list of configuration parameters
for the filters; `multitaper`

also returns the time, frequency and
corresponding spectral power.

## Details

The original source code comes from 'Prerau' Lab (see 'Github'
repository `'multitaper_toolbox'`

under user `'preraulab'`

).
The results tend to agree with their 'Python' implementation with precision
on the order of at `1E-7`

with standard deviation at most `1E-5`

.
The original copy was licensed under a Creative Commons Attribution
'NC'-'SA' 4.0 International License
(https://creativecommons.org/licenses/by-nc-sa/4.0/).

This package (`'ravetools'`

) redistributes the `multitaper`

function under minor modifications on `nfft`

. In the original copy
there is no parameter to control the exact numbers of `nfft`

, and
the `nfft`

is always the power of 2. While choosing
`nfft`

to be the power of 2 is always recommended, the modified code
allows other choices.

## Examples

```
# \donttest{
# Takes long to run
time <- seq(0, 3, by = 0.001)
x <- sin(time * 20*pi) + exp(-time^2) * cos(time * 10*pi)
res <- multitaper(
x, 1000, frequency_range = c(0,15),
time_bandwidth=1.5,
window_params=c(2,0.01)
)
image(
x = res$time,
y = res$frequency,
z = 10 * log10(res$spec),
xlab = "Time (s)",
ylab = 'Frequency (Hz)',
col = matlab_palette()
)
# }
```