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Creating & managing signals

Signals are represented by array-like data types with a series (time, frequency or space domain) per column. The SampledSignal data type wraps an array and carries sampling rate as metadata with it. This allows the API to be used without having to specify sampling rate at each call. However, if a user prefers to use other array-like data types, the sampling rate may be provided as a fs keyword argument for API calls that require sampling rate.

All code examples in this manual assume you have imported SignalAnalysis and SignalAnalysis.Units:

using SignalAnalysis, SignalAnalysis.Units

The SignalAnalysis.Units package re-exports commonly used units (s, ms, Hz, kHz, etc) from Unitful.jl. In addition, a variable 𝓈 is always exported and is an alias for the s unit from SignalAnalysis.Units. This allows indexing of signals using time in seconds:

# create a 1-second signal sampled at 20 kHz
x = signal(randn(20000), 20kHz)

# get a signal segment from 0.25 to 0.5 seconds:
y = x[0.25:0.5𝓈]

Creating and wrapping signals

Signals wrapped in a SampledSignal data type may be easily created using signal():

x = signal(data, fs)

Properties such as frame rate (sampling rate), number of channels, etc may be accessed using the SignalBase API. The signals can be treated as arrays, but carry sampling rate metadata with them. While most operations infer metadata from the input signal, some operations may be unable to automatically infer the frame rate of the output signal. We provide some handy wrappers around common DSP.jl functions to aid with rate inference:

# create a 1-second signal sampled at 20 kHz
y = signal(randn(20000), 20kHz)

# use DSP.filt() to filter a signal but retainin sampling rate
y = sfilt(lpf, y)

# use DSP.filtfilt() to filter a signal but retainin sampling rate
y = sfiltfilt(lpf, y)

# use DSP.resample() to resample a signal and infer sampling rate
y = sresample(y, 2//3)

API reference

Base.Colon — Method
(:)(start::Unitful.Time, stop::Unitful.Time)

Generates a time range index for a signal.

Examples:

julia> x = signal(randn(2000), 8kHz)
julia> x[0.2𝓈:0.201𝓈]
SampledSignal @ 8000.0 Hz, 9-element Array{Float64,1}:
 -0.08671384898800058
 -0.665143340284631
 -0.3955367460364236
  1.2386430598616671
 -0.4882254309443194
 -1.080437097803303
  0.8209785486953832
  1.3477512734963886
 -0.27722340584395494
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Base.Iterators.partition — Method
partition(x::SampledSignal, n; step=n, flush=true)

Iterates over the signal x, n samples at a time, with a step size of step. If flush is enabled, the last partition may be smaller than n samples.

When applied to a multichannel signal x, each partition contains samples from all channels.

Examples:

julia> x = signal(collect(1:10), 1.0);
julia> collect(partition(x, 5))
2-element Array{SubArray{Int64,1,Array{Int64,1},Tuple{UnitRange{Int64}},true},1}:
 [1, 2, 3, 4, 5]
 [6, 7, 8, 9, 10]

julia> collect(partition(x, 5; step=2))
5-element Array{SubArray{Int64,1,Array{Int64,1},Tuple{UnitRange{Int64}},true},1}:
 [1, 2, 3, 4, 5]
 [3, 4, 5, 6, 7]
 [5, 6, 7, 8, 9]
 [7, 8, 9, 10]
 [9, 10]

julia> collect(partition(x, 5; step=2, flush=false))
3-element Array{SubArray{Int64,1,Array{Int64,1},Tuple{UnitRange{Int64}},true},1}:
 [1, 2, 3, 4, 5]
 [3, 4, 5, 6, 7]
 [5, 6, 7, 8, 9]

julia> x = signal(hcat(collect(1:10), collect(11:20)), 1.0);
julia> collect(partition(x, 5))[1]
5×2 view(::Array{Int64,2}, 1:5, :) with eltype Int64:
 1  11
 2  12
 3  13
 4  14
 5  15
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Base.Libc.time — Method
time(i, x::SampledSignal)

Gets the time of the ith sample in the signal. The index i can be a range or an array of indices.

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SignalAnalysis.analytic — Method
analytic(s)

Converts a signal to analytic representation. The conversion preserves energy, i.e., to convert back to a real signal while conserving energy, multiply by √2.

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SignalAnalysis.issamerate — Method
issamerate(x, y)

Checks if two signals have the same sampling rate. If the sampling rate of a signal is unknown (because it is not a SampledSignal), it is assumed to have the same rate as the other signal.

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SignalAnalysis.padded — Method
padded(s::AbstractArray{T, 2}, padding; delay, fill) -> Any

Generates a padded view of a signal with optional delay/advance.

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SignalAnalysis.padded — Method
padded(s::AbstractArray{T, 1}, padding; delay, fill) -> Any

Generates a padded view of a signal with optional delay/advance.

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SignalAnalysis.samerateas — Method
samerateas(x, y)

Create a signal with samples y and sampling rate same as signal x.

Examples:

julia> x = signal(randn(100), 8kHz)
julia> y = samerateas(x, randn(5))
SampledSignal @ 8000.0 Hz, 5-element Vector{Float64}:
 -0.3053704876108388
 -0.5474123820044299
 -0.6916442204609657
 -0.5185296405433826
 -0.4598263144701988
source
SignalAnalysis.samerateas — Method
samerateas(x)

Create a signal with the same sampling rate as signal x.

Examples:

julia> x = signal(randn(100), 8kHz)
julia> y = samerateas(x)(randn(5))
SampledSignal @ 8000.0 Hz, 5-element Array{Float64,1}:
 -0.08671384898800058
 -0.665143340284631
 -0.3955367460364236
 0.8209785486953832
 1.3477512734963886
source
SignalAnalysis.signal — Method
signal(fs) -> Any

Creates a curried function that takes in an array and creates a signal with sampling rate fs.

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SignalAnalysis.signal — Method
signal(
    n::Int64,
    fs
) -> MetaArrays.MetaArray{Vector{Float64}, SignalAnalysis.SamplingInfo, Float64, 1}

Creates an empty signal of length n samples, and frame rate fs.

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SignalAnalysis.signal — Method
signal(
    n::Int64,
    ch::Int64,
    fs
) -> MetaArrays.MetaArray{Matrix{Float64}, SignalAnalysis.SamplingInfo, Float64, 2}

Creates an empty signal of length n samples, ch channels, and frame rate fs.

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SignalAnalysis.signal — Method
signal(
    x::MetaArrays.MetaArray{<:Any, SignalAnalysis.SamplingInfo, T} where T,
    fs
) -> Any

Creates a signal with frame rate fs. If the original signal's frame rate is the same as fs, this method simply returns the original signal. Otherwise, it creates a new signal with the specified frame rate and data from the original signal. Do note that this method does not resample the signal.

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SignalAnalysis.signal — Method
signal(
    T::Type,
    n::Int64,
    fs
) -> MetaArrays.MetaArray{A, SignalAnalysis.SamplingInfo, _A, 1} where {A<:(Vector), _A}

Creates an empty signal of type T, length n samples, and frame rate fs.

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SignalAnalysis.signal — Method
signal(
    T::Type,
    n::Int64,
    ch::Int64,
    fs
) -> MetaArrays.MetaArray{A, SignalAnalysis.SamplingInfo, _A, 2} where {A<:(Matrix), _A}

Creates an empty signal of type T, length n samples, ch channels, and frame rate fs.

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SignalAnalysis.toframe — Method
toframe(t, s::SampledSignal)
toframe(t, fs)

Converts time to signal frame number.

Examples:

julia> x = signal(randn(2000), 8kHz);
julia> toframe(0.2𝓈, x)
1601

julia> toframe(0.2𝓈, 8kHz)
1601

julia> toframe([0.2𝓈, 0.201𝓈], x)
2-element Array{Int64,1}:
  1601
  1609

julia> toframe(0.2:0.201𝓈, x)
1601:1609

julia> toframe((0.2, 0.201), x)
1601:1609

julia> toframe((0.2, 0.201), 8000)
1601:1609

julia> toframe(0.2:0.01:0.3, x)
11-element Array{Int64,1}:
  1601
  1681
  1761
   ⋮
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