# DSP.jl - digitalFilter - no matching method when using 'fs' as argument

**URL:** <https://discourse.julialang.org/t/dsp-jl-digitalfilter-no-matching-method-when-using-fs-as-argument/117130>\
**Category:** Signal and Image Processing\
**Tags:** dsp\
**Created:** [July 17, 2024, 9:00am UTC](https://discourse.julialang.org/t/dsp-jl-digitalfilter-no-matching-method-when-using-fs-as-argument/117130 "2024-07-17T09:00:22Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![funboat](https://avatars.discourse-cdn.com/v4/letter/f/45deac/32.png) [@funboat](https://discourse.julialang.org/u/funboat)\
**Post date:** [July 17, 2024, 9:00am UTC](https://discourse.julialang.org/t/dsp-jl-digitalfilter-no-matching-method-when-using-fs-as-argument/117130/1 "2024-07-17T09:00:22Z")

</div>

Hi,

Julia v1.10.4  
DSP v0.7.9

I was trying to create a digital filter using the example code, and get the following error.  
[https://github.com/JuliaDSP/DSP.jl/blob/76001f4f863f43df6b5ee48575e77f3dd3fe3941/docs/src/filters.md?plain=1#L216](https://github.com/JuliaDSP/DSP.jl/blob/76001f4f863f43df6b5ee48575e77f3dd3fe3941/docs/src/filters.md?plain=1#L216)

```julia
digitalfilter(responsetype, designmethod; fs=1000)

ERROR: MethodError: no method matching digitalfilter(::Bandpass{Float64}, ::ZeroPoleGain{:s, ComplexF64, ComplexF64, Float64}; fs::Int64)

Closest candidates are:
  digitalfilter(::FilterType, ::FilterCoefficients) got unsupported keyword argument "fs"
   @ DSP ~/.julia/packages/DSP/H0J1V/src/Filters/design.jl:461
  digitalfilter(::FilterType, ::FIRWindow) got unsupported keyword argument "fs"
   @ DSP ~/.julia/packages/DSP/H0J1V/src/Filters/design.jl:595

```

I tried running it without the ‘fs’ argument, and it works.  
`digitalfilter(responsetype, designmethod)`

I looked at the source, and it seems fine. I don’t understand what is wrong?  
[https://github.com/JuliaDSP/DSP.jl/blob/76001f4f863f43df6b5ee48575e77f3dd3fe3941/src/Filters/design.jl#L622](https://github.com/JuliaDSP/DSP.jl/blob/76001f4f863f43df6b5ee48575e77f3dd3fe3941/src/Filters/design.jl#L622)

Thanks for any help!

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<div class="post-metadata">

**Author:** ![mgkuhn](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mgkuhn/32/6276_2.png) [@mgkuhn](https://discourse.julialang.org/u/mgkuhn)\
**Post date:** [July 17, 2024, 10:06am UTC](https://discourse.julialang.org/t/dsp-jl-digitalfilter-no-matching-method-when-using-fs-as-argument/117130/2 "2024-07-17T10:06:44Z")

</div>

The `fs` keyword argument goes into the constructor of the `responsetype` object, e.g.

```julia
julia> digitalfilter(Lowpass(0.3; fs=1000), Butterworth(3))
ZeroPoleGain{:z, ComplexF64, ComplexF64, Float64}(ComplexF64[-1.0 + 0.0im, -1.0 + 0.0im, -1.0 + 0.0im], ComplexF64[0.9990566353334952 + 0.0016308813925406407im, 0.9990566353334952 - 0.0016308813925406407im, 0.9981168187068135 + 0.0im], 8.355936714097361e-10)

```

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<div class="post-metadata">

**Author:** ![funboat](https://avatars.discourse-cdn.com/v4/letter/f/45deac/32.png) [@funboat](https://discourse.julialang.org/u/funboat)\
**Post date:** [July 17, 2024, 11:09am UTC](https://discourse.julialang.org/t/dsp-jl-digitalfilter-no-matching-method-when-using-fs-as-argument/117130/3 "2024-07-17T11:09:59Z")

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Danke!

Can I ask a dumb follow up question.

The function definition specifies `fs` as an argument. So I’m wondering why the function doesn’t recognise it?

[https://github.com/JuliaDSP/DSP.jl/blob/76001f4f863f43df6b5ee48575e77f3dd3fe3941/src/Filters/design.jl#L622](https://github.com/JuliaDSP/DSP.jl/blob/76001f4f863f43df6b5ee48575e77f3dd3fe3941/src/Filters/design.jl#L622)

```julia
function digitalfilter(ftype::FilterType, proto::FIRWindow; fs::Real=2)
    coefs = firprototype(length(proto.window), ftype, fs)
    @assert length(proto.window) == length(coefs)
    out = coefs .* proto.window
    proto.scale ? rmul!(out, 1/scalefactor(out, ftype, fs)) : out
end

```

Also, I guess the example in the documentation is just a mistake, or just outdated?

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<div class="post-metadata">

**Author:** ![mgkuhn](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mgkuhn/32/6276_2.png) [@mgkuhn](https://discourse.julialang.org/u/mgkuhn)\
**Post date:** [July 17, 2024, 12:19pm UTC](https://discourse.julialang.org/t/dsp-jl-digitalfilter-no-matching-method-when-using-fs-as-argument/117130/4 "2024-07-17T12:19:22Z")

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This `fs` parameter was added only very recently ([PR #458](https://github.com/JuliaDSP/DSP.jl/pull/458), [commit 3ad3c1cc4689](https://github.com/JuliaDSP/DSP.jl/commit/3ad3c1cc46898cf4e000ff3926fefd9d2f98198b)) by Christian Gruber, to refactor how frequency scaling works across analogue and digital filter design. This is new to me as well.