The following example uses sequence and array operators to implement a simple digital filter. The example builds on the previous WAV I/O example. A digital filter frequently needs multiply-add operators, these may be implemented using Array.map2
> let fx = [|for x in {1..5} do yield sin((float)x)|];;
val fx : float [] =
[|0.8414709848; 0.9092974268; 0.1411200081; -0.7568024953; -0.9589242747|]
> let filter = [|1.0; -1.0; 0.0; 0.0; 0.0|];;
val filter : float [] = [|1.0; -1.0; 0.0; 0.0; 0.0|]
> Array.map2 (fun i j -> i*j) fx filter;;
val it : float [] = [|0.8414709848; -0.9092974268; 0.0; 0.0; 0.0|]
Full source:
open System
open System.IO
open System.Text
// https://ccrma.stanford.edu/courses/422/projects/WaveFormat/
type WavAudioFormat = WavPCM
exception WavUnknownFormat
exception WavUnknownNumChannels
exception WavUnknownBitsPerSample
exception WavUnknownFilter
let Sample16BitLE2Float i = float(BitConverter.ToInt16((Seq.toArray i), 0))/32768.0
let Float2Sample16BitLE s = BitConverter.GetBytes(int16(s*32768.0))
let breakBy windowSize (source: seq<_>) =
seq { let arr = Array.create windowSize (Seq.nth 0 source)
let i = ref 0
use e = source.GetEnumerator()
while e.MoveNext() do
arr.[!i] <- e.Current
i := !i + 1
if !i = windowSize then
yield Array.init windowSize (fun j -> arr.[j])
i := 0
}
type WavFile() =
let mutable AudioFormat:WavAudioFormat = WavPCM
let mutable NumChannels:int = 0
let mutable SampleRate:int = 0
let mutable BitsPerSample:int = 0
let mutable Samples:seq<float> = seq [||]
member this.Read(filename:string) =
let a = File.ReadAllBytes(filename)
if a.Length < 40 then
raise(WavUnknownFormat)
if a.[ 0.. 3] <> Encoding.ASCII.GetBytes("RIFF")
|| a.[ 8..11] <> Encoding.ASCII.GetBytes("WAVE")
|| a.[12..15] <> Encoding.ASCII.GetBytes("fmt ") then
raise(WavUnknownFormat)
let ChunkSize = BitConverter.ToInt32(a.[4..7], 0)
let SubChunk1Size = BitConverter.ToInt32(a.[16..19], 0)
let AudioFormatVal = BitConverter.ToInt16(a.[20..21], 0)
AudioFormat <- match AudioFormatVal with
| 1s -> WavPCM
| _ -> raise(WavUnknownFormat)
NumChannels <- int(BitConverter.ToInt16(a.[22..23], 0))
if NumChannels < 1 || NumChannels > 2 then raise(WavUnknownNumChannels)
SampleRate <- int(BitConverter.ToInt32(a.[24..27], 0))
BitsPerSample <- int(BitConverter.ToInt16(a.[34..35], 0))
if BitsPerSample % 8 <> 0 then raise(WavUnknownBitsPerSample)
let SubChunk2Start = 12 + 8 + SubChunk1Size
if a.[SubChunk2Start..(SubChunk2Start+3)] <>
Encoding.ASCII.GetBytes("data") then
raise(WavUnknownFormat)
let SubChunk2Size = BitConverter.ToInt32(
a.[(SubChunk2Start+4)..(SubChunk2Start+7)], 0)
let data = a.[(SubChunk2Start+8)..(SubChunk2Start+8+SubChunk2Size-1)]
Samples <- match BitsPerSample with
| 16 -> data |> breakBy 2 |> Seq.map Sample16BitLE2Float
| _ -> raise(WavUnknownBitsPerSample)
member this.Write(filename:string) =
let NumSamples = Seq.length Samples
let mutable bytes = Encoding.ASCII.GetBytes "RIFF"
bytes <- Array.append bytes (BitConverter.GetBytes
(44 + (NumSamples * BitsPerSample/8)))
bytes <- Array.append bytes (Encoding.ASCII.GetBytes "WAVE")
bytes <- Array.append bytes (Encoding.ASCII.GetBytes "fmt ")
match AudioFormat with
| WavPCM ->
bytes <- Array.append bytes (BitConverter.GetBytes 16)
bytes <- Array.append bytes (BitConverter.GetBytes (int16 1))
bytes <- Array.append bytes (BitConverter.GetBytes (int16 NumChannels))
bytes <- Array.append bytes (BitConverter.GetBytes SampleRate)
bytes <- Array.append bytes (BitConverter.GetBytes
(SampleRate*NumChannels*BitsPerSample/8))
bytes <- Array.append bytes (BitConverter.GetBytes
(int16 (NumChannels*BitsPerSample/8)))
bytes <- Array.append bytes (BitConverter.GetBytes (int16 BitsPerSample))
bytes <- Array.append bytes (Encoding.ASCII.GetBytes "data")
bytes <- Array.append bytes (BitConverter.GetBytes
(NumSamples * NumChannels * BitsPerSample/8))
let data = Samples |> Seq.map Float2Sample16BitLE
|> Seq.collect (fun i -> i) |> Seq.toArray
bytes <- Array.append bytes data
File.WriteAllBytes(filename, bytes)
member this.Print():unit =
printfn "RIFF ChunkSize= ";
printf "WAVE AudioFormat=";
if AudioFormat = WavPCM then printf "PCM "
else printf "unknown "
printfn "NumChannels=%d SampleRate=%d BitsPerSample=%d "
NumChannels SampleRate BitsPerSample
let NumSamples = Seq.length Samples
printfn "%d samples, %dmin %ds %dms playtime" NumSamples
(NumSamples/SampleRate/60)
((NumSamples/SampleRate)%60)
(((NumSamples%SampleRate)*1000)/SampleRate)
member this.GetSamples() = Samples
member this.LowPass(samples) =
// Simple sliding average window filter
let window_size = 32
let xcoeff = [| for i in 1..window_size do yield 1.0/float(window_size) |]
let buffer = ref [|for i in 1..(Array.length xcoeff) do yield 0.0|]
Seq.map (fun s ->
buffer := Array.append (!buffer).[1..((Array.length !buffer)-1)] [|s|]
Array.map2 (fun x b -> x*b) xcoeff !buffer |> Array.sum
) samples
member this.Filter(filter):unit =
match filter with
| "none" -> ()
| "lowpass" -> Samples <- this.LowPass(Samples)
| _ -> raise WavUnknownFilter
let args = System.Environment.GetCommandLineArgs()
if (Array.length args) = 4 then
let filter = args.[1]
let infilename = args.[2]
let outfilename = args.[3]
let w = new WavFile()
w.Read(infilename)
w.Print()
w.Filter(filter)
w.Write(outfilename)
else
printf "wav2.exe <filter:none,sinc> <input filename.wav> <output filename.wav>\n"
Invocation:
~> fsc.exe wav2.fs Microsoft (R) F# 2.0 Compiler build 2.0.0.0 Copyright (c) Microsoft Corporation. All Rights Reserved. ~> time ./wav2.exe lowpass /usr/share/sounds/alsa/test.wav out.wav RIFF ChunkSize= WAVE AudioFormat=PCM NumChannels=2 SampleRate=44100 BitsPerSample=16 670320 samples, 0min 15s 200ms playtime real 0m4.253s user 0m4.221s sys 0m0.095s
Further reading:
# 5--199 reserved (5--99 LSB, 100--149 distro, 150--199 appl.)
# this function returns the LSB compliants status code
# see #271198
mono_binfmt_misc_status
# NOTE: rc_status knows that we called this init script with
# "status" option and adapts its messages accordingly.
rc_status -v
;;
# probe)
# ## Optional: Probe for the necessity of a reload, print out the
# ## argument to this init script which is required for a reload.
# ## Note: probe is not (yet) part of LSB (as of 1.9)
#
# test /etc/mono.binfmt_misc/mono.binfmt_misc.conf -nt /var/run/mono.binfmt_misc.pid && echo reload
# ;;
*)
echo "Usage: $0 {start|stop|status|try-restart|restart}"
exit 1
;;
esac
rc_exit
