audio.nim

# Generate and playback a sine tone

import sdl2
import sdl2/audio
import math

# Audio settings:
const Channels = 1
const RQSizeInSamples = 4096 * Channels
const BytesPerSample = 4 
let SampleRate = 44100

# What tone to generate:
let Frequence = 440      # Hz
let Volume = 0.1          # [0..1]

# Current playback position
var x = 0 

# Generate a sine wave
let c = float(SampleRate) / float(Frequence)
proc SineAmplitude(): float32 = float32(sin(float(x) / c * 2 * PI) * Volume)
proc Envelope(): float32 = max(0.0, 1 - (x/44100))
proc Synthesis(): float32 = SineAmplitude() * Envelope()

type
  Array32Part{.unchecked.} = array[0..0, float32]
  Buffer32 = ref object
    samples: int
    bytes: int
    data: ptr Array32Part

proc AllocBuffer32(samples : int): Buffer32 =
  let bytes = samples*(sizeof(float32))
  Buffer32(samples: samples, bytes : bytes, data: cast[ptr Array32Part](alloc0(bytes)))
proc DeallocBuffer32(b : Buffer32) =
  dealloc(cast[pointer](b.data))

var buffer:Buffer32
  
proc AudioCallback_Direct(userdata: pointer; stream: ptr uint8; len: cint) {.cdecl.} =
  for i in 0..buffer.samples - 1:
    cast[ptr float32](cast[int](stream) + i * BytesPerSample)[] = Synthesis()
    inc(x)
  
proc AudioCallback_OwnBuf(userdata: pointer; stream: ptr uint8; len: cint) {.cdecl.} =
  for i in 0..buffer.samples - 1:
    buffer.data[i] = Synthesis()
    inc(x)
  copyMem(stream, addr(buffer.data[0]), buffer.bytes)

# Write amplitude to own buffer, reset hardware buffer with 0, then output buffer with MixAudio() 
proc AudioCallback_Mix(userdata: pointer; stream: ptr uint8; len: cint) {.cdecl.} = 
  for i in 0..buffer.samples - 1:
      buffer.data[i] = Synthesis()
      inc(x)
  for i in 0..buffer.samples - 1:
    cast[ptr float32](cast[int](stream) + i * BytesPerSample)[] = 0.0
  MixAudio(stream, cast[ptr uint8](addr(buffer.data[0])), uint32(buffer.bytes), SDL_MIX_MAXVOLUME)

proc main() =
  # Init audio playback
  if Init(INIT_AUDIO) != SdlSuccess:
    echo("Couldn't initialize SDL\n")
    return
  var audioSpec: TAudioSpec
  audioSpec.freq = cint(SampleRate)
  audioSpec.format = AUDIO_F32 
  audioSpec.channels = 1
  audioSpec.samples = uint16(BufferSizeInSamples)
  audioSpec.padding = 0
  audioSpec.callback = AudioCallback_Mix
  audioSpec.userdata = nil
  var obtained: TAudioSpec
  if OpenAudio(addr(audioSpec), addr(obtained)) != 0:
    echo("Couldn't open audio device. " & $GetError() & "\n")
    return
  echo("frequency: ", obtained.freq)
  echo("format: ", obtained.format)
  echo("channels: ", obtained.channels)
  echo("samples: ", obtained.samples)
  echo("padding: ", obtained.padding)
  buffer = AllocBuffer32(int(obtained.samples))
  # Playback audio for 2 seconds
  PauseAudio(0)
  Delay(2000)
  DeallocBuffer32(buffer)

main()