alsa-record-example.rb

#!/usr/bin/env ruby

# A Minimal Capture Program
# using alsa via ruby-ffi
#
# This program opens an audio interface for capture, configures it for
# stereo, 16 bit, 44.1kHz, interleaved conventional read/write
# access. Then its reads a chunk of random data from it, and exits. It
# isn't meant to be a real program.
#
# Based on C example of Paul David's tutorial : http://equalarea.com/paul/alsa-audio.html

require 'rubygems'
# Problem with 0.3.5
gem 'ffi', '= 0.3.4'
require 'ffi'

include FFI

def try_to(message, &block)
  puts message
  if (err = yield) < 0
    STDERR.puts "cannot #{message} (#{ALSA::strerror(err)})"
    exit 1
  end
end

module ALSA
  extend FFI::Library
  ffi_lib "libasound.so"

  attach_function :strerror, :snd_strerror, [:int], :string

  module PCM
    extend FFI::Library
    ffi_lib "libasound.so"

    STREAM_CAPTURE = 1
    BLOCK = 0
    attach_function :open, :snd_pcm_open, [:pointer, :string, :int, :int], :int
    attach_function :prepare, :snd_pcm_prepare, [ :pointer ], :int
    attach_function :close, :snd_pcm_close, [:pointer], :int

    attach_function :readi, :snd_pcm_readi, [ :pointer, :pointer, :ulong ], :long

    attach_function :hw_params_malloc, :snd_pcm_hw_params_malloc, [:pointer], :int
    attach_function :hw_params_free, :snd_pcm_hw_params_free, [:pointer], :int

    attach_function :hw_params, :snd_pcm_hw_params, [ :pointer, :pointer ], :int
    attach_function :hw_params_any, :snd_pcm_hw_params_any, [:pointer, :pointer], :int

    ACCESS_RW_INTERLEAVED = 3
    attach_function :hw_params_set_access, :snd_pcm_hw_params_set_access, [ :pointer, :pointer, :int ], :int

    FORMAT_S16_LE = 2
    attach_function :hw_params_set_format, :snd_pcm_hw_params_set_format, [ :pointer, :pointer, :int ], :int
    attach_function :hw_params_set_rate_near, :snd_pcm_hw_params_set_rate_near, [ :pointer, :pointer, :pointer, :pointer ], :int
    attach_function :hw_params_set_channels, :snd_pcm_hw_params_set_channels, [ :pointer, :pointer, :uint ], :int
    attach_function :hw_params_set_periods, :snd_pcm_hw_params_set_periods, [ :pointer, :pointer, :uint, :int ], :int

    attach_function :format_size, :snd_pcm_format_size, [ :int, :uint ], :int
  end
end

alsa_device = (ARGV.first or "hw:0")
capture_handle = MemoryPointer.new :pointer
hw_params = MemoryPointer.new :pointer
format = ALSA::PCM::FORMAT_S16_LE

try_to "open audio device #{alsa_device}" do
  ALSA::PCM::open capture_handle, alsa_device, ALSA::PCM::STREAM_CAPTURE, ALSA::PCM::BLOCK
end

capture_handle = capture_handle.read_pointer

try_to "allocate hardware parameter structure" do
  ALSA::PCM::hw_params_malloc hw_params
end

hw_params = hw_params.read_pointer

try_to "initialize hardware parameter structure" do
  ALSA::PCM::hw_params_any capture_handle, hw_params
end

try_to "set access type" do
  ALSA::PCM::hw_params_set_access capture_handle, hw_params, ALSA::PCM::ACCESS_RW_INTERLEAVED
end

try_to "set sample format" do
  ALSA::PCM::hw_params_set_format capture_handle, hw_params, format
end

try_to "set sample rate" do
  rate = MemoryPointer.new(:int) { |p| p.write_int(44100) }
  dir = MemoryPointer.new(:int) { |p| p.write_int(0) }

  ALSA::PCM::hw_params_set_rate_near capture_handle, hw_params, rate, dir
end

try_to "set channel count" do
  ALSA::PCM::hw_params_set_channels capture_handle, hw_params, 2
end

try_to "set hw parameters" do
  ALSA::PCM::hw_params capture_handle, hw_params
end

try_to "unallocate hw_params" do
  ALSA::PCM::hw_params_free hw_params
end

try_to "prepare audio interface to use" do
  ALSA::PCM::prepare capture_handle
end

frame_count = 128
MemoryPointer.new(ALSA::PCM::format_size(format, frame_count) * 2) do |buffer|
  10.times do 
    try_to "read from audio interface" do
      ALSA::PCM::readi(capture_handle, buffer, frame_count)
    end
  end
end

try_to "close audio device" do
  ALSA::PCM::close capture_handle
end