Lua: ProteaAudio API confuse -- How to use it?

Question

Hello everyone.

Sorry for my noob question as I'm just a non-programmer trying to learn to program with Lua.

I'm so attracted with Lua since it's indeed very simple, either in size as well as in syntax.

And I decided to explore further experiment with this Brazilian born language, like playing with sound -- as I did in Python and Ruby.

So I found this ProteaAudio and tried to play the sample scripts came within package I downloaded from here.

The package comes with two sample scripts:

1. first named example.lua to play the ogg sample file (also comes within the package)
2. and another to play function generated sound named scale.lua

The first script runs just fine on my Win 7 and Ubuntu 12.04 x86 machine.

But the second script only runs on Windows and got an error when I tried to run it on Ubuntu, generating this message:

../lua52: scale.lua:13: bad argument #1 to 'soundLoop' (number expected, got nil)

stack traceback:

[C]: in function 'soundLoop'

scale.lua:13: in function 'playNote'

scale.lua:29: in main chunk

[C]: in ?

The full original source-code from scale.lua is:

-- function creating a sine wave sample:
function sampleSine(freq, duration, sampleRate)
    local data = { }
    for i = 1,duration*sampleRate do
        data[i] = math.sin( (i*freq/sampleRate)*math.pi*2)
    end
    return proAudio.sampleFromMemory(data, sampleRate)
end

-- plays a sample shifted by a number of halftones for a definable period of time
function playNote(sample, pitch, duration, volumeL, volumeR, disparity)
    local scale = 2^(pitch/12)
    local sound = proAudio.soundLoop(sample, volumeL, volumeR, disparity, scale)
    proAudio.sleep(duration)
    proAudio.soundStop(sound)
end


-- create an audio device using default parameters and exit in case of errors
require("proAudioRt")
if not proAudio.create() then os.exit(1) end

-- generate a sample:
local sample = sampleSine(440, 0.5, 88200)

-- play scale (a major):
local duration = 0.5
for i,note in ipairs({ 0, 2, 4, 5, 7, 9, 11, 12 }) do
    playNote(sample, note, duration)
end

-- cleanup
proAudio.destroy()

And since I got confused with this ProteaAudio Lua API, I really can't get why this error comes.

Please help.

Answer 1

This is actually just a guess, but...

To play a "major" scale upwards (8 notes, jumping: full full half, full full full half) the original code does:

local duration = 0.5
for i,note in ipairs({ 0, 2, 4, 5, 7, 9, 11, 12 }) do
    playNote(sample, note, duration)
end

where the sample is a handle to a pre-generated sample created by proAudio.sampleFromMemory which is returned by function sampleSine, that passed it a calculated 'table' representing a 440hz sine-wave (concert-pitch frequency for note 'A4', the first above middle 'C').
Thus playing an 'A major scale' by changing (increasing) the 'pich' (frequency) of that sample (in 8 steps=notes). That pitch-calculation is done by function playNote.

Function playNote accepts the following arguments:
sample, pitch, duration, volumeL, volumeR, disparity,
but it currently does not receive the arguments:
volumeL, volumeR, disparity (which will then be nil).

So when function playNote tries to call:
proAudio.soundLoop(sample, volumeL, volumeR, disparity, scale),
then the call will end up like:
proAudio.soundLoop(sample, nil, nil, nil, scale),
where the sample is passed on and scale is the 'playback-pitch' of that sample, as just calculated (according to specified note) by function playNote.

Your error-message states: bad argument #1 to 'soundLoop' (number expected, got nil).
Hmm, that seems consistent with what is happening (assuming that 'bad argument #1' is the second argument, in this case volumeL).

So,
you might want to try specifying some values for volumeL, volumeR, disparity like:

local duration = 0.5
local volumeL = 1.0
local volumeR = 1.0
local disparity = 0.0
for i,note in ipairs({ 0, 2, 4, 5, 7, 9, 11, 12 }) do
    playNote(sample, note, duration, volumeL, volumeR, disparity)
end

From the proteaAudio documentation one can read about soundLoop's arguments:

sample    -            A sample handle returned by a previous load() call
volumeL   - (optional) Left volume
volumeR   - (optional) Right volume
disparity - (optional) Time difference between left and right channel in seconds. 
                       Use negative values to specify a delay for the left 
                       channel, positive for the right.
pitch     - (optional) Pitch factor for playback. 0.5 corresponds to one octave 
                       below, 2.0 to one above the original sample.

If that should do the trick, then the arguments might not be so optional on Ubuntu.

Hope this helps!