How do I decrease sound exponentially when a user press button stop on my player app ?

    % code
afr = dsp.AudioFileReader('track1.wav');
adw = audioDeviceWriter('SampleRate', afr.SampleRate)
app.dec =1; 

 while ~isDone(afr)              
         audioIn = afr();                       
         audioout = audioIn * app.dec;              
         adw(audioout);
 end

with for the stopbutton

    % code
    t = 0:1/441000:4;
    alpha = 0.1;
    app.dec = exp(- alpha *t);

Basically I just want to avoid the brutal stop (which give me bad harmonics) by decreasing the sound smoothly with an exponential when we press the button stop.

Thank you !

[Jan]

Do you want to load a WAV file, fade out the sound in the last 4 seconds and save the file? Did you draw the curve?

t = 0:1/441000:4;
alpha = 0.1;
dec = exp(- alpha *t);
plot(t, dec)

Are you sure that this is the wanted amplification?

Answer 1

[Walter Roberson]

Make counter and alpha shared variables. Initialize counter to 0 and alpha to 0. Enter your loop. In it,

t = counter + 1 : counter + length(audioin)
audioout = audioin .* exp(- alpha * t/44100);
counter = t(end);

And in the stop button callback, set the shared variables

counter = 0;
alpha = 0.1;

That is, the exp is done every time, but with alpha zero it has the effect of multiplying by 1. When the stop button is pressed then a nonzero alpha is put into effect leading to exponential drop off in volume.

Just make sure that you add something to tell it to stop looping when counter reaches or exceeds 4*44100 after the stop is pushed

[Bastien Lechat]

When I'm doing an error occur telling me :

Error using audioDeviceWriter/setup

"The number of input channels must be less than or equal to 255."

Do you know where it comes from ? Edit: I just had problem with matrix dimension

But I still have a problem where it's telling me:

Error using audioDeviceWriter/setup: A given audio device may only be opened once.

[Walter Roberson]

function go_button_Callback(hObject, event, handles)

    afr = dsp.AudioFileReader('track1.wav');
    adw = audioDeviceWriter('SampleRate', afr.SampleRate)
    app.dec =1; 

    stopping = false;  %shared variable
    counter = 0;    %shared variable
    alpha = 0.;     %shared variable

    set(handles.stop_button, 'Callback', @(hObject, event) stop_play_nested(hObject, event, handles), 'enable', 'on' );

    while ~isDone(afr)
      drawnow();   %give a chance for graphics interrupt              
      audioIn = afr();
      t = (counter + 1 : counter + size(audioIn, 1)).';
      audioout = audioin .* repmat( exp(- alpha * t/44100), 1, size(audioIn,2) );
      adw(audioout);
      counter = t(end);
      if stopping && counter >= 44100 * 4; break; end
    end

    set(handles.stop_button, 'enable', 'disable');
    delete(afr);
    delete(adw);

    function stop_play_nested(hObject, event, handles)
      stopping = true;  %shared variable
      counter = 0;      %shared variable
      alpha = 0.1;      %shared variable
    end   %end of stop_play_nested

end     %end of go_button_Callback