Question

I’m using [y, Fs, nbits, opts] = wavread(filename) to read in a wave file. Next I plot(t, y) where t = 0:1/Fs:(length(y)-1)/Fs. I play the wav via sound(y, Fs). My question is, is there a way to have a marker of some sort displayed in the plot that is synced with audio from the wav? I am hoping to listen to the audio while observing where in the plot the corresponding sound is via a marker that moves within the plot.

Answer 1

Below is a solution with good synchronisation. This works well because the audioplayer object is initiating the callback that updates the play head location. Here is the calling script . . .

fs = 44100;
durT = 3; %seconds
durS = fs*durT; %samples
x = randn(durS, 1);

dt = 1/fs;
tAxis = dt:dt:durT;

frameRate = 25; %fps
frameT = 1/frameRate;

mag = 5;

figure;
plot(tAxis, x);
ylim([-mag mag])
xlim([0 durT])
xlabel('Time [s]')

playHeadLoc = 0;
hold on; ax = plot([playHeadLoc playHeadLoc], [-mag mag], 'r', 'LineWidth', 2);

player = audioplayer(x, fs);
myStruct.playHeadLoc = playHeadLoc;
myStruct.frameT = frameT;
myStruct.ax = ax;

set(player, 'UserData', myStruct);
set(player, 'TimerFcn', @apCallback);
set(player, 'TimerPeriod', frameT);
play(player);

…and here is the callback function that you can store in a separate file …

function src = apCallback(src, eventdata)
    myStruct = get(src, 'UserData'); %//Unwrap

    newPlayHeadLoc = ...
        myStruct.playHeadLoc + ...
        myStruct.frameT;
    set(myStruct.ax, 'Xdata', [newPlayHeadLoc newPlayHeadLoc])

    myStruct.playHeadLoc = newPlayHeadLoc;
    set(src, 'UserData', myStruct); %//Rewrap
end

I have just made the signal a noise sample that you can replace with whatever you like. It is a shame that the audioplayer object has stop and resume methods without any access to a current play head location. Fortunately, there is a user data property that allows you to cram in whatever you like.