Creating a continuous tone in MATLAB whose frequency varies in real-time depending on user input - multithreading

Question

I am currently working on a graphing program in MATLAB that takes input and maps a point to x-y space using this input. However, the program should also output a continuous tone whose frequency varies depending on the location of the point.
I was able to get the tone generation done, however could not get the tone to work continuously due to the nature of the program. (Code in between tone generations) I thought I could solve this using a parfor loop with the code that alters the frequency in one iteration of the loop, and the code that generates the tone in another but cannot seem to get it due to the following error:
Warning: The temporary variable frequency will be cleared at the
beginning of each iteration of the parfor loop. Any value assigned to
it before the loop will be lost. If frequency is used before it is
assigned in the parfor loop, a runtime error will occur. See Parallel
for Loops in MATLAB, "Temporary Variables".
In multiThreadingtest at 5 Error using multiThreadingtest (line 5) Reference to a cleared variable frequency.
Caused by:
Reference to a cleared variable
frequency.
And my code:
global frequency
frequency = 100;
parfor ii=1:2
if ii==1
Fs = 1000;
nSeconds = 5;
y = 100*sin(linspace(0, nSeconds*frequency*2*pi, round(nSeconds*Fs)));
sound(y, Fs);
elseif ii==2
frequency = 100
pause(2);
frequency = 200
pause(2);
frequency = 300
pause(2);
end
end

Answer 1

The solution may not come from multithreading, but from the use of another function to output a tone(audioplayer, play, stop). 'audioplayer/play' has the ability to output sounds that overlap in time. So basically, a pseudo code would be:
get the value of the input
generate/play a corresponding 5 second tone
detect if any change in the input
if no change & elapsed time close to 5 seconds
generate/play an identical 5 second tone
if change
generate a new 5 second tone
%no overlapping
stop old
play new
%overlapping (few milliseconds)
play new
stop old
The matlab code showing the 'sound'/'play' differences.
Fs = 1000;
nSeconds = 5;
frequency = 100;
y1 = 100*sin(linspace(0, nSeconds*frequency*2*pi, round(nSeconds*Fs)));
aud1 = audioplayer(y1, Fs);
frequency = 200;
y2 = 100*sin(linspace(0, nSeconds*frequency*2*pi, round(nSeconds*Fs)));
aud2 = audioplayer(y2, Fs);
% overlapping sound impossible
sound(y1, Fs);
pause(1)
sound(y2, Fs);
% overlapping sound possible
play(aud1);
pause(1);
disp('can compute here');
play(aud2);
pause(1);
stop(aud1);
pause(1);
stop(aud2);