How to convert captured frames from webcam into avi format with 20fps.

I've used this code then it is saving the video but it's not playing when I go to that folder andplay with media player:

I'll be thankful if you may help

cd ('E:\8th semester\Machine vision LAB\Talha Data for open ended lab')

cam = webcam;

writerObj=VideoWriter('MyAVI.avi');

open(writerObj);

for i=1:60

filename = sprintf('153MC%0d.png', i);

imwrite(snapshot(cam),filename);

b=imread(['153MC',num2str(i),'.png']);

writeVideo(writerObj,b);

end

close(writerObj);