Using the 'diff' function inside the 'find' function.

Greetings,

So I am trying to identify from a wav file each time a new note is struck. looking at the 2nd derivative of it when there is a rapid change in magnitude i assume a new note is played. Now i need to write a function that says something like:

for k = 1:1:length(d2x);

if diff(d2x(k,1) and d2x((k+1),1)) > 'some-value'

store it.

end.

so it cycles through the matrix and references everytime the difference between two neighbours is bigger than a value i will decide on.

Could somebody help me actually code this idea, my matlab is getting better but im at a real loss with this.

Answer 1

[Jan]

Without a loop:

index = find(diff(d2x) > 23.74);

Answer 2

[Carlos]

Her is my guess. Why don´t you just store k in a vector? I mean something like

aux= zeros(length(d2x));
aux2=1;
for k = 1:1:length(d2x)

if diff(d2x(k,1) and d2x((k+1),1)) > 'some-value'
aux(aux2)=k;
aux2=aux2+1;
end
end

By doing this, in your vector aux you would have the indices of the positions where you detect the change.

[David Mehr]

my issue isnt simply storing it,

the line: "if diff(d2x(k,1) and d2x((k+1),1)) > 'some-value' "

Is just some pseudo code for what im trying to solve. I do however like the idea of storing the location WITH the value, deffinately something i will do.

So this doesnt answer what im trying to do fully, but it is of some help so thank you.

[Carlos]

So you want to compare d2x(k,1) and d2x((k+1),1)? If I am undersatnding well,just do:

   if (d2x(k,1)-d2x((k+1),1))> 'some-value'