how to connect the upper points of a sine curve by making them a curve?

i want to extract a curve like what is in red from the plot in blue how can i do that?

je veux extraire une courbe comme il est indiqué en rouge d'aprés la plot en bleu comment je peux aire ça?

[Jose Manuel Castiblanco Quintero]

Try to use find command, index the results and to use fitter curve app!

Answer 1

[Star Strider]

Use the envelope function (R2015b and later) in the Signal Processint Toolbox.

Perhaps:

envup = envelope(y);

The function has a number of options, so choose the one (probably 'peak') that gives the resullt you want.

.

[Aida Bouaziz]

it works with 'peak' but i want to select the upper part of the curve only

[Star Strider]

If you call envelope with no outputs, it will plot both the upper and lower envelopes. You may need to call it with outputs.

The envelope function allows envelopes of both the upper and lower parts of the curve. With one output, it should return only the upper envelope. If it returns the lower part of the envelope instead, choose both outputs and select the one you want.

EDIT — (7 Aug 2022 at 16:22)

Added example —

t = linspace(0,50,250);
y = sin(2*pi*t) .* cos(15*pi*t);

[envup, envlo] = envelope(y, 15, 'peak');

figure
plot(t, y)
hold on
plot(t, [envup; envlo], 'LineWidth',2)
hold off
grid
legend('Signal','Upper Envelope','Lower Envelope', 'Location','best')

.

[Aida Bouaziz]

great, it works

thnks

[Star Strider]

As always, my pleasure!