I'm trying to make a simple synthesizer with PureData.
but I came up to the point where I heard the sound on a MIDI keyboard. But legato is not smooth. How can you be able to legato smoothly?
No Smoothly Synth.pd

I think this should work for what you want
Smoothly Synth.pd
I think the issue you were having was that old notes you release would send the [line~] a 0. This way, every new note is checked if it is an on or off message. If it is an on message, it is sent to the [line~] and a new note is played. If it is an off message, it checks to see if the note is the same as the last on message. If it is, then it sends a 0 to [line]. If you want a monophonic behavior that remembers what notes are held down before you will need something more sophisticated though (like a FIFO stack of some kind).

I also changed [/~ 100] to [* .001] before the envelope because it's slightly more efficient.
It might make it sound more legato if you put a short [line~] before your [osc~] also

@seb-harmonik.ar
Thank You.