add_square_wave Subroutine

    subroutine add_square_wave(tape, track, t1, t2, f, Amp, envelope)
        type(tape_recorder), intent(inout) :: tape
        integer, intent(in)  :: track
        real(wp), intent(in) :: t1, t2, f, Amp
        type(ADSR_envelope), optional, intent(in) :: envelope
        ! Period in seconds:
        real(wp) :: tau
        ! Time in seconds:
        real(wp) :: t
        real(wp) :: signal
        ! ADSR Envelope value:
        real(wp) :: env
        integer  :: i, n

        env = 1._wp     ! Default value if no envelope is passed
        tau = 1.0_wp / f

        do concurrent(i = nint(t1*RATE) : nint(t2*RATE)-1)
            t = (i - nint(t1*RATE)) * dt

            if (present(envelope)) env = envelope%get_level(t1+t, t1, t2)

            ! Number of the half-period:
            n = int(t / (tau/2.0_wp))

            ! If n is even, signal is +Amp, if odd -Amp:
            if (mod(n, 2) == 0) then
                signal = +Amp * env
            else
                signal = -Amp * env
            end if

            tape%left(track,  i) = tape%left(track,  i) + signal
            tape%right(track, i) = tape%right(track, i) + signal
        end do
    end subroutine add_square_wave