add_triangle_wave Subroutine

    subroutine add_triangle_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
        real(wp) :: a, x
        integer  :: i, n

        env = 1._wp     ! Default value if no envelope is passed
        tau = 1.0_wp / f
        a = (2.0_wp * Amp) / (tau/2.0_wp)

        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))

            ! Is n even or odd ?
            if (mod(n, 2) == 0) then
                x = t - n*(tau/2.0_wp) ;
                signal = a*x - Amp
            else
                x = t - n*(tau/2.0_wp) + tau/2.0_wp ;
                signal = - a*x + 3.0_wp*Amp
            end if

            tape%left(track,  i) = tape%left(track,  i) + signal * env
            tape%right(track, i) = tape%right(track, i) + signal * env
        end do
    end subroutine add_triangle_wave