Function dds_modf 

pub unsafe extern "C" fn dds_modf(
    phase_acc: *mut u32,
    phase_rate: i32,
    scale: f32,
    phase: i32,
) -> f32

\brief Generate a floating point tone sample, with modulation. \param phase_acc A pointer to a phase accumulator value. \param phase_rate The phase increment to be applied. \param scale The scaling factor. \param phase The phase offset. \return The signal amplitude, between -1.0 and 1.0.