Trigo Time

$\large S = sin^{4}(\frac{\pi}{16})+sin^{4}(\frac{3\pi}{16})+sin^{4}(\frac{5\pi}{16})+sin^{4}(\frac{7\pi}{16})$ .

If $S$ can be represented in the form $\dfrac{a}{b}$ where $a$ and $b$ are co prime integers

Find $a \times b$