IIR coefficients for peaking EQ, how to pass them to vDSP_deq22?

Question

I have these 6 coefficients for peaking EQ:

b0 = 1 + (α ⋅ A)
b1 = −2⋅ωC
b2 = 1 - (α ⋅ A)
a0 = 1 + (α / A)
a1 = −2 ⋅ ωC
a2 = 1 − (α / A)

Wit

Answer 1

A 2nd-order filter can be expressed as the following difference equation (a and b may be interchanged depending on how you've defined them):

a0.y[n] = b0.x[n] + b1.x[n-1] + b2.x[n-2] - a1.y[n-1] - a2.y[n-2]

You can divide through by a0 to get an expression for y[n], and then you only need 5 coefficients.

The documentation I found here specifies the order (see the equation that's given). It looks like the order is:

{ b0/a0, b1/a0, b2/a0, a1/a0, a2/a0 }