NXP Semiconductors TFA9812 - page 17

TFA9812_2 © NXP B.V. 2009. All rights reserved.
Preliminary data sheet Rev. 02 — 22 January 2009 17 of 66
NXP Semiconductors TFA9812
BTL stereo Class-D audio amplifier with I2S input
In the above equation fcis the center frequency and fs is the sample frequency.
The definition of the quality factor is the center frequency divided by the 3 dB bandwidth,
see Equation 1. In parametric equalizers this is only valid when the gain is set very small
(−30 dB).
(1)
Each band filter can be programmed to perform a band-suppression (G < 1) or a
band-amplification (G > 1) function around the center frequency.
Each band of the TFA9812 equalizer has a second-order Regalia-Mitra all-pass filter
structure. The structure is shown in Figure 5.
The transfer function of this all-pass filter is shown in Equation 2:
(2)
A(z) is the second-order filter structure. The transfer function of A(z) is shown in
Equation 3:
(3)
The relationship between the programmable parameters K0, K1, and K2 and the filter
parameters G, ω, Q is shown in Equation 4 and Equation 5.
Use Equation 4 to calculate band suppression (G < 1) functions.
(4)
Use Equation 5 to calculate band amplification (G ≥ 1) functions.
Fig 5. Regalia filter flow-diagram
Qfc
f2f1 –
----------------- ;=
f1:2010 Af1
Afc
--------



log 3dB fcf1 >=
f2:2010 Af2
Afc
--------



log 3dB f2fc >,=
½
X(z)
A(z)
s
Y(z)
+
+
+
−
010aaa406
K0/2
Hz() 12⁄1Az()+()K02⁄1Az()–()⋅+⋅=
Az() K1K21K
1
+()Z1–Z2– +⋅⋅+
1K
21K 1
+()Z1–K1Z2– ⋅+⋅⋅+
---------------------------------------------------------------------------------
=
K0G=
K1ωcos–=
K22Q G ωsin–⋅()2Q G ωsin+⋅()⁄=G1<