SLES257 – SEPTEMBER 2010
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The TAS5508C has a single set of loudness controls for the eight channels. In 6-channel mode, loudness
is available to the six speaker outputs and also to the line outputs. The loudness control input uses the
maximum individual master volume (V) to control the loudness that is applied to all channels. In the
192-kHz and 176.4-kHz modes, the loudness function is active only for channels 1, 2, and 8.
V
Audio In
Audio Out
Loudness
Biquad
H(z)
Loudness Function = f(V)
V
B0017-01
Figure 2-14. Loudness Compensation Functional Block Diagram
Loudness function = f(V) = G ×[2
(Log V) × LG + LO
] + O or alternatively,
Loudness function = f(V) = G × [V
LG
× 2
LO
] + O
For example, for the default values LG = –0.5, LO = 0, G = 1, and O = 0, then:
Loudness function = 1/SQRT(V), which is the recommended transfer function for loudness. So,
Audio out = (audio in) × V + H(Z) × SQRT(V). Other transfer functions are possible.
Table 2-6. Default Loudness Compensation Parameters
LOUDNESS
TERM
V
Log V
H(Z)
DESCRIPTION
USAGE
DATA
FORMAT
5.23
5.23
5.23
I
2
C
SUB-
ADDRESS
NA
NA
0x95
DEFAULT
HEX
NA
0000 0000
b
0
= 0000 D513
b
1
= 0000 0000
b
2
= 0FFF 2AED
a
1
= 00FE 5045
a
2
= 0F81 AA27
FFC0 0000
0000 0000
0000 0000
0000 0000
FLOAT
NA
0.0
b
0
= 0.006503
b
1
= 0
b
2
= –0.006503
a
1
= 1.986825
a
2
= –0.986995
–0.5
0
0
0
Max volume
Log
2
(max volume)
Loudness biquad
Gains audio
Loudness function
Controls shape of
loudness curves
LG
LO
G
O
Gain (log space)
Offset (log space)
Gain
Offset
Loudness function
Loudness function
Switch to enable
loudness (ON = 1, OFF = 0)
Provides offset
5.23
25.23
5.23
25.23
0x91
0x92
0x93
0x94
2.9.1
Loudness Example
Problem: Due to the Fletcher-Munson phenomena, we want to compensate for low-frequency attenuation
near 60 Hz. The TAS5508C provides a loudness transfer function with EQ gain = 6, EQ center frequency
= 60 Hz, and EQ bandwidth = 60 Hz.
Solution: Using Texas Instruments ALE TAS5508C DSP tool, Matlab™, or other signal-processing tool,
develop a loudness function with the parameters listed in
32
Description
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