TAS5705
SLOS549–JUNE 2008...................................................................................................................................................................................................... www.ti.com
AM Controls
There are three controls for the AM section.
•
•
•
AM Mode Enable—This control enables and disables sample rate conversion for 38, 44.1 and 48 kHz.
AM Search Sequence—The sequence the device will search for switching frequency that avoids interference
AM Tuned Frequency—The AM tuned frequency is encoded as four binary-encoded decimal digits, held in
two registers, representing the tuned frequency in kHz. The range of valid values is from 1999 kHz to 0500
KHz.
When the TAS5705 receives a change to any of these inputs, it
1. Performs a fast (128-step) mute
2. Performs the requested AM interference avoidance operation
a. If requested, enables/disables the AM mode
b. If requested, selects the requested AM search sequence
c. Selects and sets the appropriate SRC rate
3. Performs a fast (128 step) unmute
Loudness Function
The TAS5705 provides a direct form I biquad for loudness on the subwoofer channel. The first biquad is
contained in a gain-compensation circuit that maintains the overall system gain at 1 or less to prevent clipping at
loud volume settings. This gain compensation is shown in Figure 46.
1 if Vol £ 1/G
0 if Vol ³ 1/G + 1/Scale
1 - Scale (Vol - 1/G) otherwise
To
Output
Mux
From
Input
Mux
Loudness
Biquad (0x23)
Gain = G
Biquad (0x24)
Volume
Scale = 1/(1 - 1/G)
0 if Vol £ 1/G
1 if Vol ³ 1/G + 1/Scale
Scale (Vol - 1/G) otherwise
B0273-01
Figure 46. Biquad Gain Control Structure
Table 3. Loudness Table Example for Gain = 4, 1/G = 0.25, Scale = 1.33
Volume
0.125 0.25 0.375
0.5
0.625 0.75 0.875
1
0
1
1
1.125 1.25 1.375
1.5
0
1.625 1.75 1.875
2
0
1
1
Biquad path
Direct path
Total gain
1
0
1
1
0
1
0.833 0.666
0.166 0.333
0.5
0.5
1
0.333 0.166
0.666 0.833
0
1
1
0
1
1
0
1
1
0
1
1
0
1
1
0
1
1
1
1
1
1
1
1
40
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