Datasheet

O1
INPUT
–21 dB 24.0824 dB
6.0206
0.51197555
0.1000_0011_0001_1101_0100
0x0041886A in 9.23 format
O
INPUT
+
O
DESIRED
) 24.0824 dB
6.0206
T1
SUB_ADDRESS_ENTRY
+
*64
*6.0206
+ 10.63
TAS5548
www.ti.com SLES270 –NOVEMBER 2012
T1 and T2 are the threshold settings in dB, k1 is the slope for region 1, and O2 is the offset in dB at T2. If
the user chooses to select a value of O1 that does not obey the above equation, a discontinuity at T1 is
realized.
Decreasing in volume from T2, the slope k1 remains in effect until the input level T1 is reached. If, at this
input level, the offset of the transfer function curve from the 1 : 1 transfer curve does not equal O1, there
is a discontinuity at this input level as the transfer function is snapped to the offset called for by O1. If no
discontinuity is wanted, O1 and/or k1 must be adjusted so that the value of the transfer curve at input level
T1 is offset from the 1 : 1 transfer curve by the value O1. The examples that follow illustrate both
continuous and discontinuous transfer curves at T1.
Decreasing in volume from T1, starting at offset level O1, slope k0 defines the compression/expansion
activity in the lower region of the DRC transfer curve.
3.8.2.1 Threshold Parameter Computation
For thresholds,
T
dB
= –6.0206T
INPUT
= –6.0206T
SUB_ADDRESS_ENTRY
If, for example, it is desired to set T1 = –64 dB, then the subaddress entry required to set T1 to –64 dB is:
T1 is entered as a 32-bit number in 9.23 format. Therefore:
T1 = 10.63 = 0 1010.1010 0001 0100 0111 1010 111
= 0x0550 A3D7 in 9.23 format
3.8.2.2 Offset Parameter Computation
The offsets set the boost or cut applied by the DRC-derived gain coefficient at the threshold point. An
equivalent statement is that offsets represent the departure of the actual transfer function from a 1 : 1
transfer at the threshold point. Offsets are 9.23 Formatted, 32bit logarithmic numbers. They are computed
by the following equation:
Gains or boosts are represented as negative numbers; cuts or attenuations are represented as positive
numbers. For example, to achieve a boost of 21 dB at threshold T1, the I
2
C coefficient value entered for
O1 must be:
3.8.2.3 Slope Parameter Computation
In developing the equations used to determine the subaddress of the input value required to realize a
given compression or expansion within a given region of the DRC, the following convention is adopted.
DRC transfer = Input increase : Output increase
If the DRC realizes an output increase of n dB for every dB increase in the rms value of the audio into the
DRC, a 1 : n expansion is being performed. If the DRC realizes a 1-dB increase in output level for every n-
dB increase in the rms value of the audio into the DRC, an n : 1 compression is being performed.
k = n – 1
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