TAS5548 datasheet(42/118 Pages) TI2

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0.5 : 1 compression

0.5

1 : 2 expansion

→

–

INPUT

–21 dB

24.0824 dB

6.0206

0.51197555

0.1000_0011_0001_1101_0100

0x0041886A in 9.23 format

INPUT

DESIRED

24.0824 dB

6.0206

SUB_ADDRESS_ENTRY

6.0206

10.63

–

TAS5548

SLES270A – NOVEMBER 2012 – REVISED APRIL 2015

www.ti.com

7.4.16.2.1

Threshold Parameter Computation

For thresholds,

TdB = –6.0206TINPUT= –6.0206TSUB_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

7.4.16.2.2

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 I2C coefficient value entered for O1 must be:

7.4.16.2.3

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

For n : 1 compression, the slope k can be found by:

In both expansion (1 : n) and compression (n : 1), n is implied to be greater than 1. Thus, for expansion:

k = n – 1 means k > 0 for n > 1. Likewise, for compression,

means –1 < k < 0 for n > 1. Thus, it

appears that k must always lie in the range k > –1.

The DRC imposes no such restriction and k can be programmed to values as negative as –15.999. To determine

what results when such values of k are entered, it is first helpful to note that the compression and expansion

equations for k are actually the same equation. For example, a 1 : 2 expansion is also a 0.5 : 1 compression.

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