TAS3103DBT datasheet(94/146 Pages) TI

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3.7.2.2 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 25.23 formatted 48-bit logarithmic numbers. They are computed by the following equation.

INPUT +

DESIRED )

24.0824 dB

6.0206

Gains or boosts are represented as negative numbers; cuts or attenuation 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:

INPUT +

–21 dB

) 24.0824 dB

6.0206

+ 0.51197555

+ 0.1000_0011_0001_1101_0100

+ 0x00000041886A in 25.23 format

More examples of offset computations are included in the following examples.

3.7.2.3 Slope Parameter Computation

In developing the equations used to determine the subaddress 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, a n:1 compression is being performed.

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

k = n − 1

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

+ 1

n –1

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, k

+ 1

n –1 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.

0.5 Compression

å k + 1

0.5

–1

+ 1

1 : 2 Expansion

å k + 2–1 + 1

As can be seen, the same value for k is obtained either way. The ability to choose values of k less than −1 allows the

DRC to implement negative slope transfer curves within a given region. Negative slope transfer curves are usually

not associated with compression and expansion operations, but the definition of these operations can be expanded

to include negative slope transfer functions. For example, if k = −4

Compression Equation : k

+*4 + 1

n *1 å n + –

å*0.3333 : 1 compression

Expansion Equation : k

+*4 + n–1 å n + –3 å 1: *3 expansion

With k = −4, the output decreases 3 dB for every 1 dB increase in the rms value of the audio into the DRC. As the

input increases in volume, the output decreases in volume.

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