TAS5518 datasheet(37/103 Pages) TI

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Introduction

SLES115 — August 2004

TAS5518

1.11.2.2 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.

OINPUT +

ODESIRED ) 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:

O1INPUT +

–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.

1.11.2.3 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, 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 + 1n–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 + 1n–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

1:2Expansion 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 + 1n *1 å n + –

3 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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