TAS5508 datasheet(37/105 Pages) TI

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No Discontinuity +

|T1 * T2|

k1 ) O2 For ( |T1| w |T2| )

2.10.2.1 Threshold Parameter Computation

SUB_ADDRESS_ENTRY +

*64

*6.0206

+ 10.63

2.10.2.2 Offset Parameter Computation

INPUT +

DESIRED )

24.0824 dB

6.0206

INPUT +

–21 dB ) 24.0824 dB

6.0206

+ 0.51197555

+ 0.1000_0011_0001_1101_0100

+ 0x00000041886A in 25.23 format

TAS5508

8-Channel Digital Audio PWM Processor

www.ti.com

SLES091D – FEBRUARY 2004 – REVISED JULY 2009

Threshold T2 serves as the fulcrum or pivot point in the DRC transfer function. O2 defines the boost

(> 0 dB) or cut (< 0 dB) implemented by the DRC-derived gain coefficient for an rms input level of T2. If

O2 = 0 dB, the value of the derived gain coefficient is 1 (0x0080 0000 in 5.23 format). k2 is the slope of

the DRC transfer function for rms input levels above T2, and k1 is the slope of the DRC transfer function

for rms input levels below T2 (and above T1). The labeling of T2 as the fulcrum stems from the fact that

there cannot be a discontinuity in the transfer function at T2. The user can, however, set the DRC

parameters to realize a discontinuity in the transfer function at the boundary defined by T1. If no

discontinuity is desired at T1, the value for the offset term O1 must obey the following equation.

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.

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 48-bit number in 25.23 format. Therefore:

T1 = 10.63

= 0 1010.1010 0001 0100 0111 1010 111

= 0x0000 0550 A3D7 in 25.23 format

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.

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:

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TAS5508 Datasheet(PDF) 37 Page - Texas Instruments

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