TAS5548DCAR datasheet(41/116 Pages) TI1

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No Discontinuity

|T1

T2|

O2 For ( |T1|

|T2| )

–

≥

S n(1

aa)

S n(1

ad)

–

window

S n(1

ae)

–

TAS5548

www.ti.com

SLES270A – NOVEMBER 2012 – REVISED APRIL 2015

made. For the DRC block shared by Ch1 and Ch2, the programmable parameters apply to both rms

estimators. The time window over which the rms estimation is computed can be determined by:

Care should be taken when calculating the time window for 192kHz content. Please use 96kHz as the

sampling frequency for 96kHz AND 192kHz, as the TAS5548 uses a digital decimator to do all DAP processing

at 96kHz.

ae = energy time

(1)

•

Compression/expansion coefficient computation—This DRC element converts the output of the rms estimator

to a logarithmic number, determines the region where the input resides, and then computes and outputs the

appropriate coefficient to the attack/decay element. Seven programmable parameters, T1, T2, O1, O2, k0, k1,

and k2, define the three compression/expansion regions implemented by this element.

•

Attack/decay control—This DRC element controls the transition time of changes in the coefficient computed in

the compression/expansion coefficient computation element. Four programmable parameters define the

operation of this element. Parameters ad and (1 – ad) set the decay or release time constant to be used for

volume boost (expansion). Parameters aa and (1 – aa) set the attack time constant to be used for volume

cuts. The transition time constants can be determined by:

aa = attack time

ad - decay time

(2)

7.4.16.2 Compression/Expansion Coefficient Computation Engine Parameters

Seven programmable parameters are assigned to each DRC block: two threshold parameters—T1 and T2, two

offset parameters—O1 and O2, and three slope parameters—k0, k1, and k2. The threshold parameters establish

the three regions of the DRC transfer curve, the offsets anchor the transfer curve by establishing known gain

settings at the threshold levels, and the slope parameters define whether a given region is a compression or an

expansion region.

T2 establishes the boundary between the high-volume region and the mid-volume region. T1 establishes the

boundary between the mid-volume region and the low-volume region. Both thresholds are set in logarithmic

space, and which region is active for any given rms estimator output sample is determined by the logarithmic

value of the sample.

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.

(3)

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.

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