TAS5504 datasheet(33/96 Pages) TI

33 / 96 page

Introduction

SLES123 − October 2004

TAS5504

Time

Decay

Attack

td +* 1 Fs

N(1 * D)

ta +* 1 Fs

N(1 * A)

Figure 1−19. TAS5504 Attack and Decay Definition

1.10.2

Compression/Expansion Coefficient Computation Engine Parameters

There are seven programmable parameters 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.

The audio input stream into the DRC must pass through DRC-dedicated programmable input mixers. These

mixers are provided to scale the 32-bit input into the DRC to account for the positioning of the audio data in

the 48-bit DAP word and the net gain or attenuation in signal level between the SAP input and the DRC. The

selection of threshold values must take the gain (attenuation) of these mixers into account. The DRC

implementation examples that follow illustrate the effect these mixers have on establishing the threshold

settings.

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 1.0 (0x00, 80, 00, 00 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.

TAS5504 датащи(PDF) 33 Page - Texas Instruments