TAS3103 datasheet(101/146 Pages) TI

101 / 146 page

3−41

When T1 and T2 are set equal, the following questions arise:

•

If O1

≠ O2, what roles do O1 and O2 have?

•

Which slope parameter, k0 or k1, has control of the transfer function for input levels below the common

threshold point?

•

Does k2 control the transfer function for inputs above the common threshold point?

This example addresses and answers those questions.

Table 3−6. DRC Example 3 Parameters

DRC

PARAMETER

REQUIRED (SPECIFIED) VALUE

(NET GAINSAP Input-DRC = 0 dB)

I2C COEFFICIENT VALUE

T1 and T2

−148.7 dBInput ≥ −172.7 dBDRC

−172.7/−6.0206 = 28.684849

= 0x00000E57A91F25.23 Format

−20 dB

(−20 + 24.0824)/6.0206 = 0.678072

= 0x00000056CB0F25.23 Format

10 dB

(10 + 24.0824)/6.0206 = 5.660964

= 0x000002D49A7825.23 Format

∞:1 Compression

(1/

∞) − 1 = −1 = 0xF8000005.23 Format

1:1 Transfer

(1/1) − 1 = 0 = 0x00000005.23 Format

2:1 Compression

(1/2) − 1 = −0.5 = 0xFC000005.23 Format

For this example it is assumed that a net processing gain of 24 (24 dB) is realized from the SAP input and the DRC

(which is identical to the net processing gain assumed for Example 1). The 24 gain results in reducing the 8-bit

headroom in the 48-bit DAP word to a headroom of four bits. The 32-bit data into the DRC then resides in bits 27:0,

which means that the data level into the DRC is down 24 dB with respect to the input level at the SAP. Data input into

the TAS3103 (SAP) at a level of −148.7 dB is seen as a −148.7 dB − 24 dB = −172.7-dB signal at the DRC. T1 and

T2 must be set to −172.7 dB to realize a common threshold point at an incoming signal level of −148.7 dB.

Figure 3−25 shows the transfer function resulting from entering the I2C coefficient values given in Table 3−6. At the

T1/T2 threshold, a discontinuity of 30 dB is observed. For inputs above the threshold, the transfer curve is horizontal

(infinite compression), and the horizontal line starts 20 dB above the 1:1 transfer curve at the threshold point. Thus,

for cases when T1 = T2, O2 governs the offset with regard to the starting point of the transfer curve above the common

threshold point and k2 determines the slope of the transfer curve. For inputs below the common threshold point, the

transfer curve exhibits a 2:1 compression and starts 10 dB below the 1:1 transfer curve. Thus, O1 sets the offset at

the threshold point for the transfer curve at and below the common threshold point and k0 determines the slope of

this curve. Slope parameter k1 plays no role when T1 = T2. The value of 0 (1:1 transfer) used in this example for k1

could be changed to any value and the resulting transfer function would not be altered. The change from a 2:1

compression to a 1:1 transfer at −192 dB is due to 32-bit dynamic range saturation at the DRC input.

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