TAS3103 datasheet(103/146 Pages) TI

103 / 146 page

3−43

The horizontal slope of the transfer curve above the common threshold point does not remain horizontal indefinitely.

At a point 158 dB above the common threshold point (−14.7-dB DRC input level), the transfer function has gone from

a boost of 20 dB to a cut of 138 dB. A cut of 138 dB is the maximum cut possible for a 5.23 format gain coefficient

(2−23

≥ 23 octaves × 6 dB/octave = 138 dB). Thus, at a DRC input level of −14.7 dB, minimum gain saturation has

been reached. For inputs above this saturation point, the DRC-derived gain coefficient remains constant at the

minimum gain value (2−23), and the transfer function exhibits a 1:1 transfer slope.

3.7.3.4 Example 4—Expansion/Cut/Expansion With Gain Saturation and 32-Bit Dynamic Range

Saturation

The three previous examples restricted the slope factor k to lie in the range k

≥ −1. This example illustrates the transfer

characteristic obtained using a value of k less than −1. For this example it is assumed that the net processing gain

into the DRC is 0 dB. This means that the 8-bit headroom in the 48-bit DAP processing word structure does not contain

data. Since the DRC receives the upper 32 bits of this 48-bit word, data at the DRC is down 48 dB (8 bits x 6 dB/ bit

= 48 dB) with respect to the signal level at the SAP input (TAS3103 input). The transfer function parameters for this

example are given in Table 3−7. Figure 3−26 shows the transfer function resulting from entering the I2C coefficient

values given in Table 3−7.

At the threshold point T2 (−70 dB), the transfer function is 100 dB below the 1:1 transfer slope (O2 = 100 dB). For

input levels above T2, the transfer function exhibits a 1:1.4 expansion. For input levels below T2, the transfer function

exhibits a negative slope; for every dB the input decreases, the output increases by 1 dB. At an input level 62 dB below

T2 (−132 dB), the transfer curve has risen 62 dB, for a net boost of 124 dB. The transfer curve at this input level is

24 dB above the 1:1 transfer curve. This boost value puts the DRC-derived gain coefficient into gain saturation. For

input levels below −132 dB, the gain coefficient remains constant at maximum gain and the transfer function exhibits

a 1:1 transfer slope, parallel to the 1:1 transfer curve but 24 dB above it.

At T1 (-150 dB), the transfer curve snaps back to the 1:1 transfer curve since O1 = 0 dB. The DRC gain coefficient

is no longer in gain saturation and for inputs below −150 dB, the transfer function exhibits a 1:1.5 expansion. The

change from a 1:1.5 expansion to a 1:1 transfer below −192 dB is the result of 32-bit dynamic range saturation.

TAS3103 Datasheet(PDF) 103 Page - Texas Instruments