Datasheet

3−44
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. Because the DRC receives the upper 32 bits of this 48-bit word, data at the DRC is down 48 dB (8 bits × 6 dB/bit
= 48 dB) with respect to the signal level at the SAP input (TAS3103A input). The transfer function parameters for this
example are given in Table 3−7. Figure 3−28 shows the transfer function resulting from entering the I
2
C 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 because 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.