Datasheet

3−36
3.7.3 DRC Compression/Expansion Implementation Examples
The following four examples illustrate the steps that must be taken to calculate the DRC compression/expansion
coefficients for a specified DRC transfer function. The first example is an expansion/compression/expansion
implementation without discontinuities in the transfer function and represents a typical application. This first example
also illustrates one of the three modes of DRC saturation—32-bit dynamic range limitation saturation. The second
example is a compression/expansion/compression implementation. There is no discontinuity at T1, and 32-bit
dynamic range saturation occurs at low volume levels into the DRC. Example 2 also illustrates another form of DRC
saturation—maximum gain saturation. Example 3 illustrates the concept of infinite compression. Also, in Example 3,
32-bit dynamic range saturation occurs at low volume levels and the third form of DRC saturation is
illustrated—minimum gain saturation. Example 4 illustrates the ability of the DRC to realize a negative slope transfer
function. This example also illustrates two of the three forms of saturation—32-bit dynamic range saturation at low
volume levels and minimum gain saturation.
CAUTION:
The examples presented all exhibit some form of DRC saturation. This is not intended
to imply that all (or most) DRC transfer implementations exhibit some form of saturation.
Most practical implementations do not exhibit saturation. The examples are chosen to
explain by example the three types of saturation that can be encountered. But the
phenomenon of saturation can also be used to advantage in that it effectively provides
a means to implement more than three zones or regions of operation. If saturation is
intended, the regions exhibiting the transfer characteristic set by k0, k1, and k2 provide
three regions, and the regions exhibiting saturation provide the additional regions of
operation.
3.7.3.1 Example 1—Expansion/Compression/Expansion Transfer Function With 32-Bit
Dynamic Range Saturation
For this example, the following transfer characteristics are chosen.
• Threshold point 2: T2 = −26 dB, O2 = 30 dB
• Threshold point 1: T1 = −101 dB, O1 = −7.5 dB
• Region 0 slope: k0 = 0.05 ≥ 1:1.05 Expansion
• Region 1 slope: k1 = −0.5 ≥ 2:1 Compression
• Region 2 slope: k2 = 0.1 ≥ 1:1.1 Expansion
The thresholds T1 and T2 are typically referenced, by the user, to the 0-dB signal level into the TAS3103A. But to
determine the equivalent threshold point at the DRC input, it is necessary to take into account the processing gain
(or loss) between the TAS3103A SAP input and the DRC. As an example, consider the processing gain structure
shown in Figure 3−24. Inputting the data below the 8-bit headroom in the 48-bit DAP word and then routing only the
upper 32 bits of the 48-bit word into the DRC, results in a 48-dB (8 bits x 6 dB/bit = 48 dB) attenuation of the signal
level into the DRC. Channel processing gain and use of the dedicated mixer into the DRC can revise this apparent
48-dB attenuation in signal level into the DRC. In Figure 3−24, the 2
4
mixer gain into the DRC, coupled with a net
channel gain of 0 dB, changes the net 48-dB attenuation of the signal level into the DRC to a net attenuation of 24
dB.
For slopes:
Region 0 = 1:1.05 Expansion ≥ k0 = 1.05 − 1 = 0.05
= 0 0000.0000 1100 1100 1100 1100 110
= 0x0066 666 in 5.23 format
Region 1 = 2:1 Compression ≥ k1 = 1/2 − 1 = -0.5
= 1 1111.1000 0000 0000 0000 0000 000
= 0xFC00 000 in 5.23 format
Region 2 = 1:1.1 Expansion ≥ k2 = 1.1 − 1 = 0.1
= 0 0000.0001 1001 1001 1001 1001 100
= 0x00CC CCC in 5.23 format