8.5.2.6 Two-Band Dynamic Range Control

The Dynamic Range Control (DRC) is a feed-forward mechanism that can be used to automatically control the audio signal amplitude or the dynamic range within specified limits. The dynamic range control is done by sensing the audio signal level using an estimate of the alpha filter energy then adjusting the gain based on the region and slope parameters that are defined. The Dynamic Range Control is shown in Figure 47.

Figure 47. Dynamic Range Control

The DRCs have seven programmable transfer function parameters each: k0, k1, k2, T1, T2, OFF1, and OFF2. The T1 and T2 parameters specify thresholds or boundaries of the three compression or expansion regions in terms of input level. The Parameters k0, k1, and k2 define the gains or slopes of curves for each of the three regions. The parameters OFF1 and OFF2 specify the offset shift relative 1:1 transfer function curve at the thresholds T1 and T2 respectively shown in Figure 48.

Figure 48. DRC Transfer Function Example Plot

The two-band dynamic range control is comprised of two DRCs that can be spilt into two bands using the BQ at the input of each band. The frequency where the two bands are spilt is referred to as the crossover frequency. The crossover frequency is the cut off frequency for the low pass filter used to create the low band and the cut off frequency for the high pass filter used to create the high band. It is inherent of parallel two-band DRC to have a hump at the crossover region due to the overlap of energy going through both bands of the DRC being summed in the two-band DRC output mixer.

Figure 49. DRC Attack and Decay

The DRC in each band is equipped with individual energy, attack, and decay time constants. The DRC time constants control the transition time of changes and decisions in the DRC gain during compression or expansion. The energy, attack, and decay time constants affect the sensitivity level of the DRC. The shorter the time constant, the more aggressive the DRC response and vice versa.