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So far, the HDR system has been presented like an audio limiter. With the HDR window top following exactly the level of the loudest sound, every sound above the HDR threshold is played back at the same output level when played alone, as was displayed in Figure 30.8, “Effect of HDR threshold”. As previously described, while HDR increases the perceived dynamic range between sounds relative to each other, it makes the "absolute" dynamic range smaller. It is possible to gain some of this "absolute" dynamic range back by transforming the HDR limiter into an HDR compressor. You do this by using a finite, smaller compression ratio (it is infinite by default). The compression ratio defines how closely the HDR window follows the loudest sound, and is a function of the difference between the sound's level and the HDR threshold. Figure 30.10, “Effect of HDR ratio” illustrates the effect of the ratio. Note that with non-infinite compression ratios, loud sounds may overshoot the HDR window, and thus come out of the HDR system above 0 dBFS. Give yourself enough headroom by lowering the volume of the HDR bus and/or other busses downstream in the signal path.

Figure 30.10. Effect of HDR ratio

The sound has an input level of +24 dB (left panel), and played in HDR bus with ratio 100:1, 4:1, 2:1. The corresponding output levels of 0 dBFS, + 6 dBFS and +12 dBFS are displayed on the right side. Another sound has an input level of 0 dB and is ducked down by -24 dB, -18 dB and -12 dB. At 2:1, only half of the energy above threshold is used to drive the HDR window up, hence ducking the other sounds by -12 dB. Notice that the level difference of 24 dB is preserved all along. Also, when using smaller ratios, sounds may stand above the window top, hence above 0 dBFS, so you should reserve sufficient headroom by decreasing the volume of the bus or its parents.