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Like the threshold of a typical audio limiter/compressor, the HDR threshold defines the minimum input level above which the HDR window may slide (refer to the yellow line in Figure 29.7, “HDR overview, as seen on the input side of the system”), or in other words, the minimum level above which the compressor "kicks in". When only soft sounds are playing, it directly impacts their level at the output: the farther away they are from the window top, the lower their output level is. When sounds play above this threshold, then softer sounds are ducked down automatically. It is important to understand that with an infinite compression ratio (we will discuss about the ratio later), any two sounds whose input level is above the HDR threshold will play back at the same output level when played alone, regardless of their input level as shown in Figure 29.8, “Effect of HDR threshold”.

Figure 29.8. Effect of HDR threshold

On the figure above, the left side shows input; the right side shows output. The first time, the sound is set at -21 dB with a threshold of 0 dB, and comes out at -21 dBFS. The second time, the threshold is at -12 dB and the sound comes out at -9 dBFS. The third time, the sound is above the threshold of -24 dB, and thus drives the window and comes out at 0 dBFS. The sound is replayed at an input level of 0 dB, even higher above threshold, and still comes out at 0 dBFS