The delay of the upmixed surround channels is separate from the delay set by the receiver to time-align the speakers. This is done deliberately by the Dolby process in order to take advantage of the Haas precedence effect.

Due to the way our hearing works, when we hear closely repeated versions of the same sound coming from multiple locations, we perceive it as only emanating from the direction of the closest source, providing the time delay between them is less than 40 milliseconds. Longer delays are perceived as discrete echoes, but for shorter delay values our ears/brain fuse them together and only use the more distant sources to give clues to the size of the space the sound is located in. We cannot distinctly hear identical events that are that closely spaced together. Dolby upmixers take advantage of the Haas effect by delaying the surround channels compared to the front, with a variable range from 15 to 35 milliseconds, so that the inevitable leakage of sound from the front into the rear channels will arrive at the listeners’ ears after the same sounds have already arrived from the front channels. This way it is less likely that crosstalk will influence the listener into believing that front channel cues have come from the surrounds, thus increasing immersion into the aural landscape of the film.

This system works very well when upmixing a matrixed stereo track into multiple channels. When the channels combine acoustically in the air, it sounds as it should. But when downmixing them again after this, which is never intended to happen, a hollow comb-filtered sound is hard to avoid. Shorter delay times, as I used for the first movie, sound rather worse than longer ones. The other two films are less bad in this regard because I set the surround delays longer, resulting in less phase cancellation and weirdness.