There are audible differences but its not night and day.
Dolby wrote a paper about why there are differences and they say it has nothing to do with the codecs, but instead there are other changes made to the soundtracks. (DTS hotly debates their claims.)
1. Dolby claims that they use a feature called 'diaglogue normalization' which usually reduces overall volume by 3dB. Since DTS does not use 'normalization', the DTS material may sound more dynamic to listeners.
2. Dolby claims that differences listeners notice in sound quality are down to changes made to the digital master, rather than differences in codec efficiency between DD and DTS.
3. Dolby claims differences between DD and DTS material may also be explained by remixes of the source material. Some DTS ES versions were remixed to add the extra discrete channel.
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