In deciding what audio format to encode my 4k blurays with, I ran some comparison tests between DD+ and AAC for a 5.0 system (beam + 2 x symfonisk rears). These two formats are probably the best that can be transcoded (down-mixed) for Beam compatibility which is designed for AC3 format (Dolby Digital 5.1). This was prompted after watching a movie encoded with AAC audio and being blown away with the sound. I searched for any similar tests and could not find anything this specific. The comparison was run on an LG OLED B7 connected to Beam through HDMI/ARC. I encoded the same movie with two 5.1 channel audio tracks, one in DD+ format and the other in AAC format. The movie file format was MP4 and was played off a network drive using the built-in LG ‘Photos & Video’ app. The LG TV digital audio output was set to ‘Auto’ mode, and even though the output is down-mixed for compatibility with Beam, there was a clear difference in the sounds produced by the two different audio tracks as described here:
I had previously confirmed that DD+ (960 kb) down-mixed sounds superior to straight-up AC3 DD5.1 (640 kb). The sound is much clearer and defined. For example, you can hear details such as individual low frequency rumblings from car engines or screaming giants, and clearer high-end sounds from spaceships whizzing past. However, the sound lacks beef and can be quite thin, particularly with speech.
The AAC track was noticeably thicker than DD+ down-mixed. There was less definition and a general low-frequency overtone. However, I wasn’t able to tell where the individual sounds where coming from unlike with DD+ where sounds could be clearly identified as coming from the front-placed Beam (mostly speech) or rear speakers.
While DD+ down-mixed produced excellent clarity and sound definition, it lacked that full-bodied sound that one expects in a cinema-style experience. On the other hand, AAC down-mixed produced an immersive sound that filled the room while lacking some clarity. Despite the superior clarity of DD+ down-mixed, the speaker from which a particular sound was coming from could be easily identified and thus took some of the magic away from the movie experience. After multiple tests of different movie scenes, I decided that ACC down-mixed produced a sound closer to that which I enjoy movies at. However, Beam users who prefer a clearer, defined sound are advised to encode and watch their bluray movies with a DD+ audio track that will be down-mixed. As expected, both AAC and DD+ sound better than AC3. The experience and hence conclusion here will most-likely change if a sub is added to the system to take care of the LFE channel. In this situation, DD+ down-mixed may be preferable. It is also advisable to include a lossless format audio track when encoding movies for future-proofing.
A few questions remain. Why is there a difference when both are being down-mixed to DD5.1 and what technical aspects of AAC and DD+ are responsible for the differences in sound experience? It is unknown to me if the down-mixing is occuring in the Beam or the LG TV. Any clarification or comments on this topic are greatly appreciated.